Bose Evaluation

IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 28, NO. 2, MARCHJAPRIL 1992 403

Evaluation of Modern Power Semiconductor Devices and Future Trends of Converters

Bimal K. Bose, Fellow, ZEEE

Abstract-Power semiconductor devices that constitute the heart of modern power electronics have been undergoing dy- namic evolution in recent years. Never before in the history of power semiconductor devices have we seen the emergence of so many exotic devices in such a short span of time. This paper reviews the modern power semiconductor devices that appeared in 1980s, i.e., the insulated gate bipolar transistor (IGBT), static induction transistor (SIT), static induction thyristor (SITH), and the recently introduced MOS-controlled thyristor (MCT). The characteristics of these devices have been discussed and compared from the viewpoint of power electronics applications. Although the IGBT is well known, the power electronics community is somewhat unfamiliar with the latter three devices. For completeness, a brief review of other power devices, such as the thyristor, triac, gate turn-off thyristor (GTO), bipolar transistor (BJT), and power MOSFET has also been incorporated. Finally, a perspective of future converter trends has been outlined.

I. INTRODUCTION OWER electronics is often said to have brought in the P second electronics revolution. The first electronics revo-

lution made the modern microelectronics ICs available. At the root of both revolutions was the historic invention of the transistor by Bardeen, Brattain, and Shockley in 1948. Dur- ing the recent years, we have seen widespread application of power electronics in industrial, commercial, residential, aerospace, and military applications. As the size and cost of power electronics decrease along with the improvement of performance and reliability, power electronics applications will spread practically everywhere. It has been projected that by the early twenty-first century, 60% of electrical power in the United States will flow through power electronics, and it will eventually extend to nearly 100% in the future.

Power semiconductor device is the heart of modern power electronics. In the general classification of electronics, i.e., signal electronics and power electronics, the area of power electronics incorporates not only switching mode power conversion and control but also includes linear mode class A and class B power amplifiers. A power semiconductor device is indeed the most complex, delicate, and fragile element in a converter. A power electronics engineer needs to under- stand the device thoroughly for efficient, reliable, and cost-

Paper IPCSD 91-96, approved by the Industrial Power Converter Commi- tee of the IEEE Industry Applications Society for presentation at the 1989 Industry Applications Society Annual Meeting, San Diego, CA, October

The author is with the Department of Electrical Engineering, University of Tennessee, Knoxville, TN 37996-2100 and is the Chief Scientist of the Power Electronics Applications Center, Knoxville.

1-5.

IEEE Log Number 9104081.

effective design of converter. Although the cost of power semiconductor device in a typical power electronics equipment may not exceed typically 20 to 30%, the total equipment cost is highly influenced by the price and performance of the power devices. One important trend in power electronics is that the cost of silicon-based power and control devices is continuously falling along with the improvement of performance, whereas the same for passive circuit components, such as inductor, capacitor, transformer etc. are essentially constant, and, in fact, the price is gradually increasing. Again, the bulk of size and cost of a power electronics equipment is due to passive components. Power electronics engineers are therefore searching for silicon solution of passive components. A good example is the use of resonant and quasi-resonant link principles in the modem switching mode power supplies. The advent of high-power high- frequency devices at economical price will eventually permit application of these techniques to high-power applications, such as motor drives, UPS systems, and active power line conditioners.

The age of modern power electronics began by the invention of thyristor or silicon controlled rectifier by Bell Labora- tory in 1956, and it was later commercially introduced by General Electric in 1958. Since then, we have seen the gradual emergence of other power semiconductor devices. Historically, the evolution of power electronics has generally followed the evolution of power semiconductor devices, although it is true that some generic converter topologies have been in existence from the early gas tube age. The advent of a new type of device, or quantum improvement of performance of an existing device, did create surge of R&D activities in power electronics. Fortunately, power electronics systems today incorporate power semiconductor devices as well as microelectronics ICs, both of which are digital in nature (one provides the muscle and the other gives the intelligence). The ultimate goal is to put all the silicon on the same chip.

The researchers in solid-state electronics have worked relentlessly for a long period of time to improve semiconductor processing, device fabrication, and packaging techniques, and, as a result, todays high-density high-performance high-reliability high-yield microelectronics are being available at such an economical price. All of these technologies have been extremely useful for the evolution of power semiconductor devices. Power electronics, or power device technology, would have been stalled in the primitive stage if it did not get the spinoff benefits from the solid-state research focused for todays very large-scale integration (VLSI) elec-

003-9994/92$03.00 O 1992 IEEE

IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 28, NO. 2, MARCHIAPRIL 1992 404

tronics. In fact, it will be shown later that the latest power semiconductor device (MOS-controlled thyristor, MCT) is basically a cluster of large number of microcells in parallel.

Power electronics engineers have always dreamed of using ideal switching devices in converters. Such devices should have large voltage and current ratings, zero conduction drop, zero leakage current in blocking condition, high temperature and radiation withstand capability, high mean time between failures (MTBF), and instant turn-on and turn-off characteristics. Of course, with all these ideal features, the device should be available at economical price. This dream will never materialize, but historically we have moved step by step in that direction. Let us now discuss qualitatively the benefits we get by improving the device parameters. High voltage and current ratings permit device applications in large motor drives, high-voltage (HVDC) converters, static VAR compensators (SVCs), etc., without series-parallel combination. Matching devices for series-parallel operation is always a difficult problem. The voltage rating of a power device is generally cheaper than current rating for a specified power requirement of a converter, but these parameters are normally determined by the load and source constraints. Low- conduction drop and small leakage current contribute to high efficiency of a converter, and thus the cooling requirement is small. This consideration is often more important than the energy saving aspect, especially for computer and aerospace power electronics. Again, higher junction temperature reduces the heatsink size and therefore contributes to lower size and cost of the converter. Military and aerospace applications are always looking for high-temperature power and signal electronics. The junction temperature is often derated in favor of reliability improvement of a converter. The switching speed is perhaps the most important property of a power semiconductor device. High-speed, i.e., high-frequency , devices permit size, cost, and performance improvement of the total power electronics system. At each turn-on and turn-off, a pulse of energy is dissipated in the device and therefore contributes to high average power dissipation at high switching frequency. Use of a snubber reduces the device switching loss, but the total switching loss may be increased. More- over, snubbers add to size, cost, and voltage overshoot penalties. Higher switching speed of a device gives lower switching loss and, consequently, the snubber size can be reduced. Often snubberless operation is satisfactory within the constraints of second breakdown effect and/or safe junction temperature. The device switching loss can be practically eliminated by zero voltage (ZVS) or zero current switching (ZCS) in a resonant or quasi-resonant converter. This class of converters therefore permits higher efficiency, reduced heatsink size, improved reliability, and snubberless operation. The advent of high-frequency self-controlled power devices is creating a tremendous impact on modern converter technology. The transition from the conventional phase-control (or linear)-to-PWM-to-resonant-link conversion technology has already been evident in the low end of converter applications, such as switching mode power supply. This trend is expected to continue for medium- to high-power applications also in future.

11. CONVENTIONAL POWER DEVICES

In this paper, power semiconductor devices have been grouped into two categories: the old or conventional devices that appeared before 1980, i.e., thyristor, GTO, triac, BJT, and power MOSFET and the second category of modern devices that appeared in 1980s, i.e., IGBT, SIT, SITH, and MCT. Although IGBT is well known and should have been strictly under the conventional category, the latter three devices are practically unknown to the professional community. The power diode which is universally used is excluded from the discussion.

A . Thyristor The modem age of power electronics began by the intro-

duction of thyristor slightly more than three decades ago, and it reigned almost supreme for the first two decades. Since its introduction, the thyristor has been widely applied in phase- controlled and chopper-fed dc drives, power supplies in electrochemical processes, lighting and heating control, weld- ing control, HVDC conversion, static VAR compensation (SVC), solid-state circuit breakers, and ac machine drives.

A thyristor is basically a three-junction pnpn device where pnp- and npn-component transistors are connected in regenerative feedback mode. The device can be triggered into conduction by a short gate current pulse, but once the device is conducting, the gate loses its control to turn off the device. The thyristor basically has two classifications: the slow-speed phase-control type that is commutated by ac line voltage (line commutation), and the fast inverter type that is commutated by the transient of a resonant circuit (forced commutation). The speed of the inverter-type thyristor is enhanced in asym- metrical (ASCR) and reverse conduction (RCT) devices where reverse voltage blocking capability is intentionally sup- pressed. A forward voltage-biased device can spuriously be turned on by excessive dv / dt-generated displacement current or junction temperature (T,)-generated leakage current. Shorted emitter geometry can improve these effects considerably. A conducting device carries current almost with uniform density, and the inner p and n regions become statu- rated with minority carriers. The device can regain the voltage blocking capability after clearing these minority carriers by the process of recovery and recombination. The recovery is enhanced by voltage reversal across the device, but the recombination effect is influenced by the life time of minority carriers. The lifetime can be reduced by gold/platinum doping or electron/proton irradiation, but the process adversely affects the conduction drop. The turn-on time of a device is limited by the di / dt effect, which can be improved by interdigitated gate-emitter construction. The junction temperature T, is a very crucial parameter that is usually limited to 125C in normal operating condition. The low thermal capacitance of the junction causes wide fluctua- tion of junction temperature. Within the constraint of the device can carry large current by improved cooling or at short duty cycle. For a temporary fault condition, the T,,,, can be permitted to exceed far above 125C. Since its introduction, the power ratings and characteristics of thyris-

BOSE: EVALUATION OF MODERN POWER SEMICONDUCTOR DEVICES 405

tors have continually improved over a long number of years. For example, the modern light-triggered thyristors are available with ratings up to 6000 V, 3500 A.

B. Triac A triac is essentially an integration of a pair of phase-con-

trolled thyristors connected in inverse-parallel on the same chip. The three-terminal device can be triggered into conduction in both positive and negative half cycles of supply voltage by applying positive and negative gate trigger pulses, respectively. A triac is less expensive than a pair of inverse- parallel thyristors, and the gate control circuit is somewhat simpler. However, there are a few disadvantages because of complex integration of two devices in a chip. The gate current sensitivity of a triac is poor and the turn-off time is longer due to storage charge effect. For the same reason, the reapplied du/dt rating is lower, thus making it difficult to apply with inductive load. Triac is used in the control of incandescent lamp dimming, heating, appliance-type motor drives, and solid-state relays with a supply frequency up to 400 Hz. The state-of-the-art devices are available with ratings up to 800 V, 40 A.

C. GTO A gate turn-off thyristor (GTO), as the name indicates, is

basically a thyristor-type device that can be turned on by a positive gate current pulse but, in addition, has the capability of being turned off by a negative gate current pulse. The turn-off capability of a GTO is due to highly interdigitated gate-emitter geometry that permits diversion of pnp collector current by the gate and thus break the pnp-npn regenerative feedback effect. Historically, GTO was introduced slightly after the thyristor, but the modern high-power GTOs with improved characteristics could be possible due to pioneering work of several Japanese corporations. GTOs are available with asymmetric and symmetric voltage blocking capabilities, but common GTO application is in voltage-fed converters that use asymmetric devices. A GTO has poor turn-off current gain (typically 4 or 3, and a 2000-A peak current device may need as high as 500 A negative gate current pulse. However, the energy associated with the gate current and the corresponding average power is small and can easily be absorbed by power MOSFET. The turn-off phenomena of a GTO is somewhat complex and can be explained as follows. As the anode current begins to fall sharply by negative gate current, an anode spike voltage is introduced due to finite snubber circuit leakage inductance. This spike is extremely harmful because current concentration may create hot spots, causing second breakdown failure. Moreover, during reapplied du / dt and minority carrier recombination, the anode circuit shows a long tail current that can cause large switching loss. Therefore, a well-designed snubber with large capacitor is necessary. Because of the large switching loss, the PWM frequency is usually limited within 1 to 2 KHz. In spite of these disadvantages, GTOs have practically replaced inverter-type thyristors in force-commutated voltage-fed converter applications because of overall advantages of reduced size and cost and improved efficiency. The GTOs are now

popular in ac machine drives, UPS systems, static VAR compensators, and photovoltaic and fuel cell inverters between a few hundred kilowatts to several megawatts, and this boundary is continuously improving. The state-of-the-art devices are available up to 4500 V, 3000 A ratings.

D. BJT A bipolar junction transistor (BJT), unlike thyristorlike

devices, is a continuously current-controlled bipolar two- junction device. Since the 1970s, the power ratings and characteristics of BJTs have improved dramatically, and these devices have found increasing popularity in industrial applications. An npn transistor is more common than pnp transistor because of higher mobility of electrons. Again, Darlington transistors are more popular because of higher current gain, but the disadvantages are higher leakage current, higher conduction drop, and reduced switching frequency. An important property of transistor is that its current gain varies with collector current and junction temperature. The current in a device can be increased with a lower duty cycle within the constraints of peak junction temperature, wire bond melting, and second breakdown effect. During switching, the reverse-biased collector junction may show hotspot second breakdown effects that are specified by reverse-bias safe operating area (RBSOA) and forward-bias safe operating area (FBSOA). Modern device with highly interdigitated emitter-base geometry forces more uniform current distribution and therefore considerably improves second breakdown effects. Normally, a well-designed polarized snubber constrains the device operation well within the safe operating areas (SOA). The BJT switching speed is considerably faster than thyristor-type devices because excess minority carriers in the base are almost entirely removed by negative base current (for an npn transistor). Modern high- power transistors are normally comprised of multiple matched devices in parallel within a package. Power transistor applications in industry range from a few kilowatts to several hundred kilowatts size in voltage-fed choppers and inverters with switching frequency up to 10 to 15 kHz. The state-of- the-art modules are available with ratings up to 1200 V, 800 A.

E. Power MOSFET A power MOSFET is a unipolar, majority carrier, zero

junction, voltage-controlled device. During the last decade, the power ratings and characteristics of power MOSFETs have improved dramatically with a sharp fall in prices, and it is now a key competitor to other power devices. The n-chan- ne1 enhancement mode device is common because of higher mobility of electrons. Originally, devices with surface groove technology, known as V-groove MOS (VMOS), were used but today planar diffised metal oxide semiconductor (DMOS) structure is very common. Because it is a voltage-controlled device, the gate circuit impedance is extremely high. How- ever, during fast turn-on and turn-off, the gate needs a current pulse to charge and discharge, respectively, the effective gate-source capacitance. Being a majority carrier device, there is no inherent delay and storage switching time as

406 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 28, NO. 2, MARCHIAPRIL 1992

that of BJT. The MOSFET devices, therefore, are extremely fast compared to other devices. The high switching speed causes low switching loss, and therefore snubber requirement is very minimal. Power MOSFETs have been used in converters with hundreds of kilohertz switching frequency. However, the device has a reverse body diode that is slow due to large storage charge. Although the body diode has full bypass-current capability, high-speed applications often re- quire bypassing this diode with external fast recovery diodes. The on-resistance of a device is a key parameter that deter- mines the conduction drop. The on-resistance increases with voltage rating (a Y2.5), making the device very lossy at high current. The resistance has a positive temperature coefficient and therefore permits easy paralleling of a large number of devices. The second breakdown effect of MOSFET is negli- gible due to this positive temperature coefficient effect. If localized heating occurs for any reason, an increase of resistance forces the current distribution to be uniform. The peak current of a device can, therefore, be increased on duty cycle basis. Power MOSFETs are generally used in high-frequency switching applications within the ratings of a few watts to a few kilowatts. The device is very popular in switching mode power supplies. The state-of-the-art modules are available with 500 V, 140 A ratings.

III. MODERN POWER DEVICES

A . IGBTs

An IGBT is basically a hybrid MOS-gated turn on/off bipolar transistor that combines the attributes of a MOSFET, BJT, and thyristor. The device is also known as a metal oxide semiconductor insulated gate transistor (MOSIGT), conductivity-modulated FET (COMFET), or gain-modulated FET (GEMFET), and was originally called insulated gate transistor (IGT) or insulated gate rectifier (IGR). Fig. 1 shows the basic structure of IGBT and Fig. 2 shows the equivalent circuit with the device symbol. The device was commercially introduced in 1983, and since then the ratings and characteristics have improved significantly. IGBT offers significant advantages over BJT and power MOSFET in medium-power (a few kilowatts to a few hundred kilowatts) medium- frequency (up to 50 kHz) power converter applications.

The device architecture seen in Fig. 1 is similar to that of a MOSFET except the n+ layer at the drain has been substi- tuted by a p+ layer at the collector. It is essentially identical to MOSFET in the processes after substrate fabrication. The device has the high-input impedance of a MOSFET but BJT-like conduction characteristics. If the gate is positive with respect to the emitter and the voltage is beyond the threshold value, an n channel is induced in the p region. This forward biases the base-emitter junction of the pnp transistor and holes are injected in the n- region. The holes cross the reverse-biased collector junction (p - n- ) and constitute the pnp transistor collector current. The minority carrier injection causes conductivity modulation of the n - region, giving significant improvement of conduction drop over that of a MOSFET. The device can be used as a power switch or in a

GATE

coLL,;ToR

I Pt

,c ;--,; N+ N+

1 P I

EMITTER Fig. 1. Basic structure of IGBT.

T C T< PNP t c

SHUNT

(b)

5 NpNp-l RESISTANCE AE

(a) Fig. 2. Equivalent circuit and device symbol: (a) Equivalent circuit; (b)

device symbol.

linear amplifier. Large emitter current flow gives drop in the lower p+ region, which forward biases the npn transistor and thus tends to cause thyristorlike latching action. The latching problem in a modern IGBT has been solved by proper p+ impurity concentration. In fact, the short-circuit current, if unprotected, will pull the device into an active mode where excessive dissipation will destroy the device. While the device is turned on by + 10 to + 15 V at the gate, it is turned off by zero gate voltage, which removes the conducting channel in the p region. In the reverse direction, the device does not have a conducting body diode (like a MOSFET) but gives blocking (5-10 V) due to a reverse-biased p+- n- junction. Therefore, in voltage-fed converter applications, an antiparallel diode is to be connected externally. The device has a higher current density compared to BJT and MOSFET and needs an approximately 30% die size of a MOSFET. The conduction drop is comparable to a BJT but is significantly lower than that of a MOSFET. The drop curve with current is slightly negative or flat but becomes positive at high current.

Fig. 3 shows the typical turn-on and turn-off characteristics of an IGBT where the gate is driven by a square voltage pulse through a series resistor. The device turns on very fast and the phenomena are similar to MOSFET, except that a slightly longer time is taken for the minority carriers to build

407 BOSE: EVALUATION OF MODERN POWER SEMICONDUCTOR DEVICES

TIME t Fig. 3. IGBT turn-on and turn-off characteristics.

up and complete the conductivity modulation effect. The turn-off process is somewhat complex and comprises of three intervals: 1) the delay time (t, , , ,), during which the gate voltage falls to a threshold level at which the collector current begins to fall, 2) the initial fall time (t , , ) , during which the gate drive circuit removes the charge from the gate-to-source capacitance, after which V,, builds up (the t f , is defined as the time during which the collector current falls from 90 to 20%), and 3) the fall time (t,,), during which excess minority carriers in n- base decay by the recombination process. This tail current in a modem IGBT has been significantly reduced by proton-irradiated minority carrier lifetime control and by adding the extra n+ buffer layer at the collector. The reduction of carrier lifetime causes the adverse effect of higher conduction drop. For example, commercial devices are available from the IXYS Corp. in two versions: the standard version with V,,,,, = 2.5 V and t f (t,, + tf,) = 2.0 p s , and the high-speed version with VcECs) = 3.0 V and t f = 0.5 p s .

An important property of IGBT compared to MOSFET is the significant reduction of input capacitance (Cis,). In addition, the ratio of gate-collector capacitance to gate-emitter capacitance is lower at least by a factor of three. This improves the Miller feedback effect during high d v / d t turn- on and turn-off. The FBSOA and RBSOA of the IGBT are thermally limited by q. and the device does not show any second breakdown phenomena. However, very high reapplied du / dt condition during turn-off induces lateral displacement current that can force the parasitic npn transistor to conduct a resulting loss of control and potential device failure. A well-designed polarized snubber should be used especially with inductive load.

An IGBT converter can use integrated gate drive circuits that are currently available. Fig. 4 shows a half-bridge

inverter using an IR2110 gate driver [16] developed by International Rectifier. The chip is a high-speed dual driver with independent floating rail high-side and fixed rail low- side-referenced output channels and can be used for both power MOSFET and IGBT circuits. The input signals are CMOS/LSTTL compatible, and the driver outputs with 2 A peak current capability to a device whose emitter voltage can be up to 500 V above the common pin. Both the high-side and low-side logic input signals are processed through Schmitt triggers. The high-side signal goes through a pulse generator to level-shifted outputs. The voltage V,, is booted by external diode and capacitor, and the resistor in series limits rate of voltage rise on the capacitor. If V,, is below the undervoltage limit, the uv detect will send a shutdown signal to both the channels.

Recently, IGBTs are widely popular in medium-power applications, such as dc and ac motor drives, UPS systems, power supplies, and drivers for solenoids, relays, and contac- tors. Although IGBTs are slightly more expensive than BJTs, lower gate drive requirements, along with smaller snubber and lower switching loss, make the IGBT converter more efficient with less size and cost. Recently, IGBT inverter induction motor drives using 15-20 kHz switching frequency are finding favor where audio noise is objection- able. It is expected that IGBTs will eventually oust BJTs in most applications. The state-of-the-art modules are available up to 600 V, 400 A or 1200 V, 300 A ratings, and these will be extended to 1200 V, 500 A in the near future [24].

B. SIT A SIT is a high-power high-frequency device and is essen-

tially the solid-state version of a triode vacuum tube. The device was proposed in the mid-1970s [21], but the power SIT in modern form was commercially introduced by Tokin Corp. of Japan in 1987. Fig. 5(a) shows the basic structure of SIT and (b) shows the device symbol. It is a short n-channel vertical device where the gate electrodes are buried with the drain and source n-type epi layers. The device is normally on-type (A-SIT), i.e., if V,, = 0, the majority carrier FET- like drift current will flow between the source and the drain, and the channel resistance will cause conduction drop in the device. If V,, is negative, the depletion layer of the reverse-biased p+ n junction will inhibit the drain current flow, and with higher bias the channel will be cut off completely. The device is almost identical to a junction field effect transistor (JFET) except vertical and buried-gate construction gives lower channel resistance causing a lower drop. Moreover, a lower gate-source channel resistance gives a lower gate-to-source negative feedback effect. In the active region, the device I-V characteristics are nonsaturating vacuum triodelike instead of vacuum pentode or JFET-like. The triodelike characteristics make the device useful both in active and switching modes. The device has been used in audio, VHF/UHF, and microwave amplifiers. The reliability, noise, and radiation hardness of SIT are claimed to be superior to MOSFET. Although the device conduction drop is lower than that of equivalent series parallel operation of MOSFETs, the excessively large drop of the device makes it

408 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 28, NO. 2, MARCH/APIUL 1992

+5v m HIGH SIDE

+5v m LOW SIDE

SWRCE

G

PASSIVATION LAYER

(b) -

DRAIN

(a) Fig. 5 . Basic structure of SIT and device symbol: (a) Basic structure of

SIT; (b) device symbol.

unsuitable for general power electronics applications unless justified by the need of a FET-like switching frequency. For example, a 1500-V 180-A (peak) SIT (TS300V-Tokin) has a channel resistance of 0.5 Q giving a 90-V conduction drop at 180 A. An equivalent thyristor or GTO drop may be around 2 V. Although conduction drop is abnormally high, the turn-on and turn-off times of the device are very low. A faster than MOSFET switching speed is possible because of a lower equivalent gate-to-source capacitance and resistance.

For the example device, the typical ton and to, are equal to 0.35 ps. All the other essential MOSFET characteristics are retained. Because it is a majority carrier device, SOAs are limited by junction temperature (no second breakdown problem). The positive temperature coefficient characteristic of channel resistance forces current equalization across the junction area, therefore permitting easy paralleling of devices. Besides a large conduction drop, the normally on characteristic is a definite disadvantage (normally off device is under development in Tokin). Normally, a negative bias holds the device off until a positive overdrive saturates the device. Japanese universities and industries have built promotional equipment using SITs. These include AM/FM trans- mitters, induction heaters, high-voltage low-current (loo0 V, 5 A) power supplies, ultrasonic generators, and linear power amplifiers. Fig. 6 shows a voltage-fed full-bridge resonant inverter circuit for induction heating and melting of iron [22].

.I.

Fig. 6. SIT resonant inverter for induction heating application [22].

The 12-KW 100-kHz inverter uses a pair of 2SK183 SITs (800 V, 60 A, Ron = 1.0 Q , ton = 0.25 ps, toE = 0.3 ps) in parallel for each branch, and efficiency up to 92% has been claimed. The gate drive circuit has been designed such that V,, = -40 V at the off condition and + 5 V at the on condition.

C. SITH A SITH or SI thyristor is a self-controlled GTO-like

on-off device that was commercially introduced by Toyo Electric Co. (Toyo Denki) of Japan in 1988. A similar device, known as a field-controlled thyristor (FCT) or field- controlled diode (FCD), were developed early by General Electric, but no attempt was made for commercial introduction.

Fig. 7 shows the basic structure of a SITH and the device symbol. It is essentially a p+nn+ diode with a buried p+ gridlike gate structure. The device structure is analogous to SIT except that a p+ layer has been added to the anode side. The on-off conditions of the device are explained by simplified geometry in Fig. 8. Similar to SIT, it is a normally on device, i.e., if the anode is positive and the gate voltage is zero, the device will behave like a diode, and anode current will flow freely. The forward biasing of the p+n junction will cause a hole injection into the n region and its conductivity


CATHODE

ANODE

(a) Fig. 7. Basic structure of SITH and device symbol: (a) Basic structure of

SITH; (b) device symbol.

will be modulated. If the gate is reverse biased with respect to the cathode, a depletion layer will block the anode current flow shown in Fig. 8(b). The device does not have reverse blocking capability due to emitter shorting (see Fig. 7(a)), which is needed for high-speed operation. Evidently, it is not a thyristorlike trigger-into-conduction device but shows somewhat SIT-like I-V characteristics with varying negative gate bias. The switching behavior of a SITH is explained in Fig. 9. If the negative gate voltage ( VG) is removed, and, in fact, the gate is made slightly positive, the device will turn on with delay time (td) and rise time ( t , ) as shown in Fig. 9(a). During turn-on, the gate circuit draws a pulse of capacitor- charging current. The turn-off behavior of a SITH is similar to that of a GTO, i.e., the negative gate current is large and a tail current flows in the anode circuit. If the gate voltage is negative, the minority carriers (holes) sweep out of the gate and help establish the depletion layer after storage time ( t , ) and fall time (t,). The residual holes in the n region escape through the gate slowly, causing a long tail time ( t t ) . For high-switching frequency operation, the tail time is reduced (with the penalty of higher conduction drop) by platinum diffusion. For example, the 1200-V 300-A (rms) device (TSI 802H-12) has a typical to, = 2.0 p s and to, = 9.0 p s (with t , = 5.9 p s ) and conduction drop vd = 4.0 V . The general comparison with GTO can be summarized as follows:

1) It is a normally on device unlike a GTO. 2) The conduction drop is higher. 3) The turn-off current gain is lower, typically 1 to 3

instead of 4 to 5 for GTO. 4) Both devices show a long tail current. 5) The switching frequency is higher. 6) The dv ld t and di ld t ratings are higher. There is no

spurious turn-on possibility by reapplied du 1 dt-induced displacement current and no plasma spreading problem as in GTO.

7) The SOA is improved.

Although a negative gate current of a SITH is large, the average gate power is small because of very low duty cycle; but it tends to increase with higher switching frequency. A general-power MOSFET, which has high-peak current capability, is well suited for this type of drive. Fig. 10 shows a typical gate-drive circuit recommended by Toyo Denki. The

GATE

(a) (b) Fig. 8. On and off conditions of basic SITH: (a) On condition; (b) off

condition.

Zyp- 300A I G , r T - VG t,+tf f t 31ps 5 9 p s

1, 2 ops

V, --'

(a) (b) Fie. 9. Turn-on and turn-off characteristics of SITH 1271: (a) Turn-on

7 - I

' SITH

ON

CONTROL SIGNAL

Fig. 10. SITH with gate driver [9].

primary logic signal (0 to + 15 V) is coupled to the gate driver through an opt0 coupler. At turn-on, a p-channel MOSFET is switched on that establishes a +5 V forward bias to the gate through a series resistance. The turn-off negative gate current is taken by a pair of n-channel MOS- FET's in parallel from the - 2 4 V supply. The gate circuit reverse voltage is clamped by a zener diode.

The SITH is an evolutionary device, and it is expected to appear with higher power ratings, symmetrical voltage blocking, and normally off characteristic in the near future. So far, these devices have been used by Japanese universities and industries for promotional applications, such as induction

410 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 28, NO. 2, MARCHIAPIUL 1992

heating, high-frequency-link dc-dc converter, active power line conditioners, and noiseless PWM inverter drives. Fig. 11 shows a simplified diagram of a SITH-based active power line conditioner where the APLC bridge absorbs the harmonics and lagging VAR generated by the rectifier load.

D. MCT An MCT, as the name indicates, is a thyristorlike trigger-

into-conduction device that can be turned on or off by a short pulse on the MOS gate. It is more of a GTO-like switching device except that the turn-off current gain is very high. An MCT is a high-power high-frequency low conduction drop- switching device. In switching speed, it is comparable to an IGBT but has lower conduction drop. At present, the device is not available commercially (at the time of this writing, it appears that Harris will commercially release 600 V/1200 V, 30 A (rms) and 600 V/1200 V, 60 A (rms) devices in 1992), but developmental devices were released by General Electric Co. (500 V/1000 V, 50 A/ lW A) and Harris Semiconductor (900 V, 15 A).

Unlike other power-switching devices (except power MOSFET), an MCT is basically a parallel connection of thousands of microcells on the same chip. For example, a 50-A 500-V device contains 100000 cells in parallel. The basic structure of a cell MCT is somewhat complex, and it is shown in Fig. 12. Fig. 13 shows the equivalent circuit and symbol of the device. It is turned on by a negative voltage pulse at the gate with respect to the anode and is turned off by a positive voltage pulse. The MCT has thyristorlike p-n-p-n layers between the anode and cathode, and the three junctions are labeled in Fig. 12. The pnp-npn regenerative feedback equivalent circuit with the gating MOSFET's are indicated in Fig. 13. The forward voltage on the anode is essentially blocked by the p wide base layer. In the reverse direction, the device has low voltage-blocking capability that is limited by the J3 junction.

If the gate of an MCT is negative with respect to anode, a p-channel is induced in the p-FET that causes forward biasing to the npn transistor. The resulting electron flow from the n+ layer forward biases the J , junction and the device eventually goes into saturation by positive feedback effect. The device turns on fast (typically 1.0 ps), and with a large number of devices in parallel the & / d t is high (typically 800 A/ps). At conduction, the n and p- layers are heavily saturated with minority carriers and the conduction drop is slightly more than a volt. In spite of complex geometry, the current density of an MCT is high compared to power MOSFET, BJT, and IGBT and therefore needs a smaller die area.

If the gate voltage is positive with respect to the anode, the induced n-channel of an n-FET will short circuit the emitter- base junction of the pnp transistor. This will break the positive feedback loop for thyristor operation and the device will turn off. The turn-off occurs purely by recombination of minority carriers in the n and p- layers with a typical storage time of 0.6 ps and fall time of 1.5 ps. The recombination- tailing effect is carefully controlled by proton irradiation so that the conduction drop remains small. The device has a

NONLINEAR AND LAGGING VAR LOAD

3+ AC LINE

'C

I I

Fig. 1 1 . Active power line conditioner using SITH [lo]

ANODE

OFF-FET DRAIN)

P- (NPN BASE, ON-FET DRAIN) I

I P BUFFER I

_ON - FET CHANNEL

_P (ON- FET SOURCE)

N+ SUBSTRATE

CATHODE Fig. 12. Basic structure of MCT.

large SOA capability, and snubberless operation within the constraint of T j may be permissible. Note that because the n-FET is a very low voltage device, its on-channel resistance is very small, and therefore emitter-base short circuiting effect of the pnp transistor is very effective. The property is important for successful operation of the device at high temperature because of the increase of channel resistance.


T A

JA -7v G U N-FET

(a)

Fig. 13. MCT equivalent circuit and device symbol: (a) Equivalent circuit; (b) device symbol.

Although an MCT is a voltage-controlled device, the gate circuit carries a short current pulse during turn-on and turn-off because of charging and discharging of the FET capacitors. However, unlike MOSFET, the input capacitances are fixed because of the absence of Miller effects. With the off n-FET normally on, the device is very insensitive to du/dt (typically 5000 V/ps) and T j triggering. Although commercial MCT is being rated for the Ti range of - 55 to + 150, it has been successfully operated in higher temperature ranges. Of course, at high temperature, the leakage current may be excessive and the turn-off current capability will be reduced (due to higher channel resistance of the off-FET). Moreover, the device reliability may be adversely affected. MCTs can be easily connected in series or parallel combination for higher power requirement. The typical parameters of a 60-A 600-V device are given in Table I.

The MCT, although a brand new device, shows tremendous possibility for widespread applications that include dc and ac motor drives, UPS systems, induction heating, dc-dc converters, active power line conditioners, etc. Its superior characteristics give evidence that it will challenge majority of the present devices, such as thyristors, GTOs, BJTs, IGBTs, and SITHs.

IV. COMPARISON OF MODERN DEVICES

The discussion on modem power semiconductor devices in Section ILI will now be summarized. Although not included in the comparison, readers should try to visualize this summary in the background of the other devices, i.e., thyristor, triac, GTO, BJT, and power MOSFET. Obviously, SITH is the largest power device and SIT is the highest frequency device in the present state of technology. Table I summarizes comparison of the devices where typical parameter values are shown. It is almost impossible to make apple to apple comparison because devices with compatible power ratings are not simply available, and the criteria for characterization may not be identical among the vendors. Besides, the devices, especially the MCT, are in the early stage of evolution and may show significant changes several years from now. The table highlights SIT as a very-high-frequency high-power device, but it has the serious problem of a large conduction drop. For this reason, it should be excluded from the majority of power electronics applications. SITH is the only cur-

TABLE I SUMMARY COMPARISON OF DEVICES (ONLY TYPICAL PARAMETERS ARE SHOWN)

IGBT SIT

1. Voltage rating (repetitive) (V)

2. Current rating (A) 3. No. of junctions in

forward path 4. Linear/trigger device 5. Voltage blocking 6. Voltage/current gating 7. Operating TJ (C) 8. Conduction drop (V,) 9. V, sensitivity with TJ

10. Tumoff current gain 11. Safe operating area 12. Reapplied d v l d f

13. Tumon d i l d f (Alps) 14. Delay time f , (ps) 15. Rise time 1, (ps) 16. to, = t d + t , ( p ~ ) 17. Storage time ts (ps) 18. Fall time f f (ps) 19. Tail time t, ( p s )

(V/CS)

20. to, = t , + t , + t , (as )

2 1. Switching frequency

22. Applications p)

600 50 (dc)

2 Linear

Asymmetric Voltage

3 .O Negative

(slightly positive) at high current)

7j limited

2000 600 0.05 0.3

0.35 0.25 0.3

- 20 to 150

-

-

0.55

50 ac motor drives

UPS systems

Static VAR and

800 60 (dc)

0 Linear

Asymmetric Voltage

70 Positive

- 50 to 150

- TJ limited Very high Very high - -

0.25 - - -

0.3

70 Induction heating

Ultrasonic generators

Harmonic Compensators Switching Mode Power AM/FM

Supplies Generators

SITH MPT

1. 2. 3. 4. 5. 6. I . 8. 9.

10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

1200 800 (peak)

1 Linear

Asymmetric Current

4.0 Negative

3 7j limited

2000 900 0.4 1.6 2.0 2.5 0.6 5.9 9.0 4.0

Induction heating Static VAR

compensation

-40 to + 125

600 60 0)

3 Trigger

Asymmetric Voltage

-55 to + 150 1.1

Negative

TJ limited 5000 800 0.6 0.4 1 .o 0.6 1.5

2.1 20

AC motor drives

-

-

UPS systems Static VAR and

rent-controlled device with very poor turn-off current gain, and is also slow compared to other devices. All devices presently have asymmetric voltage-blocking capability. Al- though IGBT, SIT, and SITH have somewhat linear characteristics in the active region, only IGBT will pull into a high

412 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 28, NO. 2, MARCHfAPRIL 1992

dissipation active mode for fault condition, whereas the other devices will remain saturated due to nearly vertical I-V characteristics. Apparently, MCT shows the best combination of conduction drop, junction temperature, dv l d t , di / d t , and the switching speed is only next to SIT.

V. CONVERTER TRENDS The recent evolution of power semiconductor technology is

already creating tremendous impact in the trend of modern power converters, and this trend is expected to be very dominant as new and improved devices appear in the market and their prices continue to fall. In the 34-year history of power semiconductors, the technological achievement in the last decade has been especially spectacular. Based on the present trends of power devices and converters, we can make realistic visualization of certain future converter trends, summarized as follows:

Voltage-fed converters using force-commutated thyristors are already obsolete. Nobody should plan building these now. This means that inverter-grade thyristors have no future. Most of the BJT converters are expected to gradually yield to IGBT converters. Power MOSFETs will remain as viable devices in low-voltage low-power high-frequency applications. Phase-controlled-type converters that now dominate utility systems are expected to be gradually replaced by PWM-type converters, and voltage-fed class appears to be of maximum promise. This will ultimately include large HVDC converters. More stringent power quality standards on utility systems will discourage harmonics and VAR loading by phase-controlled converters. As high-frequency high-power devices become cheaper, active power line conditioners will find favor princi- pally in retrofit applications. The new converter systems will be designed with a PWM rectifier in the front end, solving the power quality problems. Of course, in low-power ranges, the front-end diode rectifier-boost chopper method of power line conditioning will be favored. Phase-controlled cycloconverters are expected to be gradually replaced by dual voltage-fed PWM converters. This includes the presently popular cycloconverter- fed multimegawatt ac drives. The future of phase control thyristors which dominated so long in power electronics appears to be bleak. Force-commutated current-fed inverters (such as au- tosequential inverters, four-legged neutral-commutated inverters, etc .) are being rendered practically obsolete. Single or dual GTO current-fed PWM converters are their viable replacements. The load-commutated current-fed inverter for large wound-field synchronous machine drives will also be replaced by PWM-type converters (possibly with induction machine). SIT will dominate in very-high-frequency high-power applications where other devices cannot compete.

8) MCTs, when available commercially and well devel-

oped, are expected to heavily challenge all the devices except power MOSFETs. It appears to be the most dominating power device in the next generation power electronics.

VI. CONCLUSION The evolution of power semiconductor devices and their

general impact on modern power electronic converters was discussed in the beginning of this paper. Then, a general review of present generation power devices that includes the thyristor, triac, GTO, BJT, power MOSFET, IGBT, SIT, SITH, and MCT was given. Particularly, the last four devices (which are defined as modem devices) were highlighted in the discussion. Although the IGBT is somewhat well known, the latter three devices are practically unknown to the power electronics community. It is the objective of this paper to familiarize the readers with these modem devices in the perspective of the other existing devices. Finally, the general trend of power converters was outlined.

The paper will remain incomplete without some comments about the next generation materials for power semiconductor devices. Silicon material has enjoyed a monopoly over a long period of time, and this will possibly remain so in the near future. However, the new type of materials, such as gallium arsenide, silicon carbide, and diamond, shows tremendous promise in the future, in spite of processing difficulties. These materials are superior to silicon in carrier speed, band gap, and heat conduction properties. High-power high- frequency power MOSFET-like devices that can operate at higher temperatures with a lower conduction drop will have a far greater impact in future power electronics. In addition, the superconductive power control device based oh the Josephson effect also shows future promise. For this, it is indispensable to develop a new concept device technology such as three-terminal circuits with stress on the development of high-temperature superconductive materials and high criti- cal current density.

ACKNOWLEDGMENT

The author acknowledges the help of S. Y . Sotoudeh, who is a graduate student in electrical engineering at the Univer- sity of Tennessee, Knoxville.

REFERENCES B. K. Bose, Power Electronics and AC Drives. Englewood Cliffs, NJ: Prentice-Hall, 1986. B. R. Pelly, Power semiconductor devices-A status summary, in Proc. Int. Semiconductor Power Conv. Conf., 1982, pp. 1-19. B. J. Baliga et al., The insulated gate transistor, a new three-terminal MOS-controlled bipolar power device, IEEE Trans. Electron Devices, vol. ED-31, pp. 821-828, 1984. V. A. K. Temple, MOS-controlled thyristors, in IEDM Proc.,

J. Nishizawa, Application of the power static induction (SI) devices, in Proc. Int. PCIM (Tokyo, Japan), 1988, pp. 1-12. - , High frequency high power static induction transistor, IEEE Trans. Electron Devices, vol. ED-25, p. 314, 1978. J . Nishizawa et al., Performance trade-off for the static induction thyristor, in Proc. PCI Cony., 1987, pp. 1-14. Y. Nakamura et al., Very high speed static induction thyristor, IEEE Trans. Industry Applications, vol. IA-22, pp. 1OOO- 1006, Nov./Dec. 1986. J . Nishizawa et al., 60 KHz, 100 KW static induction (SI) thyristor

1984, pp. 282-285.


type voltage-controlled series resonant inverter for induction heating, in Proc. 18th Ann. IEEE-PESC Conf., 1987, pp. 508-515. M. Kohata et al., Compensator for harmonics and reactive power using static induction thyristors, in Proc. ENE, 1987, pp.

[I11 F. Goodenough, MOS-controlled thyristor turns off 1 MW in 2 p, Electron. Des., pp. 57-66, Nov. 10, 1988.

[12] MCT workshop proceeding sponsored by GE CR&D, GE Solid State, and PEACIEPRI, Nov. 30, 1988.

[13] V. A. K. Temple, Power device evolution and MOS-controlled thyristor, in Proc. PCIM, Nov. 1987, pp. 23-29.

[I41 -, Search for the perfect switch, in Proc. PCIM, June 1988,

[I51 B. K. Bose, Power electronics-An emerging technology, in Proc. IEEE-IECON, Oct. 1988, pp. 501 -508.

[16] S. Young, High speed high voltage IC driver for HEXFET or IGBT circuits, IR Application Note (AN-978), 1988.

1171 V. A. K. Temple, The MCT, a new class of power devices, IEEE Trans. Electron Devices, vol. ED-33, p. 1609, 1986.

[18] B. J. Baliga et al., The insulated gate transistor, IEEE Trans. Electron Devices, vol. ED-31, pp. 821-828, 1983.

[19] B. J. Baliga, Modern Power Devices. New York: Wiley, 1987. [20] SIT Handbook, Tohoku Metal Ind., 1987. [21] J. Nishizawa et al., Recent development of the power static induc-

tion transistors in Japan, in Proc. PCI87, 1987, pp. 118-132. [22] H. Ogiwara et al., Development of SIT high frequency resonant

inverter for metal melting uses, in Proc. PCI87, 1987, pp.

J. Nishizawa, New exploitation of the power semiconductor devices in Japan-Power SITS and SI thyristors, in Proc. PCI87, 1987,

[24] High Power Transistor (GTR modules), Toshiba Appl. Note NO.

[25] J. P. Russel et al., The COMFET-A new high conductance MOS-gated device, IEEE Electron Device Lett., vol. EDL-4, PP. 63-65, Mar. 1983.

[26] L. Reinehart, The use of MOSIGTs and MOSFETs in motor drive inverter circuits, in Proc. MOTOR-CON, 1987, pp. 175-183.

[27] SI Thyristor Appl. Note, Toyo Denki Seizo K. K., 1988. [28] Specification sheet of TSI 802H-12 Static Induction Thyristor, 1988-

04-22, Toyo Semicon. Co., 1988. [29] M. Stoisick et al., The MOS-GTO, a thyristor with MOS-controlled

emitter shorts, IEDM Tech. Digest, 1985. [30] J. S. Lai et al., An improved resonant dc link inverter for induction

motor drives, in Conf. Rec. IAS Ann. Mtg., 1988, pp. 742-748. [31] A. Cogan et al., Discrete semiconductor switches: Still improving,

in Proc. PCIM, 1986, pp. 15-22. [32] Fundamental Technologies for Progress in the 21st Century, Agency

of Industrial Science and Technology, MITI, Japan, vol. 46, 1987. [33] K. Shenai et al., Optimum semiconductors for high power electron-

ics, IEEE Trans. Electron Devices, vol. 36, pp. 1811-1823, Sept. 1989.

I341 Harris Semiconductor Developmental MCT Specification Sheets for MCTA15P90, June 16, 1989.

[35] T. M. Jahns et al., Circuit utilization characteristics of MOS-controlled thyristors, IEEE Trans. Industry Applications, vol. 27, pp. 589-597, May/June 1991.

[lo]

1265-1270.

pp. 324-335.

146- 155. [23]

pp. 453-464.

1987-12-01, 1987.

[36] B. K. Bose, Recent advances in power electronics, in Conf. Rec. IEEEIIECON90, 1990, pp. 829-838.

Bimal K. Bose (S59-M6O-SM78-F89) re- ceived the B. E. degree from Calcutta University, Calcutta, India, in 1956, the M.S. degree from the University of Wisconsin, Madison, in 1960, and the Ph.D. degree from Calcutta University in 1966.

He was a Member of the Faculty at Calcutta University (Bengal Engineering College), where he was awarded the Premchand Roychand Scholarship and the Mouat gold medal for outstanding research contributions. In 1971, he joined Rensselaer Poly- technic Institute, Troy, NY, as a member of the

faculty in the Electrical Engineering Department, where he was responsible for organizing the undergraduate and graduate programs in power electronics for five years. He served as a consultant for several industries, which included General Electric Research and Development Center, Bendix Corpo- ration, Lutron Electronics, and PCI Ozone Corporation. From 1976 to 1987, he was with General Electric Research and Development Center, Schenec- tady, NY. In 1987, he joined the University of Tennessee, Knoxville, as Professor of Electrical Engineering (Condra Chair of Excellence). He is also working as Chief Scientist of the Power Electronic Application Center (PEAC). His research interests are power converters, drive systems, and microcomputer-based performance optimization of power electronics and drives. He has published and presented over 90 papers and holds 16 U.S. patents. He edited the IEEE books Adjustable Speed AC Drive Systems (1981) and Microcomputer Control of Power Electronics and Drives (1987), which were sponsored by the IEEE Industry Applications Society, and contributed the article on ac drives in Systems and Control Encyclope- dia (New York: Pergamon, 1987). He also wrote the book Power Electron- ics and Drives (Englewood Cliffs, NJ: Prentice-Hall, 1986). Currently, an IEEE Press Book on Modem Power Electronics is in progress, and It is sponsored by the Industrial Electronics Society.

Dr. Bose was Chairman of the IEEE TRANSACTIONS REVIEW of the Static Power Converter Committee for eight years and is now Chairman of the same committee. He is an Associate Editor of the IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS and is Power Electronics Committee Chairman of the Industrial Electronics Society. He has served as a member on a number of national and international professional committees, which include the Power Electronics and Microcomputer Control Committees of the IEEE Industrial Electronics Society, Scientific Committee of the International Conference on Numerical Control of Electrical Machines, Program Commit- tees of IEEE International Static Power Converter Conference, Tokyo International Power Electronics Conference, and International Conference on Microcomputer Control of Small Machines. He is a member of the Editorial Board of the International Electrosoft Journal. He is listed in Whos Who in Technology, International Whos Who in Engineering, Personalities in America, Biography International, Directory of World Researchers, and Leading Consultants in Technology. The Institute of Electronics and Telecommunication Engineers, India, has established the Bimal Bose Award in Power Electronics, which is awarded annually to an Indian engineer for outstanding contributions to power electronics. He is a recipient of the GE publication award and the silver patent medal.

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