Unit1

UNIT IPower Semiconductor Devices

EE2301-POWER ELECTRONICSIntroductionWhat are Power Semiconductor Devices (PSD)? They are devices used as switches or rectifiers in power electronic circuits

What is the difference of PSD and low-power semiconductor device?Large voltage in the off stateHigh current capability in the on state

EE2301-POWER ELECTRONICS

EE2301-POWER ELECTRONICSClassificationFig. 1. The power semiconductor devices family


EE2301-POWER ELECTRONICSImportant ParametersBreakdown voltage. On-resistance. Trade-off between breakdown voltage and on-resistance.Rise and fall times for switching between on and off states. Safe-operating area.


EE2301-POWER ELECTRONICSPower MOSFET: Structure Power MOSFET has much higher current handling capability in ampere range and drain to source blocking voltage(50-100V) than other MOSFETs.Fig.2.Repetitive pattern of the cells structure in power MOSFET


EE2301-POWER ELECTRONICSPower MOSFET: R-V CharacteristicsAn important parameter of a power MOSFET is on resistance:

, whereFig. 3. Typical RDS versus ID characteristics of a MOSFET.


EE2301-POWER ELECTRONICSThyristor: StructureThyristor is a general class of a four-layer pnpn semiconducting device.Fig.4 (a) The basic four-layer pnpn structure. (b) Two two-transistor equivalent circuit.


EE2301-POWER ELECTRONICSThree States:Reverse BlockingForward BlockingForward Conducting

Thyristor: I-V CharacteristicsFig.5 The current-voltage characteristics of the pnpn device.


EE2301-POWER ELECTRONICSApplications Power semiconductor devices have widespread applications:Automotive Alternator, Regulator, Ignition, stereo tapeEntertainment Power supplies, stereo, radio and televisionAppliance Drill motors, Blenders, Mixers, Air conditioners and Heaters


EE2301-POWER ELECTRONICSThyristorsMost important type of power semiconductor device.Have the highest power handling capability.they have a rating of 1200V / 1500A with switching frequencies ranging from 1KHz to 20KHz.


EE2301-POWER ELECTRONICSIs inherently a slow switching device compared to BJT or MOSFET.Used as a latching switch that can be turned on by the control terminal but cannot be turned off by the gate.


EE2301-POWER ELECTRONICSDifferent types of ThyristorsSilicon Controlled Rectifier (SCR).TRIAC.DIAC.Gate Turn-Off Thyristor (GTO).


EE2301-POWER ELECTRONICSSCR

Symbol of Silicon Controlled Rectifier


EE2301-POWER ELECTRONICSStructure


EE2301-POWER ELECTRONICSDevice Operation

Simplified model of a thyristor


EE2301-POWER ELECTRONICSV-I Characteristics


EE2301-POWER ELECTRONICSEffects of gate current


EE2301-POWER ELECTRONICSTwo Transistor Model of SCR


EE2301-POWER ELECTRONICSTurn-on Characteristics


EE2301-POWER ELECTRONICSTurn-off Characteristic


EE2301-POWER ELECTRONICSMethods of Thyristor Turn-onThermal Turn-on.Light.High Voltage.Gate Current.dv/dt.


EE2301-POWER ELECTRONICSThyristor TypesPhase-control Thyristors (SCRs).Fast-switching Thyristors (SCRs).Gate-turn-off Thyristors (GTOs).Bidirectional triode Thyristors (TRIACs).Reverse-conducting Thyristors (RCTs).


EE2301-POWER ELECTRONICSStatic induction Thyristors (SITHs).Light-activated silicon-controlled rectifiers (LASCRs).FET controlled Thyristors (FET-CTHs).MOS controlled Thyristors (MCTs).


EE2301-POWER ELECTRONICSPhase Control ThyristorThese are converter thyristors.The turn-off time tq is in the order of 50 to 100sec.Used for low switching frequency.Commutation is natural commutationOn state voltage drop is 1.15V for a 600V device.


EE2301-POWER ELECTRONICSThey use amplifying gate thyristor.


EE2301-POWER ELECTRONICSFast Switching ThyristorsAlso called inverter thyristors.Used for high speed switching applications.Turn-off time tq in the range of 5 to 50sec.On-state voltage drop of typically 1.7V for 2200A, 1800V thyristor.High dv/dt and high di/dt rating.


EE2301-POWER ELECTRONICSBidirectional Triode Thyristors (TRIAC)


EE2301-POWER ELECTRONICSMode-I OperationMT2 Positive, Gate Positive


EE2301-POWER ELECTRONICSMode-II OperationMT2 Positive, Gate Negative


EE2301-POWER ELECTRONICSMode-III OperationMT2 Negative,Gate Positive


EE2301-POWER ELECTRONICSMode-IV OperationMT2 Negative,Gate Negative


EE2301-POWER ELECTRONICSTriac Characteristics


EE2301-POWER ELECTRONICSBJT structurenote: this is a current of electrons (npn case) and so theconventional current flows from collector to emitter.heavily doped ~ 10^15provides the carrierslightly doped ~ 10^8lightly doped ~ 10^6


EE2301-POWER ELECTRONICSBJT characteristics


EE2301-POWER ELECTRONICSBJT modes of operation

ModeEBJCBJCutoffReverse ReverseForward activeForward ReverseReverse activeReverseForwardSaturation ForwardForward


EE2301-POWER ELECTRONICSCutoff: In cutoff, both junctions reverse biased. There is very little current flow, which corresponds to a logical "off", or an open switch.

Forward-active (or simply, active): The emitter-base junction is forward biased and the base-collector junction is reverse biased. Most bipolar transistors are designed to afford the greatest common-emitter current gain, f in forward-active mode. If this is the case, the collector-emitter current is approximately proportional to the base current, but many times larger, for small base current variations.

Reverse-active (or inverse-active or inverted): By reversing the biasing conditions of the forward-active region, a bipolar transistor goes into reverse-active mode. In this mode, the emitter and collector regions switch roles. Since most BJTs are designed to maximise current gain in forward-active mode, the f in inverted mode is several times smaller. This transistor mode is seldom used. The reverse bias breakdown voltage to the base may be an order of magnitude lower in this region.

Saturation: With both junctions forward-biased, a BJT is in saturation mode and facilitates current conduction from the emitter to the collector. This mode corresponds to a logical "on", or a closed switch.

BJT modes of operation


EE2301-POWER ELECTRONICSBJT structure (active)current of electrons for npn transistor

conventional current flows from collector to emitter.


EE2301-POWER ELECTRONICSA GATE electrode is placed above (electrically insulated from) the silicon surface, and is used to control the resistance between the SOURCE and DRAIN regions NMOS: N-channel Metal Oxide Semiconductorp-type silicon Metal (heavily doped poly-Si)MOSFETSOURCEDRAINGATE


EE2301-POWER ELECTRONICSWithout a gate-to-source voltage applied, no current can flow between the source and drain regions.Above a certain gate-to-source voltage (threshold voltage VT), a conducting layer of mobile electrons is formed at the Si surface beneath the oxide. These electrons can carry current between the source and drain.N-channel MOSFETpDrainSourceGateIDIGIS


EE2301-POWER ELECTRONICSN-channel vs. P-channel MOSFETsFor current to flow, VGS > VTEnhancement mode: VT > 0Depletion mode: VT < 0Transistor is ON when VG=0VNMOSPMOSn+n+p+p+For current to flow, VGS < VTEnhancement mode: VT < 0Depletion mode: VT > 0Transistor is ON when VG=0V(n+ denotes very heavily doped n-type material; p+ denotes very heavily doped p-type material)


EE2301-POWER ELECTRONICSMOSFET Circuit SymbolsNMOSn+n+PMOSp+p+GGGGSSSS


EE2301-POWER ELECTRONICSThe voltage applied to the GATE terminal determines whether current can flow between the SOURCE & DRAIN terminals.For an n-channel MOSFET, the SOURCE is biased at a lower potential (often 0 V) than the DRAIN(Electrons flow from SOURCE to DRAIN when VG > VT)For a p-channel MOSFET, the SOURCE is biased at a higher potential (often the supply voltage VDD) than the DRAIN(Holes flow from SOURCE to DRAIN when VG < VT )

The BODY terminal is usually connected to a fixed potential.For an n-channel MOSFET, the BODY is connected to 0 VFor a p-channel MOSFET, the BODY is connected to VDDMOSFET Terminals


EE2301-POWER ELECTRONICSVGSSsemiconductoroxideGVDSDalways zero!IGVGSThe gate is insulated from the semiconductor, so there is no significant steady gate current.IGNMOSFET IG vs. VGS CharacteristicConsider the current IG (flowing into G) versus VGS :


EE2301-POWER ELECTRONICSVGSSsemiconductoroxideGVDSIDDIDVDSNext consider ID (flowing into D) versus VDS, as VGS is varied:Below threshold (VGS < VT): no charge no conduction Above threshold (VGS > VT): inversion layer of electrons appears, so conduction between S and D is possibleNMOSFET ID vs. VDS Characteristics


EE2301-POWER ELECTRONICSThe MOSFET as a Controlled ResistorThe MOSFET behaves as a resistor when VDS is low:Drain current ID increases linearly with VDSResistance RDS between SOURCE & DRAIN depends on VGSRDS is lowered as VGS increases above VT

NMOSFET Example:IDIDS = 0 if VGS < VTVDSVGS = 1 V > VTVGS = 2 VInversion charge density Qi(x) = -Cox[VGS-VT-V(x)]where Cox eox / toxoxide thickness tox


EE2301-POWER ELECTRONICSID vs. VDS CharacteristicsThe MOSFET ID-VDS curve consists of two regions:1) Resistive or Triode Region: 0 < VDS < VGS VT

2) Saturation Region: VDS > VGS VTprocess transconductance parameterCUTOFF region: VG < VT


Part I:Bipolar Power TransistorsThe Evolution Of IGBTBipolar Power Transistor Uses Vertical Structure For Maximizing Cross Sectional Area Rather Than Using Planar Structure

Emitter

Base

Collector

P

N+

N-

N+

Collector

Base

Emitter

Part II:Power MOSFETThe Evolution Of IGBT Power MOSFET Uses Vertical Channel Structure Versus The Lateral Channel Devices Used In IC Technology

n+

P

n-

P

n-

n+

SiO2

Gate

Source

Drain

Gate

Source

Drain

EE2301-POWER ELECTRONICSLateral MOSFET structure


The Evolution Of IGBT Discrete BJT + Discrete Power MOSFET In Darlington ConfigurationPart III: BJT(discrete) + Power MOSFET(discrete)

B

S

D

C

E

NPN

N-MOSFET

G

Part IV: BJT(physics) + Power MOSFET(physics) = IGBTThe Evolution Of IGBT More Powerful And Innovative Approach Is To Combine Physics Of BJT With The Physics Of MOSFET Within Same Semiconductor Region

This Approach Is Also Termed Functional Integration Of MOS And Bipolar Physics

Using This Concept, The Insulated Gate Bipolar Transistor (IGBT) Emerged

Superior On-State Characteristics, Reasonable Switching Speed And Excellent Safe Operating Area

The Evolution Of IGBT IGBT Fabricated Using Vertical Channels (Similar To Both The Power BJT And MOSFET)Part IV: BJT(physics) + Power MOSFET(physics) = IGBT

n+

n- - drift

p+

p - base

p+ - substrate

Emitter

Gate

Collector

E

PNP

NPN

N-MOSFET

G

C

Device Operation Operation Of IGBT Can Be Considered Like A PNP Transistor With Base Drive Current Supplied By The MOSFET

EE2301-POWER ELECTRONICSDRIVER CIRCUIT (BASE / GATE) Interface between control (low power electronics) and (high power) switch.

Functions: amplifies control signal to a level required to drive power switch

provides electrical isolation between power switch and logic level

Complexity of driver varies markedly among switches. MOSFET/IGBT drivers are simple but GTO drivers are very complicated and expensive.


EE2301-POWER ELECTRONICSELECTRICAL ISOLATION FOR DRIVERSIsolation is required to prevent damages on the high power switch to propagate back to low power electronics.

Normally opto-coupler (shown below) or high frequency magnetic materials (as shown in the thyristor case) are used.


EE2301-POWER ELECTRONICSELECTRICAL ISOLATION FOR DRIVERSPower semiconductor devices can be categorized into 3 types based on their control input requirements:

Current-driven devices BJTs, MDs, GTOsVoltage-driven devices MOSFETs, IGBTs, MCTsPulse-driven devices SCRs, TRIACs


EE2301-POWER ELECTRONICSCURRENT DRIVEN DEVICES (BJT)Power BJT devices have low current gain due to constructional consideration, leading current than would normally be expected for a given load or collector current.The main problem with this circuit is the slow turn-off time. Many standard driver chips have built-in isolation. For example TLP 250 from Toshiba, HP 3150 from Hewlett-Packard uses opto-coupling isolation.


EE2301-POWER ELECTRONICSELECTRICALLY ISOLATED DRIVE CIRCUITS


EE2301-POWER ELECTRONICSEXAMPLE: SIMPLE MOSFET GATE DRIVERNote: MOSFET requires VGS =+15V for turn on and 0V to turn off. LM311 is a simple amp with open collector output Q1.

When B1 is high, Q1 conducts. VGS is pulled to ground. MOSFET is off.

When B1 is low, Q1 will be off. VGS is pulled to VGG. If VGG is set to +15V, the MOSFET turns on.


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********************************Forward biased the PN junction of a diode has a large recombination rate and thus supports a large current.Forward biased in the BJT, the PN base to emitter junction has a low recombination rate (the base is thin and lightly doped), so theelectron proceed to the collector where they are again the majority carrier.

*Forward biased the PN junction of a diode has a large recombination rate and thus supports a large current.Forward biased in the BJT, the PN base to emitter junction has a low recombination rate (the base is thin and lightly doped), so theelectron proceed to the collector where they are again the majority carrier.

*Forward biased the PN junction of a diode has a large recombination rate and thus supports a large current.Forward biased in the BJT, the PN base to emitter junction has a low recombination rate (the base is thin and lightly doped), so theelectron proceed to the collector where they are again the majority carrier.

*Remember that the base region is deliberately made very thin and lightly doped, while the emitter is made much more heavily doped. Because of that, applying a forward bias to the emitter-base junction causes vast majority carriers to be injected into the base, and straight into the reverse-biased collector-base junction. Those carriers are actually minority carriers in the base region, because that region is of opposite semiconductor type to the emitter. To those majority-turned-minority carriers, the collector-base junction depletion region is not a barrier at all but an inviting, accelerating filed; so as soon as they reach the depletion layer, they are immediately swept into the collector region. Forward biasing the emitter-base junction causes two things to happen that might seem surprising at first: Only a relatively small current actually flows between the emitter and the base. much smaller than would flow in a normal PN diode despite the forward bias applied to the junction between them. A much larger current instead flows directly between the emitter and the collector regions, in this case, despite the fact that the collector-base junction is reversed biased.

From a practical point of view, the behavior of bipolar transistors means that, unlike the simple PN-junction diode, it is capable of amplification. In effect, a small input current made to flow between the emitter and collector. Only a small voltage--around 0.6 volts for a typical silicon transistor--is needed to produce the small input current required. ************************

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