Large Area GaN HEMT Power Devices for Power Electronic Applications: Switching and Temperature Characteristics

Naiqian Zhang', Vivek Mehrotra', Sriram Chandrasekaran', Brendan Moran', Likun Shen', Umesh Mishral, Edward Etzkorn' and David Clarke'

'Elecbical& Computer Engineering Dept. 'Rockwell Scientific Company 3Materials Department University of Califomia Santa Barbara

Santa Barbara, CA 93 106 1049 Camino Dos Rios

Thousand Oaks, CA 91360 NZhang@rfmd com vmehrotra@nusc. com moranbaengineering. ucsb.edu

lkshen@engineering. ucsb.edu sriram@rwsc. com edmaster@engineering. ucsb.edu

University of California Santa Barbara Santa Barbara, CA 93 106

mishraaece. ucsb.edu

Abstract - Large area AIGaNIGaN High Electron Mobility Transistors (HEMT) for power electronic applications have been fabricated. These power devices offer lower on-resistance and higher switching speed than Sic devices due to higher electron mobility and high channel charge density achieved by a heterojunction. The GaN epi-layers were grown on semi- insulating 4H-Sic substrate by Metal Organic Chemical Vapor Deposition (MOCVD) technique. The device structure was grown on Sic substrates due to its high thermal conductivity. The devices have been optimised with respect to electron mobility, sheet concentration, voltage breakdown, on-resistance and dispersion. Voltage breakdown of 13OOV was achieved on small devices while breakdown in the range 600-900V was achieved on packaged devices depending on the number of devices that have been paralleled. The power device figure of merit V j R / R , = 9.94~10' [V2.P'cni2], where VBR is the breakdown voltage and %. is the on-resistance, is the highest among any reported switching devices. Switching losses of large area 600V 12.5A power devices were measured using resistive and inductive loading. Switching times of< 30 ns were achieved with an on-resistance of 0.4 C l (specific on-resistance = 1.7 &.em2). The static and dynamic characteristics of GaN HEMT devices were also measured as a function of temperature up to 200OC. Finally, the temperature distributions in the active device area were measured using Raman spectroscopy (pyrospectroseopy). This technique can be used to measure temperatures with a spatial resolution of 1-2 pm. Device temperatures from both the active areas and Sic substrates have been measured.

I. INTRODUCTION

High performance power electronic circuits are expected to make a major impact on more-electric ships, submarines, aircrafts, hybrid vehicles, nuclear-powered satellites, directed energy weapons and spacecraft [1,2]. The primary benefits are in terms of reduced weight and size, fuel savings, simple thermal management and lower lifecycle costs. Si-based power devices cannot meet the temperature, voltage, switching speed, size and efficiency requirements to realize these benefits. Wide bandgap semiconductors, particularly S ic and GaN, are well suited to meet these requirements. The wide bandgap results in very low intrinsic camer concentration that provides negligible junction leakage current up to 500C. This allows high temperature operation

clarke@engineering.ucsb. edu

without excessive leakage or thermal runaway and reduces cooling requirements. The high breakdown strength of S ic and GaN results in thinner drift layers for a given blocking voltage, as compared to silicon, thus reducing the specific on- resistance and storage of minority carriers. Lastly, the high inherent thermal conductivity of these materials (Sic) or the substrates used for their growth (GaN on Sic) allows efficient heat removal.

Rapid progress has been made towards the availability of high-quality S i c substrates. This has resulted in the development and commercialisation of SIC power devices. On the other hand, tremendous progress has also been achieved in GaN microwave power devices due to improved material quality and process techniques. These advances can now be exploited to develop GaN power switching devices for power electronics.

AIGaN/GaN based High Electron Mobility Transistors (HEMTs) offer lower on-resistance and higher switching speed due to higher electron' mobility and high channel charge density achieved by a heterojunction. Electron mobility values of 1500-2000 cm2N.s have been achieved compared to -400 cm2N.s in S ic [3]. Additionally, a two- dimensional charge density of - 2 ~ 1 0 ' ~ cni2 has also been achieved in a HEMT structure. Other advantages of GaN HEMT devices include lower parasitic capacitance and a simple device fabrication (4 mask steps) [3]. In this paper, we report on the switching and temperature characteristics of large area 600V / 2.5A GaN HEMT power devices. The GaN epi-layers were grown on semi-insulating S ic substrate by Metal Organic Chemical Vapor Deposition (MOCVD) technique. The device structure was grown on Sic substrates due to its high thermal conductivity and the device layout was optimized to achieve high blocking voltages. These high performance devices lay the foundation for a high temperature and high voltage front-end converter for distributed power architecture in both commercial and military applications.

11. DEVICE DESIGN

Fig. 1 shows the cross-section of the GaN HEMT device. Room temperature hall measurements showed an electron

0-7803-7754-0/03/$17.00 02003 IEEE 233

sheet concentration n, = 8 . 3 ~ 1 0 ~ and mobility = 1500 cm2N.s with no intentional doping in the whole material system. Insulated-gate HEMT was utilized to achieve breakdown voltages in excess of 600V [4] (Fig. 1). A Si02 layer under the gate was utilized to reduce gate leakage under high drain hias, thus alleviating the leakage assisted impact ionization in the AlGaN layer and increasing the breakdown voltage. A breakdown voltage of 1300V with a specific on- resistance of 1.7 n O c m 2 (active area - 9 . 2 5 ~ 1 0 . ~ cm2) at a gate bias of 2V was achieved in GaN HEMT with Lgd = 20 pm and W, = 500 pm. The threshold voltage was measured to be -9 V. The breakdown voltage is by far the highest value achieved on all devices based on GaN material, and the on- resistance is lower than Sic switching devices reported in the literature. The power device figure of merit V i R / R , = 9.94~10 [V2.0-1cm-2], where Vsn is the breakdown voltage and &, is the on-resistance, is the highest among any reported switching devices.

0 . 9 ~ L x = . o W Ld=1 .5 -24pn *-I

SO n n

I pn 760 Ton GaN

85 nm AIN

Semi-insulating Sic

J Fig. I . Cross-section of the GaN HEMT power device

Dispersion of GaN power HEMTs was also measured since it affects the switching speed. Dispersion characteristics were measured using gate lag measurements. Fig. 2 shows a comparison of I-V curves obtained under DC and Sops pulse conditions. It can be seen that the pulsed current level is much lower than the DC level. The discrepancy between DC and pulse measurement is referred to as dispersion in microwave power electronics, which is caused by slow response of traps in the material [3]. These pulse measurements provide an indirect method to rapidly probe the device switching speed. If the device is turned-on at a speed faster than the pulse, the current carrying capability is limited and is much lower than the DC current level. For microwave applications, Si3N4 surface passivation layer has been used to solve the dispersion problem [3]. We have utilized a double gate dielectric scheme to achieve the advantages of both SiOz and Si3N4 as shown in Fig. 3. A thin layer of Si3N4 was first deposited on the device followed by a SiOl layer deposited only under the gate. The device was finished with a thick, planar SiJN4 passivation. The Si3Nd layer substantially reduces dispersion to enable high switching speed, while the Si02 layer reduces the gate

leakage. Fig. 4 shows the I-V characteristics of the device with a double gate dielectric scheme. It is clearly evident that the dispersion is negligible.

Fig. 2. I-V curves under DC (solid lines) and nulsed (dotted lines) . . conditions showing dispersion for an unoptimised device.

S

Semi-hrulsting SIC

Fig. 3. Double-gate dielectric scheme to achieve low dispersion and gate leakage.

Fig. 4. I-V curves under DC (solid lines) and pulsed (dotted lines) conditions showina nealiaible disoersion for the double-eate dielectric . .. -

S t N C N E .

The power device layout design was also optimized to achieve high breakdown voltage. In microwave devices, electric field crowding at the end of gate-fingers severely limits the device voltage blocking capability. A circular device deoign was used to distribute the electric field evenly along the gate finger (Fig. 5) . Voltage blocking measurements revealed a maximum breakdown voltage of 1050V for this design with an average voltage blocking

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capability of 900V (Fig. 6). Large area devices were fabricated using wire bonding to interconnect discrete smaller power devices. Sixty-four small devices were bonded together with a total gate length of 38.4 mm and with a current capability >2.5A (Fig. 5) . An on-resistance of 0.4Q and a breakdown voltage of 600V were obtained for a packaged device (Fig. 7). I-V curves of a large area device are shown in Fig. 8.

Fig. 5 . Circular device design for uniform electric field disuibution (left) and paralleled GaN HEMTs with a total gate length of38.4mm with

current capability >2.5A (right)

Fig. 6. I-V curves illustrating maximum voltage blocEing capability up to 105OV and average voltage blocking of 900V.

TO - nw - I

111. SWITCHING MEASUREMENTS AND TEMPERATURE EFFECTS

Switching characteristics of GaN HEMT devices were measured using the inductive (Fig. 9) and resistive loads. Figs. 10 and 11 show the turn-on and turn-off characteristics of a GaN HEMT device at a blocking voltage of >11OV and drain current of -1.4A. The gate-to-source voltage was switched fiom OV to -2OV. Turn-on and turn-off times less than 3011s were measured under these conditions. From the switching data, a turn-on loss of 0.612 pJ and a turn-off loss of 0.834 pJ was calculated that corresponds to a total switching loss of 1.45W at 1 MHz switching 6equency. The conduction losses are about 0.68W corresponding to an on- resistance of 0.4 Q. These devices have been measured up to 250V blocking voltage at a Switching current of -2.5A.

UI1"F.U

0 . 1 II

COotrDl G DUT

S

Fig. 9. Schematic circuit used to measure the dynamic switching characteristics ofGaN HEMT power devices (indicated as Device Under

Test- DLIT). The resistive loading cixuit is similar with a variable resistor replacing the diode-inductor.

Fig. 7. Packaged 600V l2.5A GaN HEMT device on Cu / Alios substrate (left) with an on-resistance ofO.40. An exploded view ofthe device is

shown on the right

~. o i 4 6 S Io I2 i 4 I6 i n

Drain Voltage (V) Fig. 8. I-V characteristics of a 600V / 2.SA GaN HEMT device.

140

120

E m 80 P B = 60

40

20

0

2.4

2.0

1.6 - 1.2 I

C m

0

s 0.8 5 0.4

0.0

0.4 4 0 -40 -20 0 20 40 60 80 100 120

Fig. IO. T u " characteristics afGaN HEMT device under resistive load showing the drain-to-source voltage (Vds) and drain current (Id).

Time (ns)

The static and dynamic characteristics of GaN power devices were also measured as a function of temperature up to 200C. Fig. 12 shows the device I-V curves for different gate voltages at 23OC and 200C. A larger gate voltage is required for complete turn-off at 200"C, presumably due to thermal activation of traps in GaN buffer. Voltage blocking charactenstics of a single device are shown in Fig. 13 at 23C

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and 200C. The dynamic characteristics of a single power device in an array of 64 devices were measured as a function of temperature using inductive loading measurements. Fig. 14 shows the turn-on and turn-off characteristics of a single device under inductive load. Effects of the resistive contribution of the probes have been corrected in this measurement. The switching losses at 200T were measured to be within 10% of the losses at 23C. These measurements demonstrate the device switching capability up to 200C. A scaled up packaged GaN HEMT device is therefore capable of operation up to 200C with low switching and conduction losses. However, further improvements in material growth are necessary to achieve a low trap density at the surface and in GaN buffer.

2.4

2.0

1.6

1.2 - 3 5

0.8 5 0

0.4

0.0

0.4

4 0 -20 0 20 40 60 80 100 120

Fig. 1 1 . Turn-off characteristics of GaN HEMT device under resistive load showing the drain-to-source voltage (Vds) and drain current (Id).

Time (nr)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

do (v) Fig. 12. Static I-V curves ofa single GaN HEMT device at 23C and

200oc.

I 4.54 1

l.Oe-5

5.0.-7

0.0

0 m 40 W 80 100 Vds 0

Fig. 13. Leakage current of large a m GaN HEMT power device at 23C and 2 0 0 T at gate-to-source voltage of -2OV.

Dynamic Characteristics at 23% and 200DC

- Id 21% ... - Id mvc

0 2 4 6 8

Time (PI Fig. 14. Dynamic charactenstics of a single GaN HEMT power device

room an m y of 64 devices under inductive load at 23% and 200C demonstrating ifs high temperahme capability.

IV. DEVICE RELIABILITY

Device degradation was observed upon exposure to ambient conditions within 4-6 weeks. The device showed longer turn-on and turn-off times and the forward drop increased rapidly. Fig. 15 shows this degradation, which is probably due to poor device passivation. In addition to improvements in materials growth to reduce trap density, improved passivation materials are a critical need for improving device reliability.

E ....... . : .

. . . . > I ...... :

. . . . .

. . ~ . ~ ~. . . . . . . .

1.6

12

3 0.8 E

5 0.4 0

0.0

I I .,oo I 400 100 300 400 em

Time (SJ

Fig. 15. Tum-on characteristics of GaN HEMT device before (top) and after degradation @onom) upon exposure to ambient for 4-6 weeks

V. TEMPERATURE MEASUREMENTS

Temperature distribution within a single power device from the large area GaN device packaged on a ceramic

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substrate (Fig. 16) was measured using Raman spectroscopy, termed pyrospectroscopy [SI. This technique allows temperature measurements from a spot size of 1-2 pm and is being used to ascertain the spatial distribution of temperature within the device for reliability prediction. Raman peak shifts from both the active device areas and Sic substrate were measured (Tdevicc and Tsubsmc). Temperatures were derived from local peak shifts using a separate calibration performed on GaN and SIC materials. Fig. 17 shows the observed Raman peak shifts as a function of dissipated power while Fig. 18 shows the derived temperatures within the drain-to-gate region of a single power device. A temperature of about 100C in the active area was measured at a dissipated power of about 12W in a packaged device.

Fig. 16. Schematic of a large area GaN HEMT device used for high resolution temperaare measurements.

GaN E2 Raman Peak Shins

m n N 567.2

0 566.8 0.0 5.0 10.0 15.0

Dissipated Power (W)

.? 777.0 1 E 4HSiC Raman Peak Shifts 22 776.5

776.0 Y

0 775.5 b 5 775.0

0.0 5.0 10.0 15.0

Dissipated Power (W) Fig. 17. GaN E2 (top) and 4H-SiC (bottom) Raman peak shifts as a function ofdissipated power from the active area of the device and

be derived from these peak shifts with a resolution of 1-2 pm. device substrate, respectively. The spatial variation of temperahue can

G lZO.O

5 80.0

f 40.0

e 100.0

f 60.0 P

+ 20.0 0.0 5.0 10.0 15.0

Dissipated Power (w) Fig. 18. Average device temperature in the drain-to-gate region ofa

single device derived from the GaN Raman peak shift (left). The location of the collected specmm from the device is also shown (right).

VI. CONCLUSIONS

600V / 2 S A GaN HEMT devices for power electronic applications have been demonstrated. AIGaN/GaN devices with a sheet charge of 8 . 3 ~ 1 0 ~ mobility of 1500 cm2N.s, and a specific on-resistance of 1.7 d a n 2 have been fabricated. Switching times less than 30 ns were achieved with an on-resistance of 0.4 0. The dynamic characteristics were measured up to 2SOV. At blocking voltages of IlOV, a turn-on loss of 0.612 pJ and a turn-off loss of 0.834 pJ was measured that corresponds to a total switching loss of 1.4SW at 1 MHz switching frequency. The static and dynamic characteristics of GaN HEMT devices were also measured as a function of temperature up to 20OoC demonstrating their applicability for high temperature power electronics. High-resolution (1-2 pm) temperature measurements were performed using Raman spectroscopy. Finally, device reliability upon prolonged exposure to ambient was investigated. Improvements in materials growth to reduce trap density and improved passivation materials are critical needs for improving device reliability.

ACKNOWLEDGMENT

This work was partially supported by the US Navy Office of Naval Research (ONR) under contract # N00014- 993-0006, N00014-03-1-0386 and COMPACT MURI.

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[3] N:Q. Zhang, Ph.D. dissertation, University of California Santa B d a r a (2002); N:Q Bang et ai, Effects ofsurface traps on breakdown voltage and switching speed of GaN power switching HEMTs, Electron Deices Meerins, 2001, EDM Tech. Digest, pp. 25.5.1 -25.5.4.

[4] N:Q. Zhang, B. M o m , S . P. DenBaars, U. K. Mishra, X. W. Wang, and T. P. Ma, Kilovolts AlGaNiGaN HEMTs as switching devices, Phys. Slat. Sol. (a), vol. 188,pp. 213-7,2001.

[5 ] J. He, V. Mehrotra and M. C. Shaw, Ultra-high resolution temperature measurement and thermal management of RF power devices using heat pipes, Inll. Symp. On Power Semiconducror Devices ond ICs, ISPSD Proc. 1999, pp. 145-148.

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