Reliability Evaluationof High Power AlGaN/GaN HEMTs on SiC Substrate

H. Kim1), V. Tilak, B. M. Green, J. A. Smart, W. J. Schaff, J. R. Shealy,

and L. F. Eastman

School of Electrical and Computer Engineering, Cornell University, Ithaca,NY 14850, USA

(Received June 22, 2001; accepted August 4, 2001)

Subject classification: 73.30.+y; 73.40.Kp; S7.14

Undoped AlGaN/GaN HEMTs grown on SiC substrates have recently demonstrated record outputpower density of 10.7 W/mm (cw) and total output power of 10 W at 10 GHz (L. F. Eastman, JointONR/MURI Review (5/15–16, 2001), CA (USA) [1]). In this paper, we present the results of relia-bility tests performed on undoped AlGaN/GaN HEMTs on SiC under dc and rf stress conditions.Undoped AlGaN/GaN HEMTs on SiC substrates have been submitted to on-wafer dc and rf stressconditions at a room temperature and the degradation in device performance induced by hot elec-tron and thermal effects have been observed.

Introduction AlGaN/GaN high-electron-mobility-transistors have demonstrated excel-lent power handling ability, compared with their lower-bandgap counterparts, due to aunique combination of material characteristics, i.e. high breakdown field (3 � 106 V/cm)and relatively large saturation and overshoot velocities (3 � 107 cm/s) [2]. In its hetero-structure with AlGaN, electron sheet density reaches 1 � 1013 cm––2 due to strong spon-taneous and piezoelectric polarization without intentional doping. Recently fabricateddevices on SiC substrates demonstrated record output power density at 10 GHz underclass B operation [3]. In a recent report, hot electron induced degradation was ob-served in undoped AlGaN/GaN HEMTs on sapphire substrates under dc and rf stressconditions, and Si3N4 was found to be a better passivation material than SiO2 for highreliability devices [4].

Undoped AlGaN/GaN HEMTs on SiC have been submitted to dc stress and rf stressconditions with high drain bias and high input drive, respectively. The stress tests havebeen performed at several different bias points to identify dominant factors in devicedegradation.

Device Fabrication Undoped AlGaN/GaN heterostructures were grown by OMVPE on200 undoped SiC substrates. They have a 1 mm unintentionally doped GaN buffer and a 10–20 nm AlGaN barrier. The Al mole fraction determined by X-ray diffraction was 30%.Device isolation was done by a mesa etch with a Cl2-based ECR plasma. Ti/Al/Ti/AuOhmic contacts, obtained with annealing at 800 �C in N2 for 30–60 s, formed source anddrain. The Ohmic contact resistance measured from TLM patterns ranged from 0.3–0.5 W mm. The mushroom gates were defined by e-beam lithography with a three-layerresist process. Ni/Au metals were used to form Schottky contacts. A Si3N4 film was de-posited as a passivation layer using PECVD at 300 �C.

1) Corresponding author; Phone: +1 607 255 1431; Fax: +1 607 255 4742;e-mail: hk91@cornell.edu

phys. stat. sol. (a) 188, No. 1, 203–206 (2001)

# WILEY-VCH Verlag Berlin GmbH, 13086 Berlin, 2001 0031-8965/01/18811-0203 $ 17.50þ.50/0

dc Stress Test Hot electron stress testwas performed on the fabricated deviceswith high VDS. The drain output currentand gate leakage currents were moni-tored every minute for 6.5 h using aHP4142B modular dc source controlledby HP VEE program. As shown in Fig. 1,

the output current was degraded with stress time, due to self-heating and increased sur-face depletion induced by hot electron effects. After stress tests, drain series resistance(RD) was increased from 16 to 18 W and source series resistance remained at 12 W,which indicates only the region between gate and drain is affected by the hot electronstress. This increase in the drain series resistance, accompanied by an increase of kneevoltage, is caused by an increased channel depletion and a decreased gate–drain elec-tric field induced by negatively charged traps. The gate leakage current was decreasedduring the test because of a reduced gate–drain electric field, due to negatively chargedtraps in the gate–drain region, which resulted in alleviated impact ionisation. dc charac-teristics of the stressed devices are shown in Fig. 2. The changes in threshold voltageand the shape of the gm curve are negligible, since the effect of traps underneath thegate is not significant.

rf Stress Test rf stress tests were performed to investigate the reliability of the devicesunder rf amplifying operation. The devices were biased closed to class B operation andload and source impedances were matched for maximum gain. rf input power was heldat the 6 dB gain compression point at 8 GHz. Gradual degradation under rf drive wasobserved as shown in Fig 3. The device stressed at higher VGD showed more significantand rapid degradation in output power and output current. To verify the effect of VGD

as a stress factor, dc stress tests were performed under different gate–drain bias. Fig. 4

204 H. Kim et al.: Reliability Evaluation of High Power AlGaN/GaN HEMTs

Fig. 1. Degradation charactersitics of outputcurrent and gate leakage current under dc biasstress at VDS = 15 V and VGS = 0 V for 6.5 h

a) b)

Fig. 2. dc characteristics of AlGaN/GaN HEMTs before and after dc stress test at VDS = 15 Vand VGS = 0 V for 6.5 h. a) I–V curve with VGS = +1, 0, ––1, ––2 . . . V. b) Transfer characteristicat VDS = 7 V

suggests that VGD, i.e. an electric field between gate and drain is a dominant stressfactor in device reliability testing and hot electron stress has a more significant effecton device degradation than self-heating.

At the hot electron stress conditions, hot electrons are created in the region betweenthe gate and drain under high rf nput drive, and are likely to tunnel into the passiva-tion layer or the AlGaN barrier layer above the channel, to generate permanent trapsthere which cause increased surface depletion, increased series resistance, and reducedgate–drain electric field [5, 6]. This degradation mechanism is shown in Fig. 5.

Conclusion Comprehensive reliability tests were performed on undoped AlGaN/GaNHEMTs on SiC. They demonstrated excellent device performance for high power appli-cations. Similar to GaAs-based devices, hot electron induced degradation such as thereduction of IDS, IG, and peak gm, and the increase of RD and knee voltage, were ob-served under dc and rf stress conditions. The hot electron stress, i.e. high reverse gate–drain bias, is found to have more dominant effect on the degradation of the devicecompared with self-heating.

phys. stat. sol. (a) 188, No. 1 (2001) 205

Fig. 3 Fig. 4

Fig. 3. Gradual degradation of output power and output current under rf input drive at VDS = 20 V,VGS = ––3 V (sample 1) and VDS = 25 V and VGS = ––4 V (sample 2)

Fig. 4. Degradation characteristics under dc stress with different VGD (VDS, VGS)

Fig. 5. Degradation mechanism inAlGaN/GaN HEMT under high stresscondition

References

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IRPS 2001, Internat. Rel. Phys. Symp. (2001) (p. 214).[5] H. Hasegawa, K. Katsukawa, T. Itoh, T. Noguchi, and Y. Kaneko, IEEE MTT-S Tech. Digest

(1996) (p. 46).[6] J. C. M. Hwang, Proc. IEEE GaAs Integrated Circuit Symp. (1995) (p. 81).

206 H. Kim et al.: Reliability Evaluation of High Power AlGaN/GaN HEMTs