IEEE ELECTROK DEVICE LETTERS. VOL. EDL-7, NO. 9, SEPTEMBER 1986 503

High-Efficiency Millimeter-Wave GaAs/GaAlAs Power HEMT’s

Abstract-The power, gain, and efficiency of 0.5-pm gate-length, 75- and 50-pm gate-width multiple heterojunction high electron mobility transistors (HEMT’s) have been evaluated from 10 to 60 GHz. At 10 GHz, with a source-to-drain voltage as low as 2.4 V, the device delivers a power density of 0.37 W/mm with 13.4-dB gain and 60.8-percent efficiency. At 60 GHz, a 50-pm device gave 0.4 W/mm with 3.6-dB gain and 14-percent efficiency. The power density and efficiency of these 0.5- pm gate-length HEMT’s above 40 GHz are the best reported for a three- terminal device, Fundamental frequency oscillations up to 104 GHz were observed when a device was bonded as a free-running oscillator.

E I. INTRODUCTION

XCELLENT low-noise performances have been demonstrated with the high electron mobility transistor

(HEMT) up to 60 GHz [l], [ 2 ] , Its usefulness for power generation has only been investigated recently [3]-[5]. The work presented here shows that 0.5-pm gate-length HEMT’s can produce useful power levels with good gain and excellent efficiency up to 60 GHz. The devices described here have a gate width of 75 and 50 pm; the gate length is 0.5 pm and the source-to-drain spacing is 2 pm.

11. DEVICE FABRICATION

The MBE grown material structure is shown in Fig. 1. It provides six layers of high mobility electrons. The AlGaAs layers are doped to over 1 X lo1* by Si and the A1 concentra- tion is 25 percent. On top of these layers a 300-A-thick layer of GaAs doped in the high 1 x 1 O l s with Si is used to facilitate ohmic contacts and reduce parasitic source and drain resist- ances. The resulting sheet carrier concentration is over 3 X 10l2 cm-2. The mask set used includes 75- and 50-pm gate- width devices of the Pi gate configuration. The 75-pm devices have, respectively, one, two, and three gate feeds. The 50-pm devices have one gate feed. Source-drain spacing is 3 pm. Fig. 2 is a photograph of a three gate-feed 75-pm device.

Device fabrication is conventional. Isolation is performed by boron implant. Ohmic contacts are formed using Au-Ge/ Ni/Au metallization alloyed at 450 C for 3 min. The 0.5-pm gates are defined and recessed before TilPtlAu evaporation. Fig. 3 shows a cross-sectional view of the HEMT. Passivation is done with silicon nitride. Metal overlay and air-bridge plating complete the front side processing. The slice is lapped to 3 mils before backside metallization and dicing.

Manuscript received May 9, 1986; revised June 16! 1986. The authors are with Texas Instruments Incorporated, Dallas, TX 75265. IEEE Log Number 8610210.

Doped AlGaAs 300 A AI = 0.2 to 0.25

Doped AIGaAs 250 A I Conductive Channels< ~~~~~ Undoped GaAs

conductive Channels= ::I:: Undoped GaAs ~-

NO A

Doped AlGaAs 150 A

Conductive Channels.< ~~~~~ Undoped GaAs 300 A ,

Doped AlGaAs 50 A

Undoped AIGaAs 0.2 prn

Buffer Undeeed 1 pm

SI Substrate

Fig. 1. Multiple HEMT structure.

Fig. 2. The 75-pm device with three gate-feeds.

Source Drain

5.1. Substrate

Fig. 3. Multiple HEMT cross sections.

0741-310618610900-0503$01.00 0 1986 IEEE

504

111. DC MEASUkEMENTS The dc characteristics for a 75-pm device are shown in Fig.

4. The saturation current is 450 mAimm and the maximum current with +0.4 V on the gate before conduction in the AlGaAs layers occurs is 600 mAimm. The maximum trans- conductance is 453 mSimm. The source resistance is about 0.28 Qmmm. Breakdown voltage is about 5-6 V. The observed variation in transconductance is much lower than in a single heterojunction HEMT. Thus lower intermodulation distortion is expected as reported in [5].

IV. MICROWAVE AND MILLIMETER-WAVE MEASUREMENTS Ali the devices are soldered on a horseshoe and source

grounding is done by multiple bond wires. For the measure- ments at 10, 15, and 20.5 GHz, a fixture with 5042 input and output lines on quartz and 3.5-mm connectors is used. Tuning is achieved by using the proper bond wire length to connect the gate and drain pad to the 5 0 4 microstrip lines.

At io .GHz, a maximum efficiency of 60.8 percent is obtained with a drain voltage of 2.4 V; the corresponding gain and power are 13.4 dB and 0.37 W/mm. Maximum power of 0.6 W/mm with 13.6-dB gain and 44.3-percent efficiency is achieved with 3.5 V on the drain.

At 15 GHz a maximum efficiency of 49 percent with 9-dB gain and 0.51 Wimm is achieved with 3-V drain bias. The corresponding small signal gain is 11 dB.

At 20.5 GHz a maximum efficiency of 42 percent is achieved with 2.5-V drain bias. The corresponding power is 0,.38 W/mm with 6-dB gain and the small signal gain is 10 dB. When the drain voltage is increased to 3.5 V, a maximum efficiency of 35 percent is ,achieved, and the corresponding power is 0.5 Wimm with 7.3-dB gain. The small signal gain is 11 dB.

For the measurements at 32.5, 40.5, 50, and 60 GHz, test fixtures with fin line transitions frdm waveguide to microstrip line are used. The 50-h transmission lines on quartz substrate are used with gate and drain bond wire length optimized for the different frequencies. Chip tuning is used for final performance optimization. At 32.5 GHz, with 5 V on the drain, a power of 0.6 Wimm with 5.4-dB gain and 30-percent efficiency is achieved. The corresponding gain compression curve is shown in Fig. 5. The small signal gain is 5.1 dB.

At 40.5 GHz, a power of 0.56 W/mm with 4.2-dB gain and 25.8-percent efficiency is obtained with a 5-V drain voltage. The small signal gain is 5.6 dB and 0.6 W/mm is obtained with 3.5-dB gain and 24-percent efficiency.

At 50 GHz, a power of 0.39 Wimm with 4.2-dB gain and 16.3-percent efficiency is achieved with a 5-V drain bias. The small-signal gain is 5.1 dB and 0.47 Wimm is obtained with 3 dB.

At 60 GHz a 50-pm device gave a 0.4 Wimm power with 3.6-dB gain and 14-percent efficiency.

A 75-pm device was bonded as a free-running oscillator. Fundamental oscillations as high as 104 GHz were observed. Although the maximum output power was only 0.03 mW, oscillations at this frequency started with a drain voltage as low as 0.5 V. No major differences were observed in the

IEEE ELECTRON DEVICE LETTERS, VOL. EDL-7, NO. 9 , SEPTEMBER 1986

Fig. 4. HEMT dc characteristics.

c

AT MAXIMUM EFFICIENCY POINT: POWER DENSITY = 0.6Wimrn

GAIN = 5 . 4 d B EFFICIENCY I 30%

! 50.0 40.0 E

E 2

30.0 B 20.0

w

410.0

10.0 4 0.0 5.0 7.0 9.0 11.0 13.0

INPUT POWER (dBm) Fig. 5 . Gain compression of a 75-pm HEMT at 32.5 GHz.

TABLE I RF PERFORMANCE SUMMARY

~~~

15

30 5.4 0.6 32.5

42 6.0 0.38 20.5

49 9.8 0.48

performances of the different 75-pm device or the device.

50-pm

Table I summarizes the results (power density, gain, and efficiency) obtained from 10 to 60 GHz.

V. CONCLUSIONS

High gain, high efficiency, and respectable power density have been demonstrated with 0.5-pm gate-length multiple structure HEMT’s from 10 to 60 GHz. Very high transcon- ductance, high current capability, and low source resistance permit these devices to operate with high efficiency at low drain voltages.

ACKNOWLEDGMENT The authors wish to thank G. Oliver for technical assistance

and D. N. McQuiddy for supporting this work.

SAUNIER AND LEE: HIGH-EFFICIENCY POWER HEhfT’s 505

REFERENCES [3] P. Saunier and H. D. Shih, “A novel double heterostructure HEMT for microwave application,” presented at WOCSEMMAD 1985.

[1] P. C. Chao, P. M. Smith, U. K. Mishra, S. C. Palmateer, K. H . G. [4] K. Hikosaka, Y. Hirachi, T. Mimura, and M. Abe, “A microwave Duh, and J. C. M. Hwang, “Quarter-micron low-noise high electron power double-heterojunction mobility transistor,” IEEE Electron mobility transistors,” in Proc. I985 IEEE/Cornell Conf. Advanced Device Lett., vol. EDL-7, no. 7 , pp. 341-342, July 1985. Concepts in High Speed Semiconductor Devices and Circuits. [ 5 ] A. K. Gupta, R. T . Chen. E. A. Sovero, and J. A. Higgins, “Power

[2] J. Berentz, “Millimiter wave HEMT technology,” presented at the saturation characteristics of GaAsiAlGaAs high electron mobility NSF Workshop on Electromagnetics, Univ. of Texas at Arlington, Jan. transistors,” presented at the IEEE 1985 Microwave and Millimiter- 29, 1986. Wave Monolithic Circuits Symp.