High-performance AlInP/AlImAs/GaInAs HEMT with a partially-doped graded pseudomorphic channel

Abstract

Despite significant improvements of the transconductance and speed of pseudomorphic AlInAs/GaInAs HEMTs, these devices suffer from low channel breakdown voltage and large outputconductance. Especially, in power applications, the drain current handling capability is limited by low channel and gate breakdown voltages [l]. This is mainly due to lower bandgap of the channel and strong localization of the 2DEG distribution at the interface. Our previous work showed that this problem can be solved by grading indium mole fraction in the pseudomorphic GaInAs channel [2]. In this work, we demonstrate a novel InP-based HEMT where partial grading of doping is employed in lower half of the pseudomorphic channel. This resulted in significant increase of the drain current handling cability without degrading channel breakdown voltage and other dc and rf performances. The device structures were grown by low-pressure OMCVD. All five device structures were identical except that the channel composition was changed in a systematic way. The total channel thickness also remained at 300 A. The Gal,,In,As channel of Devices A and By had a dorm indium composition (x) of 0.53 and 0.7, respectively. Devices C had a channel with x graded from 0.7 to 0.53. Device D was similar to Device C except the doping level in the doped AlInAs layer is reduced and the doping of the channel is graded from 0 ~m-~ to 2 x IOl7 ~m-~. For comparison, the whole channel of Device E was unformly doped (1 x lo1* ~m-~) without AlInAs doped layer above the channel. An &,2h08P ternary is used as a Schottky layer to increase the Schottky barrier height. A standard 0.7 pm gate FET processing was employed to fabricate the devices on the epitaxial wafers. All five devices had excellent Schottky diode characteristics. This is due to a high bandgap (-1.8 ev) of the high-quality &2%.8P Schottky layer. The transconductance (gm) and current-gain cutoff frequency (fT) were signrficantly increased with the increase of the indium composition as expected. Device B had 5 1 % higher gm (800 mS/mm) and 30 % higher fT (61 GHz) than Device A. However, the channel breakdown voltage (BVh=3.5V) and output conductance &=85 mS/mm) of device B became considerably poorer. However, by grading the channel composition (Device C), channel breakdown voltage (BVb=9V) and output conductance (&,do mS/mm) characteristics can be significautly improved. Final challenge is to modi6 the structure of Device C to increase the drain current handling capability, while keeping high gm and fT. As clear With Device D (graded x=0.7 to 0.6, and partially doped), doping grading significantly increases the drain current density (ID max = 1000 mA/m) and decreases go (24 mS/mm) while gm (590 mS/mm), fT (47 GHz), and BVh (7 V) are comparable to those of Device C. Device D also showed very high power-gain cutoff fiequency (f-) of 1 10 GHz. In conclusion, the studies demonstrate that the partiallydoped graded channel of InP-based pseudomorphic HEh4T's significantly enhances the advantages of these devices. [l] Y.Kwon etal. in Proc. 5th Cod. InP and related materials, WA2, pp. 465468, 1993. [2] K. B.Chough et al., IEDM Digest pp. 787-790, 1993.