Design, fabrication and characterization of indium aluminum arsenide/indium gallium arsenide/indium arsenic phosphide composite channel HEMTs

Some efforts on the growth of such InAlAs/InGaAs/InAsP HEMT structure metamorphically on GaAs have been reported, but there were no working devices reported before this work. In this study, we use pseudomorphic InAsP layer beneath InGaAs main channel to form composite channel HEMTs on InP substrate to suppress the impact ionization and simultaneously achieve excellent device performance.;The composite channel HEMT layer-structure was designed based on the simulation of band structure and electron concentration distribution by solving the by solving Poisson and Schrodinger equation self-consistently. The channel is consisted of, from bottom up, 40 A of strained InAs0.3P 0.7 doped to 2 x 1018 cm -3, 40 A of undoped InAs0.3P0.7 and 70 A In0.53Ga0.47As. 1-D theoretical simulation shows that electrons are mainly confined in the InGaAs main channel at equilibrium and begin to transfer to InAsP sub-channel under a high drain bias. The device structure was grown on InP substrate by molecular beam epitaxy by our collaborating research group. Hall measurement shows that the device wafer has a two-dimensional sheet electron density of 3 x 1012 cm -2 and an electron mobility of 7300 cm2/( V · s) at room temperature.;The composite channel devices are fabricated by a conventional mesa-isolated HEMT process. The Ge/Au/Ni/Au ohmic contacts alloyed in a furnace in N2 ambient exhibit a very low ohmic resistance of 0.03 O · mm and the specific contact resistivity of 1.2 x 10 -7 · cm-2. Sub-micron mushroom-shaped gates are patterned by electron beam lithography using a tri-layer resist scheme. The first fabricated 0.25 mum gate length HEMTs exhibited a peak extrinsic transconductance of 888 mS/mm, an fT of 115 GHz, and an fmax of 137 GHz. A 0.15mum gate device showed an fT of 195 GHz. To our knowledge, these are the highest fTs reported for composite channel HEMTs on InP substrate with the same gate length. The results indicate that our proposed composite channel structure, which combines the high electron saturation velocity of InAsP with the small conduction band offset between InAsP/InGaAs, has the strong potential in making the best composite channel devices with suppressed impact ionization.;In the research of high speed HEMTs, the scaling of channel and its high field electron velocity are two major concerns. We present a systematic study on the gate length scaling of InGaAs/InAsP composite channel HEMTs. Devices with gate lengths of 0.15, 0.34, 0.58, 0.85 and 1.13 mum were fabricated. The dependence of direct current and microwave performance on gate length is characterized. Device characterization results showed the extrinsic transconductance increased from 498 mS/mm for 1.13 mum devices to 889 mS/mm for 0.15 mum gate devices, while the unit current gain frequency increased from 24 GHz to 195 GHz. A simple delay time analysis is employed to extract the effective carrier velocity (veff) of the composite channel. The veff is determined to be 1.9 x 10 7cm/s.;To fully exploit the potential of composite channel HEMTs, we aim to improve its microwave performance by down scaling the gate length. High resolution electron beam lithography process using a ZEP/PMGI/ZEP resist stack for ultrashort T-gates were developed. One step exposure process has excellent process reliability and double exposure strategy has the potential for gate length down to 25nm. A recent InGaAs/InAsP HEMTs composite channel with a gate-length of 80 nm exhibits a unity current gain cut-off frequency (fT) of 280 GHz. For better understanding of the circuit behavior of ultra short gate composite channel HEMTs, a 10 element small signal equivalent circuit model is developed for device parameter extraction. The values of circuit elements is used in an extended delay time analysis to investigate the effect of enhanced veff at short gate length. The average electron velocity under the gate is determined to be 2.8 x 10 7cm/s in the 80 nm device.;To summarize, we systematically investigated the design, fabrication, and characteristics of InAlAs/InGaAs/InAsP composite channel HEMT devices for the first time. We demonstrated in our sample devices that this novel structure is capable to achieve ultra high speed with suppressed impact ionization. Furthermore, we conducted gate length dependence study, small signal circuit modelling and delay time analysis, which illustrated the channel electron velocity profile of this new HEMT structure. The knowledge is essential for power applications of this type of HEMT's. (Abstract shortened by UMI.)...