GaAs-based metamorphic high electron mobility transistors with high indium mole fraction quantum well channels

The layer structures of metamorphic High Electron Mobility Transistors (mHEMTs) with composite InGaAs channels of indium mole fraction up to 63% have been designed and modeled. Metamorphic HEMT epitaxial wafers were successfully grown by Molecular Beam Epitaxy (MBE) on 100-mm diameter GaAs substrates. Room temperature Hall mobility of mHEMT in excess of 9,500 cm2/Vsec was measured at sheet charge densities of 3.4--4.0 x 10 12 cm-2. This represents a 55-percent improvement in Hall mobility and a 20-percent increase in sheet charge densities over their GaAs pseudomorphic HEMT counterparts. A 0.15-mum T-shaped gate process by direct write e-beam lithography with a selective gate recess was developed to fabricate high fT and low noise mHEMTs. Both depletion- and enhancement-mode mHEMTs have also been fabricated and characterized. An mHEMT with a 0.15-mum gate length and a composite channel of In 0.53Ga0.47As/In0.63Ga0.37As/In 0.58Ga0.42As yielded an extrapolated fT of over 150 GHz at a drain bias of 1.5 volts. Furthermore, excellent noise performance was achieved when mHEMTs were biased at low drain voltages of around 0.8 to 1 volts. Very low noise figures of 0.30 and 0.82 dB were measured on 0.15-mum x 100-mum mHEMTs at 10 and 26 GHz, respectively. We have also demonstrated a process that monolithically integrates mHEMTs with PIN diodes. This new process paves the way for fabricating mHEMT transimpedance amplifiers and metamorphic PIN photodiodes on the same chip. Moreover, a 0.15-mum T-gate mHEMT Monolithic Microwave Integrated Circuit (MMIC) process with three-level gold metal interconnects on 100-mm GaAs substrates was developed for low noise and fiber-optics applications. A 77 GHz oscillator, three 40Gbit/s fiber-optics circuits, and an ultra low noise X-band LNA were successfully designed and fabricated using this single-recessed 0.15-mum mHEMT on GaAs technology. The 40-Gbit/s mHEMT Transimpedance Amplifiers (TIAs) with differential outputs exhibited state-of-the-art performance of greater than 55-dBO transimpedance and 38 GHz bandwidth. The latest fabricated and measured mHEMT MMIC is an X-band (7--11 GHz) Low-Noise Amplifier (LNA). Extremely low noise figure of 0.5 dB and very high gain of greater than 30 dB were measured across the frequency band of 7 to 11 GHz by both on-wafer RF probe and in-fixture measurement. GaAs-based mHEMTs in this study exhibited excellent fT and noise performance that is comparable to the reported results of best InP HEMTs in literature. GaAs mHEMT with relatively low manufacturing cost is an attractive device technology to replace its InP HEMT counterpart.