Abstract
Depletion-mode δ-doped In0.5Ga0.5As/In0.5Al0.5As mHEMTs have been metamorphically grown on a GaAs substrate and successfully fabricated with different kinds of gate-metal formation. The gate-metal formations include a combination of mesa- or air-type gate feeder with or without a buried gate (before or after annealing). Only the air-type mHEMT with a buried gate shows no clear kink-effect behaviour. For a 1 µm gate mesa-type (air-type) mHEMT, a maximum extrinsic transconductance of 412 (414) and 535 (472) mS mm−1 is obtained before annealing and after annealing. There is a 207 mV (205 mV) shift in VTH after the mesa-type (air-type) mHEMT is annealed. Experimental results indicate that both the gate-feeder metal and the annealing process have a significant effect on output conductance, gate leakage current, breakdown voltage, high frequency and noise performances. The peak gate leakage current density of 12 (120) µA mm−1 for the air-type (mesa-type) mHEMT before annealing is improved to 8 (55) µA mm−1 after annealing. At 2.4 (5.4) GHz, gain = 23 (20) dB can be obtained at Fmin = 1.27 (1.76) dB for the air-type mHEMT after annealing, while gain = 22 (18.5) dB is obtained at Fmin = 1.36 (2.0) dB before annealing. For the mesa-type mHEMT, these values are gain = 20 (16.5) dB at Fmin = 1.47 (2.25) dB and gain = 19.5 (14) dB at Fmin = 1.68 (3.11) dB.
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