Impact ionization and transport in the InAlAs/n/sup +/-InP HFET

We have carried out an experimental study exploring both impact ionization and electron transport in InAlAs/n/sup +/-InP HFET's. Our devices show no signature of impact ionization in the gate current, which remains below 17 /spl mu/A/mm under typical bias conditions for L/sub g/=0.8 /spl mu/m devices (60 times lower than for InAlAs/InGaAs HEMT's). The lack of impact ionization results in a drain-source breakdown voltage (BV/sub DS/) that increases as the device is turned on, displaying an off-state value of 10 V. Additionally, we find that the channel electron velocity approaches the InP saturation velocity of about 10/sup 7/ cm/s (in devices with L/sub g/<1.6 /spl mu/m) rather than reaching the material's peak velocity. We attribute this to the impact of channel doping both on the steady-state peak velocity and on the conditions necessary for velocity overshoot to take place. Our findings suggest that the InP-channel HFET benefits from channel electrons which remain cold even at large V/sub GS/ and V/sub DS/ making the device well-suited to power applications demanding small I/sub G/, low g/sub d/, and high BV/sub DS/.<
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