Impact ionization and transport in the InAlAs/n+-InPHFET

We have carried out an experimental study exploring both impact ionization and electron transport in InAlAs/n⁺-InP HFET's. Our devices show no signature of impact ionization in the gate current, which remains below 17 μA/mm under typical bias conditions for L_g=0.8 μm devices (60 times lower than for InAlAs/InGaAs HEMT's). The lack of impact ionization results in a drain-source breakdown voltage (BV_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 10⁷ cm/s (in devices with L_g<1.6 μm) 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_GS and V_DS making the device well-suited to power applications demanding small I_G, low g_d, and high BV_DS