Impact Ionization and Transport in the InAlAs/n+-InP HFET

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 p ” m under typical bias conditions for L, = 0.8 pm devices (60 t i e s lower than for InAlASnnGaAs HEMT’s). The lack of impact ionization results in a drain-source breakdown voltage (BV’s) 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 lo7 cm/s (in devices with L, < 1.6pm) 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 VGS and VDS making the device well-suited to power applications demanding small IG, low gd, and high BVDS.