The physics and technology of the InAlAs/n⁺-InP heterostructure field-effect transistor

To meet the growing demand for a high-power Heterostructure Field-Effect Transistor (HFET) family based on InP, we have developed and characterized a circuitcapable InA1As/n+-InP HFET technology employing InP not merely as a substrate but also epitaxially as the channel and as an etch-stopper in a selectively-recessed gate scheme. This thesis presents an experimental assessment of the benefits and drawbacks of exploiting InP in both roles. We find that the combination of an InP etch-stopper beneath an n+-InGaAs cap reduces the small-signal parasitic resistances significantly while simultaneously maintaining a tight threshold voltage distribution. We uncover a new mechanism in which the cap conducts in parallel with the channel at sufficiently high currents, leading to a rapid reduction in these resistances with increasing current. As a result, we achieve a peak transconductance of 200 mS/mm and a maximum drain current of 430 mA/mm for 0.8 pm gate-length devices. We also find that the wide-bandgap n+-InP channel eliminates both impact ionization and real-space transfer, leading to very low gate currents and to a drain-source breakdown voltage that increases from an off-state value of 10 V as the device is turned on. As a tradeoff, we find that the effective electron velocity approaches the InP saturation velocity of about 1.05 x 107 cm/s for gate lengths below 1.6 pm rather than achieving the material's higher peak velocity. We attribute this to the impact of doping on both the peak velocity and on the conditions necessary for velocity overshoot to take place. Our results indicate that the InA1As/n+-InP MIDFET is well-suited to applications demanding high drain current, large breakdown voltage, small gate current, low output conductance, and tight threshold voltage uniformity, and for which a moderate frequency response is an acceptable tradeoff. Thesis Supervisor: Jesuis A. del Alamo Title: Associate Professor of Electrical Engineering