| The InAs/AlSb material system has produced excellent frequency of 235 GHz for both fT and fmax in a 100 nm gate-length heterostructure field-effect transistor (HFET). However, a serious fundamental problem plagued even the best InAs device: A deleterious turn-up in the drain conductance at very low voltages that increased rapidly with increasing drain voltage. The primary objective of the present research was to submit an approach of composite channel consisting of either InAs/InAsP or InAs/InAlAs to alleviate this problem. Starting from the characterization of transport property in the strained composite channel, a high 22,500 cm 2/V-s electron mobility was observed, while 7,100 cm2/V-s mobility was observed in a single strained InAsP quantum well layer for InAsP grown at a properly lowered substrate temperature of 430°C. An InAs/InAsP composite-channel device with a dipole doping showed greatly suppressed drain conductance at a maximum ratio of 1:14 and slightly degraded microwave performance. Two InAs/InAlAs composite-channel HFETs with Te doping in top barrier and buffer respectively were also fabricated and characterized. Though both devices had very similar frequency values, only the device with Te buffer doping yielded announced suppression in drain conductance.; These results have demonstrated the application potential of antimonide-based composite-channel HFETs in improving serious breakdown problem, however high gate currents still existed in the devices at high gate biases. WKB calculation of tunneling probabilities and 2-D simulation were performed and suggested possible mechanisms for the high gate currents. Using suggested layer structures, which were designed to prevent the possible gate current paths, and with improvements in fabrication technology, we predict that the device performance will prove to be much better. |
