AlGaN/GaN HFET with composite 'slow/fast' gate
The remarkable properties of AlGaN/GaN heterojunction field effect transistors (HFET) include a record high-density two dimensional electron gas (2DEG) channel, high breakdown fields, temperature stability and chemical robustness. Radio Frequency (RF) switches based on such HFETs demonstrate very low insertion loss, high power handling capability and broad range of operating temperatures. However, the maximum operating frequency of these switches is limited by the finite off state capacitance COFF, mainly due to a small gate-channel separation. In the conventional design, when the device is in the off state, the 2DEG is depleted only under the gate, which gives rise to substantial channel-gate coupling and hence relatively large residual capacitance. To realize the best switch performance at microwave frequencies COFF minimization is critically important. In this dissertation, a novel device design defined as composite "slow/fast" gate is put forward. The "slow gate" electrode depletes the entire device channel and therefore leads to much lower intrinsic residual capacitance. RF switch MMICs using novel "slow gate" design are presented, with below 1 dB insertion loss, above 35 dB isolation and more than +35 dBm power handling capability. In addition, the developed slow/fast HFETs demonstrate higher breakdown voltage and higher switching power. The achieved performance enhancement is confirmed by experiments and simulations. Future steps to further improve the performance of slow/fast gate transistors are suggested.
Electrical and Computer Engineering, Electrical and Electronics, Engineering, microwave switches, power handling, small signal transmission
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