Gate Structure Optimization And Characteristics Research Of Millimeter-wave In InAlN/GaN HEMT

High frequency and high power are hotspots of GaN-based HEMT device all the time. And the most important for InAlN/GaN HEMT is to improve the current gain cutoff frequency(fT). InAlN has so strong spontaneous polarization effect that it can generate two-dimensional electron gas which has a higher density than AlGaN/GaN heterojunction does. Bigger aspect ratio is possible for InAlN/GaN HEMT even when the gate length is scaled to deep submicron. So it can avoid gate control ability degradation. Obviously, millimeter-wave InAlN/GaN HEMT is better than Al GaN/GaN HEMT in high frequency application.Analysis of the the reasons that cause short channel effects(SCEs) by gate length scaling have been done with two-dimensional numerical simulation tool Sentaurus TCAD. It reavels that, drain induced barrier lowering(DIBL) and two-dimensional channel potential under the gate are most responsible for SCEs. When gate length is longer than 120 nm, it is effective for gate length scaling to improve fT. But as gate length reduced to 60 nm, it is less working.In order to design applicable gate structure for f T promoting, simulating optimizations of T-gate, τ-gate and Γ-gate InAlN/GaN HEMT have been carried out, respectively. The results show that, gate field plate can improve fmax but reduce fT. Especially, increasing the length of gate field plate which is closed to source can obviously improve fmax. What is more, choosing larger gate foot height and gate-source distance can effectively improve frequency performance. However, breakdown voltage of the device will be reduced due to the increased gate-source distance. So it is significative to choose a proper gate-source distance to ensure better frequency and breakdown performance at the same time.Based on the theoretical studys and simulations above, new compound metal gate(CMG) GaN based HEMT has been proposed. The new structure makes use of the different workfunctions between metals that compose the gate to form a step like potential under the gate. The step like potential raises up the gate potential closed to drain side so as to screen the influence that drain potential does to channel. Electric field and velocity peaks will appear at the interface of different metals and increase average electron velocity under the gate. This will improve carrier transfer efficiency and drain current performance of the device. More importantly, gate capacitance will be reduced due to more metal types. This is helpful to improve frequency performance. And the performance of the device will be better as the metal type increases.The characteristics of a 200 nm gate length InAlN/GaN HEMT have been analyzed with experiment and Lombardi model. A maximum static state current about 500mA/mm is obtained at gate bias of 2V, fT is 65.8GHz and fmax is 143.6GHz. Lombardi model has been introduced to analyze the influence that interface roughness scattering does to the device. The results show that, interface roughness scattering reduces the transfer lifetime and mobility of the electron. This reduces the maximum static state current and transconductance by 57% and 69%, and reduces fT and fmax by 58% and 74%, respectively.