Mechanism Of Gan Hemts Current Collapse Effect And Experimental Study

AlGaN/GaN HEMT has high breakdown electric field, fast electron drift velocity and large electron concentration, so it has been used more and more in high frequency and large power fields. But current collapse or self-heat effects depress its performances heavily. Current collapse is the main reason of reducing device output power.Based on GaN HEMT device physics and experiment results, we found electron mobility is depend on sheet density of 2DEG and proposed a new GaN HEMT current collapse physical model. By the use of iteration method to solve Schrodinger-Poisson equations when AlGaN barrier layer doped about 1×1018cm-3, the max sheet density of 2DEG is 1×1012cm-2 and the thickness of 2DEG is increasing from 15nm to 40nm with barrier's thickness increasing.Under high drain voltage condition, the results proved that channel electrons are easily ejected into GaN buffer layer and be trapped to induce current collapse. This model described relationship of current collapse and traps in buffer layer, and the normalized product of electron mobility and 2DEG density with and without current collapses was 0 .95×θVGS. Device with 300μm width, 4μm length, -6.5V threshold voltage, 2.1×1013cm-2 2DEG sheet density, 654 cm2/V·s electron mobility has been adopted to prove this model.In GaN HEMT drain pulse current collapse experiments, drain current under pulse condition collapsed about 50% than direct current condition and the pulse signal frequency affected little on current collapse. When gate voltage is small, the relationship between pulse width and drain current is I0(δ+γT/16).In GaN HEMT gate pulse experiments, drain current under pulse conditon collapsed about 47% than direct current condition and the pulse width affected little on current collapse. The relationship between drain current and pulse frequency isμnCoxW[m+(n+k?)VGS+(n+k?)VGS2](VGS-Vth)2/L.Under pulse condition, charging and discharging of surface states between gate and drain induce GaN HEMT current collapse. Quasi-static capacitance has been measured, when drain voltage is 0V, and gate voltage changes from–5V to 0V, the surface peak...