A systematic study of device structure on DC and small‐signal characteristics of millimeter‐wave AlGaN/GaN HEMT

Abstract

AbstractIn this paper, the relationship between the gate structure, passivation structure, short‐channel effect, source‐drain distance, DC, and the small‐signal characteristic of a device is studied through simulations. By studying the gate structure and passivation layer, it is concluded that the operating frequency of a device increases as the gate length decreases. However, as the gate length decreases, the gate resistance becomes the main factor restricting the small‐signal performance. In order to solve the contradiction between the gate resistance and gate length, a T‐shaped gate structure is studied, in which the gate height, gate cap, and passivation layer thickness are optimized. It is found that when the gate height and gate cap are 120 and 500 nm, respectively, the parasitic capacitance introduced by the gate cap can be minimized. Meanwhile, the influence of gate resistance on the small‐signal gain can also be reduced. Besides, the short‐channel effect is analyzed, and the scheme for improving the short‐channel effect is proposed. By analyzing the source‐drain distance, and considering the influence of parasitic capacitance and resistance on the small‐signal characteristic, the gate‐source spacing is determined to be 0.9 μm. The design of a gate‐drain spacing is mainly based on the influence of a breakdown voltage; therefore, the gate‐drain spacing should be 1.5 to 2 μm.