Novel AlN/GaN insulated gate heterostructure field effect transistor with modulation doping and one-dimensional simulation of charge control

Abstract

We propose a novel AlN/GaN insulated gate heterostructure field effect transistor (FET) with modulation doping. The vertical structure of the FET was AlN(1)/AlGaN(2)/InGaN(3)/AlGaN(4)/AlGaN(5)/GaN(substrate)(6). The typical widths of gate insulator (1) and channel (3) are 4 and 5 nm, respectively. Charge control in the FET was simulated in one dimension by solving Poisson and Schrödinger equations self-consistently. The dependence of transconductance (Gm) and the cutoff frequency (fT) on the gate voltage (Vgs) was obtained, then the optimum structure was determined. We found: (i) In the above structure, without the electron supplying layer AlGaN(2) in the gate side, the FET has high Gm (max=2.9 S/mm) and fT (max=120 GHz; Lg (gate length)=2.5 μm) values in the broad Vgs region (about 3 V) in Gm−Vgs and fT−Vgs characteristics. (ii) Both Gm−Vgs and fT−Vgs characteristics show high values in the Vgs region, which becomes broader as the conduction band discontinuity between the channel (3) and electron supplying layers, (2) and (4), increases. (iii) The optimum channel width (w) is 2 nm⩽w⩽10 nm for the structure with only an electron supplying layer (4). This condition prevents lowering of Gm in the low Vgs region, and keeps the parasitic resistance between gate and source/drain low. (iv) There is an optimum doping concentration and an optimum width of the electron supplying layer, which depend on the conduction band discontinuity between the channel and the electron supplying layer. (v) Channel doping reduces intrinsic Gm and fT in the low Vgs region in Gm−Vgs and fT−Vgs characteristics and does not necessarily increase significantly the equilibrium two-dimensional electron gas concentration.