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 Schro¨dinger 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 highGm(max=2.9 S/mm)andfT(max=120 GHz;Lg(gate length)=2.5 &mgr;m) values in the broadVgsregion (about 3 V) inGm−VgsandfT−Vgscharacteristics. (ii) BothGm−VgsandfT−Vgscharacteristics show high values in theVgsregion, 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)is2 nm⩽w⩽10 nmfor the structure with only an electron supplying layer (4). This condition prevents lowering ofGmin the lowVgsregion, 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 intrinsicGmandfTin the lowVgsregion inGm−VgsandfT−Vgscharacteristics and does not necessarily increase significantly the equilibrium two-dimensional electron gas concentration. ©1997 American Institute of Physics.