Influence of backside doping on the nonlinear capacitances of a PHEMT affecting the VCO frequency characteristics

A local maximum in the C/sub GS/(V/sub GS/) characteristics of an AlGaAs/InGaAs/AlGaAs PHEMT is both calculated by hydrodynamic simulations and extracted from S-parameter measurements. It is found by simulation that the doping on the backside of the channel is the origin of this behavior. VCO measurements demonstrated that this C/sub GS/(V/sub GS/) characteristic can result in a partially reversed tuning behavior.