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Wide bandgap semiconductors are used to fabricate field-effect transistors with significantly improved RF output power compared to GaAs and InP-based devices. The critical electric fields for avalanche ionization in wide bandgap semiconductors, such as SiC and GaN, are about an order of magnitude greater than for traditional semiconductors such as Si, GaAs, and InP. Nitride-based HFET's can support drain bias voltages in the range of 40-50 v before breakdown occurs, and this results in the ability to generate significantly higher RF power than is possible with traditional GaAs FET's. In addition, the use of field-plate technology permits even greater drain bias to be applied. However, the operation of these devices at high drain bias introduces physical phenomena within the device that affect both dc and RF performance. In particular, the existence of a nonlinear source resistance is demonstrated and verified. Inclusion of the nonlinear source resistance in a physics-based device simulator produces excellent agreement between simulated and measured data. The nonlinear source resistance degrades RF performance and limits amplifier linearity.