Two proposed techniques simplify and improve the implementation of SPICE device models. The first technique uses nested transformation of node potentials. It avoids the discontinuity in high-order derivatives that is typical of implementations that use conditional functions. This provides the smooth mathematical form required for distortion and intermodulation analysis, and improves simulator convergence. The second technique uses state variables to implement descriptions of rate dependent and thermal transient behaviour. This simplifies model implementation by eliminating the need for extra circuit nodes. The paper demonstrates the proposed techniques by describing the implementation of an advance MESFET model. The techniques will provide models that can simulate the intermodulation and transient behaviour of high-performance communication and signal processing circuits.