Microwave-pulse-based integrated circuits are a promising solution for quantum computing. However, the cryogenic temperatures at which recent ion trap-based demonstrators operate pose a challenge for electrical circuits. There is a lack of integrated transistor models predicting the functionality accurately. This work first presents the performance of a HICUM/L2 model down to 70 K for a 130 nm technology. Secondly, a model enhancement is proposed resulting in a more accurate representation of transistor physics at cryogenic temperatures. Finally, the optimized model is used for an 18 GHz RF power amplifier design delivering 10 dBm to obtain an improved circuit behavior prediction.