Gallium Nitride (GaN) high-electron mobility transistors (HEMTs) offer advantages formicrowave power applications. Scaling GaN HEMTs for millimeter-wave (mmWave) bands introduces trade-offs involving device passivation, current collapse, and parasitic capacitance. Most applications for mmWave HEMTs require linear operation include low-noise amplifiers (LNAs) and power amplifiers (PAs). For LNAs, precise modeling of noise figure (NF) and input intercept point (IIP3) is essential for mm-wave front-end circuit simulation, particularly in the context of reducing DC power consumption (PDC). This research introduces a modified MIT Virtual Source GaN-HEMT (MVSG) model specifically for N-polar HEMTs, and demonstrates its predictive accuracy for prototype devices. The extraction methodology is developed for a mm-wave HEMT model from device characterization. The impact of different physical features of the N-polar model is quantitatively described. IV characteristics, S-parameters, and two-tone load-pull measurements were performed for experimental corroboration. The dual-threshold model has provisions for tuning physical parameters, predicting the device performance, and optimizing the linear gain efficiency (LGE). In the context of 5G applications, this work also proposes a passive load-switching topology as a generalizable tuning approach that can be applied to various device technologies. Load modulation for high average efficiency and static tuning in the presence of load variation is described. The application of POLM to GaN PAs at 28 GHz is discussed and verified through measurements.