In this work, an advanced simulation study of reliability in millimeter-wave (mm-wave) GaN Devices for power amplifier (PA) applications is performed by means of a particle-based full band Cellular Monte Carlo device simulator (CMC). The goal of the study is to obtain a systematic characterization of the performance of GaN devices operating in DC, small signal AC and large-signal radio-frequency (RF) conditions emphasizing on the microscopic properties that correlate to degradation of device performance such as generation of hot carriers, presence of material defects and self-heating effects. First, a review of concepts concerning GaN technology, devices, reliability mechanisms and PA design is presented in chapter 2. Then, in chapter 3 a study of non-idealities of AlGaN/GaN heterojunction diodes is performed, demonstrating that mole fraction variations and the presence of unintentional Schottky contacts are the main limiting factor for high current drive of the devices under study. Chapter 4 consists in a study of hot electron generation in GaN HEMTs, in terms of the accurate simulation of the electron energy distribution function (EDF) obtained under DC and RF operation, taking into account frequency and temperature variations. The calculated EDFs suggest that Class AB PAs operating at low frequency (10 GHz) are more robust to hot carrier effects than when operating under DC or high frequency RF (up to 40 GHz). Also, operation under Class A yields higher EDFs than Class AB indicating lower reliability. This study is followed in chapter 5 by the proposal of a novel π-Shaped gate contact for GaN HEMTs which effectively reduces the hot electron generation while preserving device performance. Finally, in chapter 6 the electro-thermal characterization of GaN-on-Si HEMTs is performed by means of an expanded CMC framework, where charge and heat transport are self-consistently coupled. After the electro-thermal model is validated to experimental data, the assessment of self-heating under lateral scaling is considered.