The mm-wave frequency range is being increasingly researched to close the gap between 100 to 1000 GHz, the least explored region of the electromagnetic spectrum, often termed as the 'THz Gap'. The ever increasing demand for compact, portable and reliable THz (Terahertz) devices and the huge market potential for THz system have led to an enormous amount of research and development in the area for a number of years. The Gunn Diode is expected to play a significant role in the development of low cost solid state oscillators which will form an essential part of these THz systems.Gunn and mixer diodes will "power" future THz systems. The THz frequencies generation methodology is based on a two-stage module. The initial frequency source is provided by a high frequency Gunn diode and is the main focus of this work. The output from this diode is then coupled into a multiplier module. The multiplier provides higher frequencies by the generation of harmonics of the input signal by means of a non-linear element, such as Schottky diode Varactor. A realistic Schottky diode model developed in SILVACOTM is presented in this work.This thesis describes the work done to develop predictive models for Gunn Diode devices using SILVACOTM. These physically-based simulations provide the opportunity to increase understanding of the effects of changes to the device's physical structure, theoretical concepts and its general operation. Thorough understanding of device physics was achieved to develop a reliable Gunn diode model. The model development included device physical structure building, material properties specification, physical models definition and using appropriate biasing conditions.The initial goal of the work was to develop a 2D model for a Gunn diode commercially manufactured by e2v Technologies Plc. for use in second harmonic mode 77GHz Intelligent Adaptive Cruise Control (ACC) systems for automobiles. This particular device was chosen as its operation is well understood and a wealth of data is available for validation of the developed physical model. The comparisons of modelled device results with measured results of a manufactured device are discussed in detail. Both the modelled and measured devices yielded similar I-V characteristics and so validated the choice of the physical models selected for the simulations. During the course of this research 2D, 3D rectangular, 3D cylindrical and cylindrical modelled device structures were developed and compared to measured results.The injector doping spike concentration was varied to study its influence on the electric field in the transit region, and was compared with published and measured data.Simulated DC characteristics were also compared with measured results for higher frequency devices. The devices mostly correspond to material previously grown for experimental studies in the development of D-band GaAs Gunn devices. Ambient temperature variations were also included in both simulated and measured data.Transient solutions were used to obtain a time dependent response such as determining the device oscillating frequency under biased condition. These solutions provided modelled device time-domain responses. The time-domain simulations of higher frequency devices which were developed used modelling measured approach are discussed. The studied devices include 77GHz (2nd harmonic), 125 GHz (2nd harmonic) and 100 GHz fundamental devices.During the course of this research, twelve research papers were disseminated. The results obtained have proved that the modelling techniques used, have provided predictive models for novel Transferred Electron Devices (TEDs) operating above 100GHz.