Abstract: The quickly growing semiconductor industry is becoming more demanding in improving functionality, efficiency, and in lowering the cost of semiconductor devices. This requires intensive use of computer simulation tools in modeling of devices which are now at the submicron and nano scale dimensions. Very sophisticated and convenient tools, such as Technology Computer Aided Design (TCAD) and its device simulation part DESSIS, have been developed for this purpose. Looking at device development in the future, high electron mobility transistors (HEMTs) are very promising devices especially for the rapidly advancing wireless technology. HEMT devices have been researched for more than a decade, but with the scaling of devices down to nano level they are still the subject of research. HEMT devices are based on heterostructure material configuration. At such small device dimension the effects of matching or mismatching materials have significant influence on performance. The DESSIS tool, among the others, uses the hydrodynamic (HD) model for device simulation where the information about the average carrier energy is available in a form of the carrier temperature. Many parameters depend on this average carrier energy. One of them is the energy relaxation time. The HD model does not take into account quantum effects. The HD model uses bulk material value for energy relaxation time in the channel of the device where there are really different physical conditions than in the bulk material, so it is expected that this energy relaxation time value used is not accurate. The actual value of energy relaxation time may be obtained by applying Monte Carlo simulation method on electron transport in the HEMT channel. The idea of the present work is to develop a Monte Carlo (MC) device simulator which could be used to solve the mentioned problem. For this purpose a 3D Monte Carlo particle simulator is developed and applied to a 2D pHEMT device simulation. The development of the MC simulator is achieved through several steps, starting from a MC simulator applied to a 1D GaAs resistor, through a 2D GaAs MESFET device and finally to a 2D GaAs/AlGaAs/InGaAs pHEMT device. With the MC simulator at this stage of development, one can experiment with different channel lengths and widths. Also, one can calculate the proper energy relaxation times needed in DESSIS simulations.