In this paper a two dimensional finite element heat transfer simulation has been used to optimise the standard QMIT structure for high power microwave and millimetre wave applications taking into account limitations in fabrication process. The effect of the most important parameters such as the distance between active device and substrate (W), front side metallization, glue thermal conductivity (k epoxy) and use of a heat spreader to decrease the thermal resistance are investigated in details. It is well known that by using dry etching, the hole dimensions on the front side of Si-wafer are uniform, accurate and reproducible. There are two other possible structures, one by using the full dry etching and through a combination of wet etching and dry etching. The simulations for all of these three structures have been done and the results are described. In order to compare the results for standard QMIT structure to the other monolithic technologies, simulations for the same structure and dimensions have been done in which all materials were assumed to be GaAs or Si. When the whole structure is made of GaAs, thermal resistance is 20.14 [°C/W] which is equal to that of QMIT with kheat spreader=67.5 [W/mK] and when it is totally made of Si, however, the GaAs active device is embedded, thermal resistance is 11.68 [°C/W] which is equal to that of QMIT with kheat spreader=237.8 [W/mK]

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