The metal-oxide semiconductor field-effect transistor (MOSFET) is a device mainly used for amplifying electronic signals. It is the cornerstone of all integrated circuits and can also be used in analogue circuits. A Double-diffused metal oxide semiconductor (DMOS) is a power MOSFET that will most certainly be found in an audio amplifier. The 'metal' in the name of these devices has been kept for historical reasons. In reality the metal part has been replaced by polycrystalline silicon. Data for operating characteristic parameters like specific on-resistance, threshold voltage and leakage current of such semiconductor power devices are usually available only for temperatures below 150oC. Nowadays though, in a number of high-tech industrial fields, circuits operate in 'hot' environments, with temperatures often exceeding 200oC. Aerospace, avionics, ship and oil extraction industries are few examples. The ATHIS project has conducted an in depth research on the behaviour of commercially available vertical DMOS (VDMOS) transistors at temperatures higher than 150oC. The VDMOS transistor is a special form of DMOS transistor with two types of doping elements, usually phosphorus and arsenic which are co-implanted in the drain region. VDMOS have the advantage of a lower on-resistance and a smaller lateral size compared to a lateral DMOS transistor. Due to the high breakdown voltage, this type of transistor is commonly used for power devices, for example in automotive electronics. In the course of their investigation, project partners have found many low voltage, low on resistance MOSFET devices with an operating temperature up to 175oC. To raise the operating temperature to 200oC, numerical simulations for the basic cell of a MOSFET indicated the required modifications. Alteration of the cell pitch distance, reduction of the body-epitaxy junction area, high dose implantation in the channel region and a more efficient package resulted in successful operation at the required temperature.