MOSFET is a device in which current at two electrodes drain and source, is controlled by the action of electric field at another electrode gate. The gate terminal is electrically insulated from the channel, having a dielectric layer. The metal area of gate in conjunction with the insulating dielectric oxide layer and semiconductor channel forms a parallel plate capacitor. Due to insulating layer, the device is called Insulating Gate Field Effect Transistor. The ISFET methodology for ion measurement is developed on the basis of MOSFET. The fundamental working of MOSFET is the control of current between source and drain via the potential of the gate electrode but in the ISFET, is determined by the hydrogen ion concentration present at gate terminal.The n-channel MOSFET consists of a lightly doped p-type substrate into which two highly doped n+ regions are diffused, act as source and drain. There is a contact to body or bulk of device and gate is separated from Si by dielectric layer. In modern devices for gate terminal, heavily doped poly-Si is used. To examine the behavior of MOSFET, source and bulk terminals are grounded and gate and drain potentials are varied.In this paper, by using different dielectric layers, the response of MOSFET is observed. The electrostatic relations between potentials, charges and capacitances are much simpler, when the gate dielectric is a perfect insulator like SiO2, while for a high gate dielectric it can be very complicated.Process Simulation was carried out in AthenaTM environment. Silicon wafer with orientation <100> was used. Firstly, the field oxide of 0.854 ?m thickness was grown over the wafer surface. Then patterning of field oxide was done. Growth of gate oxide and deposition of dielectric with a thickness of 0.05 ?m and 0.08 ?m respectively, was achieved. Then patterning of gate oxide and dielectric was done for defining gate region. In the diffusion process, doping of phosphorous was done for source and drain regions, followed by drive-in process. The sheet resistance and junction depth was calculated. For electrical contacts, sputtering of aluminum metal was done. Figs. 2 and 3 show the complete device structure, structure with doping profile as obtained by simulation in Athena.