Definition of Enhancement MOSFET and its Working

Enhancement MOSFET

A p-channel enhancement MOSFET consists of lightly doped n-substrate. Into which two heavily doped p⁺ regions are diffused. These two p⁺  source and the drain. A thin layer of siO₂ is grown over the surface of the entire assembly. Holes are cut into this Si O₂ layer for making contact with p⁺  source and drain regions. On the SiO₂layer, a metal (aluminium) layer is overlaid coveting the entire channel region from source to drain. This aluminium layer constitutes the gate. The area of MOSFET is typically 5 square miles or less. This area is extremely small being only about 5% of the area required for a bipolar junction transistor.

A parallel plate capacitor is formed with the metal area of the gate and the semiconductor channel acting as the electrodes of the capacitor. The oxide layer acts as the dielectric between the electrodes.

Working of Enhancement MOSFET

                                                         P-channel enhancement MOSFET

                                                                 Drain Charecteristics of Enhancement MOSFET
                            

The above fig shows the p-channel enhancement MOSGET. The substrate will be connected to the common terminal i.e. to the ground terminal. A negative potential will be applied to the gate. This results in the formation of an electric field normal the Si O₂ layer this electric field originates from the induced positive charges on the semiconductor side on the lower surface of the SI O₂ layer. The induced positive charge become minority carriers in the n-type of substrate. They form an inversion layer.

The magnitude of these induced positive charges increase with the increase of the magnitude of the gate negative voltage. The region below the oxide layer now will be having p-type carriers. The conductivity of the region increase and the current flow through the induced channel from source to drain also increase. It is observed that the drain current has been enhanced on application of negative gate voltage. This is the reason for calling it as enhancement MOSFET.

The fig shows the drain characteristics of a enhancement MOSFET. By increasing the gate potential, pinch off voltage and drain currents are increased. The curves are similar to drain characteristics of JFET.