Q. Show Basic Construction of MOSFET?
The basic construction of the n-channel depletion-type MOSFET is provided in Figure. A slab of p-type material is formed from a silicon base and is referred to as the substrate-Jet is the foundation upon which the device will be constructed In some cases, thesubstrate is internally connected to the source terminal However, many discrete devices provide an additional terminal labeled SS, resulting in a four-terminal device,. The source and drain terminals are connected through metallic contacts to n-doped regions lirlked by an n- channel as shown in the figure. The gate is also connected to a metal contact surface but remains insulated from the n-channel by a very thin silicon dioxide SiO2layer. SiO2is Ii particular type of insulator referred to as a dielectric that sets up opposing (as revealed by the prefix di-) electric fields within the dielectric when exposed to an externally applied field.
The fact that the SiO2 layer is an insulating layer reveals the following fact:
There is no direct electrical connection between the gate terminal and the channel of a MOSFET:
In addition:
It is the insulating layer of SiO2 in the MOSFET construction that accounts for the very desirable high input impedance of the device. In fact, the input resistance of a MOSFET is often that of the typical JFET, even though the input impedance of most JFETs is sufficiently high for most applications. The very high input impedance continues to fully support the fact that the gate current is essentially zero amperes for dc-biased configurations.
The reason for the label metal-oxide-semiconductor FET is now fairly obvious: metal for the drain, source, and gate connections to the proper surface-in particular, the gate terminal and the control to be offered by the surface area of the contact, the oxide for the silicon dioxide insulating layer, and the semiconductor for the basic structure on which the n- and p-type regions are diffused. The insulating layer between the gate and channel has resulted in another name for the device: insulated- gate FET or IGFE1; although this label is used less and less in current literature.