If the line is dotted or broken then it represents the Enhancement type of MOSFET which is normally in an OFF state as zero drain current flows with zero gate potential. If the line is unbroken then it represents the Depletion type of MOSFET which is normally in an ON state as the drain current can flow with no gate biasing potential. The line between the drain (D) and source (S) connections show the semiconductive channel of the transistor. When such is the case it is removed from the symbol altogether. It is at times connected internally to the source terminal. It is used to connect the main semiconductive channel (n or p) through a diode junction to the metal layer of the MOSFET. The substrate terminal is neither an input nor an output terminal but a terminal present to ground the substrate. The symbols show an additional terminal called the substrate. If it is an n-channel semiconductor device then electrons will flow and if it is a p-channel then holes will flow through the drain-source channel.įigure 3 shows the four MOSFET symbols. The depletion and enhancement MOSFET make use of an electric field which is produced by the gate voltage to alter the flow of charge carriers. This makes the overall MOSFET input resistance very high which can cause large amounts of static charge to accumulate resulting in damaging the transistor. This is because the current flowing through the main channel between the source and drain is proportional to the input resistance. The characteristics of the MOSFET resemble a voltage-controlled resistor. Therefore, no current flows into the gate. This insulation prevents the current flow from the gate to the main current-carrying channel between the drain and source terminal. No current flows through the oxide layer under all the static biasing conditions as the oxide is a perfect insulator. There are no charge centres present in the oxide layer or oxide semiconductor interface. This property of the MOSFET makes it a viable candidate for electronic switches or for making logic gates as with no bias and a high input resistance little or no input current is required. This increases the overall input resistance of the MOSFET, therefore, making it almost infinite. The insulated gate acts as one plate of a capacitor. This means that it is possible to bias the gate of the MOSFET to either positive or negative polarity without any limitations. The gate is thick enough so that under AC or DC biasing conditions it can be an equipotential region.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |