Principle of Operation of Depletion type MOSFET
i) Negative Gate Operation
In the figure given below a negative bias is applied to the gate
The negative voltage depletes the conducting channel of majority carriers electrons and controls their flow.
The gate and channel forms a parallel plate capacitor with silicon dioxide layer as dielectric.The negative gate bias causes concentration of electrons on the gate.These electrons repel the conduction band electrons in the n-channel leaving a positive ions layer as shown.More negative gate voltage,the greater is the depletion of electrons in n-channel.
ii) Positive Gate Operation
A depletion mode MOSFET when gate bias is positive is shown in figure below.
The gate and channel can again be thought of as a capacitor positive charge on gate induce negative charge in n-channel.These negative charges add to those already present in the channel.Thus positive gate voltage increases or enhances the conductivity of the channel.More positive the gate voltage,greater the conduction from source to drain.
In both positive and negative operations,the gate current is negligible because the gate is insulated.Therefore input resistance is very high.It is necessary to connect the substrate of an n-channel MOSFET to the negative terminal of a battery.So that the p-n junction doesnot becomes forward biased.If this happens the device will cease to act as MOSFET.
In p-channel device,the substrate is connected to the negative terminal of the battery.
The four terminal gates,source,drain and substrate are denoted by G,S,D,B respectively.Sometimes substrate is intensively connected to source so that only three terminals G,S and D are brought out.