Introduction :
A field effect transistor that can be operated in the enhancement -mode is called a MOSFET. A MOSFET is a important semiconductor device and can be used in any of the circuits covered for JFET.
The main draw back of a JFET is that gate must be reverse biased for proper operation of the device which means that negative gate operation for n-channel and positive gate operation for p-channel.This means that we can only decrease width of the channel.This type of operation is called depletion mode operation.There fore JFET can only operate in the depletion mode.
Symbols for MOSFET :
Types of MOSFET :
there are two basic types available in MOSFET
1.D-MOSFET or Depletion type MOSFET .This type of MOSFET can be operated in both depletion mode and enhancement mode.
2.E-MOSFET or Enhancement type MOSFET.This type of MOSFET can be operated only in enhancement -mode.
1.D-MOSFET :
The above figure shows that n-channel MOSFET is a piece of n-type material with p-type region on the right and insulated gate on the left side then free electrons are moving from source to drain must passes through the narrow channel between the gate and the n-type region.
It is usual practice to connect the substrate to the source internally so that a MOSFET has three terminals which are source,drain,gate.
gate insulated from the channel we can apply either positive or negative voltage to the gate.therefore D-MOSFET can be operated in both modes then JFET can operated only one mode which means that depletion mode.
The figure given in below the n-channel D-MOSFET
The E-MOSFET has no channel between source and drain unlike the D-MOSFET. The E-MOSFET requires a proper gate voltage to form a channel .It is reminded that E-MOSFET can operate only enhancement mode only.The constructional details of a n-channel E-MOSFET figure given in below.
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