The MOSFET is a four terminal device and the terminals are:
BODY or SUBSTRATE
The Terminal Body or Substrate is generally internal to the device and is connected to the Source terminal.
As has been discussed earlier the MOSFET is either an N - Channel MOSFET or a P- Channel MOSFET.
There are two basic forms of MOSFET available:
ENHANCEMENT Type MOSFET
DEPLETION Type MOSFET.
Enhancement Type Mosfet:
The Enhancement Type MOSFET Transistor requires a Gate-Source Voltage VGS to switch ON the device.
The Enhancement Type MOSFET Transistor is equivalent to a Normally Open Switch.
Depletion Type Mosfet:
The Depletion type MOSFET Transistor requires a Gate - Source Voltage VGS o switch OFF the device.
The Depletion type MOSFET Transistor is equivalent to a Normally Closed Switch
Symbols Of Mosfet:
The symbols for the MOSFET for both Enhancement Type and Depletion Type MOSFET are shown in the figures 1 and 2 below:
The terminal Substrate is connected to the Source terminal.
It may be noted from the Figure 2 above that in Depletion Type MOSFET symbol, the line representing the Drain and Source connections is Thick
It may be noted from the Figure 1 above that in Enhancement Type MOSFET symbol, the line representing the Drain and Source connections is Thin
The Terminal Substrate is neither connected neither to the input nor to the output, but is used for grounding.
It is connected to the main semiconductor channel through a diode junction to the body or metal tab of the MOSFET.
The terminal Substrate is generally connected to the Source Terminal as it is at the lowest potential.
Normally -On Transistor :
The Depletion Type MOSFET is the same as a Normally -On Transistor. This is so because Drain current flows in the MOSFET even in the absence of any Gate biasing or potential i.e. VGS =0
Normally -Off Transistor :
The Enhancement Type MOSFET is the same as a Normally -Off Transistor. This is so because NO Drain current flows in the MOSFET when no Gate biasing or potential is applied i.e. VGS =0