DAY 9 OF ELECTRONICS

METAL OXIDE SEMICONDUCTOR FIELD EFFECT TRANSISTOR (MOSFET)

MOSFETs are another category of FETs. It also has three terminals drain, source and gate.
It is similar to that of JFET except that the Gate is insulated from the channel; as a result it doesn't form a P-N junction.
Because of this MOSFET is also called as IGFET (Insulated Gate Field Effect Transistor).
The Gate is insulated from channel by means of introducing a layer of silicon dioxide in between Gate and channel.
There are two types of MOSFETs:
    1. Depletion MOSFET (D-MOSFET)
    2. Enhancement MOSFET (E-MOSFET)
It also has N-channel and P-channel. Normally we use N-channel in most of the cases.

1. DEPLETION MOSFET (D-MOSFET):

CONSTRUCTION:

An N-channel D-MOSFET consists of highly doped P-type substrate.
In this substrate, two heavily doped N-regions are diffused.
These two regions are used as Source and Drain.
A thin layer of silicon dioxide is applied on the surface of this structure. An ohmic contact brought out from this layer called as Gate.
This SiO2 layer acts as a insulator between Gate and the Channel.
It should be noted that the gate and channel do not form PN junction as like in other cases, because the Gate is insulated from channel by means of SiO2 layer.
Hence it is named as Insulated Gate Field Effect Transistor (IGFET).
The construction of P-channel D-MOSFET is also similar to that of N-channel, with the difference that the substrate is of N-type and heavily doped region is P-type.

OPERATION:

 

As the Gate is separated by means of insulating material, the Gate and channel acts as two plates of a parallel-plate capacitor.
The SiO2 layer acts as dielectric between these two plates.
When a voltage of particular polarity is applied to the gate, the charges with opposite polarity are induced in the channel by the principle of parallel-plate capacitor.
The voltage may be either +ve or -ve, hence two modes are possible
        Depletion mode - for negative gate voltage
        Enhancement mode - for positive gate voltage

DEPLETION MODE - If negative voltage is applied to the gate, then D-MOSFET operates in depletion mode.
The negative voltage at the gate produces positive charges on another plate called channel.
Due to this the channel is depleted of electrons (i.e. positive charges produce) and current flow reduces.
If Drain-Source voltage is applied the positive charges near channel are influenced towards drain.
As the negative voltage near gate increases, positive charge near channel keeps on increasing. This reduces drain current.
ENHANCEMENT MODE - If positive voltage is applied to the gate, then D-MOSFET operates in enhancement mode.
The positive voltage at the gate produces negative charges on channel.
Due to this more and more electrons produced in the channel and the current flow enhances.
Hence this mode is called as enhancement mode.

2. ENHANCEMENT MOSFET (E-MOSFET):

Construction:

 

An N-channel E-MOSFET consists of highly doped P-type substrate.
In this substrate, two heavily doped N-regions are diffused.
These two regions are used as Source and Drain.
A thin layer of silicon dioxide is applied between two N-regions. An ohmic contact brought out from this layer called as Gate.
The construction of E-MOSFET and D-MOSFET are similar. The only difference is that in E-MOSFET a channel is not formed as in the case of D-MOSFET.

OPERATION:

 

The operation of E-MOSFET is similar to that of enhancement mode of D-MOSFET.
The difference is the induced negative charge on the other side forms channel between two N-regions.
After the formation of channel only the current starts to flow before that current will be zero.
So that other operations are similar to that of D-MOSFET.