MOSFET Full Form – Metal Oxide Semiconductor Field Effect Transistor

MOSFET (Metal Oxide Semiconductor Field Effect Transistor)

Metal Oxide Silicon Field Effect Transistor commonly known as MOSFET is electronic devices used to switch or amplify voltages in circuits. It is a voltage controlled device

Like JFET, it has a source, Drain and Gate. It is also called IGFET (Insulated Gate FET) because gate terminal is insulated from channel. Therefore it has extremely high input resistance.

MOSFET Types

It has two types

1.Depletion mode MOSFET

2. Enhancement mode MOSFET

1. Depletion mode MOSFET

a. N-channel

b. P-channel

2. Enhancement mode MOSFET

a. N-channel

b. P-channel

The enhancement-type MOSFET is usually referred to as an EMOSFET, and the depletion type, a DMOSFET. The drain current in a MOSFET is controlled by the gate-source voltage VGS.

1. Depletion mode MOSFET [DMOSFET]

The bias voltage on the gate in the depletion mode of operation lowers the drain current ID by reducing the number of charge carriers in the channel. It is capable of both depletion and enhancement.

DMOSFET Symbol

DMOSFET Construction

Two highly doped n-regions are diffused within a lightly doped p-type substrate.

Through metallic contacts, the source and drain terminals are linked to n-doped regions that are connected by an n-channel. A very thin silicon dioxide (SiO2) layer separates the gate from the n-channel while still allowing it to be connected to a metal contact surface. When exposed to an applied external field, a specific kind of insulator known as SiO2 creates opposing (as indicated by the prefix di-) electric fields within the dielectric.

Then the thin layer of metal aluminum is formed over the Sio2 layer. This metal overs the entire channel region and it forms the gate(G).

Operation of N-channel DMOSFET

Case (i) “when and is increased from zero”

Here N-base (Drain) is connected to positive supply. It act as a reverse bias. Due to this, depletion region gets increases.

Free electron from n-channel are attracted towards positive potential of drain terminal. This establishes current through channel flows from drain to source and denoted as IDSS.

Pinch off voltage

The junction’s charge carrier depletion voltage is known as the pinch off voltage.

Case (ii) “when and is increased from zero”

The negative charge on gate repels conduction electrons from the channel and attract holes from the p-type substrate.

Due to this electron-hole recombination occurs and reduce the number of free electrons in the channel available for conduction, reducing Drain current (ID).

When negative voltage of is increased the pinch of voltage decreased. When is further increased the channel is fully depleted and no current flows through it.

The negative voltage depletion MOSFET.

DMOSFET Characteristics curve

Two types

Drain characteristics

Transfer characteristics

DMOSFET are biased to operate in two modes: depletion or enhancement mode.

2. ENHANCEMENT MODE MOSFET [EMOSFET]

In this mode bias on the gate increases the number of charge carriers in the channel and increases the drain current (ID).

It operates only in the enhancement mode and has no depletion mode of operation. It has no physical channel.

EMOSFET Symbol

EMOSFET Basic Construction

The n-channel enhancement-type MOSFET’s basic structure consists of a silicon base that is formed into a slab of p-type material, also referred to as the substrate. Similar to a depletion-type MOSFET, the substrate can occasionally be internally connected to the source terminal, but in other cases, a fourth lead is provided so that the potential level can be controlled externally.

The SiO2 layer is still present to isolate the gate metallic platform from the region between the drain and source, but now it is simply separated from a section of the p-type material.

In summary, therefore, the construction of an Enhancement-type MOSFET is quite similar to that of the Depletion-type MOSFET, except for the absence of a channel between the drain and source terminals.

EMOSFET Operation

If VGS is set at 0 V and a voltage applied between the drain and source of the device, the absence of an N-channel (with its generous number of free carriers) will result in a current of Effectively zero amperes—quite different from the Depletion- type MOSFET and JFET where ID – IDSS.

It does not reach the same level of saturation as the JFET and depletion-type MOSFET.

The device is referred to as an Enhancement MOSFET because the positive bias voltage on the gate increases the Conductivity of the channel. EMOSFETs are Frequently referred to as OFF MOSFETs.

EMOSFET Characteristics

Drain Characteristics curve