Introduction to MOSFET

Course Details

Field of Study: Electronics Engineering
Course Name: Applied Electronics
Subtitle: Introduction to MOSFET

We will begin the discussion from the second major type of transistor called the field effect transistor(FET).
In particular, we will examine in detail the metal oxide semiconductor FET (MOSFET).
This is an extremely popular type of transistor.
MOSFETs have similar uses as BJTs. They can be used as signal amplifiers and electronic switches.

Types of MOSFET
There are two major types of MOSFETS, called
Enhancement type and depletion type.
Each of these types can be manufactured with a so-called n channel or p channel:
In this presentation, we will discuss the Enhancement Type, N Channel MOSFET since it is the most widely used FET.

Physical structure of MOSFET

Basic Operation of MOSFET
With no bias voltage applied to the gate terminal, there exists two back-to-back pn junctions between the drain and the source.
No current flows from drain to source.
In order to obtain current flow the MOSFET needs to be biased, similar to what is required for BJTs.
For the MOSFET, however, we apply a voltage to the gate with respect to the source: VGS
Because of the oxide layer under the gate electrode, the gate current will be essentially zero.


Two things happen when VGS is applied:
1. Free holes in the p-type substrate are repelled from the region under the gate.
2. Electrons from the heavily doped N regions (the drain and source) are attracted under the gate.
These effects create an n-type channel
Notice that this bias voltage VGS is required in order to create the channel: no VGS, no channel.
The VGS required to accumulate sufficient numbers of mobile electrons in the channel is called the threshold voltage, Vt.
Now, if a voltage is applied between the drain and source, VDS we will have a flow of electrons from source to drain(i.e., a current).
For small VDS, MOSFET is behaving like a resister between the drain and source terminals whose resistance is controlled by VGS.

MOSFET when small VDS is applied


MOSFET when VDS is increased
The behavior of the MOSFET changes considerably when VDS increases beyond small values.
In these circumstances, an additional electric field is created from drain to source that is large enough to alter the shape of the channel.
This produces a thicker channel near the source than the drain.
Because of this tapered shape of the channel, the resistance of the channel increases.
Hence, the iD-VDS characteristic curve is no longer a straight line.



Regions of MOSFET Operation
There are three regions of operation for a MOSFET:
1. Off or cutoff region, where iD = 0
2. “Triode” region, where VDS < VDS sat = VGS – Vt
3. “Saturation” region, where VDS > VDS sat.

Derivation of iD-vDS Relationship

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