N channel E-MOSFET in the easiest way possible-: Construction and Working+Drain characteristics and Transfer characteristics-FULL 8 MARKS in exam

All about Mosfet for electronics devices and circuits.

MOSFET is called a metal oxide field effect transistor because oxide layer insulates the gate electrode from device body

 

Why JFET is useless compared to MOSFET?

Because MOSFET has high impedance at input than JFET. So JFET is useless.

 

Types of MOSFET

  • N-channel
  • P-channel

And

  • enhancement type
  • depletion type

In enhancement type MOSFET, channel is initially absent so it has to be enhanced.

But in depletion type MOSFET, channel is already present so it can be depleted too.

 

Put image-: of mosfet

Construction of MOSFET-:

We can see the 4 terminals in MOSFET

  • source (S)
  • drain(D)
  • gate(G)
  • body(B)

initially, p-type substrate is put on transistor to make it a MOSFET. It helps for the following things-:

 

  • That gives rise to two n+ regions namely S & D respectively.
  • It helps to form SiO2 which acts as an insulator to gate G.

 

 

Working of n-channel mosfet

 

The working of n-channel mosfet can be classified into 4 different cases.

  • VGS=0
  • VGS=positive
  • VDS small
  • VDS bigger

They are all explained as follows-:

  • VGS=0

This mean no potential difference or voltage applied to the gate terminal.

(VG-VS=0).

 

So no current flows (from S to D.)

 

  • VGS=+ve

In this case, holes that are near gate G are repelled because positive potential is applied as shown in figure.

 

 

 

So the remaining charges must be electrons. Those electrons starts to collect at one point. And that is called “a channel is formed-specially a n-channel is formed”.

 

This channel connects S and D. This channel is enhanced day per day (joking; not day per day, but gradually). Obviously the channel will not be induced as soon as we applied VGS=+ve, but after that VGS ­goes to a certain value called threshold voltage VT.

 

Now the role of VGS is finished since the n-channel is already formed.

Now comes the role of VDS.

  • VDS=small but positive

In the case we have small VDS with VGS>VT(we assumed that VGS=VT has already happened).

Now say that VDS=0.1 V to 0.2V (i.e VDS is small and positive); now due to the positive VDS, { Note-:This is just a mnemonic to understand it better. And n+ region connected to both drain and source will cause forward bias and} allows the flow of drain current.

By experiments, it was found that the drain current is linearly dependent to drain to source voltage.

Its graph is shown below.

the above figure will not be called as drain characteristics in  many books but it is absolutely a part of drain characteristics. the only difference with those books is that the saturation current is not drawn in this. so better draw the below diagram for exam to not create any confusion.

 

 

 

  • VDS is increased

When VDS is increased the following things will happen.

 

We know that VDS means voltage from drain to source.

Now say we have increased VDS to a voltage 6V.

 

VDS =6=VD-VS

i.e voltage at drain is higer than at source.

This means that the voltage goes on decreasing from drain to source.

This is the reason for the channel being tapered in shape.

If VDS  is increased in such a way that it has the power to cancel the voltage VGS-VT (VGS>VT); then the channel is said to be at pinch off region.

(in the pinch off region, the channel depth is almost zero)

 

And at this point no further drain current stops increasing. That is to say that the drain current saturates.

 

Draw the transfer and drain characteristics of n-channel e-mosfet

Transfer characteristics of n channel e-mosfet

The graph between iD and VGS is called transfer characteristics.

 

ID=K(VGS-VGST)

is the equation followed by the above curve (the best way to remember this transfer characteristics is just this-don’t search further information they will just confuse you)

2) Drain characteristics of n-channel E-MOSFET

 

VDS=VGS-VT

Is important because it separates 2 regions called as triode region and saturation region.

 

Cut off region also exist there when VGS<VT.

I have tried to explain this in a best possible way without bombarding information. 

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