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Characteristics Of MOSFET

 In the last post, we studied some basic concepts of MOSFET like different types of MOSFET, its construction, and its working. I hope you understood each and everything in that post. Now, in this post, we will study the characteristics of both types. 
1) Characteristics of N-type Depletion MOSFET:-
In the last post, we concluded that, although the internal structure of JFET and MOSFET are not similar, N-type Depletion MOSFET is almost similar to the N-type JFET.
Now, since their working is similar therefore we can say that their characteristics should also be similar. Therefore we proved that characteristic of N-type JFET is the same as N-type Depletion MOSFET.
That means the equation for output current(Id) would also remain the same.
Id = Idss*(1 - Vgs/Vp)^2

So in this post, we will fully focus on the characteristic of n-channel enhancement type MOSFET.

2) Characteristics of N-channel Enhancement MOSFET:-
Before studying characteristics of Enhancement type MOSFET let's recall the working of the BJT transistor. When the input voltage of BJT is less than 0.7 volts i.e. (Vin < Vbe), BJT works in the cutoff region or is off state. and when Vin is greater than 0.7 volts, i.e. (Vin > Vbe), BJT might work in an active or saturation mode depending on its biasing state.
So with the same logic, we will try to analyze the characteristic of N-channel Enhancement type MOSFET.
1) Transfer Characteristics:-
When a graph is plotted between the input voltage and output current by keeping the output voltage constant, that plot or that graph is known as Transfer characteristic.
In the last post, we saw the working of enhancement type MOSFET in great depth and there I used one term i.e. Vth. So the minimum gate to source voltage required to accumulate a sufficient number of the carrier and to form a channel between source and drain beneath the gate terminal is known as Vth or threshold voltage.
When we provide a positive voltage to the gate terminal or Vgs at first electrons (minority carriers) present in the p-substrate will move towards the gate terminal. When we increase Vgs such that it is greater than Vth i.e. (Vgs  > Vth), an n-channel is formed between drain and source and beneath the gate terminal.
Now as we keep on increasing Vgs so we end up getting the graph as shown below.

Diagram:- Graph of transfer characteristic of E-MOSFET.
So from the above graph, you can easily see that when Vgs < Vth MOSFET operates in the cutoff region and when Vgs > Vth it works in the saturation region, and thus the slope obtained from the graph is known as transconductance(gm). If you want to know what exactly is transconductance then I suggest reading the post of JFET where I had explained this concept in great depth.
So we can say that concept or logic of Vth is the same as Vbe in the BJT transistor. So to get your concept more clear I explained the BJT transistor and then I compared it with Vth in E-MOSFET.  
Also, from this graph, we end up getting one equation showing the relation between Id and Vgs.
Id = k × (Vgs - Vth)^2 
Id is drain output or Drain current
Vgs is Gate to Source voltage
Vth is the threshold voltage
K is a conduction parameter
Thus we can say that Id is directly proportional to Vgs
Note that the values of k and Vth vary from one E-MOSFET to the next and can not be physically changed. This is because they are of specific specification relating to the material and device geometry in-built during the transistor's fabrication.

2) Drain Source characteristics:-
When a graph is plotted between the output voltage and output current by keeping input voltage constant, that graph is known as drain-source characteristics. 
As you can see in the above diagram, these characteristics remain the same for JFET and MOSFET. So now we will understand that which regions E-MOSFET can be operated in. Look at the image down below.

So as you can see similar to D-MOSFET, E-MOSFET can be operated in 3 different regions. Although I had not explained these 3 different regions in D-MOSFET but I expect that you might have gone through my post on JFET there I had explained these 3 regions in depth. Since the working of the D-MOSFET is similar to that of JFET therefore I'm not explaining the D-MOSFET in that depth. 
The 3 regions of MOSFET are:-
1) Ohmic Region
2) Cuttoff Region and
3) Saturation Region
1) In the ohmic region MOSFET can be operated as a resistor but generally, in practical usage, we neglect the operation of the MOSFET in this region since it takes external energy to operate in a linear region whereas resistors don't take external energy to operate in the linear region. Also, resistors are very cheap as compared to MOSFET.
2) In the cutoff region it acts as an Open sitch or open circuit.
3) In the saturation region it can be operated as a closed switch or closed circuit and as an amplifier. 

So I hope you understood all the concepts which I discussed in this post if you have any doubts just post them in the comments section.


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