In the last post, we had seen Ac analysis of JFET and there we had completed the whole JFET. Until now we had studied all the concepts of BJT and JFET in great depth. Now from this post, we will be starting with MOSFET. The full form of MOSFET is Metal Oxide Semiconductor Field Effect Transistors. In the end, it will be very much clear to you, why it is called metal-oxide field-effect transistors.
1) Types of MOSFET:-
There is a total of 2 types of MOSFET i.e. Depletion-MOSFET (D-MOSFET) and Enhancement-MOSFET (E-MOSFET). Further, both are divided into 2 i.e. N-type and P-type. While explaining the construction part of MOSFET you will understand these 2 types of MOSFET in great depth but for now, just remember the symbol of both the MOSFET.
Symbol:-
NOTE:- MOSFET can be a 3 terminal device or 4 terminal device. Sometimes substrate is shorted with the source internally at that time we can say that it is a 3 terminal device if the substrate is not connected with the source internally then we can see 4- terminals.
2) Construction of MOSFET:-
Since there are 2 types of MOSFET therefore we will try to understand D-MOSFET and at the end E-MOSFET.
Now, as I had explained in BJT that while dealing with transistors we are mostly concerned with N-type transistors because they are very much faster as compared to P-type but up till the end of MOSFET, you will be getting detailed information on n-type depletion MOSFET and n-channel enhancement type MOSFET. Since you are getting detailed information on N-type depletion MOSFET and n-channel enhancement type MOSFET therefore I hope you will learn P-type depletion MOSFET and p-channel enhancement type MOSFET on your own.
# Depletion type MOSFET:-
As you can see above that both drain and source terminal is made up of highly doped n-type material(electrons) while the substrate is made up of p-type material(holes) and an extra layer of n-type material is doped between 2 n-type and 1 p-type dopant. This extra n-type dopant is lightly doped with electrons but still contains more electrons as compared to the P-type region while the gate is insulated from this extra n-type dopant by a thin SiO2 layer. SiO2 is a type of insulator referred to as dielectric, which sets up an opposing electric field within the dielectric when exposed to an externally applied field. Drain and Source terminals are connected to the metal surface and this metal surface is connected directly to the highly doped n-type dopant. Since SiO2 acts as a good insulator, therefore, MOSFET provides a very high input impedance than JFET.
# Enhancement type MOSFET:-
As you can see above that the construction of n-channel-E-MOSFET is somewhat similar to N-type D-MOSFET. The major difference between both is that there is no extra n-type dopant that was present in N-type D-MOSFET near to gate terminal or in between drain, source, and a gate terminal. Thus apart from that extra n-type dopant rest, all the things are the same when compared to N-type D-MOSFET.
3) Working of MOSFET:-
I hope you understood the construction of MOSFET because if you want to understand it's working then its construction part must be very much clear.
# Depletion MOSFET:-
1) When we apply a positive voltage to the gate terminal:-
As you can see in the internal structure that when we are providing positive voltage at the gate terminal then more electrons are swept from source to gate terminal and further these electrons are moved to the drain terminal because positive voltage(VDD) is applied to the drain terminal. Since electrons move very quickly from source to drain and both the voltages are positive therefore current flowing through the MOSFET would be maximum such that it would be greater than drain to source saturation current i.e. Id > Idss. Due to all these reasons we never provide a positive voltage at the gate terminal of N-type D-MOSFET.
2) When the gate terminal is grounded (Vg = 0v):-
As you can see in the internal structure that when the gate terminal is grounded then the electrons in that lightly doped n-type region are repelled towards the p-type region due to the negative voltage of VDD and they move away from the gate terminal but in this process, they don't enter in the p-type region because of the small depletion region. Now, due to the positive voltage at the drain terminal, the electrons get attracted towards the drain terminal which means the electrons from the source terminal are swept to the drain terminal. Since the majority of electrons are swept from source to drain therefore maximum current flows from source to drain when the gate terminal is grounded.
Now if we apply a negative voltage at the gate terminal then the electrons present in that less doped n-type region will move towards the p-type region but in this case, the number of electrons moving towards the p-type region would be more as compared, when the gate terminal was grounded. Since more electrons are repelled by the gate terminal, therefore, the majority of electrons take part in the increasing depletion region while few electrons take part in conduction which means the current I will decrease and thus we can say that the gate terminal is acting as a variable resistor. Similarly, if we keep on decreasing gate voltage i.e. if we make gate voltage more negative then the current decreases and gradually becomes zero because of the increase in the depletion region.
Conclusion of D-MOSFET:-
1) When we apply positive voltage at the gate terminal:- Maximum current will flow (I > Idss)
2) When we apply Zero voltage at the gate terminal:- Current I will flow such that, (0 < I < Idss)
3) When we apply negative voltage at the gate terminal:- Minimum current flow (I = Imin almost zero) and if we keep on making it more negative then no current will flow.
From the conclusion part, we can say that the working of N-type depletion MOSFET is almost similar to the N-type JFET.
# Enhancement MOSFET:-
1) When we apply negative voltage at the gate terminal:-
As you can see in the internal structure that when we are providing negative voltage at the gate terminal then the holes present in a p-type substrate are attracted towards negative voltage provided at the gate terminal and move towards the gate terminal. Now if you can see clearly that the source terminal has an n-type dopant and the gate terminal has a p-type dopant and we are providing positive voltage at the source terminal and negative voltage at the gate terminal i.e. we can say that it will act like a p-n junction diode which is reverse biased. Therefore current flowing from source to drain (I = 0mA) is zero.
2) When we apply positive voltage at the gate terminal:-
As you can see in the internal structure that when we are providing positive voltage at the gate terminal then the minority carriers in p-type substrate i.e. electrons move towards the gate terminal. As you can see in the above figure, an extra n-type layer is created between the gate terminal and the p-type substrate but for that Vgs should be greater than or equal to Vth (In the next post I will explain the concept of Vth in depth). We can say that we have converted P-type E-MOSFET into N-type D-MOSFET just by applying a positive voltage at the gate terminal. Thus due to the introduction of positive voltage at the drain terminal electrons from the source terminal are swept to the drain terminal with the help of that n-type layer which we have created by applying a positive voltage at the gate terminal. Therefore current will flow from drain to source and in this case, the value of the current would be maximum i.e. (I = Imax).
Now if we keep on increasing positive voltage at the drain terminal and the substrate is grounded that means we are providing high voltage to the n-type region and less voltage to the p-type region thus we can say that it acts like a reverse-biased p-n junction diode. Therefore the depletion region near the drain terminal increases due to which the electrons can't move from source to drain and the current gradually decreases and eventually it becomes zero (I = Imin).
Conclusion of E-MOSFET:-
1) When we apply negative voltage at the gate terminal:- No current will flow (I = 0mA)
2) When we apply positive voltage at the gate terminal:- Maximum current will flow (I = Imax)
3) When we keep on increasing positive voltage at the drain terminal:- Minimum current flow (I = Imin) and at a certain point, the current becomes zero (I = 0mA)
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