In this tutorial, we are going to learn about MOSFET constructions and working principles.
MOSFET – This MOSFET is of two types, one is Enhancement type MOSFET and other is Depletion type MOSFET. These both types of MOSFET’s are of two types like N channel and P channel MOSFET are available. In this tutorial we are going to learn on only N channels MOSFET. Firstly we will discuss on Enhancement type MOSFET and then we will discuss on Depletion type MOSFET.
Construction of MOSFET:
As per above image can see, the body of Enhancement n channel type is made from P-type material and it is known as substrate. On this body two N wells are form from there N type of material is diffuse. So this is N type material and other is P type material so the junction of P type material and N type material forms junction called PN junction. In the lower part of substrate a metal contact make from there a terminal releases its known as substrate terminal and body terminal. Same type of metal contact is available above N wells also with them two terminals are also available one is known as source terminal and other is known as drain terminal and in between both these terminal one thin layer is available it’s made from silicon dioxide. Silicon dioxide is insulator and if we say according to capacitance topic than it is dielectric and above the thin layer of silicon dioxide the one metal contact is presents from there one more terminal extends and it’s known as gate terminal. So in MOSFET four cases are available. But in most of the MOSFETS these source and substrate are internally connected due to this its converts into one terminal. So if in MOSFETS this type of construction is available than we see three terminals.
Working of MOSFET:
As per above circuit, these two terminals are ground and these two terminals are connected are ground therefore this side, this terminal side always on zero volt. so the construction of this is like that current flow is not possible basically in the field effect transistor the current is flowing from drain to source but there the current cannot flow from drain to source because there two p junctions are available. if you attaches the drain and source with a battery keep drain in higher potential and source in lower potential so the tendency of current flow is from drain to source but there this will be reverse biased because n type of material is available in higher potential and p type material is in lower potential so the current cannot flow from there the depletion region will be broad the barriers potential will be more so the current cannot flow in same way if you keep source in higher potential and drain in lower potential than also the current cannot flow because there the barrier potential will be increases so now this comes in reverse biased so in this way current cannot flow. so if want that current can flow for that we want different channel the creation of that channel is very interesting and for the creation of this channel connect the gate and source with battery its known as v genes. There gate is in higher potential and source is in lower potential and because source is grounded therefore it always in zero volt but gate is comes in positive potential higher potential. Due to this reason the channel forms is very interesting. see this body is made from p type material means according to majority charge carriers there many holes are available and on basis of minority charge carriers there many less fray electrons available now when this gate is comes in higher potential and this is in zero volt and this side is also available in zero volt but this side is comes in higher potential due to this these fray electrons move towards this so the electric field forms from upward to downwards so in the opposite direction of electric field these electrons push. So electron there collects now see the layer of silicon dioxide working you understand. The first function of this because it is insulator so the free electron not moving outside means this free electron there assembles this is the one function of silicon dioxide and the other function is also available if you think about parallel plate capacitor. A parallel plate capacitor connects with a battery than what happens in the internal faces of both plates there some positive charges and there some negative charges available but in between of these a dielectric inserts than what happens. than the capacitance of this parallel plate capacitor increases now the voltage source is same but still the more charges accumulate there therefore the capacitance of this increases in same way the silicon dioxide layer works there that this can accumulate more and more charges there as in case of parallel plate capacitor dielectric helps in accumulation of more charges. The same effect is available there also. so the reason of using silicon dioxide layer mainly two which you understands so free electrons firstly there accumulates when VGS value very lower due to this the P behavior of this decreases because there some free electrons comes the free electrons concentration increases so the P behavior slightly lower this happens when we apply VGS, then VGS value very low and if increase the value of this voltage source and places the more voltage battery than there electric field more wades, when electric field there is stronger than in this place atoms are available from them outer shell the electron moves outside.