Enhancement type MOSFET - construction, working, characteristic

Enhancement type MOSFET 

In this article , we will learn about the construction,working and the characteristics of the enhancement type of MOSFET. (you can check out for JFET and FET

Construction of enhancement type MOSFET (n channel) 

    Now, if we talk about the construction then in some aspects it is similar to the depletion type of MOSFET. 

Construction of enhancement type MOSFET

    So, if we see the construction of the n-channel enhancement type of MOSFET then the substrate is made up of p-type material and through the metallic contacts, the drain and the source terminals are connected to this n-type material. 
  And similar to the depletion type of MOSFET,the oxide layer isolates the gate terminal and the p-type substrate. But here unlike the depletion type of MOSFET,there is no channel between this drain and the source terminal.
 So, whenever we apply the control voltage,between the gate and the source terminal, then the channel is formed between  drain and the source terminal.
  So, here in this type of MOSFET, basically the application of the control voltage enhances the number of charge carrier . And due to that, the channel is getting created. 
   And that is why this type of MOSFET is known as the enhancement type of MOSFET. 

    Working of then-channel enhancement type of MOSFET 

let's understand how the channel is formed whenever we apply the control voltage. 
   
Working of enhancement type MOSFET

So, initially, here if Vgs is kept is zero and the voltage is applied between this drain and the source terminal ,then due to the absenceof the channel, there will not be any flow of current through this MOSFET. 
   So, whenever Vgs is zero, then this MOSFET will remain in the OFF condition.         when we apply the positive value of Vgs. , the substrate and the source terminals are connected together. And they are connected to the ground terminal. 
    The  positive voltage is applied between the gate and the source terminal. And for a moment, let's assume that  voltage Vds is equal to zero. 
    Now, holes are the majority carriers in the p-type substrate. And whenever we apply the positive voltage at this gate terminal, then the holes which are near the oxide layer will be pushed awayfrom the gate terminal. 
   And at the same time, the electrons which are the minority carriers in the p-type substrate will also get attracted towards the gate terminal.
    But at the lower voltage of Vgs, these electrons will get recombined with the majority charge carriers. 
   Now, as we keep on increasing this voltage Vgs, then the holes will be pushed more and more deeper into the substrate  and the electrons will be able to overcome the recombination with these holes. 
    And they will be rushed towards the gate terminal. But due to this insulating layer, they will not be able to cross this oxide layer. And they will start accumulating near the oxide layer. 
  So, eventually, the inversion layer of free electrons will get created near this oxide layer. And now this inversion layer will act as a channel between this drain and the source terminal. 
     now, suppose if we apply the voltage between the drain and the source terminal, then the current can flow through this channel. 
  So, the value of the gate to source voltage at which this inversion layer is getting created is known as the threshold voltage. below this threshold voltage, there will not be any flow of current through the MOSFET. 
    whenever the Vgs is greater than this threshold voltage, then the width of the channel will increase. So, in this way, due to the application of the voltage Vgs, the channel is formed between the drain and the source terminal. 
   But along with this channel, there will also be a depletion layer around this channel. Because if you observe, there are two PN junctionswhich are reversed biased. So, the first PN -junction is formed between the drain and the substrate. And the second PN - junction is formed between the substrate and the source terminal. And if you observe,  both PN -junctions are reverse biased. So, now we will consider that this voltage Vgs is already greater than the threshold voltage. 
    now let's see what happens when we apply the voltage Vds. 
  So, when we apply the voltage Vds, then through the channel electrons get attracted towards this positive terminal. And in this way, the current will establish in this circuit. 
  And the conventional current will flow from the drain terminal towards the source terminal. But now if you observe, the width of the channelhas been reduced towards the drain side. Because now, due to the positive voltage at the drain terminal, the PN junction will get more reversed biased. And due to that, the width of the depletion region will increase. So, because of that, the effective channel width towards the drain terminal will reduce. 
   And the same phenomenon can be also explained in another way. So, once we apply the drain to source voltage,then the voltage difference between this gate and the drain terminal will reduce. So, the voltage difference between these two terminals will be equal to Vgs - Vds
    as the source or the substrate terminal is grounded, so we can say that the difference will be equal to Vg - Vd. So, as the value of the voltage Vd will increase,then the difference between these two voltages will reduce. On the other end, this source terminal is connected to the ground terminal. So, the voltage difference between the gate and the source terminal will remain as it is. 
   So, due to that, the gate terminal which is towards the drain side will be less positive than the other side. And hence, this region will attract few erelectrons compared to the other side. And due to this reason, the channel width gets narrower as we go from the source terminal towards the drain terminal. 

    And as we keep on increasing the  voltage Vds, then at one particular voltage, the pinch-off condition will occur. So, at that particular voltage, the drain current which is flowing through the circuit will get saturated. So, the voltage Vds, at which this pinch-off condition occurs is known as the saturation voltage. 
saturation voltage can be expressed as
       Vgs - Vt. 
  Where Vt is the threshold voltage. 
   That means the pinch-off condition will occur whenever the difference between the gate and the drain terminal is just equal to the threshold voltage. 
  At  the threshold voltage, the channel is just getting created between the drain and the source terminal. So, for the fixed value of Vgs, if we further increase the value of Vds, the voltage difference between the gate and the drain terminal willbe even lesser than this threshold voltage. And due to that, the channel will not get formed towards the drain terminal. 
    So, it appears that the current through the channel should become zero. But actually if you see, still the currentwill flow through this channel and the current 'Id' will get saturated. Because the electrons which are passing through this channel can still be able to cross this depletion layer due to the electric force.
    So, once the pinch-off condition occurs, then the current Id gets saturated. And even if we increase the value of voltage Vds, still the current through this circuit will remain almost constant.

 Drain curves or the Id vs Vds curves for the different value of Vgs

Drain characteristic


       As the value of Vgs will increase, the current Id will also increase. 
    this parabolic curve shows the locus of the voltage Vds, where the drain current Id will get saturated. 
  In this graph, the left region of curve   is known as either linear of the ohmic region. So, in this region, the MOSFET can be operated as a voltage controlled resistor.    For the fixed value of Vds, as we change the value of voltage Vgs, then the width of the channel will change. Or we can say that effectively the channel resistance will change.
     So, whenever the Vds is less than Vgs - Vt, and Vgs is greater than Vt,  in that case, the MOSFET is operated in this linear region. And in this region, it can be operated as a voltage controlled resistor

    Then if we talk about the next region, thenit is the cut-off region(the axis of Vds region). 
    So, whenever, this voltage Vgs is less thanVt, in that case, the current through the MOSFET is zero. Or we can say that the MOSFET will remainin the OFF condition. 

  Region of operation/ saturation region 

  Whenever the MOSFET is operated on the right-hand side of the locus, then we can say that it is operating in the saturation region. 
    Mathematically we can say that whenever this voltage Vds is greater than or equal to Vgs - Vt, at that time it is operating in this saturation region.
   

 Transfer characteristics

   
Transfer characteristic

Transfer characteristic shows the relationship between the input voltage Vgs and the output drain current Id. 
  So, basically, it shows how the drain current Id changes as we change the value of voltage Vgs.
    While plotting this characteristic the voltage Vds has been kept constant. 
    So, as you can see, up to the threshold voltage,the drain current Id is zero. And after that, as we increase the value of voltage Vgs, then the drain current Id will increase. 
  The relationship between the current and the voltage Vgs can be given as 
       Id = k (Vgs - Vt)^2. 
Where K is the device constant and it depends on the physical parameters of the device.
     So, using this expression, we can find the value of drain current for the fixed value of Vgs. Alright, so far we have discussed the n-channel MOSFET. 
   This is all about n channel enhancement type MOSFET. Thank you. 
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