Depletion type MOSFET : construction , working and characteristics

 In the earlier article  of the field effect transistor, we have briefly discussed about the different types of FET. And in detail we have already discussed about the JFET.

      So in this article  let discuss  the second type of FET, which is known as IGFET and here this IGFET stands for insulated gate field effect transistor .

     So  in this IGFET, the gate terminal is isolated from the channel using the insulating layer and the MOSFET is the most common type of IGFET. 

    So here this MOSFET stands for metal-oxide-semiconductor field-effecttransistor.  

   Classification of MOSFET 

   MOSFET can be further classified as 

  •  depletion type of MOSFET 
  •  enhancement type of MOSFET. 

 Depletion type  MOSFET 


N channel depletion type MOSFET 

Construction of n channel depletion type MOSFET 

Construction of depletion type of MOSFET

  •       If you see this n-channel depletion type of MOSFET then the channel is made up of n-type material and the substrate is p-type material.  
  •      Through the metallic contacts the drain and the source terminals are connected to this n-channel and similarly the gate terminal is also connected through this metallic contact. 
  •   There is no direct connection between  N channel and gate terminal. And the gate terminal is isolated from the channel using this SiO2 layer. 
  •    MOSFET  is consists of the metallic contacts for  drain, gate and the source terminals, then the  insulating layer and the conducting channel which is made up of the semiconductor material. And that is why this MOSFET is known as themetal-oxide-semiconductor field-effect transistor. 
  •    Now due to this insulating layer there will not be any flow of current through this gate terminal. Or we can say that the input impedance of the  gate terminal is very high and in fact it is even higher than the JFETs. (And that is why these MOSFETs are used inthe application where the minimum power consumption is required). 

 Working of depletion type MOSFET

Working of depletion type of MOSFET

 Initially let us assume that the gate and the source terminals are connected together. And they are connected to the ground terminal, means initially let us assume that Vgs is equal to zero volt. 

When VDS is positive , of depletion type MOSFET 

  • The positive voltage is applied between this drain and the source terminal. So as soon as we apply the positive voltage then the electrons in this N channel will get attracted towards the positive terminal.
  •  the electron starts moving towards the drain terminal from source terminal.  And in this way the current will establish in  N channel. 
  •  if we keep on increasing the voltage between the  drain and the source terminal then the current which is flowing through the channel will increase. 
  • this process will continue until all the electrons in this channel will contributes in the flow of current.And then after if we increase the voltage then the current ,which is flowing through the channel , will become constant.
  •  so if you see the direction ofthe conventional current then it will flow from the drain terminal towards the source terminal. 
  •  For the Vgs is equal to zero, the output or the drain characteristic then it will look like this. 

Depletion type MOSFET output characteristic

That means as we keep on increasing the value of voltage VDS then the drain current ID will increase.And after certain voltage, the drain current ID will become constant.


          The value of the saturation current for Vgs is equal to zero is known as the IDSS.

    

When VGS is negative , of depletion type MOSFET 


Depletion type MOSFET at negative voltage

  •       when the voltage Vgs is negative, the negative voltage , the gate terminal will push the electrons towards the substrate and at the same time the holes in the p-type substrate will also get attracted towards these electrons.
  •  So ,due to the negative voltage at the gate terminal the electrons in the channel will get recombined with this holes. And the rate of the recombination will depend on the applied negative voltage.
  •     so as we increase this negative voltage then the rate of recombination will increase. And that will reduce the number of free electrons which is available in this n-channel. And effectively it reduces the flow of current.

Depletion type MOSFET output characteristic

     So as you can see from the graph, as the value of VGS  will become more and more negative,then the value of drain current will reduce.

[ What is pinch-off voltage? 

     And at one voltage this drain current will become zero. so this voltage Vgs is known as the pinch-off voltage. ]

 So if you see the drain or the output characteristic of the MOSFET then it looks quite similar to the JFET. But this MOSFET also works for the positive values of  Vgs. 

   what happens when we apply the positive voltage (VGS) ?

Whenever we apply the positive voltage at the gate terminal then the electrons which are minority carriers in p-type substrate will also get attracted towards this n-channel. And due to that,the number of free electrons in this N channel will increase. so effectively we can say that the flow of current in this n-channel will increase. so for the positive value of voltage Vgs the drain current ID will be even more than this IDSS

Transfer characteristic of depletion type MOSFET 

       Transfer characteristic is defines as  the relationship between the input and the output quantity. so basically it defines the relationship between the drain current ID and  VGS for the fixed value of VDS

     so if you see the transfer characteristic then it will be similar to the JFET. But now you will also get the value of current ID for the positive values of VGS.  So due to that the curve will get extended towards the right-hand side.

Transfer characteristic of depletion type MOSFET

      Now as we have seen whenever this  VGS is positive then the number of free electrons in the channel will increase and due to that this region where the VGS is positive, is known as the enhancement region and the region where the  VGS   is negative is known as depletion region.

     But still the relationship between this current ID and the voltage VGS can be expressed by the same expression 

Expression for transfer characteristic


       So using this expression we can find the value ofdrain current ID for the given value of VGS . 

P channel depletion type MOSFET 

P channel depletion type MOSFET

So similarly let us briefly discuss about the p-channel type of MOSFET . So in case of a p-channel depletion type MOSFET the channel is made up of p-type semiconductor material and the substrate is n-type.  

     For the P channel MOSFET, now the polarity of the applied voltage will also get reversed that means this voltage VDS will be negative and this voltage VGS will be positive .

    let us see how the current will flow whenever  VGS is equal to 0. So when Vgs is equal to 0 and Vds is applied in this fashion that means when Vds is negative then the holes in this p-type channelwill get attracted towards the negative terminal and the flow of holes will be established in this fashion.

P channel depletion type MOSFET

 And in this case the conventional current will also flow in the same direction. now whenever we apply the positive value of voltage VGS then the holes will be pushed towards the n-type substrate and at the same time the electrons in this n-type substrate will also get attracted towards the p-type channel.

        Due to that this holes and the electrons will get recombined and as we keep on increasing this voltage VGS  then the number of holes in this p-type channel will reduce and effectively the flow of current in this p-type channel will reduce.

Output characteristic of p channel depletion type MOSFET

      So if you see the drain or the output characteristic of this p-channel MOSFET then it will look like this. buthere this voltage VDS is negative and the voltage VGS is positive. So as you can see as we keep on increasing this voltage VGS then the drain current ID will reduce and at the pinch off voltage this drain current  will become zero. and whenever this VGS is negative then the value of drain current will be even higher than the VDSS


Transfer characteristic of p channel depletion type MOSFET

        similarly if you see the transfer characteristic then it will look like this. 

Symbol of depletion type MOSFET 

      So now let us seethe electronic symbols of this n-channel and p-channel MOSFETs. 

Symbol of depletion type MOSFET

So if you see the symbols of depletion type of MOSFET then they resembles the actual construction of the MOSFET. 

     it consists of a three terminals that is gate, drain and the source .

 there is a space between this gate terminal and this channel.

    line which connects the drain and the source terminal represents the channel. 

 the space between this gate terminal and the channel represents that the gate terminal is isolated from the channel. 

   Now if you observe the n-channel and the p-channel MOSFETs then the only difference between the two symbol is the direction of the arrow. 

    If it is going inward then it indicates the n-channel MOSFET and if it is going outwards then it represents the p-channel MOSFET. 

basically it indicates the direction of the flow of current whenever the PN Junction which is formed by the channel and the substrate is forward biased. 

      So incase of the N channel MOSFET whenever this PN Junction is forward biased then the current will flow in this direction and similarly for the P channel MOSFET whenever this PN Junction is forward bias then current will flow in the outward direction. So basically by the direction of the arrow we can differentiate these two symbols. 

  So I hope in this article  you understood the construction, working and the different characteristic of this depletion type MOSFET. So similarly in the upcoming articles we will learn about the enhancement type of MOSFET. So if you have any question or suggestion do let me know here in the comment section below. If you like this article hit comment below  and do email  subscribe  for more such amazing article.  

Previous Post
Next Post

post written by:

0 comments:

Please do not enter any spam link in the comments