Let us discuss MOSFET in detail
MOSFET is an electronic device most widely used in ICs. It has three terminals named as source, drain and gate
Construction of basic design of MOSFET is quite simple. Just take a block of p-type semiconductor
This block will be your base for MOSFET. We also call it as substrate
And the entire MOSFET we will fabricate on this base. Now two portions on the same side of this p-type substrate
are made highly doped with n-type impurity as these are shown here in the figure
Source and drain terminals are brought out from these two end regions as shown
Now the entire surface of the substrate is coated with a layer of silicon dioxide
Silicon dioxide will behave here as insulation. On this layer of silicon dioxide a thin metallic plate is placed
This metallic plate will act as gate contact to the substrate. The gate terminal is brought out from this metallic plate
Hence it is seen from the figure that gate contact is insulated from the substrate
but drain and source contacts are directly connected to respective n-type regions
This was the constructional feature of a MOSFET. Now we will come to the operational features of MOSFET
For that we have to connect on DC voltage source between these two n-type regions
The n-type region connected with positive terminal of the source is called as drain
and the n-region connected with negative terminal of the source is called as source
As soon as the drain and source are connected with DC voltage source
there will be two p-n junctions formed. One is in between n-type drain and p-type base
and the other is in between p-type base and n-type source. As drain is connected with positive terminal of the source
the drain is in higher voltage than base. Hence this p-n junction is reverse biased
On the other hand as source is connected with negative terminal of the battery
p-type base will be at higher potential than n-type source. Hence this p-n junction is forward biased
As between drain and source there will be one reverse biased and one forward biased junction
There will be no significant current flowing from the drain to source
But a tiny current continues to flow due to minority carriers. We can say that virtually there is no continuous path or channel between drain and source
Now let us see what happens if one suitable positive voltage is applied at gate terminal of the device
In that case due to attraction or positive potential of gate contact plate
negative free electrons i.e. minority carriers in p-type base will be accumulated towards the contact plate
and form a channel of negative charge carriers even inside p-type semiconductor
This channel due to higher concentration of free electrons will behave as n-type semiconductor
although entire base is doped with p-type or trivalent impurity. It is obvious that the number of free electrons accumulated towards gate contact plate
depends on the strength of positive potential given to the gate terminal
Hence width of n-channel depends on the voltage applied at gate terminal
More the applied voltage, greater the width of the channel means greater the conductance between drain and source
Here we have seen that there is no direct connection between gate contact and substrate
Instead these are insulated from each other. But due to electrostatic attraction
the conducting channel between drain and source is formed and because of that the channel is also called induced channel
and the device is called induced channel MOSFET. It is also clear that when there is no voltage applied to the gate of the device
there will be no current from drain to source There will be no current from drain to source
except negligible amount of saturation current due to minority carriers. But if the gate voltage is gradually increased
by keeping fixed drain to source voltage after certain minimum applied gate voltage
drain to source current will start flowing and this voltage is called threshold voltage
So below threshold voltage the device will behave as non-conducting or in off mode
and for gate voltage greater than threshold voltage and for gate voltage greater than threshold value
the device will be in conducting or on mode. So this type of MOSFET is suitably used as a gate control switching device
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