Lets connect a dc voltage source between drain and
source as shown.
The gate is kept open.
Due to the voltage VDD ,the pn junction of
drain terminal will be reverse biased and the pn junction of source will be
forward biased.
Therefore, the equivalent circuit looks like as shown.
Therefore, no current flows between drain and source.
Although, a very very small reverse saturation current flows due to minority
carriers. This small current is negligible. So, we can say that there is no
continuous path between the drain and the source. The device is OFF.
Lets see what happens when a dc voltage source is
connected between the gate and the source as shown.
If you focus on gate terminal, you may find that the
gate metallic plate, silicon dioxide layer below it, and the p-region between two n-regions, makes a
capacitor. Therefore, the gate of a MOSFET behaves pretty much like a
capacitor.
So, a current will flow between the source and the
gate when VDD is connected and the capacitor charges as shown in above
figure. When the capacitor gets fully charged, the current flow between the source
and the gate stops.
As the , p-region between the two n-regions , acts as
one plate of the capacitor , therefore, negative charge (electrons) is accumulated
in it. These electrons formed a channel between the two n-regions and now
current can flow from drain to source.
Obviously, from capacitor basics ( charge = capacitance × Potential difference
) , the number of free electrons accumulated in p-substrate depends on the strength of the +ve
potential given to the gate terminal.
Hence, width of the induced n-channel
depends on the voltage applied at the gate terminal. More the applied voltage,
greater the width of the channel (lesser the resistance of the channel), means
greater the current from drain to source. So, the voltage applied to the gate
controls the current through the device. That is why MOSFET is called a voltage-controlled device.
The minimum value of VGS due to which the channel is completely formed
and the drain current just starts to flow is called the threshold voltage (VT) of a MOSFET. The voltage VGS must be greater than VT .
This is the working of a MOSFET in general or more specifically working
principle of a N-channel enhancement MOSFET.
