Question

2. (CLO_1): (Cognitive Level C2, i.e., Comprehension) (PLO_1, i.e., Engineering Knowledge) Explain the following devices with the help of the construction diagram and device operation
   1. n-channel JFET
   2. n-channel Depletion MOSFET III-     n-channel Depletion MESFET

Answer 1

I) n-channel JFET

The n-chnnel JFET is made of mostly n type semiconductor. The drain and source of the JFET is placed at mutually opposite and they are made by n type semiconductor. There are 2 p-regions present which are connected to each other and form gate terminal. So, the source and drain terminal are made of n type and the gate terminal is made of p type semiconductor.

To turn on the JFET we have to apply a positive voltage at the drain terminal with no voltage applied on the gate terminal. So due to this postive voltage at the drain, the electron will flow and it will create drain to source current.So, when the gate voltage is 0,i.e Vg = 0 v then the drain to source current is maximum which will make the transistor ON or in saturation region.

So if the Drain voltage Vd > 0 V and Gate voltage Vg = 0 volt the JFET will be turned ON and maximum current will flow through it.

To turn off the JFET either we have to give no bias voltage at drain terminal then the transistor will not turn ON or we can apply a certain negative gate voltage, to make the transitor off. If we apply negative voltage then the drain current will start to decrease and if we keep decreasing the gate voltage then the transistor will reach cutoff region and drain current will be zero.

There are 4 region of operation in JFET

i) Cutoff Region : - In this region the JFET transistor is OFF so, I_D = 0 .

ii) Ohmic Region: - In this region the drain current will face some resistance and the drain crrent is statrted to flow from drain to source.

iii) saturation region - In this region the Drain current is maximum and the transistor is fully ON.

iv) Breakdown Region - In this region the drain applied voltage exceeds the maximum voltage and will cause a high current flow from drain to source which will cause the breakdown of the JFET.

II) n-channel Depletion MOSFET

It consists of a highly doped P-type substrate and two highly doped n type substrate which will form source and drain. An n-channel is formed by diffusion between the source and drain. Now a thin layer of SiO₂ dielectric is grown over the entire surface. Metal is deposited to form drain and source terminals, and on the surface between drain and source, a metal plate is deposited.

Deplition MOSFET can be operated negative gate voltage. So at negative gate voltage the MOSFET will operate at depletion mode.

At negative gate voltage gate repels some of the negative charge carriers out of the n-channel. This creates a depletion region in the channel, which increases the channel resistance and reduces the drain current. The more negative the gate, the less the drain current. In this mode of operation the device is referred to as a deplition-mode MOSFET.

III)  n-channel Depletion MESFET:

In depletion mode MESFETs by making the gate more negative the depletion channel is enlarged, because negative electrons are repelled by the negative gate voltage, in P-channel devices making the gate voltage more positive achieves this. In enhancement mode devices the depletion zone normally covers the whole channel at Vg=Vs and no drain current flows, only when a positive is applied the depletion channel reduces, allowing for current flow.