Test: MOSFET-1 - Electronics and Communication Engineering (ECE) MCQ

Thyristor (Controlled & Semi-controlled device):

• Thyristors are a broad classification of bipolar-conducting semiconductor devices having four (or more) alternating N-P-N-P layers.
• It acts exclusively as a bistable switch and starts conducting when the gate receives a current trigger, and continues to conduct until the voltage across the device is reversed biased, or until the voltage is removed by some external circuitry.

Classification of thyristor family:

• Among the following options, MOSFET does not belong to the Controlled & Semi-controlled device family.

In a series connection, the IV characteristic is obtained by the intersection of current and union of voltage.

Therefore, option 4 is correct.

FET is classified based on the channel in each type.
Based on the channel there are two types:
1) n – channel FET
2) p – channel FET
Based on the formation of the channel
1) JFET
2) Depletion  MOSFET
3) Enhancement MOSFET
Field Effect Transistor also has the fourth terminal which is called as “ Body or Substrate”
n – channel JFET

p – Channel JFET

p – channel Depletion MOSFET

n – Channel Enhancement MOSFET

p – channel Enhancement MOSFET

As shown in the above figure in a power MOSFET, pinch-off occurs when V_DS = V_GS - V_T
Where, V_DS = Drain to source voltage
V_GS = Gate to source voltage
V_T = Threshold voltage
In power MOSFET when V_DS < V_GS - V_T, then power MOSFET works in the triode region.
In power MOSFET when V_DS > V_GS - V_T, then power MOSFET works in the saturation region.

MOSFET:
MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor.
It is a majority carrier device and also called as Voltage controlled current device (VGS control the current ID)

Operation of the MOSFET:
The drain characteristics or the plot between ID and VDS is shown below

The drain characteristics are shown in below tabular form.

V_GS = Gate-Source voltage
I_D = Drain current
V_DS = Drain-Source voltage

• MOSFETs are majority carrier devices that mean flow of current inside the device is carried out either flow of electrons (N-Channel MOSFET) or flow of holes (P-Channel MOSFET).
• So, when the device turns off, the reverse recombination process will not happen. It leads to short turn ON/OFF times.
• As switching time is less, loss associated with it less and hence it gives the highest switching speed.

For a MOSFET in saturation, the current is given by:

W = Width of the Gate
Cox = Oxide Capacitance
μ = Mobility of the carrier
L = Channel Length
Vth = Threshold voltage

Calculation:
The drain conductance (gd) is calculated as the rate of change of drain current with respect to the Drain to source voltage, i.e.

Putting on the respective values, we get:

At V_gs = 0, no current flows through the MOS transistors channel because the field effect around the gate is insufficient to create or open the n-type channel. Then the transistor is in its cut-off region acting as an open switch.

As we now gradually increase the positive gate-source voltage Vgs, the field effect begins to enhance the channel regions conductivity and there becomes a point where the channel starts to conduct. This point is known as the threshold voltage V_th. As we increase V_gs more positive, the conductive channel becomes wider (less resistance) with the amount of drain current (Id).

Therefore, the n-channel enhancement MOSFET will be in its cut-off mode when the gate-source voltage (VGS) is less than its threshold voltage level (V_th) and its channel conducts or saturates when V_gs is above this threshold level.

FET is classified based on the channel in each type.

Based on the channel there are two types:

1) n – channel FET

2) p – channel FET

Based on the formation of the channel

1) JFET

2) Depletion  MOSFET

3) Enhancement MOSFET

Field Effect Transistor also has the fourth terminal which is called as “ Body or Substrate”

n – channel JFET

p – Channel JFET

n – channel Depletion MOSFET

p – channel Depletion MOSFET

n – Channel Enhancement MOSFET

p – channel Enhancement MOSFET

In Depletion MOSFET channel is initially formed whereas in Enhancement MOSFET it is not. So channel in Enhancement MOSFET is represented by the dotted symbol.
Conclusion: From the given figure as the channel is thick line it is Depletion MOSFET. The direction of the arrow is inside, so it represents n – channel.

For a MOSFET operating in the linear region, the current is given by:

 

W = Width of the Gate
C_ox = Oxide Capacitance
μ = Mobility of the carrier
L = Channel Length
V_th = Threshold voltage
The transconductance is defined as the change in drain current for a given change in Gate-to-source voltage, i.e.

Differentiation equation (1) with V_GS we get:

Hence option (2) is the correct answer.