Test: MOSFET - 2 - Electronics and Communication Engineering (ECE) MCQ

The drain current in saturation for a JFET is given by:

I_DSS = Saturation Drain Current
V_p = Pinch Off Voltage
The transconductance (g_m) is defined as the change in I_D with respect to a change in the gate to source voltage (V_GS), i.e.

Taking the differentiation of Equation (1), we get:

The maximum value of g_m occurs when V_GS = 0

Calculation:
Given:
I_DSS = 1 mA, Vp = -8V
We can write:
Maximum transconductance

 

The current equations for the MOSFET in different regions are shown below:
When the V_gs < V_t
NMOS operates in the cutoff region and current I_D = 0
When V_gs > V_t and V_ds < V_gs - V_t 
NMOS operates in the linear region
The current is defined as:

NMOS operates in the saturation region
The current is defined as:

Hence, we can see that in the saturation region the relation between I_d and V_gs is quadratic.
Analysis:

The current equation for a depletion type JFET is given by:

I_DSS = Saturation Current of the JFET
V_GS = Gate to source voltage applied
V_P = Pinch off voltage at which I_D = I_DSS, which is also the voltage at which the channel ceases to exist. It is also denoted by V_GS(off).
The transconductance (gm) is defined as the change in ID with respect to a change in the gate to source voltage (V_GS), i.e.

Taking the differentiation of Equation (1), we get:

The maximum value of gm occurs when V_gs = 0

g_m0 is the maximum transconductance at V_GS = 0

The characteristic graph for a change in V_GS is as shown:

The drain current in MOSFET in saturation is given by:

Observation:
From the equation, we can observe that the MOSFET’s drain current depends on:
1) Device parameters W/L
2) Gate to Source Voltage V_GS
To realize this physically, as the Gate to source voltage is increased, the channel becomes deeper, hence there is more area through which electrons can move.

MOSFET (Metal Oxide Semiconductor Field Effect Transistor):
MOSFET transistor is a semiconductor device that is used for amplifying and switching electronic signals in electronic devices.

MOSFET is of two types:
1. Enhancement MOSFET:

• In this kind of MOSFET, there is no predefined channel. The channel is constructed using the gate to source applied voltage.
• The more is the voltage on the gate, the better the device can conduct.

2. Depletion mode MOSFET:

• In this type of MOSFET, the channel (between drain and source) is predefined and the MOSFET conducts without any application of the gate voltage.
• As the voltage on the gate is either positive or negative,  the channel conductivity decreases.
• Depletion MOSFET can work in both depletion and enhancement mode.

MOSFET can be used in multiple ways such as:

• MOSFET as a switch for LED.
• MOSFET as a switch for Arduino.
• MOSFET switch for ac load.
• MOSFET switch for dc motor.
• MOSFET switch for negative voltage.
• MOSFET as a switch with a microcontroller.
• MOSFET switch with hysteresis.
• MOSFET as switch diode and active resistor.
• MOSFET as a switch equation.
• MOSFET switch for airsoft.
• MOSFET as switch gate resistor.
• MOSFET as a switching solenoid.
• MOSFET switch using an optocoupler.

• Ideal enhancement MOSFET is a MOSFET which saturates when V_DS ≥ V_GS - V_T and allows a constant current to flow across it even after a further increase in V_DS.
• But for a non-ideal MOSFET (i.e. MOSFET in which channel length modulation effect is significant), a change in V_DS beyond V_GS - V_T, will result in a further increase in current beyond saturation. This effect is known as channel length modulation.
• This is explained with the help of the following diagram:

Also, the output characteristics will be as shown:

now is no longer infinite.

Complementary Metal-oxide-semiconductor (CMOS) uses complementary & symmetrical pair of P-type & n-type MOSFETS.

• The two important characteristics of CMOS devices are high noise immunity and low power dissipation.
• CMOS devices dissipate less power than NMOS devices because the CMOS dissipates power only when switching (“dynamic power), whereas N channel MOSFET dissipates power whenever the transistor is on because there is a current path from V_dd to V_ss.
• In a CMOS, only one MOSFET is switched on at a time. Thus, there is no path from voltage source to ground so that a current can flow. Current flows in a MOSFET only during switching.
• Thus, compared to N-channel MOSFET has the advantage of lower drain current from the power supply, thereby causing less power dissipation.

The O in a MOSFET stands for Oxide layer which provides high input impedance to the device.

MOSFET:
A Metal Oxide Semiconductor Field Effect Transistor (MOSFET) has 4 terminals Gate, Drain, Source, and Substrate (Body) terminal. But in many practical circuits MOSFET is used by connecting three terminals Gate, Drain, and Source while connecting the Substrate (Body) terminal to the source.

drain, source, the gate is the terminal for MOSFET.

MOSFET:

• MOSFET can carry DC current in both directions.
• The MOSFET channel itself can conduct current in either direction.
• And in addition, it has this built-in body diode that can conduct a negative current.
• Due to the itegral body diode, most discrete MOSFETs cannot block in the reverse direction, but the channel will conduct in either direction when the gate is biased "ON".
• So the MOSFET by itself can be a current bidirectional two-quadrant switch.

• The body diode of practical MOSFETs may not be a very good diode.
• So if we have a slow body diode we might not want to let it conduct and have to switch it off because of its slow switching time.
• Thus we have to put an external diode in series with a MOSFET that only lets the current flow in the positive direction.
• And then put in externally an antiparallel diode to get a current bidirectional switch.