Test: Types of MOSFETs - Electrical Engineering (EE) MCQ

Concept:

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 ID = 0

When Vgs > V_t and V_ds < V_gs - V_t 

NMOS operates in the linear region

The current is defined as:

Calculation:

Given values are I_d = 5 mA, V_gs = 2 V, V_ds = 4 V and V_t = 0.8 V. If the thickness of oxide is 500 A°

For silicon, the mobility is 0.13 m²/V-s

we know that value of oxide capacitance is:

The aspect ratio is approximately 25.

• A MOSFET (metal–oxide–semiconductor FET) is one kind of field-effect transistor with an insulated gate mainly used for amplifying or switching signals. 
• This is used in Digital ICs.MOS ICs use enhancement MOSFETs exclusively.
• The features of these MOSFETs are low power dissipation, simple manufacturing, and small geometry. So these features will make them used within integrated circuits
• There is no pathway in between the drain (D) and source (S) of this MOSFET when no voltage is applied in between the gate & source terminals.
• So, applying a voltage at gate-to-source will enhance the channel, making it capable of conducting current. This property is the main reason to call this device an enhancement-mode MOSFET.

Concept:

• MOSFET (Metal Oxide Silicon Field Effect Transistor) is an electronic device used to switch or amplify voltages in circuits.
• MOSFET is a three-terminal device namely Source, Drain, and Gate. 
• It is a Voltage controlled device. 

MOSFET is divided mainly into two types

Depletion MOSFET (D-MOSFET):

• In D-MOSFET one channel is inserted near the gate terminal at the time of manufacturing of the device.
• In Depletion type MOSFET when the voltage across the gate terminal is applied either positive or negative depending upon its type whether N-Channel or P-Channel, its channel width can be changed.
• This type of MOSFET can work in both enhancement mode and in depletion mode.

Enhancement MOSFET (E-MOSFET):

• In E-MOSFET there is no inbuilt channel is present, as we apply voltage at the gate terminal one channel near the gate terminal starts enhancing.
• This type of MOSFET can only work in enhancement mode.

Concept:

For an n-channel MOSFET, the current in triode and saturation region is given by:

V_DS = Drain to Source Voltage

V_GS = Gate to source voltage

V_th = Threshold Voltage

K_n = Conduction parameter

Analysis:

And the MOSFET is in saturation.

I_D = 0.249 (2V_TN - V_TN)²

= 0.249 (V_TN)²

= 0.249 (0.75)²

= 0.14 mA

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

In the linear region of operation, 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 of a MOSFET is defined as the change in drain current(I_D) with respect to the corresponding change in gate voltage (V_GS), i.e. 

Concept:

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:

• 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 Vdd to Vss.
• 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.

• 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.

Important Points

The turn-off times of different power electronic devices are given below:

• MOSFET has the lowest switching off time in the order of nanoseconds.
• BJT has the turn-off time in the order of nanoseconds to microseconds.
• IGBT has the turn-off time in the order of microseconds (about 1 μs).
• Thyristor (SCR) has the turn-off time in the order of microseconds (about 5 μs).

Therefore, the increasing order of turn-off times is:

MOSFET > BJT > IGBT > Thyristor (SCR)

MOSFET:

The MOSFET is a voltage-controlled field-effect transistor.

It has a “Metal Oxide” Gate electrode which is electrically insulated from the main semiconductor n-channel or p-channel by a very thin layer of insulating material usually silicon dioxide (SiO2), commonly known as glass.

Insulating SiO2 layer provides High input impedance.

MOSFET is of two types:

Depletion type (D-MOSFET):

• Presence of physical layer between source and drain.
• We can apply both positive and negative between source and gate.

Enhancement type (e-MOSFET):

• Absence of physical layer between source and drain
• Only one type of polarity voltage between gate and source such that physical layer should form.

Symbol representation of MOSFET: