MOSFET MCQ Quiz - Objective Question with Answer for MOSFET - Download Free PDF

Explanation:

Power MOSFETs

Definition: Power MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) are a type of transistor used for amplifying or switching electronic signals. They are widely used in power electronics due to their high efficiency, high switching speed, and ability to handle significant power levels.

Working Principle: Power MOSFETs operate as majority carrier devices. This means they rely on the majority charge carriers (electrons in N-channel or holes in P-channel devices) to conduct current. When a voltage is applied to the gate terminal, it creates an electric field that allows or prevents the flow of these carriers between the source and drain terminals.

Advantages:

• High input impedance, which means they require very little input current to operate.
• Fast switching speeds due to the majority carrier operation.
• High efficiency in switching applications, reducing power losses.
• Good thermal stability and ability to handle high power levels.

Disadvantages:

• As the voltage rating increases above 200 V, the conduction characteristics tend to degrade. This is due to the increase in on-resistance (R_DS(on)), which leads to higher conduction losses.
• More complex driving circuitry compared to other types of transistors like BJTs (Bipolar Junction Transistors).

Applications: Power MOSFETs are used in various applications, including power supplies, motor drives, DC-DC converters, and other power management systems.

Correct Option Analysis:

The correct option is:

Option 2: Both Statement 1 and Statement 2 are true.

This option correctly identifies that Power MOSFETs are majority carrier devices and acknowledges that their conduction characteristics become inferior as the voltage rating increases above 200 V. The increase in on-resistance with higher voltage ratings is a well-known limitation of Power MOSFETs, leading to greater conduction losses and reduced efficiency.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: Statement 2 is true and Statement 1 is false.

This option is incorrect because Statement 1 is true. Power MOSFETs are indeed majority carrier devices, which is an essential characteristic of their operation and contributes to their high switching speeds and efficiency.

Option 3: Both Statement 1 and Statement 2 are false.

This option is incorrect as both statements are true. Power MOSFETs are majority carrier devices, and their conduction characteristics do degrade with increasing voltage ratings above 200 V due to higher on-resistance.

Option 4: Statement 1 is true and Statement 2 is false.

This option is incorrect because Statement 2 is true. While it is true that Power MOSFETs are majority carrier devices, it is also true that their conduction characteristics become inferior as the voltage rating increases above 200 V, leading to higher conduction losses.

Conclusion:

Understanding the operational characteristics and limitations of Power MOSFETs is crucial for their effective application in power electronics. The correct analysis acknowledges that Power MOSFETs are majority carrier devices, and their efficiency can be affected by higher voltage ratings due to increased conduction losses. This knowledge helps in selecting the appropriate components for specific applications, ensuring optimal performance and reliability.

Concept

An n-channel enhancement-type MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) requires a positive gate-to-source voltage (V_GS) to create a conductive channel between the drain and source terminals. The threshold voltage (V_T) is the minimum gate-to-source voltage required to turn the MOSFET on.

When V_GS < V_T, the MOSFET is in the cutoff region, meaning no conductive channel is formed and no current flows between the drain and the source.

Solution

Given the condition V_GS < V_T, let's analyze the options:

1. The MOSFET conducts maximum current. - This is incorrect because the MOSFET cannot conduct any current if V_GS is less than the threshold voltage (V_T).
2. The MOSFET is in the cutoff mode and no current flows between the drain and the source. - This is correct. When V_GS is less than V_T, the MOSFET is in the cutoff region, resulting in no current flow between the drain and source.
3. The MOSFET is in the saturation mode. - This is incorrect. The MOSFET can only enter saturation mode if V_GS is greater than V_T and V_DS is sufficiently large. In saturation mode, the MOSFET conducts current, which contradicts the given condition V_GS < V_T.
4. The MOSFET is in the triode region. - This is incorrect. The MOSFET enters the triode region when V_GS > V_T and V_DS is small. In the triode region, the MOSFET conducts current, which contradicts the given condition V_GS < V_T.

Therefore, the correct answer is option 2.

Concept

In a single-stage MOSFET amplifier, the Miller capacitance is the capacitance between the input and output terminals of the MOSFET, multiplied by the gain of the amplifier. This effect is known as the Miller effect.

The equivalent capacitance seen at the input is given by:

\(C_{in(Miller)} = C_{gd}(1 + A_v)\)

where,

• \(C_{gd}\) is the capacitance between gate and drain.
• \(A_v\) is the voltage gain of the amplifier.

Solution

The Miller capacitance in a single-stage MOSFET amplifier plays the role of reducing the high-frequency gain of the amplifier. This happens because the Miller capacitance effectively increases the input capacitance of the amplifier, which in turn affects the high-frequency response.

Therefore, the correct answer is option 3.

Additional Information

Option 1: It increases the high-frequency gain.

This statement is incorrect. The Miller capacitance actually decreases the high-frequency gain by introducing an additional capacitance at the input, which reduces the overall bandwidth.

Option 2: It enhances the linearity of the amplifier.

This statement is incorrect. The Miller capacitance does not have a significant impact on the linearity of the amplifier. Linearity is more related to the operating region of the MOSFET and the design of the amplifier circuit.

Option 3: It decreases the high-frequency gain.

This statement is correct. As explained above, the Miller capacitance increases the input capacitance, which in turn reduces the high-frequency gain of the amplifier.

Option 4: It stabilizes the DC operating point.

This statement is incorrect. The Miller capacitance primarily affects the AC response of the amplifier and does not have a significant impact on the DC operating point.

Concept

A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a key component in many electronic circuits, including amplifiers. The unity-gain bandwidth of a single-stage amplifier is an important parameter that determines the frequency range over which the amplifier can provide gain.

The unity-gain bandwidth (f_T) is given by the expression:

\( f_T = \frac{g_m}{2\pi C_{gs}} \)

where:

• g_m is the transconductance of the MOSFET.
• C_gs is the gate-source capacitance of the MOSFET.

From this equation, it is clear that the transconductance (g_m) is crucial in determining the unity-gain bandwidth of a single-stage amplifier. Therefore, the correct option is:

Correct Option: 3) Transconductance (g_m)

 Additional Information

Option 1: Oxide capacitance per unit area (C_ox)

Oxide capacitance per unit area (C_ox) is a parameter that affects the gate capacitance (C_gs) of the MOSFET. However, it is not the most direct parameter in determining the unity-gain bandwidth. C_ox influences the device's input capacitance, but the unity-gain bandwidth is more directly influenced by the transconductance (g_m).

Option 2: Channel length modulation

Channel length modulation is a secondary effect in MOSFETs that affects the output characteristics, such as the output conductance. While it does impact the overall performance of the amplifier, it is not the primary parameter for determining the unity-gain bandwidth.

Option 4: Threshold voltage (V_th)

The threshold voltage is the minimum gate-to-source voltage that is required to create a conductive channel between the source and drain terminals. It is an important parameter for the switching behavior of MOSFETs, but it does not directly determine the unity-gain bandwidth.

The correct answer is Both I and II.

Key Points

• Statement I: The switching speed is very high and the switching times are of the order of nano-seconds.
  • Power MOSFETs are known for their high switching speeds.
  • The switching times of Power MOSFETs are typically in the nano-second range.
  • This characteristic makes them highly suitable for applications requiring fast switching.

  Hence, statement I is correct.

• Statement II: Power MOSFETs find increasing applications in low - power high frequency converters.
  • Power MOSFETs are increasingly used in low-power high-frequency converters.
  • They are preferred in such applications due to their high efficiency and fast switching capabilities.
  • These features allow for more efficient power conversion at high frequencies.

  Hence, statement II is correct.

Additional Information

• Power MOSFET Characteristics:
  • Power MOSFETs are a type of metal-oxide-semiconductor field-effect transistor (MOSFET) designed to handle significant power levels.
  • They are widely used in power electronics due to their efficiency and reliability.
  • Power MOSFETs are characterized by their high input impedance and fast switching speeds.
• Applications:
  • Common applications include switching power supplies, DC-DC converters, and motor controllers.
  • They are also used in audio amplifiers and RF amplifiers due to their high-frequency performance.
• Advantages:
  • Power MOSFETs offer several advantages such as low on-resistance, high efficiency, and thermal stability.
  • Their ability to operate at high frequencies makes them ideal for modern power electronic systems.
• Key Terms:
  • Switching Speed: Refers to the time it takes for the MOSFET to switch from on to off states or vice versa.
  • High Frequency Converters: Electronic devices that convert power at high frequencies, often used in power supplies and inverters.

Transistor can be acted as

1) Resistor in current mirror

2) Capacitor in level shifter

3) Closed or ON switch in saturation region

4) Inverter in cutoff and saturation region

5) Amplifier in active region

Explanation:

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

NOTE: The difference between Depletion and Enhancement MOSFET is the formation of the channel.

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.

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.

In on-state, there is a conducting channel between the Drain and the Source

The structure of MOS is like:

• GATE (metal)
• Oxide (SiO₂) layer
• Conductive channel

The structure consists of 2 conductive regions separated by a dielectric which is equivalent to a resistor

Note:

It is also a voltage-controlled resistor, in which the channel thickness depends on the gate to source voltage.

MOSFET:

MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor.

It is a majority carrier device and also called as Voltage controlled current device (V_GS control the current I_D)

Operation of the MOSFET:

The drain characteristics or the plot between I_D and V_DS 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

• MOSFET acts as a constant current source in the saturation region.
• This is because after increasing V_DS to such a level that pinch-off occurs, the gate and the drain voltages lose its control over the current flowing.
• So, beyond that value of V_DS, the current is almost constant.

This is explained in the following characteristics of N-MOS:

Concept:

Cutoff region

• VGS < Vth
• ID = 0

Active/ linear/ Ohmic/ Triode region:

• VGS > Vth
• VDS < VGS – Vth.

Saturation region:

• VGS > Vth
• VDS > VGS – Vth.

Where

VG = Gate voltage

VD = drain voltage

VS = Source voltage

VGS = Gate to source voltage.

VDS = Drain to source voltage.

Vth = Threshold voltage.

ID = Drain current.

Application:

The condition to turn ON the device is:

VGS > Vth

V_GS > 5V

• 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.

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

\({I_{D\left( {sat} \right)}} = \frac{{W{μ _x}{C_{ox}}}}{{2L}}{\left( {{V_{GS}} - {V_{th}}} \right)^2}\)

In the linear region of operation, the current is given by:

\({I_D} = {\mu _n}{C_{ox}} \times \frac{W}{L}\left[ {\left( {{V_{GS}} - {V_T}} \right){V_{DS}} - \frac{{V_{DS}^2}}{2}} \right]\)

W = Width of the Gate

Cox = Oxide Capacitance

μ = Mobility of the carrier

L = Channel Length

Vth = Threshold voltage

The transconductance of a MOSFET is defined as the change in drain current(ID) with respect to the corresponding change in gate voltage (VGS), i.e. 

\({g_m} = \frac{{\partial {I_D}}}{{\partial {V_{GS}}}}\)

\(g_m = \frac{{W{μ _x}{C_{ox}}}}{{L}}{\left( {{V_{GS}} - {V_{th}}} \right)}\)

Trans-conductance gm for the linear region will be:

\({g_{m\left( {linear} \right)}} = \frac{{\partial {I_D}}}{{\partial {V_{GS}}}} = \frac{{{\mu _n}{C_{ox}}W}}{L} \times {V_{DS}}\)

• 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)