In enhancement type of MOSFET, at zero gate bias: A. Channel conductance is very low B. Channel conductance is very high C. Channel resistance is very low D. Channel resistance is very high Choose the correct answer from the options given below:
A and D only
Understanding Enhancement-Type MOSFETs at Zero Gate Bias
Let's analyze the behavior of an enhancement-type MOSFET when the gate-source voltage ($V_{GS}$) is zero. An enhancement-type MOSFET is a type of MOSFET that is normally off when the gate-source voltage is zero. This means there is no conductive channel between the source and drain terminals.
For an n-channel enhancement-type MOSFET, a positive gate-source voltage greater than a certain threshold voltage ($V_T$) is required to create an n-type conductive channel between the source and drain. Similarly, for a p-channel enhancement-type MOSFET, a negative gate-source voltage less than the threshold voltage is required to create a p-type channel.
At zero gate bias ($V_{GS} = 0$), no channel is induced in the enhancement-type MOSFET. The region between the source and drain is essentially the substrate material, which is typically highly resistive compared to a formed channel.
Analyzing Channel Conductance and Resistance
The conductivity of a material is its ability to conduct electric current. Resistance is the opposition to the flow of current. Conductance is the reciprocal of resistance, usually denoted by $G$, where $G = \frac{1}{R}$.
When there is no conductive channel formed between the source and drain in an enhancement-type MOSFET at zero gate bias, the resistance between these terminals is very high. This is because the only current flow would be due to leakage through the substrate, which is minimal.
Since conductance is the reciprocal of resistance, if the resistance is very high, the conductance must be very low.
Let's consider the given statements:
- Statement A: Channel conductance is very low. This aligns with our understanding that resistance is very high when no channel is formed.
- Statement B: Channel conductance is very high. This is the opposite of what we expect.
- Statement C: Channel resistance is very low. This is also the opposite of what we expect; resistance is high when no channel exists.
- Statement D: Channel resistance is very high. This aligns with our understanding that at zero bias, there is no conductive path (channel) between source and drain.
Therefore, statements A and D accurately describe the condition of an enhancement-type MOSFET at zero gate bias.
Evaluating the Options
We found that statements A and D are correct. We need to choose the option that includes both A and D.
- Option 1: A and B only. Incorrect, because B is false.
- Option 2: A and C only. Incorrect, because C is false.
- Option 3: B and D only. Incorrect, because B is false.
- Option 4: A and D only. Correct, as both A and D are true.
Based on the analysis, at zero gate bias, an enhancement-type MOSFET has a very high channel resistance and consequently a very low channel conductance.
The final answer is therefore the option that states A and D only.
| Parameter | Value/State |
|---|---|
| Channel Formation | No channel formed |
| Channel Resistance ($R_{DS}$) | Very High |
| Channel Conductance ($G_{DS}$) | Very Low |
Revision Table: MOSFET Basics
| Feature | Enhancement Type | Depletion Type |
|---|---|---|
| Channel at $V_{GS}=0$ | No channel (normally off) | Exists (normally on) |
| Channel Formation Method | Requires $V_{GS} > V_T$ (N-channel) or $V_{GS} < V_T$ (P-channel) to induce channel. | Existing channel is depleted by applying $V_{GS}$. |
| Threshold Voltage ($V_T$) | Positive (N-channel), Negative (P-channel) | Negative (N-channel), Positive (P-channel) |
Additional Information: MOSFET Operation Regions
A MOSFET can operate in different regions depending on the applied gate-source voltage ($V_{GS}$) and drain-source voltage ($V_{DS}$). For an n-channel enhancement MOSFET:
- Cutoff Region: When $V_{GS} < V_T$. No channel exists, so the drain current ($I_D$) is approximately zero. This is the state at $V_{GS}=0$ for enhancement type.
- Linear (Triode) Region: When $V_{GS} > V_T$ and $V_{DS} < (V_{GS} - V_T)$. A conductive channel exists, and the MOSFET behaves like a voltage-controlled resistor. The drain current is roughly proportional to $V_{DS}$.
- Saturation Region: When $V_{GS} > V_T$ and $V_{DS} \ge (V_{GS} - V_T)$. The channel is 'pinched off' near the drain end. The drain current is relatively constant and independent of $V_{DS}$, primarily controlled by $V_{GS}$.
At zero gate bias ($V_{GS}=0$), the enhancement-type MOSFET is in the cutoff region, where resistance is very high and conductance is very low.
Important Questions from MOSFET - Teaching
The maximum operating frequency of a MOSFET is
A. \(\rm \frac{\omega_m}{2\pi}\)
B. \(\rm \frac{\mu_nV_D}{2\pi L^2}\)
C. \(\rm \frac{\mu_nL^2}{2\pi}\)
D. \(\rm \frac{\mu_nV_D}{2\pi L}\)
Choose the correct answer from the options given below:
In a MOSFET, the transconductance in linear region ean be expressed as:
For a thermally grown silicon dioxide layer, in a MOSFET, the leakage current between the gate and channel is:
If N is the doping level of substrate and V is the effective voltage across the junction in a MOSFET, then the depletion region width for the junction is given by:
For n MOSFET fabrication the substrate required is:
