An N-channel D-MOSFET with a positive VGS is operating in _______

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Examoogle · Accepted Answer

The enhancement-mode

Explanation: An N-channel Depletion-type MOSFET (D-MOSFET) operates in the enhancement mode when a positive gate-to-source voltage ($V_{GS} > 0$) is applied. This voltage attracts additional electrons into the N-channel, effectively increasing its conductivity beyond its initial state.

Working Principle: In a D-MOSFET, a physical channel of N-type material exists between the drain and source even at $V_{GS} = 0$. When $V_{GS}$ is positive, it creates an electric field that draws more minority carriers (electrons) into the N-channel from the substrate, increasing the carrier density and channel current ($I_D$), which is defined as the enhancement mode.

Key Formulas: $I_D = I_{DSS} \left( 1 - \frac{V_{GS}}{V_P} \right)^2$ — Shockley equation for MOSFET current; $g_m = \frac{2I_{DSS}}{|V_P|} \left( 1 - \frac{V_{GS}}{V_P} \right)$ — Transconductance

Key Points: D-MOSFETs can operate in both depletion and enhancement modes.; A negative $V_{GS}$ reduces channel conductivity (depletion mode).; A positive $V_{GS}$ increases channel conductivity (enhancement mode).; Unlike E-MOSFETs, D-MOSFETs possess a pre-fabricated physical channel.

Advantages: Versatility in circuit design (can operate with positive or negative bias).; High input impedance due to insulated gate structure.

Disadvantages: More complex manufacturing process than standard JFETs.; Insulated gate can be susceptible to static discharge damage.

Applications: RF amplifiers; Automatic gain control (AGC) circuits

Additional Info: The transition point between depletion and enhancement mode in a D-MOSFET is exactly $V_{GS} = 0V$.. Option A is incorrect because depletion mode occurs when $V_{GS}$ is negative for an N-channel device.

Answer: B is correct — Applying a positive $V_{GS}$ to an N-channel D-MOSFET attracts more electrons to the channel, enhancing its conductivity.

Exam Tip: Remember: For N-channel D-MOSFETs, think of 'Positive $V_{GS}$' as 'Adding more electrons' (Enhancement) and 'Negative $V_{GS}$' as 'Removing electrons' (Depletion).

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