MOSFET (metal-oxide semiconducting field-effect transistor) (A-level only) Flashcards – AQA A-level Physics (7408)

What are the three terminals of an N-channel enhancement-mode MOSFET?

Drain, source and gate.

In a simplified N-channel enhancement-mode MOSFET, what are the semiconductor types of the source, drain and surrounding material?

The source and drain are N-type regions in a P-type semiconductor.

Why does a MOSFET gate have very high input resistance, and what is the consequence for loading?

An insulating oxide separates the gate from the semiconductor. Negligible steady current enters the gate, so a voltage source controls it with little loading.

What do $V_{\mathrm{GS}}$ and $V_{\mathrm{DS}}$ represent in a MOSFET?

$V_{\mathrm{GS}}$ is the controlling gate–source voltage; $V_{\mathrm{DS}}$ is the drain–source voltage.

What is the conducting channel state of an ideal enhancement-mode MOSFET when $V_{\mathrm{GS}}=0$?

There is no conducting channel.

How does a sufficiently positive gate voltage create a channel in an N-channel enhancement-mode MOSFET?

It attracts electrons beneath the gate, forming an N-type channel between source and drain.

What is the threshold voltage $V_{\mathrm{th}}$ of an enhancement-mode MOSFET?

The gate–source voltage at which the channel begins to conduct appreciably.

With positive $V_{\mathrm{DS}}$ and $V_{\mathrm{GS}}$ above threshold, which way does conventional drain current flow in an N-channel MOSFET?

From drain to source.

What effect does increasing $V_{\mathrm{GS}}$ have on an N-channel MOSFET's channel and drain current?

It strengthens the channel, allowing a greater drain current.

At a fixed conducting $V_{\mathrm{GS}}$, how does MOSFET drain current vary as $V_{\mathrm{DS}}$ increases?

It rises, then tends towards a saturation region.

What is $I_{\mathrm{DSS}}$, and what value is expected for an enhancement-mode MOSFET?

It is the drain current at zero $V_{\mathrm{GS}}$ and a stated $V_{\mathrm{DS}}$. It is ideally zero, though a small leakage current can flow.

Which voltage controls an N-channel enhancement-mode MOSFET switch?

The gate–source voltage: the gate voltage relative to the source.

What is the drain–source state of an N-channel enhancement-mode MOSFET when its gate–source voltage is below threshold?

It is off and ideally carries no current.

What happens when an N-channel enhancement-mode MOSFET has a sufficiently positive gate–source voltage?

It turns on, allowing current through the drain–source path and connected load.

How does the drain–source resistance of a switching MOSFET compare in its on and off states?

It is low when on and high when off.

Is exceeding the gate–source threshold voltage enough to guarantee that a MOSFET has a sufficiently low on-state resistance for its load?

No. Exceeding threshold alone does not guarantee a low enough resistance for the intended load.