N-MOS

electronics transistors mosfet

N-MOS

An N-MOS transistor is an N-channel MOSFET where the channel consists of electrons as majority carriers. It operates in enhancement mode: applying a positive gate-source voltage $V_{GS}>V_{th}$ (where $V_{th}>0$) creates an inversion layer, forming a conducting channel between source and drain.

Structure

Substrate: p-type semiconductor (lightly doped). Source/Drain: Heavily doped n+ regions diffused into the substrate. Gate: Metal or polysilicon electrode separated from the substrate by a thin oxide layer. Channel: Forms beneath the gate when $V_{GS}>V_{th}$ (positive threshold).

Operation

Off State ($V_{GS}<V_{th}$): No conducting channel exists; drain current $I_D\approx 0$. The transistor acts as an open switch.

On State ($V_{GS}>V_{th}$): Positive gate voltage attracts electrons to the surface, creating an n-type inversion layer (channel) connecting source to drain. Current flows from drain (higher potential) to source (lower potential).

Example

Consider an N-MOS with $V_{th}=0.7\text{V}$:

Condition	$V_{GS}$	State	$I_D$
$V_G=0\text{V}$, $V_S=0\text{V}$	$0\text{V}$	Off	$\approx 0$
$V_G=3.3\text{V}$, $V_S=0\text{V}$	$3.3\text{V}$	On	$>0$

When used as a low-side switch: source connected to ground ($0\text{V}$), gate driven high ($3.3\text{V}$) to turn on, allowing current to flow from the load through the drain to ground.