13 Jul 2026

Drain and Transfer Characteristics of an Enhancement MOSFET

practical pg-ii mosfet characteristics

Aim

To obtain the drain and transfer characteristics of an n-channel enhancement MOSFET and determine its threshold voltage and transconductance.

Apparatus

Enhancement MOSFET, regulated gate and drain supplies, milliammeter, voltmeters, resistors, breadboard, and connecting leads.

Experimental arrangement

The drain supply controls $V_{DS}$, the gate supply controls $V_{GS}$, and the drain current is measured in the device path.

Theory

In an enhancement MOSFET, no conducting channel exists at $V_{GS}=0$. A positive gate voltage attracts electrons towards the oxide-semiconductor interface and creates a channel between source and drain. Drain current begins only when $V_{GS}$ exceeds the threshold voltage $V_T$.

In the saturation region, the transfer characteristic is approximately

\[I_D=K(V_{GS}-V_T)^2,\]

where $K$ depends on the device construction. The transconductance is $g_m=\Delta I_D/\Delta V_{GS}$ at constant $V_{DS}$.

Observations

$V_{GS}$ (V)	$I_D$ (mA) at $V_{DS}=6$ V
1.5	0.0
2.0	0.4
2.5	1.6
3.0	3.4
3.5	5.8

Result

The threshold voltage from the onset of drain current is approximately $V_T=1.9$ V. The drain current increases quadratically with gate voltage in the saturation region.

Viva Questions

1. Why is gate current nearly zero? The gate is insulated from the channel by a thin oxide layer.
2. What is threshold voltage? The minimum gate-source voltage needed to form a conducting channel.
3. Why must the MOSFET be protected from static charge? The thin gate oxide can be damaged by a large electrostatic voltage.

Maxima Code

Download the PG-II electronics calculation file.