13 Jul 2026

Tunnelling Effect in a Tunnel Diode

practical ug-vi semiconductor tunnel-diode tunnelling

Experimental arrangement

The tunnel diode is biased through a current-limiting resistor while the ammeter and voltmeter trace its complete V-I curve.

Aim

To study the current-voltage characteristic of a tunnel diode and identify its negative-resistance region.

Apparatus

Tunnel diode, regulated supply, microammeter, voltmeter, series resistor, and connecting wires.

Theory

Heavy doping makes the depletion layer extremely thin, allowing carriers to tunnel through the barrier. The current first increases to a peak, then decreases with increasing voltage, and finally increases again by ordinary conduction. The negative differential resistance is

\[r_d=\frac{dV}{dI}<0.\]

Observations

Voltage (V)	Current (mA)
0.00	0.0
0.05	1.8
0.10	3.4
0.15	2.6
0.20	1.4
0.30	2.0
0.40	4.2

Peak point: $(V_p,I_p)=(0.10\,\text{V},3.4\,\text{mA})$; valley point: $(V_v,I_v)=(0.20\,\text{V},1.4\,\text{mA})$.

Result

The negative-resistance region extends approximately from $0.10$ to $0.20\,\text{V}$.

Precautions

1. Use a low-current range.
2. Increase voltage in small steps near peak and valley points.
3. Do not exceed the diode rating.

Viva Questions

1. What causes tunnelling? Quantum-mechanical transmission through a very thin potential barrier.
2. What is negative resistance? A region in which current decreases as voltage increases.
3. Where are tunnel diodes used? High-frequency oscillators, switching circuits, and microwave devices.