Draw and explain IV characteristics of PN junction diode

A PN junction diode is formed when a P-type semiconductor (with holes as majority carriers) is joined to an N-type semiconductor (with electrons as majority carriers). At the junction, a depletion region forms where positive and negative ions create an internal electric field (potential barrier of ~0.3 V for germanium, ~0.7 V for silicon).

When the positive terminal of the battery is connected to the P-side and negative to the N-side:

What happens: The external voltage opposes the internal potential barrier. As the applied voltage approaches the threshold voltage ($V_T$ ≈ 0.7 V for Si, 0.3 V for Ge), the barrier is overcome. Current begins to flow exponentially.

I-V characteristic (forward bias):

• For $V < V_T$: Very small current (essentially zero — barrier not overcome)
• At $V = V_T$: Current begins to rise sharply
• For $V > V_T$: Current rises steeply (exponentially) with small increase in voltage
• The I-V curve is non-linear — the diode doesn’t obey Ohm’s law

The diode acts like a closed switch above threshold — it conducts heavily.

Equation: $I = I_0(e^{V/\eta V_T} - 1)$ where $I_0$ is the reverse saturation current and $\eta$ is the ideality factor.

When the negative terminal is connected to the P-side and positive to the N-side:

What happens: The external voltage adds to the internal barrier, widening the depletion region. Majority carriers cannot cross. Only minority carriers (few thermally generated electrons in P-type, holes in N-type) cross, giving a very small reverse saturation current $I_0$ (order of µA or nA).

I-V characteristic (reverse bias):

• Current is almost constant (very small) regardless of increasing reverse voltage — this is $I_0$
• At very high reverse voltage, the junction breaks down — either:
  • Zener breakdown (heavily doped, low voltage): occurs at a specific Zener voltage $V_Z$
  • Avalanche breakdown (lightly doped, high voltage): impact ionisation

At breakdown, current increases sharply. The Zener diode is specially designed to operate in this region for voltage regulation.

The diode acts like an open switch in reverse bias (before breakdown).

The complete I-V curve occupies two quadrants:

Quadrant I (forward bias): Voltage positive, current positive — rises steeply after threshold.

Quadrant III (reverse bias): Voltage negative, current negative — very small magnitude (flat at $-I_0$) until breakdown at $-V_{BR}$, where current suddenly increases.

Key values to label on the diagram:

• Threshold voltage ($V_T$): ~0.7 V (Si), ~0.3 V (Ge)
• Reverse saturation current ($I_0$): Very small, temperature dependent
• Breakdown voltage ($V_{BR}$): Typically 5–200 V depending on diode type