reverse bias operation

reverse bias operation

1. What Reverse Bias Is

Reverse bias = external voltage with p → -, n → + (opposite to forward), which aids built-in potential $V_{bi}$, increases barrier height ($V_{bi}+|V_R|$) and widens depletion width $W$, blocking majority diffusion ⇒ tiny reverse current (minority drift).

2. Operation Details

• Effective barrier:

increases, $W$ grows as

, electric field in depletion strengthens (from n to p).

• Currents: Diffusion (majority) suppressed by higher barrier ⇒ negligible. Drift of minority carriers (electrons in p, holes in n) that wander into depletion are swept by field ⇒ reverse saturation current

(also called leakage, nA–µA, voltage-independent until breakdown, temperature-sensitive doubling every 10°C).

• Capacitance: junction capacitance (depletion)

dominates reverse (varactor).

3. I-V in Reverse

• For $V_R$ from 0 to breakdown $V_{BR}$:

(constant, small). In Shockley:

with $V<0$ large ⇒

.

• At $V_{BR}$ (e.g., -50V for regular diode, -5V for Zener): avalanche/Zener breakdown ⇒ current shoots to large negative (needs limiting resistor).

4. Worked Checks

(a) Si diode V_R=-5V: barrier 0.7+5=5.7V, W≈√(5.7/0.7)=2.85× wider than zero bias ⇒ C_j drops.

(b) I0=10nA at 25°C ⇒ at 35°C ~20nA (doubles) ⇒ reverse leakage heats further ⇒ thermal runaway if not limited.

(c) Need to block 100V? Choose diode with V_BR >100V.

(d) Why reverse current small? Minority concentration low (n_i²/N_D) ⇒ few carriers to drift.

(e) Photodiode reverse bias for detection: light creates extra minority carriers ⇒ I_R = I0 + I_photo.

5. Exam Points

• Define reverse bias p -, n +, $V_{eff}=V_{bi}+|V_R|$, $W$ increases — 5 marks, very frequent (almost certain to ask reverse bias description).
• Show $I≈-I_0$ (saturation) and breakdown at $V_{BR}$ — 5 marks, frequent.
• Mention $C_j∝1/√(V_{bi}+V_R)$ — 3-5 marks, frequent.
• Distinguish from forward (diffusion vs drift) — 3-5 marks, frequent.