Diode Reverse Bias

A p-n junction diode is a one-way valve for electric current. When you connect the positive terminal of a battery to the n-type side and the negative terminal to the p-type side, you are applying what is called a reverse bias. Instead of letting current flow freely, the diode almost completely blocks it — this is the defining feature that makes diodes so useful in circuits like power supplies, signal detectors, and protection circuits. To understand why reverse bias blocks current, you need to think about what happens to the depletion region — the thin zone at the p-n junction where mobile charge carriers have recombined and left behind fixed ions. In forward bias, this region shrinks, lowering the built-in potential barrier. In reverse bias, the opposite happens: the electric field you apply adds to the existing built-in field, widening the depletion region and raising the barrier so high that virtually no charge carriers can cross it. Virtually no current flows — but not exactly zero. A tiny current called the reverse saturation current (symbol I₀ or I_s) does leak through, caused by thermally generated minority carriers that happen to wander into the depletion region. This current is typically in the range of nanoamperes to microamperes and is nearly independent of the applied reverse voltage, until the voltage becomes large enough to cause breakdown. Understanding reverse bias is essential to grasping how rectifier circuits, Zener diodes, and photodiodes all operate.

Key takeaways
  • The Built-In Potential at Equilibrium
  • Applying the Reverse Voltage
  • Widening of the Depletion Region
  • Reverse Saturation Current
  • Breakdown

The Built-In Potential at Equilibrium

Even with no external voltage, a p-n junction has a built-in electric field pointing from the n-side to the p-side across the depletion region. This field arises from positive donor ions on the n-side and negative acceptor ions on the p-side that are left behind after free carriers recombine. The built-in potential V₀ is typically 0.6–0.7 V for silicon.

Applying the Reverse Voltage

When you connect the positive battery terminal to the n-side and the negative terminal to the p-side, the external electric field points in the same direction as the built-in field — from n to p. This reinforces the barrier. The holes in the p-region are pulled further away from the junction toward the negative terminal, and the electrons in the n-region are pulled away toward the positive terminal. Both types of majority carriers move away from the junction.

Widening of the Depletion Region

As majority carriers retreat from the junction, more donor and acceptor ions are exposed on each side, widening the depletion region. The width W of the depletion region grows approximately as W ∝ √(V₀ + V_R), where V_R is the magnitude of the reverse voltage. A wider depletion region means a taller and broader energy barrier — majority carriers simply cannot climb over it.

Reverse Saturation Current

Thermal energy constantly generates electron-hole pairs everywhere in the semiconductor. Minority carriers — electrons in the p-region and holes in the n-region — that are generated within about one diffusion length of the depletion region get swept across the junction by the strong internal field. This produces a small but measurable reverse saturation current I₀. Because this current depends on the rate of thermal generation rather than the applied voltage, it stays nearly constant as V_R increases — hence the name 'saturation.'

Breakdown

If the reverse voltage becomes large enough, two mechanisms can cause a sudden large current. In Zener breakdown (dominant below ~5 V), the strong field directly tears electrons out of covalent bonds by quantum tunnelling. In avalanche breakdown (dominant above ~5 V), a minority carrier accelerated by the field collides with a lattice atom hard enough to ionize it, releasing another electron-hole pair — those carriers then do the same, creating a multiplication cascade. In ordinary diodes this is destructive, but Zener diodes are engineered to operate safely in this regime for voltage regulation.

The Key Equation

I=I0(eqVnkBT−1)I = I_0 \left( e^{\frac{qV}{nk_BT}} - 1 \right)
  • I — Diode current (A): Net conventional current through the diode; negative in reverse bias, indicating it flows opposite to the assumed positive direction.
  • I_0 — Reverse saturation current (A): The small current due to thermally generated minority carriers; typically 10⁻⁹ to 10⁻⁶ A in silicon.
  • q — Elementary charge (C): Magnitude of the electron charge: 1.602 × 10⁻¹⁹ C.
  • V — Applied voltage (V): Voltage across the diode (positive = forward bias, negative = reverse bias).
  • n — Ideality factor (dimensionless): A factor between 1 and 2 accounting for recombination mechanisms; n = 1 for ideal diffusion-dominated diodes.
  • k_B — Boltzmann constant (J K⁻¹): Relates thermal energy to temperature: 1.381 × 10⁻²³ J K⁻¹.
  • T — Absolute temperature (K): Temperature of the semiconductor in Kelvin; at room temperature T ≈ 293 K.

Worked Example

Problem. A silicon diode has a reverse saturation current I₀ = 2.0 nA. The ideality factor n = 1. At room temperature (T = 300 K), calculate the reverse current when a reverse bias of V = −5.0 V is applied. Use k_B = 1.381 × 10⁻²³ J K⁻¹ and q = 1.602 × 10⁻¹⁹ C.

I≈−I0=−2.0 nAI \approx -I_0 = -2.0 \text{ nA}

Answer. The reverse current is approximately −2.0 nA, equal to −I₀.

Figure
Diagram
p-n Junction Under Reverse Biasp-typen-typeDepletionRegion(widened)−−−−−++++++++holes retreate⁻e⁻e⁻electrons retreat−+External field reinforces built-in field → barrier grows← Net Electric Field (n→p direction, reinforced) →
Origins · 1 minWilliam Shockley and how this was discovered
WS
William Shockley
1910–1989 · American (British-born)

Shockley co-invented the transistor at Bell Labs in 1947 alongside John Bardeen and Walter Brattain, winning the Nobel Prize in Physics in 1956. He also developed the theoretical framework for p-n junction behavior, including the Shockley diode equation that governs both forward and reverse bias characteristics. His mathematical description of minority-carrier injection and depletion region physics remains the foundation of semiconductor device theory taught in universities worldwide. Shockley was a passionate mountaineer who climbed the John Muir Trail in California. His competitive and often abrasive personality led to a mass resignation of talented engineers from his company — those engineers then founded Fairchild Semiconductor, which in turn spawned much of Silicon Valley, including Intel.

  • Diode Forward Bias — Forward bias is the opposite operating condition, where the barrier is reduced and large current flows — understanding both modes gives a complete picture of diode behavior.
  • P-N Junction and Depletion Region — The depletion region and built-in potential are the physical origin of the reverse-bias barrier; this concept explains how they form at equilibrium.
  • Zener Diode and Breakdown — Zener diodes are engineered to operate in controlled reverse breakdown, making reverse bias a useful rather than destructive regime.
  • Rectifier Circuits — Rectifiers exploit the asymmetry between forward and reverse bias to convert AC to DC, a direct application of reverse-bias blocking.