Reverse Bias

Part of The Transistor

Reverse bias applies voltage across a PN junction in the direction that widens the depletion region and blocks current — the condition that makes diodes rectify, transistors switch off, and collector-base junctions provide the electric field that drives transistor action.

Why This Matters

Reverse bias is half the story of semiconductor operation. Forward bias allows current to flow; reverse bias blocks it. Together, they give the PN junction its rectifying property. But reverse bias is not merely the absence of conduction — it plays an active, essential role in transistor operation.

In every NPN transistor operating as an amplifier or switch, the collector-base junction is deliberately reverse biased. This reverse-biased junction creates the strong electric field that sweeps electrons from the base into the collector, turning the base into a control electrode. Without this reverse-biased junction, the transistor would not amplify — it would just short-circuit the supply through two forward-biased junctions.

Understanding reverse bias also explains why transistors break. Exceeding the breakdown voltage of a reverse-biased junction destroys the device catastrophically. Knowing what breakdown voltage is and how to avoid exceeding it is essential for practical circuit design.

Physical Effect of Reverse Bias

When a voltage is applied with positive terminal to the n-side and negative terminal to the p-side:

The depletion region widens. The external voltage adds to the built-in electric field (which points from n to p). This stronger field drives free carriers away from the junction on both sides:

• Electrons on the n-side are pushed further into the n-bulk, away from the junction
• Holes on the p-side are pushed further into the p-bulk, away from the junction

The depletion region — the zone empty of free carriers — grows wider. The junction becomes a better insulator.

Depletion width vs. reverse voltage:

W ∝ √(V_bi + V_reverse)

Where V_bi is the built-in voltage (~0.6 V for silicon). Applying 10V reverse bias adds 10/0.6 ≈ 16× to the built-in voltage, widening the depletion region by √17 ≈ 4× compared to zero bias.

This widening has two useful effects:

1. Reduces junction capacitance (C ∝ 1/W) — useful for voltage-controlled capacitors (varactors)
2. Increases breakdown voltage tolerance (wider depletion, lower peak electric field — up to a point)

Reverse Leakage Current

A perfect reverse-biased junction would allow zero current. Real junctions allow a small reverse saturation current I₀ from thermally generated minority carriers:

Device	I₀ at 25°C
Small silicon diode (1N4148)	25 nA typical
Silicon rectifier (1N4001)	5 µA max
Small germanium diode	1–50 µA typical
Germanium transistor (I_CBO)	1–20 µA
Silicon transistor (I_CBO)	1–100 nA

Temperature dependence: Reverse leakage doubles approximately every 10°C for silicon, and every 8°C for germanium. At 85°C, a germanium transistor’s leakage may be 100× its room-temperature value, potentially dominating circuit behavior.

This is why germanium transistors require careful temperature compensation and why silicon replaced germanium in most applications — silicon’s lower intrinsic carrier concentration means 100–1,000× less leakage at room temperature.

Reverse Bias in Transistors

In a properly operating NPN transistor:

Collector-base junction: reverse biased (typically 5–20 V)

This reverse bias is not a bug — it is essential. Here is why:

1. Electrons injected from the emitter diffuse across the thin base
2. When they reach the depletion region of the reverse-biased collector junction, the strong electric field accelerates them into the collector
3. This is the mechanism that transfers electrons from emitter to collector with nearly 100% efficiency
4. The higher the reverse bias on the collector, the stronger this sweeping field, and the more efficiently electrons are collected

What happens without collector reverse bias:

• If V_CB = 0 (base-collector junction at zero bias): some electrons still make it to the collector, but many recombine in the base — lower gain
• If V_CB is forward-biased: transistor enters saturation mode — both junctions forward-biased, collector voltage nearly equals emitter voltage, gain collapses
• If V_CB reverse bias exceeds breakdown: junction fails catastrophically

Breakdown Mechanisms

Two distinct mechanisms cause reverse-biased junction breakdown:

Avalanche Breakdown

At high reverse voltage, carriers in the depletion region gain enough kinetic energy to ionize silicon atoms — creating new electron-hole pairs that themselves accelerate and create more pairs. This avalanche multiplication causes current to increase explosively.

Characteristics:

• Occurs in lightly doped junctions with wide depletion regions
• Breakdown voltage: 10–1,000 V depending on doping
• Temperature coefficient: positive (breakdown voltage increases with temperature)
• Potentially destructive if current is not limited externally
• In Zener diodes rated >~6V, this is the dominant breakdown mechanism

Zener Breakdown

At very high electric fields in heavily doped junctions (narrow depletion regions), quantum mechanical tunneling allows electrons to cross the energy gap directly:

Characteristics:

• Occurs in heavily doped junctions with very narrow depletion regions
• Breakdown voltage: 2–6 V for typical Zener diodes
• Temperature coefficient: negative (breakdown voltage decreases with temperature)
• Reversible — no damage if current is limited
• Used intentionally in Zener voltage regulator diodes

Transistor Breakdown Voltages

Rating	Meaning
V_CEO	Maximum collector-emitter voltage with base open — most commonly specified
V_CBO	Maximum collector-base voltage with emitter open — usually higher than V_CEO
V_EBO	Maximum emitter-base reverse voltage — typically 5–7 V for most transistors

Emitter-Base Breakdown

V_EBO is often only 5–7 V for silicon transistors. If you apply more than this in reverse to the base-emitter junction, the transistor fails permanently even if the collector circuit is fine. This is a common failure mode when using transistors to switch inductive loads with insufficient flyback protection.

Using Reverse Bias Intentionally

Varactor Diode (Voltage-Controlled Capacitor)

The depletion region capacitance varies with reverse voltage:

• C ∝ 1/√(V_bi + V_reverse)
• A varactor diode uses this property to tune oscillators electrically

For a simple AM transmitter or receiver, a varactor eliminates the need for a mechanical tuning capacitor. Apply 1–10 V reverse bias from a potentiometer to tune through the AM band.

Zener Voltage Regulation

A Zener diode in reverse bias breakdown conducts at a stable voltage. Connect in reverse bias with a series resistor from a higher voltage supply to get a stable reference voltage.

Reverse Bias as a Test

Measuring reverse resistance of a PN junction reveals junction quality:

• Good silicon junction: >1 MΩ reverse resistance
• Good germanium junction: >100 kΩ
• Leaky junction (damaged, contaminated, or heavily doped): <10 kΩ
• Shorted junction: <100 Ω

Practical Precautions

Stay below V_CEO: Design circuits so the maximum collector-emitter voltage (at peak signal swing) remains 20–30% below the transistor’s rated V_CEO. Inductive load switching can produce brief spikes exceeding the supply voltage.

Protect the base-emitter junction: In circuits driving inductive loads, ensure the collector voltage spike cannot forward-bias the base-emitter junction in reverse. Flyback diodes protect the collector; emitter-base protection may require a dedicated diode or clamp.

Watch temperature in germanium circuits: High temperature dramatically increases leakage. If a germanium transistor runs warm, check that operating point has not drifted toward saturation due to increased I_CBO.

Summary

Reverse Bias — At a Glance

• Reverse bias (+ to n-side, − to p-side) widens the depletion region and blocks current
• Small leakage current (I₀) flows from thermally generated minority carriers — 1,000× larger in germanium than silicon
• Collector-base junction is reverse-biased in active mode transistors — this creates the field that sweeps electrons from base to collector
• Avalanche breakdown: voltage-triggered current multiplication in lightly doped junctions (reversible with current limiting)
• Zener breakdown: quantum tunneling in heavily doped junctions <6V — used intentionally for voltage regulation
• V_EBO (emitter-base breakdown, typically 5–7 V) is easily exceeded by reverse input pulses — a common failure mode
• Test junction quality: good silicon junction >1 MΩ reverse resistance