What happens in forward bias?

In forward bias the applied voltage opposes the built-in barrier, so the depletion region narrows and the barrier falls. Once the voltage exceeds the knee voltage, about 0.7 volts for silicon and 0.3 volts for germanium, majority carriers cross the junction and a large forward current flows.

What happens in reverse bias?

In reverse bias the applied voltage adds to the barrier, so the depletion region widens and the barrier rises. Almost no current flows, only a tiny reverse saturation or leakage current, until the voltage reaches the breakdown value where the diode conducts suddenly in reverse.

What is the knee or threshold voltage of a diode?

The knee, threshold or cut-in voltage is the minimum forward voltage at which a diode starts to conduct significantly. It is about 0.7 volts for a silicon diode and about 0.3 volts for a germanium diode, set by the junction barrier potential.

What is the diode I-V characteristic curve?

The I-V characteristic plots diode current against applied voltage. In the forward region the current stays small until the knee voltage, then rises sharply. In the reverse region only a tiny leakage current flows until reverse breakdown, where the current increases rapidly in the negative direction.

How do you calculate diode current in forward bias?

For a simple circuit with a series resistor, subtract the diode drop from the supply and divide by the resistance: I equals (source voltage minus 0.7 volts) divided by R for a silicon diode. For example 5 volts, a 1 kilo-ohm resistor and a silicon diode give about 4.3 milliamps.

Why does a reverse-biased diode still pass a tiny current?

Thermally generated minority carriers are swept across the junction by the field, giving a very small reverse saturation or leakage current, typically nanoamps to microamps. It roughly doubles for every 10 degrees Celsius rise in temperature.

Forward & Reverse Bias: Diode Biasing, I-V Curve & Knee Voltage

Q: What is forward and reverse bias?

Biasing means applying a voltage across a diode. In forward bias the positive terminal goes to the anode (P side), which lowers the junction barrier so the diode conducts. In reverse bias the positive terminal goes to the cathode (N side), which raises the barrier so the diode blocks current apart from a tiny leakage.

Q: What is the difference between forward and reverse bias?

Forward bias connects positive to the anode, lowers the barrier, narrows the depletion region, gives low resistance and large current, and makes the diode act like a closed switch. Reverse bias connects positive to the cathode, raises the barrier, widens the depletion region, gives very high resistance and only leakage current, and makes the diode act like an open switch.

What is Biasing?

Biasing means applying an external DC voltage across a diode to control its depletion region. The direction of that voltage decides everything: a diode is a one-way valve — it conducts one way and blocks the other.

Connect the supply one way (forward bias) and the diode conducts like a closed switch. Reverse the connection (reverse bias) and it blocks like an open switch. This single behaviour is the foundation of rectifiers, logic, and every semiconductor device.

Forward bias circuit with the bulb lit and reverse bias circuit with the bulb off, showing a diode conducts one way and blocks the other

A diode is a one-way valve: forward bias (+ to anode) lets current flow and the bulb lights; reverse bias (+ to cathode) blocks it and the bulb stays off.

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Diode: anode → cathode

FWD

Conducts (ON)

REV

Blocks (OFF)

0.7 V

Si knee voltage

Anode and cathode

The anode is the P side, the cathode the N side (marked by the band on the body). Current flows anode→cathode only — in the direction of the diode’s arrow.

Forward Bias: the Diode Conducts

In forward bias the battery’s + goes to the P side (anode) and − to the N side (cathode). This applied voltage opposes the built-in barrier, so the depletion region narrows and the barrier falls.

Forward-biased diode: battery plus to P, barrier lowered, depletion region narrow, large forward current flows

Forward bias lowers the barrier and narrows the depletion region — above the knee voltage, majority carriers flood across and a large forward current flows.

I_F = (V_S − V_knee) / R (simple series circuit)

Once V_S > ~0.7 V (Si), current is limited mainly by the series resistor

Worked example

A silicon diode, 5 V supply, 1 kΩ series resistor:

I_F = (5 − 0.7) / 1000 = 4.3 mA. The diode drops ~0.7 V; the resistor takes the rest.

Reverse Bias: the Diode Blocks

In reverse bias the battery’s + goes to the N side (cathode) and − to the P side (anode). Now the applied voltage adds to the barrier, so the depletion region widens and the barrier rises.

Reverse-biased diode: battery plus to N, barrier raised, depletion region wide, only a tiny leakage current flows

Reverse bias raises the barrier and widens the depletion region — the diode blocks, passing only a tiny reverse saturation (leakage) current I_S.

Reverse breakdown

Beyond the breakdown voltage the diode suddenly conducts in reverse (avalanche or Zener effect). It is destructive for ordinary diodes but deliberately used in Zener diodes for voltage regulation.

The Diode I-V Characteristic

Plot current against voltage and the whole story appears in one curve — the diode’s I-V characteristic.

Diode I-V characteristic curve: forward current rises past the knee voltage, reverse current is tiny until breakdown

Forward: current is small until the knee (~0.7 V), then rises steeply. Reverse: only a tiny leakage until breakdown.

I = I_S(e^V/nV_T − 1) (Shockley diode equation)

V_T ≈ 26 mV at room temperature; n is the ideality factor (1–2)

The four regions

1) Forward below knee (small I) · 2) Forward above knee (large I) · 3) Reverse (tiny leakage) · 4) Reverse breakdown (large reverse I).

Knee / Threshold Voltage (Si vs Ge)

Every diode needs a minimum forward voltage — the knee, threshold or cut-in voltage — before it conducts properly. It equals the junction barrier potential.

Forward characteristic knee voltage: germanium conducts near 0.3 V and silicon near 0.7 V

Germanium turns on near 0.3 V, silicon near 0.7 V — the knee/threshold voltage of each material.

Material	Knee voltage	Notes
Silicon (Si)	≈ 0.7 V	Most common; lower leakage
Germanium (Ge)	≈ 0.3 V	Lower drop; higher leakage
Schottky	≈ 0.2–0.4 V	Very fast, low drop
LED	≈ 1.8–3.3 V	Depends on colour

Forward vs Reverse Bias

Same diode, opposite behaviour. This side-by-side is the summary to remember.

Comparison of forward and reverse bias: battery polarity, barrier, resistance, current and switch behaviour

Forward bias = a closed switch that conducts; reverse bias = an open switch that blocks.

Memory aid

“P to +, current flows.” If the P side (anode) is at the higher potential, the diode is forward-biased and conducts.

Where Biasing Is Used

Controlling when a junction conducts is the basis of countless circuits.

Rectifiers

Diodes conduct only on forward half-cycles to turn AC into DC in every rectifier.

Protection

Reverse-biased diodes block wrong polarity and clamp inductive spikes (flyback).

LEDs

Forward-biased LEDs emit light above their threshold voltage.

Zener regulation

A reverse-biased Zener in breakdown holds a steady voltage.

Clippers & clampers

Bias sets the level where a diode starts to conduct and reshape a signal.

Transistor biasing

BJT junctions are biased (forward B-E, reverse B-C) to set the operating point.

Key Terms at a Glance

The essential biasing vocabulary students and engineers search for.

Bias

External DC voltage across a diode.

Forward bias

+ to anode (P); conducts.

Reverse bias

+ to cathode (N); blocks.

Knee voltage

Turn-on: 0.7 V (Si), 0.3 V (Ge).

I-V characteristic

Current vs voltage curve.

Leakage current I_S

Tiny reverse current.

Breakdown

Reverse conduction (avalanche/Zener).

Anode / cathode

P side / N side of the diode.

Frequently Asked Questions

Quick, expert answers to the questions people ask most about forward and reverse bias.

What is forward and reverse bias in simple words?

Forward bias means connecting the battery so the diode conducts (+ to the anode); reverse bias means connecting it the other way so the diode blocks (+ to the cathode). A diode only lets current through one way.

Which way does current flow through a diode?

Only from anode to cathode — the direction the diode symbol’s triangle points — and only when forward-biased above the knee voltage.

What is the knee (threshold) voltage?

The minimum forward voltage to start real conduction: about 0.7 V for silicon and 0.3 V for germanium. Below it, only a tiny current flows.

What is the difference between forward and reverse bias?

Forward: + to anode, barrier low, depletion narrow, low resistance, large current, acts as a closed switch. Reverse: + to cathode, barrier high, depletion wide, very high resistance, only leakage, acts as an open switch.

Does a reverse-biased diode carry any current?

Only a tiny reverse saturation (leakage) current, from thermally generated minority carriers — typically nanoamps to microamps, roughly doubling every 10°C.

How do I calculate the current in a forward-biased diode circuit?

Subtract the diode drop from the supply and divide by the series resistor: I = (V_S − 0.7) / R for silicon. E.g. 5 V, 1 kΩ → 4.3 mA.

What is reverse breakdown?

At a high enough reverse voltage the diode suddenly conducts in reverse (avalanche or Zener). Ordinary diodes can be damaged; Zener diodes use it on purpose to regulate voltage.

Why is a diode called a one-way switch?

Because forward bias makes it a low-resistance closed switch and reverse bias makes it a high-resistance open switch — it passes current in only one direction.

Conclusion & Key Takeaways

Bias direction decides whether a diode conducts or blocks — the one-way behaviour behind all diode circuits.

+ to anode → ON

Forward bias conducts.

+ to cathode → OFF

Reverse bias blocks.

Knee 0.7/0.3 V

Si / Ge turn-on.

Reverse = leakage

Tiny I_S until breakdown.

I-V curve

Knee, leakage, breakdown.

One-way switch

Closed vs open.

Forward & Reverse Bias

Complete Learning Path — Forward & Reverse Bias