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Forward Bias vs Reverse Bias: Depletion Width and Current

A PN junction diode behaves very differently depending on which way the voltage is applied. Forward bias means making the p side positive and the n side negative. The junction barrier drops, majority carriers are injected across the junction, and the current rises exponentially with voltage. Reverse bias means making the p side negative and the n side positive. The barrier rises, the depletion region widens, and only a tiny reverse current flows. Forward bias is used for rectification and for making LEDs emit light; reverse bias is used for light detection, voltage-controlled capacitance and, with breakdown, voltage regulation.

AspectForward biasReverse bias
How the voltage is appliedp side (anode) positive, n side (cathode) negativep side (anode) negative, n side (cathode) positive
Depletion widthNarrower; proportional to √(Vbi − V), where Vbi is the built-in potentialWider; proportional to √(Vbi + VR). When VR is much larger than the built-in potential, four times the voltage gives about twice the width
Barrier heightLowered to q(Vbi − V)Raised to q(Vbi + VR)
Current and carrier motionMajority carriers cross the barrier, are injected into the other side and diffuse as minority carriers (diffusion current)A very small current of thermally generated minority carriers swept across by the field in and near the depletion region
Current-voltage relationI ≈ Is·exp(qV/nkT). With ideality factor n = 1, the current rises tenfold for about every 60 mV at room temperature. Silicon typically conducts significant current from about 0.6 to 0.7 VA small current that hardly depends on voltage (ideally −Is; in practice generation current in the depletion region adds to it). It rises sharply with temperature
Dominant capacitanceDiffusion capacitance, from stored injected minority carriers; it grows roughly in proportion to current (Cd ≈ τI/(nkT/q))Junction (depletion) capacitance, C = εA/W; it falls as the reverse bias increases
Worked example (abrupt junction, p side 1×10¹⁹ cm⁻³, n side 1×10¹⁶ cm⁻³, room temperature, depletion approximation)Built-in potential about 0.89 V. Depletion width about 0.34 µm at 0 V and about 0.23 µm at 0.5 V forward biasDepletion width about 0.87 µm at 5 V reverse bias and about 1.6 µm at 20 V. Junction capacitance at 5 V is about 0.39 times its value at 0 V
BreakdownDoes not occur; at high current, series resistance and heating limit the currentAbove a certain voltage, avalanche or Zener breakdown makes the current rise sharply
Typical usesThe conducting direction in rectifier circuits, light emission in LEDs and laser diodes, the base-emitter junction of a BJT, temperature sensorsPhotodiodes, varactor (variable-capacitance) diodes, Zener diodes (operated in breakdown), avalanche photodiodes, junction isolation between devices

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

Forward bias
The condition for passing current: the conducting half-cycle of a rectifier diode, a lit LED and the base-emitter junction of a BJT. Forward voltage typically falls by about 2 mV/°C as temperature rises, so it is also used for temperature sensing. In fast switching, watch the diffusion capacitance and reverse recovery time caused by stored minority carriers.
Reverse bias
The condition for blocking current, or for using the depletion region itself. A photodiode collects light-generated carriers with the depletion-region field, and a varactor changes its capacitance by changing the depletion width with voltage. A Zener diode is operated near its breakdown voltage to hold a fixed voltage. In integrated circuits, well and diffusion junctions are kept reverse biased to isolate devices from one another.

FAQ

Why does a little current still flow under reverse bias?

Electrons and holes generated thermally in and near the depletion region are swept to the other side by its field. The amount is set by how many minority carriers are generated thermally, so it barely increases with voltage but rises sharply with temperature. In silicon it is very small, but it matters as dark current in photodiodes and as the leakage that limits data retention time in DRAM.

Does the depletion region disappear completely under forward bias?

No. The width shrinks in proportion to √(Vbi − V), but the current becomes large before the voltage approaches the built-in potential, and the drop across series resistance grows, so the voltage across the junction itself stays below the built-in potential. For a one-sided abrupt junction with the n side at 1×10¹⁶ cm⁻³, for example, the width goes from about 0.34 µm at 0 V to about 0.23 µm at 0.5 V forward bias (a depletion-approximation estimate).

Why is the dominant capacitance different in forward and reverse bias?

The charge behind the capacitance is different. Under reverse bias, the charge of ionized dopants on both sides of the depletion region changes with voltage, so the junction capacitance, which behaves like a parallel-plate capacitor, dominates. Under forward bias, the charge of minority carriers injected across the junction and stored in the neutral regions grows with current, so the diffusion capacitance dominates. Diffusion capacitance becomes large at high current and slows down fast switching of diodes and bipolar transistors.

Why are photodiodes operated under reverse bias?

A wider depletion region collects light-generated electrons and holes from a larger volume, and the strong field separates them quickly into a current. The junction capacitance also gets smaller, which speeds up the response. By contrast, a solar cell delivers power with no applied bias, and an LED emits light when carriers injected under forward bias recombine.

Check your understanding

How is voltage applied to forward-bias a PN junction diode?

  1. AThe p side positive and the n side negative
  2. BThe p side negative and the n side positive
  3. CThe p side and n side at the same potential
Show answer and explanation

Answer: A. The p side positive and the n side negative

Making the p side (anode) positive and the n side (cathode) negative lets the applied voltage oppose the built-in potential, lowering the barrier so majority carriers are injected across the junction. This is forward bias, and current grows exponentially with voltage. Connecting it the other way raises the barrier, leaving only a tiny current of thermally generated minority carriers. At equal potential, diffusion and drift balance and the current is zero.

What happens to the width of the depletion layer when the reverse bias applied to a PN junction is increased?

  1. AIt narrows
  2. BIt does not change
  3. CIt widens
Show answer and explanation

Answer: C. It widens

Under reverse bias the applied voltage adds to the built-in potential, pulling carriers further from the junction, so the depletion region widens. Its width scales roughly as √(Vbi + V), so when V is much larger than Vbi, quadrupling the voltage about doubles it. A wider depletion region lowers the junction capacitance (C = εA/W), the property used in varactor diodes, and spreading the field over a wider region also helps withstand voltage.

Which property does a variable-capacitance (varactor) diode use?

  1. AEmitting light with forward current
  2. BResistance changing with temperature
  3. CJunction capacitance changing with reverse bias voltage
Show answer and explanation

Answer: C. Junction capacitance changing with reverse bias voltage

Increasing reverse bias widens the depletion region and lowers the junction capacitance. A varactor diode exploits this to make capacitance voltage-controlled. Hyperabrupt junctions with steep doping profiles change capacitance strongly with voltage, widening the tuning range. Because the resonant frequency f = 1/(2π√(LC)) can then be set by voltage, varactors are used in tuners, voltage-controlled oscillators (VCOs) and PLLs.

  1. How is a photodiode generally used to detect light?
  2. In a forward-biased PN junction, which capacitance arises from the storage of injected minority carriers?
  3. How does an LED emit light?

Practice PN junctions and diodes questions

Key terms explained

Forward bias
Applying a positive voltage to the p side and a negative voltage to the n side. The barrier drops and current flows readily. The opposite is reverse bias, under which almost no current flows.
Depletion layer
A region near a PN junction boundary that contains almost no carriers. It widens under reverse bias.
Built-in potential
The potential difference across the depletion layer of a PN junction even with no external voltage applied. About 0.6 to 0.9 V in silicon, it forms the barrier that stops further carrier diffusion.
Junction capacitance (depletion capacitance)
Capacitance arising because the depletion layer of a PN junction acts like a parallel-plate capacitor. It decreases as reverse bias widens the depletion layer; varactor diodes use this property.
Diffusion capacitance
Capacitance in a forward-biased PN junction caused by injected minority carriers stored in the neutral regions. It grows in proportion to forward current and limits high-speed operation.
Zener diode
A diode that provides a constant voltage by using the nearly fixed reverse breakdown voltage.
Photodiode
A diode that generates charge (current) when it receives light. Used as the light-receiving element in image sensor pixels.
Avalanche breakdown
A phenomenon in which carriers accelerated by a strong reverse field strike atoms and create new carriers, which multiply in a chain reaction so that a large current flows.

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