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medium difficulty11 min

Forward and Reverse Bias Characteristics of a p-n Diode

The p-n diode characteristic is the graph of current through a diode against the voltage applied across it. In forward bias the diode offers low resistance after the knee voltage, while in reverse bias it offers high resistance with a small reverse saturation current until breakdown.

In one line: Forward bias narrows the barrier; reverse bias raises it.

Concept Practice Coming Soon

Understand it

  • A diode conducts mainly in one direction because external bias changes the depletion region and potential barrier.
  • In forward bias, the p-side is connected to the positive terminal and the n-side to the negative terminal.
  • The barrier reduces, the depletion region narrows, and majority carriers cross the junction more easily.
  • Current rises sharply after the knee voltage.
  • In reverse bias, the p-side is connected to the negative terminal and the n-side to the positive terminal.
  • The barrier increases, the depletion region widens, and only a small reverse saturation current flows due to minority carriers until breakdown occurs at high reverse voltage.

Example

For a silicon diode in forward bias, current remains small at very low voltage but increases sharply near about 0.7 V. In reverse bias, the current stays very small for a wide voltage range and rises suddenly only if breakdown voltage is reached.

Formula and rule

Circuit condition: forward bias has p connected to positive and n connected to negative; reverse bias has p connected to negative and n connected to positive.

Graph relation: current is non-linear with voltage.

The dynamic resistance in a small region is rd = delta V / delta I, where V is voltage in volt, I is current in ampere, and rd is in ohm.

Use this only for a small portion of the I-V curve, not for the entire non-linear graph.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

A diode current changes from 8 mA to 18 mA when the forward voltage changes from 0.68 V to 0.72 V in a small operating region.

  1. 1Dynamic resistance rd = delta V / delta I = (0.72 - 0.68) V / (18 - 8) mA = 0.04 V / 0.010 A = 4 ohm.
  2. 2This means the diode offers small incremental resistance in that forward-biased region.

Method you can reuse

  1. 1First identify diode terminals and battery polarity.
  2. 2Decide whether the junction is forward or reverse biased.
  3. 3Describe the effect on depletion width and barrier potential.
  4. 4Then connect this to the size and direction of current.
  5. 5For graph questions, read voltage on the x-axis and current on the y-axis before calculating ratios.

Sketch it in your answer

Show the circuit for forward and reverse bias and the I-V characteristic graph. Graph labels must include voltage V on the horizontal axis, current I on the vertical axis, forward region, reverse region, knee voltage, reverse saturation current, and breakdown voltage.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why does diode current increase sharply after the knee voltage in forward bias?

Think first, then show answer
After the knee voltage, the external forward voltage has reduced the potential barrier enough for many majority carriers to cross the junction. This sharply increases the diode current, so the diode behaves as a low-resistance path in forward bias.

Common wrong answer

Using V/I from the origin as a constant diode resistance is usually wrong because a diode is non-ohmic. For a small part of the curve, use delta V divided by delta I.

Don't mix this up

Students often reverse the battery connections for forward bias. Forward bias means p-side to positive terminal and n-side to negative terminal.

Exam tip

In graph questions, label both axes with units, mark forward and reverse regions, show knee voltage in forward bias, and show breakdown region on the reverse side if required.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Students may be asked to draw the I-V curve, compare forward and reverse bias, identify knee voltage or breakdown voltage, calculate dynamic resistance from graph data, or explain why a diode is unidirectional.
  1. 1

    Enough for 1 mark

    The p-n diode characteristic is the graph of current through a diode against the voltage applied across it. In forward bias the diode offers low resistance after the knee voltage, while in reverse bias it offers high resistance with a small reverse saturation current until breakdown.

  2. 2

    Add for 2 marks

    Example: For a silicon diode in forward bias, current remains small at very low voltage but increases sharply near about 0.7 V. In reverse bias, the current stays very small for a wide voltage range and rises suddenly only if breakdown voltage is reached.

  3. 3

    Add for 3 marks

    A diode conducts mainly in one direction because external bias changes the depletion region and potential barrier.

Practice

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