Backward Diode Tutorial

- the backward diode uses much the same structure as the tunnel diode, but it has a different IV characteristic making it useful in a variety of applications.

The backward diode, sometimes also called the back diode is a form of PN junction diode that is very similar to the tunnel diode in its operation.

Although not widely used, it does have some advantages that mean it can be used in some specialist or particular applications where its level of performance is needed.


A backward diode is a form of tunnel diode where one side of the junction is less heavily doped than the other.

This doping profile results in a diode that shares a number of characteristics with the tunnel diode, but modifies others. It means that in the reverse direction, the tunnelling effect means that the diode has a characteristic similar to a normal forward biased PN junction diode.

In the forward direction the tunnelling effect is much reduced and it follows virtually the same characteristic as a normal PN junction diode.

The IV characteristic of the backward diode showing the important voltage turning points and the negative resistance region
Backward diode IV characteristic

The fact that the diode can be used 'backward' way round gives rise to its name.

Additionally the reverse characteristic is very similar to that of a zener diode, although the voltages are much lower. It has a very flat curve where voltage remains relatively constant independent of the level of the current.

Breakdown diode circuit symbol

Despite the operation of the breakdown diode, its circuit symbol is based on that for the standard diode, but has 'tails' added to the bar element visible either side of the bar to indicate its operation and to differentiate it from other forms of PN junction diode.

The circuit symbol for the backward diode showing the slightly different arrangement on the bar to differentiate from other forms of diode
Backward diode circuit symbol

Typically it is annotated to show which side is P type and which is N. This is useful because the current flows mainly from N to P, in the opposite direction to that expected with a standard PN junction diode or rectifier.

Backward diode applications

The characteristics of the backward diode make it suitable for a limited number of applications where other diodes may not perform as well.

  • Detector :   The backward diode provides a linear detection characteristic for small signals. Additionally the fact that there is no charge storage in its mode of operation means that it can be used for signals with frequencies extending to 50 GHz and more.
  • Rectifier:   The diode is suitable for rectifying signals with peak voltages between about 0.1 and 0.6 volts
  • Switch:   In view of its speed of operation, the diode is sometimes used for very high speed switching applications. It can be used as a switch within an RF mixer or multiplier where it provides excellent signal performance at microwave frequencies.

Although not widely used, the backward diode can be very effective in applications where its characteristics can be fully utilised.

By Ian Poole

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