The backward diode, is sometimes also called the back diode. The backward diode is a form of PN junction diode that has many similarities to the tunnel diode.
The backward diode is not widely used, and availability is an issue but it has some useful characteristics that mean it can be used in some specialist applications where its level of performance is needed. Typically it provides excellent performance at microwave frequencies.
Backward diode basics
A backward diode is essentially 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.
- Forward direction: In the forward direction the tunnelling effect is much reduced and it follows virtually the same characteristic as a normal PN junction diode.
- Reverse direction: In the reverse direction, the tunnelling effect means that the backward diode has a characteristic similar to a normal forward biased PN junction diode.
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.
Backward diode circuit symbol
Despite the operation of the backward diode, its circuit symbol is based on that for the standard diode, but it is differentiated from the basic PN junction diode circuit symbol by the modification to the bar section of the diode symbol.
Typically the circuit symbol 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.
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.
- 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 backward 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.
- Detector : The backward diode provides a linear detection characteristic for small signals. As there is no charge storage in the backward diode, it means that it can be used for signals with frequencies extending to 50 GHz and more.
The backward diode is not commonly used, but it can be usefully used in a limited number of occasions. The main issue with the backward diode is its availability.