The height of the potential barrier formed in the diode – which prevents diffusion of the electrons from n-type silicon to p-type silicon – can be modified by external electric field. The height of the potential barrier can be changed as a function of how the two electrodes of the diode are connected to the electrodes of the external direct power supply source.

Zero bias
Forward biased diode
Reverse biased diode

The height of the potential barrier can be reduced by opening or direct polarisation, which results in a flow of electricity through the diode, or can be increased by reverse biassing. In the latter case, however, practically no conduction will take place.

If the p-type half crystal of the semiconductor is connected to the positive potential electrode of the power supply source, and the n-type half to the negative potential electrode, the strength of the external electric field (Eexternal) will be of opposite direction as the Einternal formed in the blocking layer. As long as the size of Eexternal does not reach that of Einternal, no current will flow through the diode, but as soon as Eexternal becomes greater than Einternal, conduction will start. This is called forward bias.

In this case potential conditions are changed, the potential of the p-range is increased, that of the n-range declines, and the original Vi (built-in potential) relative height of the potential hill is reduced to (Vi-Ve) level. (Ve external voltage.)

Forward biased diode

In the opposite case, that is when the p-type half of the diode crystal is connected to the negative potential electrode of the power supply source, and the n-type half crystal to the positive potential electrode, the strength of the external electric field (Eexternal) will be identical in direction with the Einternal formed in the depletion layer. As a result, the potential barrier would be increased, the depletion layer becomes wider and they jointly obstruct electron migration from n-type silicon the p-type silicon. No current will flow through the diode. This is the reverse bias.

In this case the potential of the p-range is decreased due to the changes in the potential conditions, that of the n-range grows, and the original Vi relative height of the potential hill is also increased to (Vi-Ve) level, caused by the Ve external voltage of the battery. In spite of the reverse bias there is still a low level of current present (Io), created by the minority charge carriers, since for them polarisation would be forward bias.

In other words the diode will conduct the electric current only in one direction, when forward bias is applied.

Conductivity of the p-n junction (depletion layer) formed in the diode reacts asymmetrically in face of the connected Ve voltage, therefore the diode can be used to rectify alternating current voltage. In the case of forward bias the current strength increases exponentially, while in the case of a reverse bias it converges towards the Io saturation level. Any substantial current will flow through the diode only in case of a forward bias.

Diode: V-I Characteristic
Reverse biased diode

Key statements

1. Einternal  built up in the depletion layer of the diode would prevent (thermal) diffusion of electrons from the n-type silicon to the p-type silicon.

2. Using external electric field the size of the Einternal  formed in the depletion layer can be changed.

3. When the diode is forward biased (connecting positive potential to p-type silicon and negative potential to n-type silicon) the Einternal will be eliminated, if the electric field of the external power supply source is strong enough. Current will flow trough the diode.

4. In the case of reverse bias of the diode (i.e. p-type silicon connected to the negative potential, n-type to the positive one) the depletion layer and hence, the Einternal  is widened up as an impact of the electric field of the external power supply source. No current will flow trough the diode.

5. The diode lets current through only in one direction, in return of forward biases, this is why the diode can be used to rectify alternating current.