Rami Arieli: "The Laser Adventure" Chapter 6.3 Diode lasers Page 7.
Applying Voltage over p-n junction
When voltage is applied across the p-n junction, the population of the energy bands changes.

Voltage can be applied to the junction in two possible configurations:

  1. Forward Biased Voltage - means that the negative pole of the voltage is connected to the "n" side of the junction, and the positive side is connected to the "p" type semiconductor, as shown in figure 6.23:
Figure 6.23: Energy band of a p-n junction which is attached to a forward bias voltage.
    Forward biased voltage creates extra charge carriers in the junction, lowers the potential barrier, and causes injection of charge carriers, through the junction, to the other side.

    When an electron from the conduction band in the "n" side is injected through the junction to an empty "hole" in the valence band on the "p" side, a process of Recombination of (electron + hole) takes place. As a result of this recombination process, energy is released.
    For diode lasers we are interested in the specific cases when this energy is released  in the form of laser radiation. A sharp increase in conductivity occurs when the forward bias voltage is approximately equal to the semiconductor energy gap.

  1. Reverse (Backward) Voltage - which causes an increase in the potential barrier of the junction, and lowers the possibility for an electron to cross the junction to the other side. Increasing the reversed bias voltage to high values (tens of volts), can cause a voltage breakdown (Avalanche) of the junction.

If you want to play with an interactive simulation about the energy level diagram of a Diode Laser, click here. (VQM Simulation - Diode Laser)(Requires Shockwave)