Page 163 - Photonics Essentials an introduction with experiments

Lasers

Lasers 157

pletion region decreases. There are other important changes, to be
sure. The nonequilibrium carrier density at the edge of the depletion
region rises exponentially, and there is current in the diode. However,
for a moment, concentrate on the first two changes.
In forward bias, the Fermi level on the n-side of the diode is at high-
er energy than the Fermi level on the p-side. Electrons will move from
the n-side to p-side in an attempt to redress this difference, and there
will be current in the diode. In Fig. 7.8 we show the situation as the
applied voltage is close to the magnitude of the built-in potential.
The effect of forward bias in the diode is to create a population inver-
sion in a four-level system. It does this by bringing the populations of
electrons and holes into physical spatial overlap. Recombination of
electrons and holes requires that the electrons and holes be “in the
same place at the same time,” that is, within a de Broglie wavelength
of each other at the same time. This is a distance of about 10 nm. Of
course, energy and momentum must be conserved. This condition is as-
sured by choosing a diode made from direct band gap materials such as
GaAs, InP, or direct bandgap alloys made from these materials.

Electrons
C B

ENERGY V B w

Holes

DISTANCE
Figure 7.8. Energy level diagram of a degenerately doped p-n junction diode in forward
bias. Note that the concentration of electrons in the conduction band lies higher in en-
ergy than the concentration of holes in the valence band. Note also that the distance w
becomes smaller and smaller as the bias voltage increases. This feature brings the con-
centrations of electrons and holes into spatial overlap.

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