Page 162 - Photonics Essentials an introduction with experiments

Lasers

156 Photonic Devices

nhfg(f)
k = (N 2 – N 1 )P 21 (7.11)
c
An optically excited semiconductor can be used to make a laser, but
the really interesting application of semiconductors is the use electrical
current to turn the laser on. You know that current injection into a p-n
junction diode creates light. In order to turn this light into laser light,
we need gain, and to get gain, we need a population inversion. In order
to see how this can be achieved, we show in Fig. 7.7 the energy level di-
agram for a p-n junction. This p-n junction is different from others we
have looked at. It is heavily doped on both the p-side and the n-side so
that the Fermi level actually lies in the conduction band on the n-side
and in the valence band on the p-side. This is called degenerate doping.
It is not a requirement, but it does make lasing easier to obtain.
From the work you have done to characterize diodes, you know that
there are two important things that happen when a diode is subjected
to forward bias. One is that the energy difference between the conduc-
tion band on the p-side and the conduction band on the n-side be-
comes smaller. The other thing that occurs is that the width of the de-

C B

Electrons

V B
ENERGY Fermi Level

Holes

DISTANCE

Figure 7.7. Energy level diagram for a p-n junction with degenerate doping on both the
p- and the n-sides. V B refers to the valence band and C B refers to the conduction band.
The Fermi level is constant throughout, so no bias is applied to the diode and there is
no current.

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