Page 329 - Electrical Properties of Materials

Semiconductor lasers 311

) a ( ) b (

eU
E E 1
E C C
F
Fig. 12.11
An AlGaAs–GaAs heterojunction
eU
1 (a) at thermal equilibrium, (b) at a
E
V E V forward bias of eU 1 .

+
p – Ga As

p – Ga Al As

p – Ga As

n – Ga Al As
Fig. 12.12
Schematic representation of a double

n – Ga As heterojunction GaAlAs laser with
external Bragg reflector mirrors.

higher. The threshold current will also be small on account of the small
thickness, d, of the active region [see eqn (12.38)] which is 100 nm in our
example.
Can we further decrease the threshold current? Yes, both common sense and
eqn (12.38) tell us that we need higher reflectivity mirrors. One way of doing
this is to use an external Bragg reflector, as shown in Fig. 12.12. Each slight
corrugation will cause a small reflection which all add up in phase at the right
∗
wavelength. By these means threshold currents as small as 0.5 mA have been ∗ The corrugations (or slight bumps) can
achieved. actually be inside the laser, in which
It is very nice, indeed, to reduce the threshold current because that will case we talk about a Distributed Bragg
Reflector laser or DBR.
reduce the power consumption of devices (e.g. compact disk players) using
semiconductor lasers. But those lasers have to deliver a certain amount of
power. There is no way of getting out a fair amount of power without putting
in a fair amount of power, so it is also of crucial importance how the output
power increases as the current exceeds its threshold value.
Let us now come back to the role of d, the thickness of the active region. As
we reduce it, the threshold current decreases simply because fewer electrons
need to be supplied to make up for spontaneous emission. Fewer electrons
being available will also reduce the achievable power output. For this reason
d cannot be usefully reduced to a value smaller than about 100 nm. Actually,
if our aim is to reduce the thickness further without losing output power, we
could simply increase the number of wells, say by a factor of 10, and make
each of them a thickness of d/10.