Page 359 - Electrical Properties of Materials

Light emitting diodes (LEDs) 341

materials to utilize the longer wavelengths in sunlight, including a GINA alloy
mentioned in Section 12.7.3. The expected conversion efficiency is 41% com-
pared with about 30% for a single junction device. The four-stage device is
made by MOCVD so is more expensive than coal, but it can be used with mir-
rors or lenses to concentrate light up to 500 suns, where its expected efficiency
is over 50%. Possibly some hope for the terrestrial market as the technology
advances.
So far we have talked about light detectors but since the principles are more
∗
or less the same, this is also the place to treat infrared detectors. The primary ∗ Purists might object saying that the
form of detection in that region too is the change in the conductivity of the ma- command ‘let there be light’ was restric-
ted to the visible region, but we think that
terial induced by incident radiation. The materials are again semiconductors.
is a too narrow interpretation.
The devices rely on band-to-band transition up to 10 μm, requiring small en-
ergy gap semiconductors. Semiconductors of even smaller energy gap can be
found, so in principle, band-to-band transition could be used for even longer
wavelengths, but in practice this range from 10–100 μm is covered by impurity
semiconductors in which the increased conductivity is obtained by exciting an
electron from a donor level into the conduction band (or from the valence band
into an acceptor level).
†
Infrared radiation between 100 μm and 1 mm is usually detected with the † Some care needs to be exercised with
aid of the so-called ‘free carrier absorption’. This is concerned with the excit- terminology. This region used to be
ation of electrons from lower to higher energy levels in the conduction band. called the submillimetre region. The
latest fashion is to talk about frequency
The number of electrons available for conduction does not change, but the mo-
rather than wavelength. 300 GHz would
bility does, owing to the perturbed energy distribution of the electrons. The correspond to 1 mm wavelength, 1 THz
change (not necessarily increase) in conductivity may then be related to the to 0.3 mm and 100 THz to 3 μm.
strength of the incident infrared radiation. Since electrons may be excited to
higher energy levels by lattice vibrations as well (thus masking the effect of
the input infrared radiation) the crystal is usually cooled.
Relative newcomers to the family are the QWIPs (Quantum Well Infrared
Photodetectors) and the QDIPs (Quantum Dot Infrared Photodetectors) which
work by exciting electrons from discrete energy levels into the high energy
continuum.

13.3 Light emitting diodes (LEDs)

Semiconductor lasers have been discussed in some detail in Section 12.7.
We came to the conclusion that it is a good thing to have a good semicon-
ductor laser: it has many applications. Are bad lasers good for anything? Yes,
certainly, as long as they can emit light. How do we make a bad laser? By
depriving a good laser of its cavity. The injected carriers may still recombine
and emit light but in the absence of the cavity there is no agent to take care of
the phase relationships and consequently the emitted light will not be coherent
and will not emerge as a narrow beam. But that is exactly what we want if
the aim is to use the semiconductor diode for lighting: we want a diffuse light
coming out in all directions. This light source does indeed exist. It is called a
light emitting diode, abbreviated as an LED.
In fact, LEDs were invented in 1960, a few years before semiconductor
lasers. The first specimen emitted red light. They were made initially of GaAsP
alloys (GaAs has an infrared energy gap, which is increased by adding the
lighter element P). Too much P induces an indirect gap, so the other alloy