Chemistry Reference
In-Depth Information
600
Ternary QW
Quaternary QW
500
400
300
200
100
0
-3
-2
-1
0
1
2
Tension
Strain (%)
Compression
Figure 5.17
Summary of threshold current density,
J
th
, per well deduced for infinite
cavity length 1.5
µ
m lasers versus the strain in the InGaAs(P) quantum
wells, using data reported in the literature (after Thijs
et al
. © 1994 IEEE).
5.8 Tunnelling structures and devices
When a classical particle is incident on a barrier of height
V
, it has a 100
per cent probabilityof transmission if its energy
E
>
V
, andwill be reflected
with 100 per cent probability if
E
V
. By contrast, this is not the case in
quantum mechanics where, because of its wave-like nature, an incident
electron has a finite probability of tunnelling through a thin barrier even
when
E
<
V
.
The requirements for creating tunnelling structures in III-V semiconduc-
tors are not very different to those for quantum wells: a single barrier for
tunnelling can be formed by sandwiching a thin layer of wide band gap
material between two narrower gap regions, as illustrated in fig. 5.18(a).
Because the tunnelling probability, and hence the current, varies expo-
nentially with both barrier height and width, and can also be modified
by impurities, careful growth control is considerably more critical for
tunnelling structures than for quantum well devices, so that tunnelling
devices have not been as widely commercialised. Nevertheless, much ele-
gant physics andmanyuseful effects have beendemonstrated in tunnelling
structures, both for electronic and optoelectronic applications (Kelly 1995).
We first review here the principles of quantum mechanical tunnelling
calculations and then conclude this chapter by describing briefly two
of the most significant tunnelling devices: first, double barrier resonant
tunnelling devices, which can display negative differential resistance
<
