Biomedical Engineering Reference
In-Depth Information
Figure 3.16c shows the implementation of the OR and NAND functions.
Although structurally more complex than the previous gates, it is a very
symmetric design; and therefore, synchronization should be easier to obtain.
On the other hand, the effects of the various splitters, combiners, and cross-
overs will need to be optimized.
A major problem with cascading the Double-Y logic gate is that an “on”
output from a following gate cannot be generated by inversion of an “off”
output from a previous gate. This is because the individual gates inherently
use “on” control pulses. As these are progressively canceled, subsequent
gates have fewer operations that can be performed. This is markedly dif-
ferent from electrical logic gates where a source to invert an “off” value is
always available.
The MQW Oscillator of interest for optical computing is a hybrid device,
which relies on an electrical signal to change the absorptive properties of
the material. This change in absorption, caused by the QCSE [42] is a very
important effect in MQW structures. These structures are crystals grown
by alternating very thin layers of two materials which have similar lattice
constants but different band gap energies. A schematic diagram of the mate-
rial structure and the band diagram of a typical MQW structure is shown in
Figure 3.17. The interesting quantum effect arises from the confinement of
excitons in the narrow QWs of the material. Confining these excitons, which
normally have a radius of about 300 Å, produces two effects. The first is that
the exciton absorption line, which normally broadens into the band gap
absorption with increasing temperature, remains resolvable even at room
temperature. This effect is illustrated in Figure 3.18. The second effect is an
induced anisotropy in the effect of an applied electrical field. The exciton
absorption line is shifted by an electric field applied perpendicular to the
structure layers. This is due to the QW's enhancement of the exciton binding
Quantum wells
One period
∆
E
c
E
c
E
g
E
g
ε
Al
x
Ga
1-
x
As
Al
y
Ga
1-
y
As
E
v
∆
E
v
a
a
b
b
One period
Material structure
x
>
y
Band structure
FIGURE 3.17
Multiple quantum well structure.
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