could be substantially lower for the same transmission distance than for ten-micron wavelength
transmission. Unfortunately, if the nanomorphic cell is used in an in vivo application, one encounters
a limitation due to the strong absorbance of water in this range of wavelengths. Another grand
challenge for the utilization of THz radiation is that the integrated circuit technology has not yet been
developed to support signal generation and detection.
The other possibility that avoids the peak of the thermal noise spectrum is the use of optical
wavelengths, e.g., on the order of one micron (near-infrared). Existing optoelectronic technologies
could in principle support operation atw1
m
m wavelength. However, omnidirectional communication
by the nanomorphic cell is practically forbidden for this regime, since the energy required to send a bit
of information at
l
¼ 1
m
m is close to the total energy budget of the nanomorphic cell. Different
schemes of dir ectional transmission coul d reduce the number of photons in the packet and therefore
the total energy consumed. However, the orientation problem must be solved, which might require
additional energy expenditure. Alternatively, directional transm ission could be augmented by using as
many external detecto rs as possible or even a surround of receivers (e.g. MRI-like). This certainly
limits the application space but a general conclusion is that the optical wavelengths do not allow
ubiquitous communication by the nanomorphic cell. However, in contrast with the THz regime,
generation and detection of optical (w1
m
m) radiation allows for the use of devices whose physical
size is commensurate with the dimensions of the nanomorphic cell. A brief review of the physics of the
operation of optoelectronic devices, such as light-emitting diodes and photodiodes, was given in this
chapter followed by an analysis of the scaling limits for optoelectronic devices. While there are many
factors limiting the efficiency of optoelectronic devices, the scaling limits of these devices universally
depend on the ability of light to pass through an aperture/optical window under which they are placed.
It was argued that the size of the optical window should be larger than the radiation wavelength for an
efficient light transmission. The estimates of the minimum energy for the 1-
m
m directional commu-
nication are 6–7 orders of magnitude less than for the omnidirectional communication schemes.
However, the energy to send a bit is still about two orders of magnitude larger than the energy required
to operate one logic switch. Thus, comparisons of the energy costs to ‘process’ one bit with the
communication energy costs to transmit one bit of information suggests that the overall design goal
should be to minimize communication and to maximize the ‘intelligence’ of the nanomorphic cell.
LIST OF SYMBOLS
Symbol Meaning
c Velocity of light in vacuum, c ¼ 3 10
8
m/s
C Capacitance
C
pn
pn-junction capacitance
C
wire
Wire capacitance
d Diameter
e Electron charge, e ¼ 1.6 10
19
C
E Energy
148 CHAPTER 5 Nanomorphic cell communication unit
Symbol Meaning
E
b
Energy barrier height
E
com
Communication energy per bit
E
g
Semiconductor bandgap
E
LED
LED turn-on energy
E
ph
Photon energy
E
stored
Stored energy
E
SW
Switching energy
h Planck’s constant, h ¼ 6.63 10
34
J$s
I, I
0
Current
k Wave number, k ¼ 2
p
/
l
k
B
Boltzmann constant, k
B
¼ 1.38 10
23
J/K
K Dielectric constant
L Distance, length
L
ant
Length of antenna
L
cell
Length of nanomorphic cell
L
opt
Optimum antenna length
n Integer
N
4
p
Number of photons emitted into solid angle of 4
p
steradians
N
bit
Number of bits
N
ph
Number of photons
p, p
0
Radiation power density (intensity)
P
in
Input power
P
loss
Loss power
P
n
Noise power
P
ph
Detected photon power
P
rad
Radiation power
P
s
Signal power
r Radius
R Resistance
R
loss
Loss resistance
R
rad
Radiation resistance
R
l
/2
Radiation resistance of half-wave antenna, R
l
/2
¼ 73.1
U
SNR Signal-to-noise ratio
t,
s
Time, time interval
t
H
Heisenberg time
t
s
Signal time
T Absolute temperature
u Velocity
V, V(t), V
D
,V
0
Voltage
V
th
Threshold/turn-on voltage
W Barrier length/depletion length
(Continued)
List of symbols 149
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