Major Areas of Opportunity - Implications of Emerging Micro-and Nanotechnologies - page 67

are more complex, requiring local oscillators, but have inherently greater sensi-
tivity. Both systems require a nonlinear element in the front end as the interface
between the RF and electronic environments. Although diodes are generally
used, it is also possible to make use of active devices such as field-effect transis-
tors, or high electron mobility transistors (HEMTs).
The fundamental limitation to the frequency performance of a rectifying
front-end element can be described by the cutoff frequency, which can be ex-
pressed as

1

f =

c

2 p
where R j is the resistance of the diode junction and C j is the diode capacitance. For
optimum performance these devices are designed so that the maximum amount of
RF energy is coupled into the diode junction area and directed across R j . The diode
capacitance is a parasitic parameter that acts to shunt RF energy around the diode
junction, thereby degrading conversion efficiency. For this reason, it is desirable
to minimize the device capacitance. This can be accomplished by scaling the diode
area to very small dimensions. Diodes with cutoff frequencies in the terahertz
regime can be produced. For example, GaAs Schottky barrier diodes with diam-
eters of 0.15 micrometer have a cutoff frequency of 3.4 terahertz. 109 Nano-
technology scaling can push these cutoff frequencies to over 10 terahertz.
Nanotechnology offers the potential for significant improvements in RF sen-
sor performance. Improvements will come from two directions. First, nano-
technology advances will result in improved technology for material growth that
will permit ultrasmall devices to be produced with atomic-level control of the
semiconductor layer thickness. For example, Schulman et al. 110 have demon-
strated a Sb-heterostructure diode that can provide temperature-insensitive per-
formance at frequencies exceeding W-band. These diodes were fabricated using
an InAs/AlSb/GaAlSb heterostructure in a lattice matched configuration grown
by MBE and had an area of 4 square micrometers. These devices can be used as
backward and zero-bias diodes with both high sensitivity and low direct current
power requirements. 111 RTD designs can push frequency performance well into
the terahertz region and still maintain good detection sensitivity. Second, nano-
technology offers the potential to integrate intelligence into structures at the
system front end. That is, a certain amount of processing can be integrated into
the nonlinear element right at the point of RF to electronic conversion. At this
location, the process of extracting useful information from the RF signal can be
performed with high efficiency, thereby avoiding losses associated with transfer-
ring the signal further into the system. This paradigm would also permit simplifi-
cation of the processing circuitry. Nanotechnology offers the potential to build
intelligent processes into the semiconductor devices structure. These approaches
are in their infancy but are likely to provide advances for numerous applications
of interest to the Air Force.

RC

j j