Electro-Optics - Applied Optics Fundamentals and Device Applications - page 11

Another key consideration is that of operating frequency. As the radar
operating frequency increases, the size of the antenna decreases, thereby
enhancing the mobility of the system; however, since the antenna is smaller,
either the size of the T/R module in Figure 2.4 must decrease proportionally
or each source must feed a larger number of radiating elements.
The preceding considerations indicate much of the difficulty in phased
radar development is mechanical in nature. One additional mechanical
consideration leads to perhaps the primary reason for the lack of wide-
spread phased array deployment. Each of the several thousand radiating
elements in a phased array must be manufactured to exacting tolerances—
each shifted to a high degree of accuracy. The sheer number of specialized
elements makes cost the primary design driver. Tang and Brown [13] cite
the following factors as the major contributors to the high cost of system
development:
1. A large number of discrete components in conventional array
antenna.
2. Poor production yields of high-power amplifiers.
3. High labor costs due to tight manufacturing tolerances.
4. Lack of dedicated production lines due to limited quantities.
The MMIC technology promises to overcome these problems, so that the
phased array technology can come to full fruition.
2.2.2 GaAs Field Effect Transistor Technology
The advantages of GaAs are utilized in many recent developments in dis-
crete GaAs field effect transistor (GASFET) technology. Gain and relatively
low-noise characteristics were demonstrated by several manufacturers into
the millimeter wave region. (In comparison, silicon devices are typically
limited to applications below X-band.) In addition to its proven microwave
performance, GASFET technology is readily integrated onto a common sub-
strate with optical and optoelectronic components, permitting the develop-
ment of optical monolithic microwave integrated circuit (OMMIC) systems.
Since the GASFET is the basic building block of the MMIC technology, a
discussion of FET structures and processes follows.
The FET is a three-terminal unipolar device in which the current through
two terminals is controlled by the voltage present at the third. The term “uni-
polar” indicates that the FET uses only majority carriers to handle the cur-
rent flow. This characteristic provides the FET with several advantages over
bipolar transistors [14]:
1. It may have high-voltage gain in addition to current gain.
2. Its efficiency is higher.