Electro-Optics - Applied Optics Fundamentals and Device Applications - page 16

smaller and lighter than their electronic counterparts—coaxial cable and
waveguide. This weight and space reduction is very important when con-
sidering transportable (man or vehicle) or airborne equipment. Finally, opti-
cal control is inherently immune to electromagnetic interference (EMI). This
feature is becoming more important each day with secure transmissions and
electronic warfare/countermeasures.
There are basically two types of optical control of microwave devices and
systems. One is control of passive components such as microstrip lines or
dielectric resonators. Examples of theses are switches, phase-shifters, atten-
uators, and dielectric resonator oscillators (DROs). The second type is the
control of active devices such as IMPATT diodes [21], TRAPATT diodes [22],
MESFETs [23], and transistor oscillators [24]. Regardless of whether one is
controlling passive or active devices, optical control is governed by the fol-
lowing: (1) illumination of photosensitive material, (2) absorption of the illu-
mination, and (3) generation of free carriers due to the illumination.
2.2.3.1 Optical Control of Active Devices: IMPATT Oscillators
One type of active device that has been used quite extensively in experiments
to illustrate optical control capability is the IMPATT oscillator. IMPATT
stands for IMP act Ionization A valanche T ransit T Time. The IMPATT diode
uses impact ionization to generate free carriers that then travel down the
drift region. The avalanche delay and the transit time delay allow the volt-
age and current to be 180° out of phase that creates the negative resistance
needed for oscillation.
Figures 2.9 through 2.11 show the basic characteristics of a Read diode.
The Read diode is one of several diode types that can be used as an IMPATT
diode. Other types are PIN diodes, one-sided abrupt p-n junction, and modi-
fied Read diodes. The following discussion deals only with the Read diode
structure and the theory of operation.
Figure 2.9 shows the basic device structure, doping profile, electric field
distribution, and avalanche breakdown region for a p + -n-i-n + Read diode.
Note the electric field distribution. At the p + -n junction, the electric field is
a maximum and decreases linearly until the intrinsic region is reached. The
electric field is then constant throughout the intrinsic region. At the point of
maximum electric field, the generation of electron-hole pairs occurs through
avalanche breakdown. This region is called the avalanche region and is
shown in Figure 2.9c. The free carriers travel across the region of constant
electric field, called the drift region.
Figure 2.10 shows the Read diode connected across a reverse biased DC
voltage, as shown in Figure 2.10d. The DC voltage is equal to the reverse
breakdown voltage of the diode, so that breakdown occurs during the posi-
tive half cycle of the AC voltage avalanche; during the negative half cycle
of the AC voltage, avalanche breakdown has ceased and the carriers drift
at their saturation velocity. Looking at Figure 2.10e, note how the injected