الفهرس 
Historical Review
Temperature EffectOnly 14 pages are availabe for public view
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Abstract
The aim of the present thesis is to shed further light on the
characterization, operation and applications of one of the most important
electronic devices; namely optocoupler in different operating (biasing
voltages and frequency) and environmental conditions (temperature, and
nuclear radiation). In short words, in electronics, an optoisolator, also
called an optocoupler, which is a component that transfers electrical
signals between two isolated circuits using light. In this concern, an
optocoupler of the type 4N25 was investigated, which is, basically,
consists of a gallium arsenide (GaAs) infrared light emitting diode (LED)
coupled with a silicon NPN-phototransistor.
Detailed experimental and simulation studies of the initial electrical
charac-teristics of the input LED and output phototransistor, as well as,
the current transfer characteristics of the optocouoler were investigated.
Where, the obtained results from both techniques were found to be in
excellent agreement. For the input light emitting diode and output
phototransistor, their forward and reverse (C–V) characteristics were
investigated and plotted at different applied bias voltage-and frequencylevels. In this concern, the diffusion -and transition -capacitances, impedance, quality-and dissipation-factors and the phase angle were investigated.
As an application, the design, analysis and operation of switching
system based on the optocoupler were investigated. Where, it is found
that the switching times depend on the load resistance, bias voltage,
forward current, and base emitter resistance. from which, it is proved that
insertion of an emitter base resistance, in the order of 50 kΩ, improves pronouncedly the fall time of the switching system, where its decreases
down to 22.1 μs, although its value at open emitter is 110 μs.
The thesis was extended to include a detailed study of the
temperature and gamma irradiation dependences on the static-and dynamic-characteristics of the optocoupler. For the input light emitting diode,
its electrical characteristics are plotted at different temperature levels
ranging from -175 C up to +100 C. from which, it is clear that the
forward current of the device is a direct increasing function of the
temperature. Besides, its threshold voltage is shown to decay linearly,
where its initial value of o.83 Volt, measured at room temperature (30
°C), is shown to be decreased down to 0.72 Volt, measured at 100 °C. On
the other hand, for very low temperature environments, its value was
increased up to 1.04 Volts, measured at -175 °C. On the other hand,
concerning the output phototransistor, it is found that, the temperature
dependence of the collector current, plotted at different -LED forward
current -and collector -emitter voltage -values, was shown to follow a
Gaussian distribution, with peaks at around -55°C, -45°C, and -30°C,
respectively. from which, it is clear that the temperature dependence is a
direct function of the LED forward current rather than its dependence on
bias voltage.
Concerning the optocoupler, the temperature dependences of its
gain and current transfer ratio are investigated and plotted at different
LED input forward currents (IF= 10 mA, 20 mA, and 50 mA) and at two
different emitter collector voltages of 0.6 Volt and 10 Volts. from which,
it is clearly shown that the temperature dependence of gain and current
transfer ratio of the optocoupler follows a Gaussian distribution with
peaks at around -55°C, -45°C, -30°C respectively. from which, it is clear that the temperature dependence is a direct function of the LED forward
current rather than its dependence on bias voltage.
Also, the temperature dependence of the junction characteristics of
the optocoupler were investigated and plotted for both the input lighting
emitting diode and the output phototransistor. Concerning, the light
emitting diode, its diffusion capacitance shows a pronounced temperature
dependence, where its value increases from 160 pF up to 376 pF in the
temperature range from -175 ºC up to +100 ºC, while its value at room
temperature is 354 pF. On the other side, considering the transition
capacitance, it is clearly shown that, starting from cryogenic level up to -
45oC a slight increase on the capacitance value, from 15.40 pF up to
24.41 pF. For higher temperature levels, from -45 ºC up to 100 ºC, a
pronounced increase in the capacitance was observed, where its value was
increased from 24.41 pF up to 100.28 pF.
Concerning, the phototransistor, its emitter -and collector -capacitances are studied and plotted as a function of investigated temperature
range. from which, it is clear that both capacitances are a direct functions
of temperature. For both parameters, and starting from temperature range
from -175 oC, up to around -50 oC, their temperature dependences were a
little bit negligible, where their values were increased from 920 pf, and
53.45 pF up to 983.9 pf, and 77.7 pF, respectively. On the other hand,
and for higher temperatures range, from -50 oC up to 100 oC, both
capacitances are shown to increase pronouncedly, from 983.5 pF, and
77.7 pF up to 1459.3 pF, and 899 pF, respectively.
Concerning the temperature dependence on the transit switching
times, it is observed that both the rise -and fall -times show great variations with temperature, where their values of 0.097 μs and 3.9 μs,
measured at -175 ºC, reach values of 1.09 μs and 29 μs, measured at 100
ºC, respectively. On the other hand, it is clearly shown that both the
delay- and storage -times are insensitive to the variation in temperature.
The study is extended to include gamma radiation effects on the
characteristics of the light emitting diode, the phototransistor, and the
proposed optocoupler. In this concern, for the light emitting diodes, it is
observed that, for low gamma-doses, up to around 1.0 kGy, their (I-V)
characteristic curve shifts toward a lower forward bias voltage. But for
higher gamma-dose levels, up to 15 kGy, the threshold voltage tends to
increase. On the other side, exposing the phototransistors to gamma-rays
while plotting their dc-output characteristic curves, it is observed that
after two gamma-exposure doses of 1.0 kGy and 100 kGy, at constant
light illumination levels of 500 lux and 1000 lux, maximum damage
effects appear to be at collector current values of around 10 mA. Finally,
as a result of the previously mentioned effects of the gamma-irradiation, it
is proved that CTR decreased as gamma irradiation dose increased.
The study was extended to include the gamma-irradiation
dependence of the junction characteristics of the proposed optocoupler.
For the input light emitting diode and output phototransistor, their C–V
characteristics, on both forward and reverse-biasing conditions, were
investigated and plotted at different γ-radiation doses. In this concern, the
diffusion-and transition-capacitances, impedance, quality-and dissipationfactors, and the phase angle were investigated. Considering the (C-V)
characteristics of the input LED, it is found that both the transition-and
diffusion-capacitances were shown to be decreased as gamma dose levels
increases from 0.0 up to 8.0 kGy. Considering the phototransistor, it is found that the diffusion capacitance is more sensitive to γ-exposure than
the collector transition one.