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INTRODUCTION
This chapter is devoted to describe the relevant principles of
optoelectronic p-n junction devices that are used in the realization of
electro-optical hybrid logic circuits. They include light sources like Light
Emitting Diodes (LED’s), three terminal light detectors like
phototransistors and integrated light sources and detectors like
optocouplers.
2.1 RADIATIVE TRANSITIONS
There are basically three processes for interaction between a photon
and an electron in a solid [17]. They are 1)Absorption, 2)Spontaneous
emission and 3)Stimulated emission. Consider a simple system with two
energy levels E1 and E2 of an atom, where E1 corresponds to the ground
state and E2 corresponds to an excited state, to understand these
processes. Any transition between these states involves the emission or
absorption of a photon with frequency ν12 is given by, hν12 = E2–E1. At
room temperature most of the atoms in the solid are at ground state.
2.1.1 Absorption
When a photon of energy exactly equal to hν12 falls on a material,
an atom in the ground state E1 absorbs the photon and goes to an excited
state E2. This is known as the absorption process and plays a dominant
role in solar cells and photo detector devices.
INTRODUCTION TO LIGHT SOURCES
The two important semiconductor light sources widely used are
LED’s and LASER diodes. The dominant operating process for the LED is
spontaneous emission and for the LASER, it is stimulated emission.
Under forward biased conditions, carriers acquire enough energy to
h ν12
BEFORE AFTER
E2
E1
E2
E1
excited state
ground state
h ν12
BEFORE AFTER
E2
E1
E2
E1
excited state
ground state
h ν12
h ν12
12
overcome the potential barrier existing at the p-n junction. When forward
voltage is applied, minority carriers are injected across the junction and
they recombine with majority carriers. These recombination’s may be
either radiative or nonradiative. Radiative recombination’s emit light and
nonradiative recombination’s produce heat.
The most suitable materials for the emission of light are the direct
band gap semiconductors [18]. In direct band gap semiconductors, the
electrons at the bottom of the conduction band and the holes at the top of
the valence band on either side of the forbidden energy gap have the same
value of the crystal momentum. Thus, when the electron-hole
recombination takes place, the momentum of both types of carriers,
remains the same and the energy which corresponds to the band gap
energy (Eg) is emitted as light. Some of the direct band gap
semiconductors are GaAs, GaSb, InAs and InSb. The band gap energy (Eg)
for GaAs, GaSb, InAs and InSb are 1.43, 0.73, 0.35 and 0.18 eV
respectively.
The rate of radiative recombination Rr, defined as the number of
photons emitted per volume per second, is proportional to n and p as
follows:
Rr =Brecpn ⋅⋅⋅Eq.(2-1)
where,
p - majority carrier concentration,
n - minority carrier concentration,
Brec - recombination coefficient.
From Eq.(2-1), it may be noted that the radiative recombination rate
increases with the minority carrier density and the majority carrier
concentration. The majority carrier concentration (p) may be increased by
increasing the impurity concentration. The minority carrier density (n)
may be increased by injection of the charge carrier.
GENERAL CHARACTERISTICS OF LIGHT SOURCES
Some of the important characteristics of light sources have been
discussed in the following.
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2.3.1 Drive current vs light output
When injected carriers cross the junction, they can recombine by a
radiative process which produces light or by a nonradiative process which
produces heat. The ratio between these two processes is dependent on the
current density (Amp/cm2) of semiconductor junction area.
At low forward voltages, the diode current is dominated by the nonradiative
recombination current, mainly due to surface recombination’s
near the perimeter of the LED chip. At higher forward voltages, the diode
current is dominated by the radiative diffusion current. At even higher
voltages, the diode current will be limited by the series resistance.
Also, at high forward drive currents the junction temperature of the
semiconductor increases due to significant power dissipation. This
increase in temperature results in a decrease in the radiative
recombination efficiency. As the current density is further increased,
internal series resistance effects will also tend to reduce the light
generating efficiency of the light source.
2.3.2 Quantum efficiency
Quantum efficiency (η) is defined as the ratio of the radiative
recombination rate (Rr) to total recombination rate (Rt) and is given by
η=Rr/Rt ⋅⋅⋅Eq.(2-2)
2.3.3 Switching speed
The term switching speed refers to how fast a light source can be
turned on and off by an electrical signal to produce a corresponding
optical output pattern. Laser diodes offer faster switching speeds than
LED’s.
2.3.4 Peak spectral wavelength
The wavelength at which the maximum amount of light is generated
is called the peak wavelength (λP). It is determined by the energy band gap
of the semiconductor material used.
Spectral width
The range of wavelengths over which a light source emits light. The
light source needs to emit light within a narrow spectral width. The
spectral width of the LASER diode is very narrow compared to LED.
2.4 PRINCIPLE OF OPERATION OF LED
LED’s are p-n junction devices which emit light when forward
biased. The p-n junction is formed using direct band gap materials such
as gallium arsenide (GaAs), gallium antimonide (GaSb), indium arsenide
(InAs), indium antimonide (InSb) etc.
The two basic structures of LED [19] are shown in Fig.2.4 and
Fig.2.5. Generally, LED’s which emit red light are fabricated on GaAs
substrates as shown in Fig.2.4 and LED’s which emit orange/
yellow/green light are fabricated on GaP substrates as shown in
Fig.2.5. The device structure consists of two layers of GaAsP grown on
GaAs substrate. GaAsP grown on GaAs substrate may produce lattice
mismatches. Hence, a graded alloy GaAs1-yPy layer is grown epitaxially
between the substrate and the active region to minimize the nonradiative
centers at the interface that result from lattice mismatch. For red LED,
the n-layer is doped with terillium (Te) and for p-type layer, Zn is the best
dopant. As shown in Fig.2.4, an n-type substrate and top Zn diffused
layer are the most common configuration because of the fact that Zn can
easily be incorporated by diffusion. The diffusion is done selectively
through a mask of Si3N4 to form p-n junction. Contact layers are usually
formed using Al and AlSn on p- and n- sides respectively.
LED’s emit light when they are forward biased. Under forward
biased conditions, carriers acquire enough energy to overcome the
potential barrier existing at the junction. When forward voltage is applied,
minority carriers are injected across the junction and they recombine with
majority carriers. These recombination’s may be either radiative or
nonradiative. Radiative recombination’s emit light and nonradiative
recombination’s produce heat.
Double Heterojunction (DH) LED’s
The homojunction LED structure discussed in the previous section
has a very low quantum efficiency and low radiance. In order to achieve a
high radiance and a high quantum efficiency, the LED structure must be
capable of confining charge carriers in the active region of the p-n
junction, where radiative recombination’s takes place [20]. In addition, the
LED structure needs to confine the light so that the emitted photons are
not absorbed by the material surrounding the p-n junction. An effective
structure for achieving carrier and optical confinement is the double
hetero structure LED. It is referred to as double hetero structure device
because of the two different alloy layers on each side of the active region.
The band gap differences of adjacent layers confine the charge carriers,
while the differences in the indices of refraction of the adjoining layers
confine the emitted photons to the central active layer.
The Double Heterojunction (DH) LED structure consists of p-type
GaAs layer sandwiched between a p-type AlGaAs and an n+-type AlGaAs
layers as shown in Fig.2.6. When a forward bias is applied to the DH
LED, the electrons from the n+-type layer are injected into the central ptype
GaAs layer and recombine with the holes. Due to electron-hole
recombination’s, photons are generated with energy corresponding to the
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band gap energy (Eg) of the central p-type GaAs layer. The injected
electrons are prevented from diffusing into the p-type AlGaAs layer
because of the potential barrier at the p-p hetero junction. The difference
between the band gap energy of the adjacent layers of the p-p structure
confines the charge carriers, while the difference between the refractive
index of the adjacent layer confines the optical power in the central GaAs
due to total internal reflection.
These double confinements give rise to both high quantum
efficiency and high radiance. Hetero structure LEDs can also be
fabricated using InGaAsP/InGaAs/InGaAsP material which emits light at
longer wavelengths (typically 1.3μm).
PHOTOTRANSISTORS
Most widely used photodetectors are p-n junction photodiode, PIN
photodiode, Avalanche photodiode (APD) and phototransistor.
Phototransistor is a 3-terminal photo detector and has an internal gain.
Phototransitor can be operated with either electrical signal or optical
signal. Hence, phototransistor has been used in implementing hybrid
circuits. The device structure and the basic principle of operation of
phototransistor has been discussed in the following.
2.5.1 Principle of operation of phototransistor
The structure of phototransistor [21] and its circuit symbol is
shown in Fig.2.8. To understand the operation of the phototransistor,
Forward Bias
(Light Emission)
VBR
Reverse Bias
IF
VF
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consider that the emitter junction is forward biased and collector junction
is reverse biased i.e., the phototransistor is biased in the active region.
Assume that there is no radiation excitation. Under these circumstances,
minority carriers are generated thermally, and the electrons crossing from
the base to the collector, as well as the holes crossing from the collector to
the base, constitute the reverse saturation collector current (Ico). When the
base terminal of the phototransistor is open,
V-I characteristics of phototransistor
The current-voltage characteristics of the phototransistor are
similar to conventional npn BJT’s, with the major exception that incident
light provides the base drive current (IB). Typical V-I characteristics of
phototransistor is shown with base current (IB) and illumination intensity
as parameters in Fig.2.9. If the radiation is concentrated on the region
near the collector junction with the base terminal open, additional
minority carriers are photo generated and these contribute to the reverse
saturation current. The phototransistor input and output characteristics
along with the circuit diagram are discussed in 4.2 section.
Spectral response
The output of a phototransistor is dependent upon the wavelength
of incident light. Phototransistors respond to light over a broad range of
wavelengths from the near UV, through the visible region, and into the
near IR part of the spectrum. The spectral response of Silica (Si)
phototransistor is shown in Fig.2.10. The peak spectral response is in the
near IR at approximately 840nm. Phototransistors respond to fluorescent
or incandescent light sources but display better optical coupling
efficiencies when used with IR LED’s.
Collector-Emitter saturation voltage
By definition, saturation is the condition in which both the emitterbase
and the collector-base junctions of a phototransistor become forward
based. From a practical standpoint, the collector-emitter saturation
voltage VCESAT, is the parameter which indicates how closely the photo
detector approximates a closed switch. This is because VCESAT is the
voltage drop across the detector when it is in its “ON” state. VCESAT is
usually given as the maximum collector-emitter voltage allowed at a given
light intensity and for a specified value of collector current.
2.5.6 Dark current
When the phototransistor is placed in the dark and a reverse biased
voltage is applied from collector to emitter, a certain amount of current
will flow. This current is called the dark current (lD). This current consists
of the leakage current of the collector-base junction multiplied by the DC
current gain of the transistor. The presence of this current prevents the
phototransistor from being considered completely “OFF”, or being an ideal
“open” switch. Dark current is specified as the maximum collector current
permitted to flow at a given collector-emitter test voltage. The dark current
is a function of the applied collector-emitter voltage and ambient
temperature.
Breakdown voltage
Phototransistors must be properly biased in order to operate.
However, when voltages are applied to the phototransistor, care must be
taken not to exceed the collector-emitter breakdown voltage (VBRECO).
Exceeding the breakdown voltage can cause permanent damage to the
phototransistor.
2.5.8 Speed of response
The speed of response of a phototransistor is dominated almost
totally by the capacitance of the collector-base junction and the value of
the load resistance. These dominate due to the Miller Effect which
multiplies the value of the RC time constant by the current gain of the
phototransistor. This leads to the general rule that for devices with the
same active area, the higher the gain of the photo detector, the slower will
be its speed of response.
A phototransistor takes a certain amount of time to respond to a
sudden changes in light intensity. This response time is usually expressed
by the rise time (tr) and fall time (tf) of the detector where tr is the time
required for the output to rise from 10% to 90% of its on state value, tf is
the time required for the output to fall from 90% to 10% of its on state
value.
As long as the light source driving the phototransistor is not intense
enough to cause optical saturation, characterized by the storage of
excessive amounts of charge carriers in the base region, rise time equals
fall time. If optical saturation occurs, tf can become much larger than tr.
2.6 OPTOCOUPLERS
There are many occasions where the optical output from LED’s are
to be directly coupled to photo detectors (to avoid capacitive coupling
between circuits). This has lead to the development of optocouplers.
Optocouplers typically come in a small 6-pin IC package, and are
essentially a combination of two different devices. The input section of all
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optocouplers is an infrared LED (IRLED), and the output section can be of
different devices depending on the required application. Because IRLEDs
emit at wavelengths which provide a close match to the peak spectral
response of silicon photo detectors, both GaAs and GaAIAs IRLEDs are
often used with phototransistors. The output section may consist of a
photodiode, phototransistor, or photo darlington pair