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Phototransistor
The phototransistor is a semiconductor light sensor
formed from a basic transistor with a transparent
cover that provides much better sensitivity than a
photodiode
Phototransistor structure
• The photo transistor has much larger base and collector areas than
would be used for a normal transistor. These devices were generally
made using diffusion or ion implantation.
•
• Early photo transistors used germanium or silicon throughout the
device giving a homo-junction structure. The more modern
phototransistors use type III-V materials such as gallium arsenide and
the like
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• Heterostructures that use different materials either side of the p-n
junction are also popular because they provide a high conversion
efficiency.
• These are generally fabricated using epitaxial growth of materials that
have matching lattice structures.
• These photo transistors generally use a mesa structure.
• Sometimes a Schottky (metal semiconductor) junction can be used
for the collector within a phototransistor, although this practice is less
common these days because other structures offer better levels of
performance.
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• In order to ensure the optimum conversion and hence sensitivity, the
emitter contact is often offset within the phototransistor structure.
This ensures that the maximum amount of light reaches the active
region within the phototransistor.
Phototransistor operation
• Photo transistors are operated in their active region
• For operation the bias conditions are quite simple. The collector of
an n-p-n transistor is made positive with respect to the emitter or
negative for a p-n-p transistor.
• The light enters the base region of the phototransistor where it
causes hole electron pairs to be generated. This mainly occurs in the
reverse biased base-collector junction.
•
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• The hole-electron pairs move under the influence of the electric field
and provide the base current, causing electrons to be injected into
the emitter.
Phototransistor characteristics
• The photo transistor has a high level of gain resulting from the transistor
action.
-For homo-structures, i.e. ones using the same material throughout
the device, this may be of the order of about 50 up to a few hundred.
-However for the hetero-structure devices, the levels of gain may
rise to ten thousand.
-Despite their high level of gain the hetero-structure devices are not
widely used because they are considerably more costly to manufacture.
• The characteristics of the photo-transistor under different light intensities.
They are very similar to the characteristics of a conventional bipolar
transistor, but with the different levels of base current replaced by the
different levels of light intensity.
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• There is a small amount of current that flows in the photo transistor
even when no light is present. This is called the dark current, and
represents the small number of carriers that are injected into the
emitter.
ADVANTAGES
• A further advantage of all phototransistors when compared to the
avalanche photodiode, another device that offers gain, is that the
phototransistor has a much lower level of noise.
DISADVANTAGE
• One of the main disadvantages of the phototransistor is the fact that
it does not have a particularly good high frequency response.
- This arises from the large capacitance associated with the base-
collector junction. This junction is designed to be relatively large to
enable it to pick up sufficient quantities of light.
- For a typical homo-structure device the bandwidth may be
limited to about 250 kHz.
-Hetero-junction devices have a much higher limit and some can
be operated at frequencies as high as 1 GHz.
Phototransistor symbol
• The phototransistor symbol for use in electronic circuit diagrams is
very straightforward. It is formed from the basic transistor symbol
with arrows point in to it to indicate that it is light sensitive.
• The phototransistor symbol often has two arrows pointing towards it,
but other phototransistor symbols show a jagged arrow. Both
versions of the phototransistor symbol are acceptable and
understood.
Phototransistor circuit configurations
• The phototransistor can be used in a variety of different circuit
configurations. Like more conventional transistors, the
phototransistor can be used in common emitter and common
collector circuits. Common base circuits are not normally used
because the base connection is often left floating.
• The choice of common emitter or common collector phototransistor
circuit configuration depends upon the requirements for the circuit.
The two phototransistor circuit configurations have slightly different
operating characteristics and these may determine the circuit used.
•
Common emitter phototransistor circuit
• The common emitter phototransistor circuit configuration is possibly
the most widely used, like its more conventional straight transistor
circuit. The collector is taken to the supply voltage via a collector load
resistor, and the output is taken from the collector connection on the
phototransistor. The circuit generates an output that moves from a
high voltage state to a low voltage state when light is detected.
• The circuit actually acts as an amplifier. The current generated by the
light affects the base region. This is amplified by the current gain of
the transistor in the normal way.
Common collector phototransistor circuit
• The common collector, or emitter follower phototransistor circuit
configuration has effectively the same topology as the normal
common emitter transistor circuit - the emitter is taken to ground via
a load resistor, and the output for the circuit being taken from the
emitter connection of the device.
• The circuit generates an output that moves from the low state to a
high state when light is detected.
Phototransistor circuit operation
• The phototransistor circuits can be used on one of two basic modes of
operation. They are called active or linear mode and a switch mode.
• Operation in the "linear" or active mode provides a response that is very
broadly proportional to the light stimulus. In reality the phototransistor
does not give a particularly linear output to the input stimulus and it is
for this reason that this mode of operation is more correctly termed the
active mode.
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• The operation of the phototransistor circuit in the switch mode is
more widely used in view of the non-linear response of the
phototransistor to light. When there is little or no light, virtually no
current will flow in the transistor, and it can be said to be in the "off"
state. However as the level of light increases, current starts to flow.
Eventually a point is reached where the phototransistor becomes
saturated and the level of current cannot increase. In this situation
the phototransistor is said to be saturated. The switch mode,
therefore has two levels: - "on" and "off" as in a digital or logic
system. This type of phototransistor mode is useful for detecting
objects, sending data or reading encoders, etc.
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• With most circuits not using the base connection (even if it is available),
the only way to change the mode of operation of the circuit is to change
the value of the load resistor. This is set by estimating the maximum
current anticipated from the light levels encountered.
• Using this assumption, the following equations can be used:
• Active mode VCC > RL x Ic
• Switch mode: VCC < RL x Ic
• Where
• RL = load resistor (i.e. Rc or Re in the diagrams above).
IC = maximum anticipated current.
VCC = supply voltage
