Phototransistor or photo transistor
- a summary or tutorial giving information about the basics of the Photo
Transistor or Phototransistor as well as the Photodarlington and PhotoFet.
There is a wide selection of photosensitive devices that are
available to the electronic designer. Whilst photo-diodes fulfil many
requirements, phototransistors or photo transistors are also available, and are
more suitable in some applications. Providing high levels of gain, and standard
devices are low cost, these phototransistors can be used in many applications.
The idea of the photo transistor has been known for many
years. William Shockley first proposed the idea in 1951, not long after the
ordinary transistor had been discovered. It was then only two years before the
photo transistor was demonstrated. Since then phototransistors have been used in
a variety of applications, and their development has continued ever since.
Structure
Although ordinary transistors exhibit the photosensitive
effects if they are exposed to light, the structure of the phototransistor is
specifically optimised for photo applications. 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, however
the more modern phototransistors use type III-V materials such as gallium
arsenide and the like. 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, although this practice is less common these days because
other structures offer better levels of performance.
Phototransistor operation
Photo transistors are operated in their active regime,
although the base connection is left open circuit or disconnected because it is
not required. The base of the photo transistor would only be used to bias the
transistor so that additional collector current was flowing and this would mask
any current flowing as a result of the photo-action. 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. 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
As already mentioned 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. 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.
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.
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.
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. Like
the photo-generated carriers this is also subject to the amplification by the
transistor action.
Photodarlington
Another form of photo transistor that is often seen and
belongs to the same family is called the Darlington phototransistor, or
photodarlington. It is essentially a pair of transistors in a Darlington
arrangement where the first transistor acts as the photodetector, and its
emitter is coupled into the base of the second transistor. This gives a very
much higher level of gain, but it is very much slower than the ordinary
phototransistor, having a maximum frequency of around 20 kHz.
Another alternative is to use an avalanche phototransistor.
Here the collector base junction is biased so that a very high field exists in
this region and avalanche multiplication occurs. This increases the gain of the
photo transistor significantly.
PhotoFET
There are a number of types of photosensitive FETs that can
be used. The simplest mode in which a photo-FET can operate is based on
photo-conductivity. Here light generates additional carriers that are used to
increase the level of conductivity. The various types of junction FET can use
the fact that the diode formed at the reverse biased junction between the gate
and channel can act as a photodiode. Gate current will flow if the gate is
connected to an external resistor and the resultant current amplified by the FET.
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