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.