What is a Phototransistor: tutorial & primer

Phototransistors are a form of transistor that is sensitive to light - having the gain of a transistor they are much more sensitive than photodiodes.

Phototransistor Includes:
Phototransistor basics     Applications & circuits     Photodarlington     Optocoupler / optoisolator    

The phototransistor is a device that is able to sense light levels and alter the current flowing between emitter and collector according to the level of light it receives.

Phototransistors and photodiodes can both be used for sensing light, but the phototransistor is more sensitive in view of the gain provided by the transistor. This makes phototransistors more suitable in a number of 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.

Phototransistor operation

The phototransistor uses the basic transistor concept as the basis of its operation. In fact a phototransistor can be made by exposing the semiconductor of an ordinary transistor to light. Very early photo transistors were made by not covering the plastic encapsulation of the transistor with black paint.

Image of an old OC71 transistor - this type was introduced by Mullard in Britain in 1954, but production continued well beyond this date into the 1960s
Old OC71 transistor - by removing the black paint it exhibited phototransistor characteristics

The photo transistor operates because light striking the semiconductor frees electronics / holes and causes current to flow in the base region.

Photo transistors are operated in their active regime, although the base connection is generally left open circuit or disconnected because it is often 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 where it causes hole electron pairs to be generated. This generation 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. As a result the photodiode current is multiplied by the current gain β of the transistor.

The performance of the phototransistor can be superior to that of the photodiode for some applications in view of its gain. As a rough guide, where a photodiode may enable a current flow of around 1µA under typical room conditions, a phototransistor may allow a current of 100µA to flow. These are very rough approximations, but show the order of magnitude of the various values and comparisons.

One of the drawbacks of the phototransistor is that is particularly slow and its high frequency response is very poor

Phototransistor circuit symbol

The phototransistor symbol consists of the basic transistor symbol with two arrows pointing towards the junction of the transistor. This diagrammatically represents the operation of the phototransistor.

Phototransistor circuit symbol - NPN
Phototransistor circuit symbol (NPN)

It is perfectly possible to have a PNP phototransistor, and for this the direction of the arrow on the emitter is reversed in the normal way.

It can be seen that the phototransistor symbol shown does not give a base connection. Often the base is left disconnected as the light is used to enable the current flow through the phototransistor. In some instances the base may be biased to set the required operating point. In this case the base will be shown in the normal way on the phototransistor symbol.

Phototransistor 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.

Planar phototransistor structure
Homojunction planar phototransistor structure

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. 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.

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 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.

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