Phototransistors

There is a wide selection of photosensitive devices that are available to the electronic designer. Whilst photodiodes fulfill many requirements, phototransistors or phototransistors 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.

Phototransistor structure

Although ordinary transistors exhibit the photosensitive effects if they are exposed to light, the structure of the phototransistor is specifically optimized for photo applications. The phototransistor 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. Hetero structures that use different materials either side of the p-n junction are also popular because they provide high conversion efficiency. These are generally fabricated using epitaxial growth of materials that have matching lattice structures. These phototransistors 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.

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 bas e of the phototransistor 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 dis advantages 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 basecollector 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.

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.

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.

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. With most circuits not using the bas e 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 res is tor. This is set by estimating the maximum current anticipated from the light levels encountered.

Phototransistor Frequency Response

All silicon photo sensors (phototransistors, etc.) respond to the entire visible radiation range as well as to infrared.

How It Works

The actual operation of a phototransistor depends on the biasing arrangement and light frequency. For instance, if a PN junction is forward biased, the increased current through the junctions due to incident light will be relatively insignificant. On the other hand, if the same junction is reverse biased, the increase in current flow will be considerable and is a function of the light intensity. Therefore, reverse bias is the normal mode of operation.

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