Phototransistors

A bipolar phototransistor is essentially an electronic transistor structure, where light from outside can hit the base–collector junction. The created electrical carriers are injected into the base, thus having a similar effect as a base current injected through a wire in an ordinary transistor. The resulting collector current exhibits substantial amplification, easily by a factor of 100 or more.

The responsivity of the device is correspondingly higher than that of a photodiode. However, this does not necessarily lead to a higher sensitivity of detection, i.e., to a lower noise-equivalent power, because photocurrent noise is also amplified. The same holds for the dark current, i.e., the current flowing even without any incident light.

A substantial the drawback of the phototransistor concept is that it generally leads to a substantially lower speed (detection bandwidth), limited by the electrical capacitance of the collector–base junction. Typical rise and fall times are of the order of a few microseconds.

Phototransistors can be based on different semiconductor materials such as germanium, silicon and gallium arsenide. Due to their different bandgap energies, they are usable in different wavelength regions – similar to those of photodiodes based on the same materials.

Note that an ordinary transistor would also be light-sensitive, if it were not equipped with a non-transparent housing. However, phototransistors are not just ordinary transistors with a transparent housing; they have further optimized structures, e.g. for efficient illumination of the collector–base junction.

In most cases, a phototransistor has only two pins (for a emitter and collector), but there are also devices with an additional base pin. The former type can be used in the same kind of electronic circuits like photodiodes, e.g. simply in series with a resistor connected to a constant voltage. The resulting voltage drop on the resistor is then approximately proportional to the incident light intensity. The use of a transimpedance amplifier offers better performance, however.

If there is an additional base pin, one may connect this via an additional resistor with the emitter. The applied resistance influences both the responsivity and the dark current.

The linearity of response (photocurrent vs. light intensity) of a phototransistor is substantially less precise than that of a photodiode, because the gain of the phototransistor depends on the collector current.

The photocurrent also exhibits a substantially higher temperature dependence than for a photodiode, because the transistor gain depends on temperature.

For even higher responsivity, there are photodarlingtons. In analogy to conventional Darlington transistors, there is an additional internal transistor for further current multiplication. Unfortunately, the detection bandwidth of such devices is further reduced. Rise and fall times are typically tens to hundreds of microseconds.

Avalanche photodiodes also offer a substantially increased responsivity and are in that sense similar to phototransistors. However, the operation principle is quite different. In an avalanche photodiodes, one exploits carrier multiplication due to a carrier avalanche within a depletion region, not a transistor-like amplification. For that, significantly higher operation voltages are required, and the multiplication factor depends quite sensitively on the operation voltage. Also, the detection bandwidth can be much higher. In contrast, phototransistors can be operated with only a few volts, and the dependence of the responsivity on the drive voltage is not that strong.

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