The phototransistor, as illustrated in Figure 1, is a photodiode-transistor combination, that is the base current of the transistor is generated by irradiating the reverse biased base-collector junction which acts as an in-built photodiode.
The output current (Ic or Ie, depending on the configuration) therefore relates to this base photocurrent Ip via the d.c. current amplification hFE, as shown in eqn (3.3):For some applications the base may need to be biased to a specific operating point (e.g. to operate the transistor in a more linear part of the transfer characteristic) in the which case the transistor output current will be proportional to the algebraic sum of the bias current IB and the photocurrent Ip.
Phototransistors are therefore more sensitive than photo diodes but do have worse output tolerances and a less linear transfer characteristic due to the hFE variations with temperature, bias voltage and collector current. The speed of response of a phototransistor also differs from that of a photodiode due to the dominant effect of the input R-C time constant on the output rise and fall times (i.e. the bandwidth of a phototransistor is governed by the effective ‘input’ R-C value rather than the actual transistor switching speed) as well as the effect of the current amplification factor hFE, as shown in eqn (3.4):
Phototransistors are therefore mostly used in low cost, medium speed and medium sensitivity light detection but are slowly being replaced by photodiode-operational amplifier combinations due to the market availability of low cost, high gain operational amplifiers whose performance can be selected to suit the application.