Photodarlington Working:

The Photodarlington Working shown in (Fig. 20-33) consists of a phototransistor connected in Darlington arrangement with another transistor. This device is capable of producing much higher output currents than a phototransistor, and so it has a greater sensitivity to illumination levels than either a phototransistor or a photodiode. With the additional transistor involved, the Photodarlington Working has a considerably longer switching time than a phototransistor.

PhotoFET:

A photoFET is a JFET designed to have its gate-channel junction illuminated. The illumination controls the level of the device drain current. Consider the n-channel JFET and the photoFET in Fig. 20­-34. The gate-source leakage current (I_GSS) is the reverse saturation current at a pn-junction.

The voltage drop across R_G produced by I_GSS is normally too small to affect the JFET circuit. In the photo­FET, the junction reverse current (λI_G) is susceptible to light. Illumination on the junction generates additional minority charge carriers, thus increasing λI_G. This current flows through the bias resistance (R_G) and produces a voltage drop (V_RG), as illustrated.

If the gate bias voltage (-V_G) is just sufficient to bias the device off when dark, then when the junction is illuminated, V_RG can raise the level of the gate voltage to bias the photoFET on.

The external bias voltage (-V_G) might be selected at a level that biases the device on, so that light level variations cause I_D to increase and decrease. In a photoFET, λI_G is termed the gate current, and the normal I_GSS at the junction is the dark gate-leakage current. The light-controlled drain current is designated λI_D.