The Unijunction Transistor

The unijunction transistor(UJT)is shown in Fig. 6.7. It has only one p-n junction The n -region (the base), which has ohmic contacts B₁ and B₂ at its ends, is not a channel that would change its resistance due to changes in its cross-sectional area. The p ⁺ -type emitter and the base form a p ⁺ -n junction which is for­ward-biased externally. The output current flowing through the base produces across the portion from the emitter to contact B₁ a voltage drop U_B1 which acts as a reverse-bias voltage for the emitter and turns it off. When the external forward voltage equal to U_EB1 exceeds U_B1 the resultant voltage across the junctionbecomes forward-biasing, the junction is turned on, and holes are injected from emitter into base. As a result, the base resistance decreases. A change in the input voltage brings about a change in the injection level, the base resistance decrease and, UJT switch.

Fig. 6.7. Unijunction transistor

Optoelectronic Devices

Photodiodes

The incident light flux Ф controls the reverse current of a photodiode. When photons of energy greater than the energy gap of the device material are absorbed in the device, hole-electron pairs are generated, the conduc­tivity of the diode increases, and the reverse current builds up. This is the current mode of operation (Fig. 7.1).

Fig. 7.1. Connection of a photodiode for operation in the current mode

The current-voltage cha­racteristics, at Ф = const for the cur­rent mode of operation (Fig. 7.2) are not unlike the output characteristics of a bipolar transistor connected in a common-base circuit.

Fig. 7.2. Current-voltage characteristics of a photodiode in the current mode

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