A Uni-junction transistor (UJT) is a switching device.It switches from blocking to conducting state when the applied voltage reaches a critical value. It is a bar of high resistivity semiconductor with ohmic contacts at each end. The bar is usually of n-type material. A p-type emitter is alloyed at an intermediate position along the length. (In complementary UJTs the bar is p-type and the junction is n-type). When the junction is open or reverse biased, the resistance between base 1 and base 2 is a few kilo-ohms (fig. 1).
When a positive voltage VBB is applied between B2 and B1 , the potential of point E will be ηVBB. The factor η is known as the intrinsic stand off ratio and depends on internal resistance RB1 and RB2 of the bar. If VE is less than VC , the p-n junction will be reverse biased, the emitter current will be negative and equal in magnitude to the small reverse saturation current. The V-I characteristics are shown in Fig. 2. When VE = ηVBB + VD ,where VD is the forward voltage drop of the diode, the emitter base junction will become forward biased. The emitter current will become positive. The point A is known as peak point. The corresponding voltage and current are Vp and IP respectively. The holes injected by the emitter drift towards B1. Therefore, the number of charge carriers in the lower half region of the bar increases and the resistance R1 decreases. Therefore, the potential of point C will decrease and the p-n junction will experience a higher forward bias. Therefore, the potential of point C will decrease and the p-n junction will experience a higher forward bias.
Fig: 1: The Symbol, Construction & Equivalent circuit of UJT
When a positive voltage VBB is applied between B2 and B1 , the potential of point E will be ηVBB. The factor η is known as the intrinsic stand off ratio and depends on internal resistance RB1 and RB2 of the bar. If VE is less than VC , the p-n junction will be reverse biased, the emitter current will be negative and equal in magnitude to the small reverse saturation current. The V-I characteristics are shown in Fig. 2. When VE = ηVBB + VD ,where VD is the forward voltage drop of the diode, the emitter base junction will become forward biased. The emitter current will become positive. The point A is known as peak point. The corresponding voltage and current are Vp and IP respectively. The holes injected by the emitter drift towards B1. Therefore, the number of charge carriers in the lower half region of the bar increases and the resistance R1 decreases. Therefore, the potential of point C will decrease and the p-n junction will experience a higher forward bias. Therefore, the potential of point C will decrease and the p-n junction will experience a higher forward bias.
Fig: 2: Characteristics Curve of UJT
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