Principle of single junction transistor

Unijunction transistor (UJT for short), also known as base diode, is a semiconductor device with only one PN junction and two resistive contact electrodes. Its substrate is a strip-shaped high-resistance N-type silicon wafer, with two terminals at both ends. The ohmic contact leads to two bases b1 and b2. Use the alloy method to make a P region as the emitter e in the middle of the silicon chip slightly on the b2 side. Its structure, symbols and equivalent electrical calls are shown in Figure 1. 1. Characteristics of single-junction transistor It can be seen from Figure 1 that the resistance between the two bases b1 and b2 is called the base resistance: rbb=rb1+rb2 where: rb1----the first base and the emitter junction The resistance between them changes with the emitter current ie. rb2 is the resistance between the second base and the emitter junction, and its value has nothing to do with ie; the emitter junction is a PN junction, which is equivalent to a diode. If a positive voltage Vbb is added between the two-sided three-pole base b2 and b1, the voltage at point A is: VA=[rb1/(rb1+rb2)]vbb=(rb1/rbb)vbb=ηVbb where: η---- It is called the partial voltage ratio, and its value is generally between 0.3---0.85. If the emitter voltage VE gradually increases from zero, the volt-ampere characteristics of the single-junction transistor can be measured. See Figure 2. Figure 2. Single-junction transistor Voltage-ampere characteristics (1) When Ve < eta Vbb, the emitter junction is reverse biased, the tube is turned off, and the emitter has only a small leakage current Iceo. (2) When Ve≥η Vbb+VD VD is the forward voltage drop of the diode (about 0.7 volts), the PN junction conducts forward, Ie increases significantly, the resistance of rb1 decreases rapidly, and Ve decreases accordingly. This voltage changes with the current The characteristic that increases but decreases is called negative resistance characteristic. The critical P where the tube enters the negative resistance area from the cut-off area is called the peak point, and the corresponding emitter voltage and current are called the peak point voltage Vp, peak point current Ip, and peak point current Ip respectively. Ip is the forward leakage current, which is the minimum current required to turn on the unijunction transistor. Obviously Vp = ηVbb (3) As the emitter current ie continues to rise, Ve continues to fall. After falling to the V point, Ve no longer falls. , this point V is called the valley point, and its corresponding emitter voltage and current are called the valley point voltage, Vv and valley point current Iv. (4) After passing the V point, the carriers in the semiconductor between the emitter and the first base reach a saturated state, so when uc continues to increase, ie rises slowly. Obviously, Vv maintains the conduction of the unijunction transistor. Minimum emitter voltage, if Ve