**Unijunction transistor:** Although a unijunction transistor is not a thyristor, this device can trigger larger thyristors with a pulse at base B1. A *unijunction transistor* is composed of a bar of N-type silicon having a P-type connection in the middle. See Figure below (a). The connections at the ends of the bar are known as bases B1 and B2; the P-type mid-point is the emitter. With the emitter disconnected, the total resistance R_{BBO}, a datasheet item, is the sum of R_{B1} and R_{B2} as shown in Figure below (b). R_{BBO} ranges from 4-12kΩ for different device types. The intrinsic standoff ratio η is the ratio of R_{B1} to R_{BBO}. It varies from 0.4 to 0.8 for different devices. The schematic symbol is Figure below (c)

The Unijunction emitter current vs voltage characteristic curve (Figure below(a) ) shows that as V_{E} increases, current I_{E} increases up I_{P} at the peak point. Beyond the peak point, current increases as voltage decreases in the negative resistance region. The voltage reaches a minimum at the valley point. The resistance of R_{B1}, the saturation resistance is lowest at the valley point.

I_{P} and I_{V}, are datasheet parameters; For a 2n2647, I_{P} and I_{V} are 2µA and 4mA, respectively. [AMS] V_{P} is the voltage drop across R_{B1} plus a 0.7V diode drop; see Figure below(b). V_{V} is estimated to be approximately 10% of V_{BB}.

The relaxation oscillator in Figure below is an application of the unijunction oscillator. R_{E} charges C_{E} until the peak point. The unijunction emitter terminal has no effect on the capacitor until this point is reached. Once the capacitor voltage, V_{E}, reaches the peak voltage point V_{P}, the lowered emitter-base1 E-B1 resistance quickly discharges the capacitor. Once the capacitor discharges below the valley point V_{V}, the E-RB1 resistance reverts back to high resistance, and the capacitor is free to charge again.

During capacitor discharge through the E-B1 saturation resistance, a pulse may be seen on the external B1 and B2 load resistors, Figure above. The load resistor at B1 needs to be low to not affect the discharge time. The external resistor at B2 is optional. It may be replaced by a short circuit. The approximate frequency is given by 1/f = T = RC. A more accurate expression for frequency is given in Figure above.

The charging resistor R_{E} must fall within certain limits. It must be small enough to allow I_{P} to flow based on the V_{BB} supply less V_{P}. It must be large enough to supply I_{V} based on the V_{BB} supply less V_{V}. [MHW] The equations and an example for a 2n2647:

Article extracted from Tony Kuphaldt’s Lesson in Electric circuits Volume III Semiconductors under the terms and conditions of Design Science License