Unijunction Transistor Features and Operation

The unijunction transistor (UJT) is a three terminal, single-junction device which exhibits negative resistance and switching characteristics totally unlike those of conventional bipolar transistors.

The UJT consists of a bar of n-type having ohmic contacts designated Base 1 (B₁) and Base 2 (B₂) on either side of a single PN junction designated the emitter. An illustration of this is shown below:

The Operation of Unijunction Transistor

In operation, a positive voltage is applied to B₂ and B₁ is placed at ground potential. The B₂ – E – B₁ junctions then act like a voltage divider which reverse-biases the emitter junction. An external voltage having a potential higher than this reverse bias will forward-bias the emitter and inject holes into the silicon bar which moves toward B₁. The emitter B₁ resistance then decreases and this, in turn, causes the emitter voltage to decrease as the emitter current increases, and a negative resistance characteristic is obtained. This is illustrated in the characteristic curve below:

The region to the left of the peak point is termed to as the cutoff region, and in this region the emitter is reverse biased and only a small leakage current flows. The region between the peak point and the valley point is referred to as the negative resistance region. The region to the right of the valley point is the saturation region, and this region, the resistance is positive.

Related: The Junction Field Effect Transistors (JFET)

One of the commonly used UJT devices is 2N2646 and is employed in the design of pulse and sawtooth generators, analog-to-digital converters, frequency dividers, and relay time delay circuits. We also have programmable UJTs, which can have their parameters set by external components such as resistors and capacitors. Example in this case is Device 2N6027

Also Read: Metal Oxide Semiconductor FET (MOSFET)