Working Regions of a Unijunction Transistor

A unijunction transistor (UJT) is a three-layered semiconductor device that has three terminals: the emitter (E), the base 1 (B1), and the base 2 (B2). It is primarily used as a switching device or as a relaxation oscillator.

The working of a UJT is characterized by different regions of operation, each with its own unique behavior. The working regions of a UJT are as follows:

1. Cut-off Region:
In the cut-off region, the UJT behaves as an open circuit. There is no current flow between the emitter and the base 1 terminals. The UJT remains in this region until the voltage across its emitter terminal (Ve) reaches the peak-point voltage (VP), which is typically around 0.6V.

2. Negative Resistance Region:
Once the voltage across the emitter terminal exceeds the peak-point voltage (VP), the UJT enters the negative resistance region. In this region, the UJT exhibits a negative resistance characteristic, meaning that as the current increases, the voltage decreases. This behavior is due to the presence of the intrinsic standoff ratio (η) of the UJT.

The negative resistance region is important in applications such as relaxation oscillators, where the UJT is used to generate a sawtooth waveform. In this region, the UJT can be used as a relaxation oscillator by connecting a resistor and a capacitor to the emitter terminal.

3. Saturation Region:
As the voltage across the emitter terminal continues to increase, the UJT eventually enters the saturation region. In this region, the UJT behaves as a closed switch, allowing current to flow freely between the emitter and the base 1 terminals. The UJT remains in the saturation region until the current through it drops below a certain level called the valley-point current (IV).

4. Linear Region:
The linear region is the transition region between the negative resistance region and the saturation region. In this region, the UJT exhibits a linear relationship between the voltage across the emitter terminal and the current flowing through it. The UJT operates as a variable resistor, with the resistance controlled by the voltage across the emitter terminal.

Conclusion:
Among the given options, the correct answer is option 'B', which represents the negative resistance region of a unijunction transistor. This region is characterized by the UJT's unique negative resistance behavior, which is essential in various applications such as relaxation oscillators. Understanding the working regions of a UJT is crucial in designing and analyzing circuits involving this semiconductor device.