WHY UJT IS CALLED AS NEGATIVE RESISTANCE DEVICE

Vacuum tubes were the common workhorses in electronics before transistors came along in the 1950s. Vacuum tubes had limitations such as large size, low efficiency, low reliability, slow switching speed, high operating voltages, and poor ruggedness. Vacuum tubes were replaced by transistors which were small in size, more efficient, more reliable, had much faster switching speeds, longer life, operated on low voltages, and were very rugged.

How Did UJTs Come About?

As transistors became popular in electronics, researchers looked for other improved versions of transistors with unique properties. One of the outcomes of this research was the unijunction transistor or UJT. This was a special type of transistor that had only one junction and three terminals: the emitter, base 1, and base 2. This unique structure gave the UJT unique characteristics that were not found in traditional bipolar transistors.

Why is it Called a Negative Resistance Device?

The UJT has a peculiar property that gives it its name. When the voltage between the emitter and base 1 (V_EB1) is increased, the current between the emitter and base 1 (I_EB1) initially increases. However, after reaching a certain point, the current starts to decrease as the voltage continues to increase. This region of operation is where the UJT exhibits negative resistance. In this region, an increase in voltage results in a decrease in current; this is the opposite of the normal behavior of a conductor.

What Causes the Negative Resistance?

The negative resistance behavior of the UJT is due to its unique structure and the way it is biased. When V_EB1 is increased, the holes injected from the emitter into the base region initially increase, causing I_EB1 to increase. Further increase in V_EB1 causes the depletion region between the emitter and base 1 to widen, reducing the number of holes that can cross the junction. As a result, I_EB1 starts to decrease even though V_EB1 continues to increase. This results in the negative resistance region.

Applications of UJTs

Despite its negative resistance characteristic, the UJT has found applications in various electronic circuits. Some of its applications include:

1. Relaxation Oscillators: UJTs are used to build relaxation oscillators, which are circuits that generate periodic waveforms.

2. Pulse Generators: UJTs are used in pulse generators to create pulses of a specific duration and repetition rate.

3. Timing Circuits: UJTs are used in timing circuits to generate accurate time delays or intervals.

4. Trigger Circuits: UJTs are used in trigger circuits to initiate or control other electronic circuits.

Conclusion

The UJT is a unique electronic component with a negative resistance characteristic. This peculiar property makes it useful in various applications, including relaxation oscillators, pulse generators, timing circuits, and trigger circuits. Despite the availability of more modern electronic devices, the UJT still finds use in specific applications where its unique characteristics are advantageous.

FAQs

1. What is the main characteristic of a UJT?

The main characteristic of a UJT is its negative resistance, where an increase in voltage results in a decrease in current.

2. What causes the negative resistance in a UJT?

The negative resistance in a UJT is due to the widening of the depletion region between the emitter and base 1 as V_EB1 increases.

3. What are some applications of UJTs?

UJTs are used in relaxation oscillators, pulse generators, timing circuits, and trigger circuits.

4. Why is the UJT called a unijunction transistor?

The UJT is called a unijunction transistor because it has only one junction, unlike a bipolar transistor which has two junctions.

5. What are the advantages of UJTs over bipolar transistors?

UJTs offer some advantages over bipolar transistors, such as higher input impedance, lower noise, and greater temperature stability.