Union Junction Transistors (UJT), also known as Double-base Diodes, are semiconductor devices characterized by a unique negative resistance region in their input-output characteristics. This distinct feature sets them apart from conventional transistors and has earned them the title of "negative resistance devices." Let's delve into the reasons behind this intriguing property.

1. Understanding Negative Resistance

In conventional devices like resistors, diodes, and bipolar junction transistors (BJTs), the current (I) through the device increases as the voltage (V) applied across it increases. This relationship is represented by a positive slope on a graph, indicating a positive resistance. However, in UJTs, there exists a voltage range where an increase in voltage leads to a decrease in current. This unusual behavior results in a negative slope on the I-V graph, signifying negative resistance.

2. Structure of UJT

A UJT consists of a silicon bar with ohmic contacts at both ends and a third terminal called the base 1 or emitter terminal, which is connected to the bar at a point closer to one end. This structure divides the bar into two regions: the emitter region, connected to base 1, and the base 2 region, connected to one of the ohmic contacts.

3. Operation and Negative Resistance Region

When a voltage is applied between the base 1 and base 2 terminals, a small current flows through the emitter region due to the presence of minority carriers. As the voltage increases, more minority carriers are injected, causing the emitter current (IE) to rise steeply. However, at a certain voltage, the electric field in the base 2 region widens the depletion region, reducing the number of minority carriers available for conduction. This results in a decrease in IE, creating the negative resistance region.

4. Applications of Negative Resistance

The negative resistance region in UJTs finds application in various electronic circuits, including relaxation oscillators, sawtooth generators, and phase-locked loops. The negative resistance helps generate stable oscillations or waveforms by providing energy to the circuit, counteracting losses and maintaining the oscillations.

5. UJT as a Switch

Due to its negative resistance, UJTs can also function as switching devices. When the voltage across the UJT reaches the negative resistance region, a small increase in voltage causes a significant decrease in current. This sudden change in current can be utilized to trigger a switching action, making UJTs suitable for use in electronic switches and trigger circuits.

Conclusion

The unique negative resistance region exhibited by Union Junction Transistors (UJT) sets them apart from conventional transistors and has earned them the title of "negative resistance devices." This intriguing property arises from the structure and operation of UJT, where a specific voltage range causes a decrease in current instead of an increase. The negative resistance region finds applications in relaxation oscillators, sawtooth generators, phase-locked loops, and switching circuits, making UJTs valuable components in electronic systems.

Frequently Asked Questions (FAQs)

1. What is negative resistance?

Negative resistance is an unusual behavior observed in certain electronic devices like UJTs, where an increase in voltage leads to a decrease in current, resulting in a negative slope on the I-V graph.

2. How does UJT exhibit negative resistance?

In UJTs, the negative resistance region occurs due to the widening of the depletion region in the base 2 region, which reduces the number of minority carriers available for conduction, leading to a decrease in emitter current.

3. What are the applications of UJTs?

UJTs are commonly employed in relaxation oscillators, sawtooth generators, phase-locked loops, and switching circuits, utilizing their negative resistance region to generate stable oscillations or trigger switching actions.

4. Why is UJT also called a double-base diode?

UJT is referred to as a double-base diode because it has two ohmic contacts at both ends of the silicon bar, similar to two diodes connected back-to-back.

5. What is the significance of the emitter region in UJT?

The emitter region in UJT is the region connected to base 1. When a voltage is applied between base 1 and base 2, minority carriers are injected into the emitter region, contributing to the emitter current.