UJT

The Unijunction Transistor (UJT) is a three-terminal semiconductor device that exhibits a unique negative resistance characteristic in its emitter I-V curve, making it inherently suited for relaxation oscillators, timing circuits, and trigger pulse generators. Despite the name transistor, it does not amplify signals; instead it functions as a switching and waveform generation device.

Core Concept Explanation

The UJT consists of a lightly doped n-type silicon bar with ohmic contacts at each end called Base 1 (B1) and Base 2 (B2). A small p-type pellet (emitter) is alloyed onto one side of the bar, positioned closer to B2. The n-type bar between B1 and B2 acts as a resistive element with total interbase resistance RBB typically 4 to 10 kilohms. When a supply VBB is connected across B1 and B2, this resistance acts as a voltage divider.

The voltage at the emitter junction point on the n-bar is eta * VBB, where eta (intrinsic standoff ratio) is the ratio RB1 / (RB1 + RB2), typically 0.5 to 0.8. This is the threshold voltage that the emitter must exceed to turn the device on. When emitter voltage VE exceeds (eta * VBB + VD), where VD is the diode forward drop approximately 0.7 V, the p-n junction becomes forward biased.

Once forward biased, the emitter injects holes into the n-type bar between the emitter and B1. These holes increase the conductivity of the RB1 portion dramatically (conductivity modulation), reducing RB1 and thus the emitter voltage. This means current increases while voltage drops, giving the negative resistance region in the emitter I-V curve. The device remains on (low resistance path between E and B1) until the emitter current falls below the valley current Iv.

Mathematical Expression

The key design parameter is the intrinsic standoff ratio eta. It determines the peak point voltage at which the UJT fires:

VP = eta * VBB + VD, where VP is the peak emitter voltage, VBB is the supply across the base terminals, and VD is approximately 0.7 V. The intrinsic standoff ratio is fixed by geometry: eta = RB1 / RBB. Since RB1 decreases when the device fires, the voltage VP drops to the valley voltage VV (typically 1 to 2 V). This snap-back action is what generates sharp trigger pulses.

In the relaxation oscillator configuration, a capacitor C charges through a resistor R toward VBB. When VC reaches VP the UJT fires, discharging C rapidly through B1 until VC reaches VV. The oscillation frequency is approximately f = 1 / (R * C * ln(1 / (1 - eta))), which shows that frequency depends on R, C, and the intrinsic standoff ratio.

Practical Understanding

The most common application of UJT is the relaxation oscillator used to generate sawtooth waveforms and trigger pulses for SCR or thyristor gate firing circuits. In power electronics, the UJT oscillator provides the periodic trigger signal to control the firing angle of thyristors in AC power control circuits.

The Programmable UJT (PUT) is a variant where the effective standoff ratio can be set externally using a voltage divider, providing more flexible design control. PUTs are more common in modern designs than conventional UJTs which have been largely replaced, though UJT theory remains a standard examination topic.

Given:
UJT relaxation oscillator: VBB = 12 V, eta = 0.65, R = 10 kohm, C = 0.1 uF

Why this formula applies:
Capacitor charges to VP then UJT fires, frequency set by RC and eta

Formula:
VP = eta * VBB + VD
f = 1 / (R * C * ln(1/(1 - eta)))

Substitution:
VP = 0.65 * 12 + 0.7 = 7.8 + 0.7 = 8.5 V

f = 1 / (10000 * 0.0000001 * ln(1/(1 - 0.65)))
f = 1 / (0.001 * ln(1/0.35))
f = 1 / (0.001 * ln(2.857))
f = 1 / (0.001 * 1.0498)

Final Answer: f = 952.5 Hz, UJT fires when VC reaches 8.5 V

Exam Tip: Intrinsic standoff ratio eta = RB1/RBB and is between 0.5 and 0.8. Peak voltage VP = eta*VBB + 0.7V. For GATE oscillator frequency questions, the formula is f = 1/(RC * ln(1/(1-eta))). Do not confuse UJT with BJT; UJT does NOT amplify.

• When VE is below VP, the emitter junction is reverse biased and only small leakage current flows (cut-off region).
• At VE = VP = eta*VBB + 0.7V, the junction becomes forward biased and holes are injected into RB1 region.
• Injected holes reduce RB1 (conductivity modulation), dropping VE while IE increases, giving negative resistance.
• The capacitor in the relaxation oscillator charges through R, fires UJT at VP, discharges rapidly to VV, and the cycle repeats.
• The output at B1 is a series of sharp positive pulses ideal for triggering thyristors.

Quick Revision

• UJT has one emitter (p+) and two bases (B1, B2) on an n-type silicon bar.
• Intrinsic standoff ratio eta = RB1/RBB, value 0.5 to 0.8.
• Peak voltage VP = eta*VBB + VD (about 0.7V).
• Negative resistance: after firing, RB1 drops due to hole injection, voltage decreases as current increases.
• Relaxation oscillator frequency: f = 1/(RC * ln(1/(1-eta))).
• Application: trigger pulse for SCR/thyristor firing, sawtooth generators.
• Exam trap: UJT is a switching device, NOT an amplifier. Negative resistance here is due to conductivity modulation, not quantum tunneling.