Unijunction transistors (UJTs) are semiconductor devices with one PN junction. Hence, the word unijunction in its name. Since it has got three terminals like a transistor, it is called unijunction transistor.
2N 2646 is the most commonly used UJT type number.
Figure below shows the construction cross-section of a UJT. UJT comprises a lightly doped N-type silicon bar with two ohmic contacts called base terminals, B122 to form a PN junction.
Construction of a UJT
Figure below shows the electrical equivalent circuit of a UJT. It comprises a potential divider arrangement of two resistors and a PN junction diode. Resistor RBB1 represents the resistance of base bar between B1 terminal and the PN junction. ResistorRBB2 represents resistance of the base bar between B2 terminal and the PN junction. RBB1 is a variable resistance as its value depends upon the emitter current flowing through the PN junction when it is forward-biased. For IE = 0, total resistance of the base bar (= RBB1 + RBB2) is termed as RBB.RBB lies in the range of 4–10 k Ω.
Electrical equivalent circuit of UJT
Intrinsic stand-off ratio η is equal to [RBB1/(RBB1 + RBB2)] with value of RBB1 taken at IE = 0. η is in the range of 0.5–0.8 and is typically 0.7, which also signifies that the emitter terminal is closer to B2 terminal.
Figure below electrical circuit of a UJT with different currents and voltages.
Electrical circuit of UJT
The working of UJT is explained below.
• B2 terminal is made more positive with respect to B1 terminal for normal operation. Also, it is essential to forward bias a PN junction diode by a voltage equal to its cut-in voltage Vg (0.35–0.7 V) for any significant emitter current IE to flow through the PN junction. In the absence of the required forward bias, voltage across RBB1 is given by η VBB. For the PN junction to be forward-biased, the externally applied voltage to the emitter terminal must at least be equal to the sum of η VBB and the cut-in voltage of the diode junction. The emitter voltage VE at which diode starts conducting is termed as VP.
• As the emitter voltage VE is increased, initially the current remains negligibly small in magnitude until a voltage (= VP) is reached at which the diode is forward-biased.
• As VE is increased beyond VP, current increases rapidly. It is seen that as the current increases, the voltage decreases giving a negative resistance region. This is because due to increase in forward current, the resistance of E–B1 region falls rapidly resulting in reduction in the voltage.
• When IE becomes very large, it may be considered to be much greater than IB2. Under these conditions, the curve approaches asymptotically the curve for IB2 = 0 giving rise to a valley point.
• The curve for IB2 = 0 is the same as it would be for ordinary PN junction diode.
• Figure below shows VE–IE characteristics.
VE–IE characteristics of UJT
In the VE-IE characteristics of a UJT, the region before the peak point (VP, IP) is called cut-off region as the input diode remains reverse-biased in this region.
From the V–I characteristics of a UJT, we can see that the curve is a single-valued function of current and a multi-valued function of voltage. A single-valued function of current implies that for each current value, there is a unique voltage. Owing to single-valued function of current nature of its V–I characteristics, a UJT is called a current-controllable device
Figure below shows the basic UJT-based relaxation oscillator circuit.
UJT relaxation oscillator circuit
Initially UJT is in the cut-off region. The capacitor starts charging from voltage +V through resistor R. When the voltage Vout across the capacitor becomes large enough to forward bias the input diode, the capacitor starts discharging through the low resistance between the emitter-base B1 region and resistor R1. This discharge process continues until it reaches a point where input diode is again reverse-biased. At this point, the capacitor starts charging again. The process of charging through R, which is comparatively a higher resistance, and discharging through low forward resistance of input diode and R1 continues and gives rise to a waveform as shown in Figure below
Waveform across the capacitor in basic UJT relaxation oscillator
The frequency of oscillation is given by
If we replace the resistor R with a constant current source (refer to figure below) then the voltage across the capacitor will be a sawtooth waveform.
Constant current charging in UJT relaxation circuit
The sawtooth waveform is shown in figure below.
Waveform across the capacitor in UJT relaxation oscillator having constant current source
During the time UJT is ON, a train of positive pulses appear at B1 and a train of negative pulses appear at B2 as shown in figures below.
Waveforms at B1 terminal
waveforms at B2 terminal
It may be mentioned here that UJT is no longer a popular device for building oscillators. It has been largely replaced by opamp- and timer IC-based circuits
