The basic structure of a unijunction transistor (UJT) is shown in Fig.1. It is essentially a bar of N type semiconductor material into which P type material has been diffused somewhere along its length. Contacts are then made to the device as shown; these are referred to as the emitter, base 1 and base 2 respectively. Fig.2 shows the schematic symbol used to denote a UJT in circuit diagrams. For ease of manufacture alternative methods of making contact with the bar have been developed, giving rise to the two types of structure - bar and cube.

DIAGRAMS..

The equivalent circuit shown in Fig.4 has been developed to explain how the device works, and it is necessary to define the terms used in this explanation.

R_{BB} is known as the __interbase resistance__, and is the sum of R_{B1} and R_{B2}:

R_{BB} = R_{B1} + R_{B2}

N.B. This is only true when the emitter is open circuit.

V_{RB1} is the voltage developed across R_{B1}; this is given by the voltage divider rule:

RSince the denominator of equation 2 is equal to equation 1, the former can be rewritten as:_{B1}

V_{RB1}=

R_{B1}+ R_{B2}

RThe ratio R_{B1}

VRB1 =x V_{BB}

R_{BB}

_{B1}/ R

_{BB}is referred to as the

__intrinsic standoff ratio__and is denoted by (the Greek letter eta).

If an external voltage V_{e} is connected to the emitter, the equivalent circuit can be redrawn as shown in Fig..

If Ve is less than V_{RB1}, the diode is reverse biased and the circuit behaves as though the emitter was open circuit. If however V_{e} is increased so that it exceeds V_{RB1} by at least 0.7V, the diode becomes forward biased and emitter current Ie flows into the base 1 region. Because of this, the value of R_{B1} decreases. It has been suggested that this is due to the presence of additional charge carriers (holes) in the bar. Further increase in V_{e} causes the emitter current to increase which in turn reduces R_{B1} and this causes a further increase in current. This runaway effect is termed __regeneration__. The value of emitter voltage at which this occurs is known as the peak voltage V_{P} and is given by: V_{P} = _{AV}V_{BB} + V_{D}

The characteristics of the UJT are illustrated by the graph of emitter voltage against emitter current.

As the emitter voltage is increased, the current is very small - just a few microamps. When the peak point is reached, the current rises rapidly, until at the valley point the device runs into saturation. At this point R_{B1} is at its lowest value, which is known as the __saturation resistance__.

The simplest application of a UJT is as a __relaxation oscillator__, which is defined as one in which a capacitor is charged gradually and then discharged rapidly. The basic circuit is shown in Fig.7; in the practical circuit of Fig.8 R3 limits the emitter current and provides a voltage pulse, while R2 provides a measure of temperature compensation. Fig. 9 shows the waveforms occurring at the emitter and base 1; the first is an approximation to a sawtooth and the second is a pulse of short duration.

The operation of the circuit is as follows: C1 charges through R1 until the voltage across it reaches the peak point. The emitter current then rises rapidly, discharging C1 through the base 1 region and R3. The sudden rise of current through R3 produces the voltage pulse. When the current falls to I

_{V}the UJT switches off and the cycle is repeated.

It can be shown that the time t between successive pulses is given by:

V_{BB}- V_{V}

t + R1C lnsecs (5) Megaohms. C in ÂµF.

V_{BB}- V_{P}

The oscillator uses a 2N2646 UJT, which is the most readily available device, and is to operate from a 10V D.C. power supply.

From the relevant data sheet the specifications for the 2N2646 are:

VIt is important that the value of R1 is small enough to allow the emitter current to reach I_{EB2O}I_{E}(peak) P_{TOT}(max) I_{P}(max) I_{V}(max) Case style TO18

30V 2A 300mw 5ÂµA 4ma 0.56 - 0.75

_{P}when the capacitor voltage reaches V

_{P}and large enough so that the emitter current is less than I

_{V}when the capacitor discharges to V

_{V}. The limiting values for R1 are given by:

VFrom the specifications for the 2N2646 the average value of is 0.56 + 0.75/2 = 0.655. Substituting this value in equation (4) and assuming V_{BB}- V_{P}V_{BB}- V_{V}

R1(max) =and R2(min) =

I_{P}I_{V}

_{D}= 0/7V: V

_{P}= 0.655 x 10 + 0.7 = 7.25V.

So R1(max) = 10 - 7.25/5ÂµA = 550K, and if VIf we choose a value for R1 somewhere between these limits, e.g. lOK, the value of C can be calculated from equation._{V}= approx VBB/10,

R1(min) = 10 - 1/4mA = 2.25K.

If f = 1MHz, t = 1/f = 1msec. V_{BB} - V_{P} = 10 - 7.25 = 2.75 and V_{BB} - V_{V} = 10 - 1 = 9

tBecause of component and UJT tolerances it is sufficient in most circumstances to use an approximate formula: f = 1/CR, which assumes that is 0.63 - well within 5% of the average value for the 2N2646. In practice one would use a variable resistance (or a variable resistance in series with a fixed resistance) for R1 so that the frequency of oscillation could be adjusted to give the required value.

Rearranging equation(5) to make C the subject: C = V_{BB}- V_{V}

R1 ln

V_{BB}- V_{P}

0.001

so C == approx 84nF.

10^{4}ln (9/2.75)

R2 is not essential; if it is included, a value of 470 ohms is appropriate for the 2N2646. The value of R3 should be small in comparison with R_{BB}, with which it is in series, so as to prevent it from affecting the value of the peak voltage. A value of 47 ohms or thereabouts is satisfactory.