Unijunction Transistor

INTRODUCTION:-
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:

         RB1
VRB1 =         
      RB1 + RB2  

Since the denominator of equation 2 is equal to equation 1, the former can be rewritten as:

       RB1 
VRB1 =    x VBB 
       RBB

The ratio R_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 = _AVV_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:

           VBB - VV
t + R1C ln         secs (5) Megaohms. C in µF. 
           VBB - VP

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:

VEB2O IE(peak) PTOT(max) IP(max) IV(max)            Case style TO18
 30V  2A       300mw     5µA    4ma    0.56 - 0.75 

It is important that the value of R1 is small enough to allow the emitter current to reach I_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:

          VBB - VP               VBB - VV
R1(max) =         and R2(min) =         
             IP                    IV

From 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_D = 0/7V: V_P = 0.655 x 10 + 0.7 = 7.25V.

So R1(max) = 10 - 7.25/5µA = 550K, and if VV = approx VBB/10, 
   R1(min) = 10 - 1/4mA = 2.25K.

If we choose a value for R1 somewhere between these limits, e.g. lOK, the value of C can be calculated from equation.

If f = 1MHz, t = 1/f = 1msec. V_BB - V_P = 10 - 7.25 = 2.75 and V_BB - V_V = 10 - 1 = 9

                                                          t 
                                                                
Rearranging equation(5) to make C the subject: C =        VBB - VV
                                                   R1 ln          
                                                          VBB - VP

           0.001
so C =                = approx 84nF. 
       104 ln (9/2.75)

Because 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.

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.

Posted in: Electron devices