The Unijunction Transistor

Unijunction Transistor
by Matthew H. Williams
Back to the British Amateur Electronics Club.
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 - shown in Fig.3
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.
RBB is known as the interbase resistance, and is the sum of RB1 and RB2:
RBB = RB1 + RB2 (1)
N.B. This is only true when the emitter is open circuit.
VRB1 is the voltage developed across RB1; this is given by the voltage divider rule:
        RB1
VRB1 =         (2)
      RB1 + RB2  
Since the denominator of equation 2 is equal to equation 1, the former can be rewritten as:
      RB1
VRB1 =     x VBB (3)
      RBB
The ratio RB1 / RBB is referred to as the intrinsic standoff ratio and is denoted by (the Greek letter eta).
If an external voltage Ve is connected to the emitter, the equivalent circuit can be redrawn as shown in Fig.5.
If Ve is less than VRB1, the diode is reverse biased and the circuit behaves as though the emitter was open circuit. If however Ve is increased so that it exceeds VRB1 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 RB1 decreases. It has been suggested that this is due to the presence of additional charge carriers (holes) in the...