Unijunction Transistor (UJT)

Unijunction Transistor (UJT)

• It is a three terminal device having two layers. It consists of lightly doped n type silicon slab to which aluminium rod is alloyed at the one end with a p type material, forming a p‒n junction.

• At other end of slab, two base contacts B₁ and B₂ are attached. The third terminal emitter E is taken out from aluminium rod.

• Fig. 6.5.1 shows the construction and symbol of UJT.

• It has only one p‒n junction p‒n junction hence called unijunction.

• The p‒n junction can be treated as a diode D while internal resistances of two bases are denoted as R_B1 and R_B2. The resistance R_B1 is greater than R_B2.

• When emitter diode is not conducting, the resistance between two bases is called interbase resistance given by R_BB = R_B1 + R_B2.

• Fig. 6.5.2 shows the equivalent circuit of UJT.

• When I_E = 0, then the voltage drop across R_B1 is given by,

 = Intrinsic stand off ratio

• The value of η is between 0.5 to 0.8.

Working of UJT:

• The supply voltage V_BB is connected between B₁ and B₂ while variable emitter voltage V_E is applied to emitter. This is shown in Fig. 6.5.3.

• The V_E is used to forward bias the diode. The drop across diode is V_D.'

• The potential of A is decided by η and is equal to η V_BB.

Case 1: V_E < V_A

• As long as V_E is less than V_A, the p‒n junction is reverse biased. Hence emitter current I_E will not flow. Thus UJT is said to be OFF.

Case 2: V_E > V_P

• The diode drop V_D is generally between 0.3 to 0.7 V. Hence we can write,

V_P = V_A + V_D =  ηV_BB + V_D

• When V_E becomes equal to or greater than V_P the p‒n junction becomes forward biased and current I_E flows.

• Due to this the charge carriers are injected in the R_BI region of the bar.

• Due to these additional charge carriers, the conductivity of the R_B1 region increases i.e. resistance and due to which the drop across it also decreases.

• This makes the p‒n junction more forward biased which further increases the current and more charge carriers are injected.

• The current I_E is increases to a value determined by the source resistance.

• Under these conditions, the UJT is said to be ON and remains in this condition till the input is open or the current I_E gets reduced to very low value.

1. UJT Charateristics

• The graph of emitter current against emitter voltage plotted for a particular value of V_BB is called the characteristics of UJT.

• For a particular fixed fixed value of V_BB such characteristics is shown in Fig. 6.5.4.

• The characteristics can be divided into three main regions which are,

1. Cut‒off region: The emitter voltage V_E is less than V_P and the p‒n junction is reverse biased. A small amount of reverse saturation current I_EO flows through the device, which is negligibly small of the order of μA. This condition remains till the peak point.

2. Negative resistance region: When the emitter voltage V_E becomes equal to V_P the p‒n junction becomes forward biased and I_E starts flowing. The voltage across the device decreases in this region, though the current through the device increases. Hence the region is called negative resistance region. This decreases the resistance R_B1. This region is stable and used in many applications. This region continues till valley point.

3. Saturation region: Increase in I_E further valley point current I_V drives the device in the saturation region. The voltage corresponding to valley point is called valley point voltage denoted as V_V. In this region, further decrease in voltage does not take place. The characteristic is similar to that of a semiconductor diode, in this region.

• The active region i.e. negative resistance region, the holes which are large in number on p‒side, get injected into n‒side. This causes increase in free electrons in the n‒type slab. This increases the conductivity i.e. decreases the resistivity. Hence the resistance R_B1 decreases in this region.

• As the V_BB increases, the potential V_P corresponding to peak point will increase.

• The typical UJT emitter characteristics for I_B2 = 0, V_BB = 20 V and V_BB = 5V are shown in Fig. 6.5.5.

2. Applications of UJT

• The various applications of UJT are,

1. Triggering of other devices like SCR.

2. In a sawtooth waveform generator.

3. In a relaxation oscillator.

4. In timing circuits.

5. In automobile ignition circuits.

UJT relaxation oscillator: Fig. 6.5.6 shows the circuit of UJT relaxation oscillator.

• R₁ and R₂ are biasing resistors. R_T and C_T decide the oscillating rate.

• The value of R_T is so selected that UJT operates in negative resistance region.

• C_T gets charged to V_BB through R_T.

• When its voltage is more than V_P, UJT fires. Then Ст Cr starts discharging through R_B1 + R₁. This discharge is very fast due to small R₁.

• This produces pulse across R₁.

• Then capacitor voltage becomes less than V_V, UJT becomes OFF and charging of capacitor starts again and cycle repeats.

• The waveforms of UJT relaxation oscillator are shown in Fig. 6.5.7.

Review Questions

1. Explain the basic construction and working of UJT. Draw its equivalent circuit.

2. Explain the equivalent circuit and define intrinsic stand off ratio of UJT.

3. Draw and explain the characteristics of UJT.

4. Define negative resistance region and valley point current with respect to UJT.

5. Sketch the UJT emitter characteristics for I_B2 = 0, V_BB = 20 V and V_BB = 5 V. Identify each region and important points on the characteristics.

6. Explain how a UJT functions as relaxation oscillator.

7. Describe the construction, operation and characteristics of UJT.