Unijunction Transistor (UJT)

Unijunction Transistor (UJT)

• A unijunction transistor (UJT) is a device which does not belong to thyristor family but is used to turn ON SCRs.

 

1. Construction

• It is a three terminal device, having two layers. It consists of a slab of lightly doped n type silicon material. The two base contacts are attached to both the ends of this n type surface. These are denoted as B1 and B2 respectively. A p-type material is used to form a p-n junction at the boundary of the aluminium rod and n type silicon slab. The third terminal called emitter (E) is taken out from this p-type material. The n-type is lightly doped while p type is heavily doped. The basic construction is shown in the Fig. 5.1.1.

• As n type is lightly doped, it provides high resistivity and p-type as heavily doped, provides low resistivity.

• The symbolic representation of UJT is shown in the Fig. 5.1.2. The emitter is shown by an arrow which is at an angle to the vertical line representing n-type material. This arrow indicates the direction of flow of conventional current when the UJT is forward biased.

 

2. Equivalent Circuit of UJT

• The Fig. 5.1.3 (a) shows the basic structure of UJT while the Fig. 5.1.3 (b) shows the equivalent circuit of UJT.

• The internal resistances of the two bases are represented as R_B1 and R_B2. In the actual construction, the terminal E is closer to B₂ as compared to Br Hence resistance RB1 is more than the resistance R_B2. The p-n junction is represented by a normal diode with V_D as the drop across it.

• When the emitter diode is not conducting then the resistance between the two bases B₁ and B₂ is called interbase resistance denoted as R_BB.

R_BB = R_B1 + R_B2

• Its value ranges between 4 kΩ and 12 kΩ.

 

3. Intrinsic Stand Off Ratio (η)

• Consider UJT as shown in the Fig. 5.1.4 to which supply V_BB is connected. With I_E = 0 i.e. emitter diode is not conducting,

R_BB = R_B1 + R_B2

• Then the voltage drop across R_BI can be obtained by using potential divider rule.

• The typical range of r| is from 0.5 to 0.8. The voltage VRB1 is called intrinsic stand off voltage because it keeps the emitter diode reversed biased for all the emitter voltages less than V_RB1.

 

4. Principle of Operation

• While operating an UJT, the supply V_BB is applied between B₂ and B₁ while the variable emitter voltage V_E is applied across the emitter terminals. This arrangement is shown in the Fig. 5.1.5.

• Let us see the effect of change in V_E. 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 IE will not flow. Thus UJT is said to be OFF.

Case 2 : V_E > V_r

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

V_P = V_A + V_D = η V_BB + V_D

• When VE becomes equal to or greater than Vp the p-n junction becomes forward biased and current IE flows. The UJT is said to be ON.

 

5. UJT Characteristics

• The graph of emitter current against emitter voltage plotted for a particular value of VBB called the characteristics of UJT. For a particular fixed value of VBB such characteristics is shown in the Fig. 5.1.6.

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

1. Cut-off region : The emitter voltage VE is less than Vp and the p-n junction is reverse biased. A small amount of reverse saturation current IEO flows through the device, which is negligibly small of the order of u.A. This condition remains till the peak point.

2. Negative resistance region : When the emitter voltage VE becomes equal to VP the p-n junction becomes forward biased and IE 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 RB1. This region is stable and used in many applications. This region continues till valley point.

3. Saturation region : Increase in IE further valley point current IV drives the device in the saturation region. The voltage corresponding to valley point is called valley point voltage denoted as Vv 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 RB1 decreases in this region.

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

 

6. Applications

• The UJT is mainly used in the triggering of other devices such as SCR. It is also used in the sawtooth wave generators and some timing circuits. The most popular application of UJT is as a relaxation oscillator to obtain short pulses for triggering of SCRs. Let us discuss UJT relaxation oscillator in detail.

 

7. UJT Relaxation Oscillator

• The pulse signal required to drive the digital circuits can be obtained from a single stage oscillator circuits using a particular device like unijunction transistor. Such a oscillator which uses UJT is called UJT relaxation oscillator. The basic circuit of UJT relaxation oscillator is shown in the Fig. 5.1.7. This is also called UJT based sawtooth oscillator.

• The R₁ and R₂ are biasing resistances which are selected such that they are lower than interbase resistances R_BI and R_B2 . The resistance RT and the capacitance CT decide the oscillating rate. The value of RT is so selected that the operating point of UJT remains in the negative resistance region. The UJT characteristics and the negative resistance region of the characteristics are shown in the Fig. 5.1.8 The characteristics of UJT shows the variation between V_E and I_E, where VE is emitter voltage and IE is emitter current.

a. Operation

• Capacitor C_T gets charged through the resistance R_T towards supply voltage VBB. As long as the capacitor voltage is less than peak voltage VP, the emitter appears as an open circuit.

V_P = η V_BB + V_D … (5.1.1)

where            η = Stand off ratio of UJT

V_D = Cut-in voltage of diode 

• When the capacitor voltage VC exceeds the voltage Vp, the UJT fires. The capacitor starts discharging through R1 + RB1 where RB1 internal base resistance. As RBI is assumed negligible and hence capacitor discharges through R1.

• Due to the design of R1z this discharge is very fast, and it produces a pulse across Rr When the capacitor voltage falls below VV i.e. VC = VE = VV, the UJT gets turned OFF. The capacitor starts charging again.

• The discharge time of the pulse is controlled by the time constant CTR1 while the charging time constant by RTCT- The waveforms are shown in the Fig. 5.1.9.

• There is voltage drop across R2 voltage rise across R1, when UJT fires.

The charging equation of the capacitor is given by,

 

Ex. 5.1.1 In a UJT relaxation oscillator, RT = 5kΩ, CT = 0.1 µF and n = 0.58. Find the frequency of the oscillations.

Sol :

 

Ex. 5.1.2 The intrinsic stand-off ratio for a UJT is 0.6. It the inter base resistance is 10 kΩ, what are the value of R_B1 and R _B2

Sol :

Review Questions

1. Explain the working of UJT as a relaxation oscillator with necessary waveforms and derive the expression for frequency of oscillations.

AU : Dec.-09, 10, 11, 16, Marks 8, May-13, Marks 16

2. Explain the construction, equivalent circuit and the operation of UJT and draw its characteristics.

AU : May-08, 09, 10, 15, Dec.-ll, 15, Marks 8

3. Draw the equivalent circuit of UJT and define the intrinsic stand off ratio.

4. Write a detailed note on UJT based sawtooth oscillator.

5. Explain the operation of UJT.

6. Discuss the characteristics and applications of UJT.