Unijunction transistor (ujt)

Notes

1. Unijunction Transistor (UJT) Miss Sushama V. Nikam Assist. Professor Y. C. Institute of Science Satara

2. It is three terminal semiconductor device having only one PN junction. Hence it is known as Unijunction Transistor. It can be used more efficiently in switching application as compare to BJT.

3. Structure of UJT Schematic Symbol Basic Structure

4. UJT consist of lightly doped N type semiconductor bar. On both sides of semiconductor bar, there are two terminals Base 1(B 1 ) and Base 2 (B 2 ) A PN junction is formed very close to Base B2 as shown in fig. The terminal attached to heavily doped P type region is known as Emitter. Thus UJT has three terminals B 1 , B 2 and Emitter.

5. UJT Equivalent Circuit R B1 = dynamic resistance of the silicon bar between the emitter and base 1. It varies inversely with emitter current I E , and therefore it is shown as a variable resistor. R B2 = dynamic resistance between the emitter and base 2 R BB = The total resistance between base B 1 and B 2 is called as Interbase resistance (R BB ). R BB = R B1 + R B2 The PN junction is formed in such a way that R B1 /R B2 = 2/3

6. Equivalent Circuit Intrinsic standoff Ratio The ratio of resistance between emitter and Base 1 to the total resistance of UJT is defined Intrinsic standoff Ratio. It is denoted by η . η = R B1 /R BB = R B1 /( R B1 + R B2 ) Normally the value of η is between 0.5 to 0.85

7. Working of UJT

8. When an external voltage V BB is applied between Base B 1 and Base B 2 , it develops uniform voltage drop. The voltage between Emitter and Base B 1 is known as Peak Point Voltage V P and it is given as, V P = η . V BB When emitter voltage V E is less than Peak Point Voltage V P , the PN junction is in reverse bias mode and UJT is off. When emitter voltage V E exceeds Peak Point Voltage by 0.7 V, the PN junction becomes forward bias and UJT is turn ON

9. When UJT turn ON it offers low resistance and it’s Base resistance R B1 decreases with η . Now, UJT is said to be in Negative resistance. UJT is also known as Negative resistance device.

10. IV characteristics of UJT

11. Above fig shows the circuit arrangement for plotting IV characteristics of UJT. The DC voltage is applied between Base 1 and Base 2. The emitter current I E is measured by varying emitter voltage V E . Following graph shows the IV characteristics of UJT.

12. There are two important points on the curve 1) Peak Point 2) Valley Point These two points divides the curve in three regions as explained bellow Cutoff Region: In this region emitter voltage is below peak point voltage V P . Therefore UJT is in OFF position. Negative Resistance Region: The region between peak point and valley point is called negative resistance region. In this region emitter voltage decreases from peak point to valley point. In this region UJT work as an Oscillator . Saturation Region: The region beyond the valley point is called as saturation region. In this region, the device is in it’s ON position. The emitter voltage V E remains constant with the increasing emitter current.

13. Applications Trigger device for SCR’s and triac Nonsinusoidal (relaxation) oscillators, Sawtooth g e nerators Phase control Timing circuits