Power Electronics - UJT.pptx

An introduction to Uni-junction transistor (UJT)

1. Power Electronics – I Uni -junction transistor (UJT) Arunachalam .M

2. Power Electronics - Definition Power Electronics is the branch that deals with the control of power In many applications it is required to control the power fed to the load. The semiconductor devices used to fine control the power are Uni -junction transistor (UJT) Silicon Controlled Rectifier (SCR) Diac Triac

3. Uni -junction Transistor (UJT) UJT is a three terminal (Emitter, Base1 and Base2) two layer semiconductor switching device. It consists of n – type silicon bar doped with p-type material at the middle. It has only one p-n junction Hence the name Uni -junction transistor It exhibits negative resistance Characteristics Used for switching applications

4. UJT – Symbol& Construction Construction

5. Construction of UJT The channel consists of lightly doped N type P type material is infused at the middle and doped heavily Thus it forms a single PN junction The terminals Base1 (B1) ,Base2 (B2) are taken from the N channel through the ohmic contacts Emitter taken from P type material. Emitter terminal is closer to the B2

6. UJT - Circuit action (Fig.) Normally B2 is positive with respect to B1

7. Equivalent circuit - UJT

8. Normally B2 is positive with respect to B1 Voltage V BB is applied between B1 and B2 with emitter (E) open Voltage V1 between E and B1 produces reverse bias at the pn junction and E is cut off Now the current through the N bar will produce a voltage gradient Now the voltage between B1 and E is V1 = η V BB Here η = R1/ R BB = R1 /(R1 + R2) η is the intrinsic stand-off ratio R1 - Resistance between B1 and Emitter junction R2- Resistance between Emitter junction and B2 R BB - Resistance between B1 and B2 (Normally 4 to 10k Ω )

9. Apply a positive voltage V E between the emitter (E) and Base 1 As long as V E is less than V1 the pn junction is reverse biased and there is no emitter current (I E ) When the input voltage V E becomes greater than V1 pn junction will be forward biased Now holes are injected into n region. These holes are repelled by B2 and attracted by B1 Now the conductivity of the n channel increases causing large in increase in emitter current (I E ) Now UJT is in ON state and (I E ) is controlled by the emitter power supply (V E )

10. Negative resistance Greater conductivity between B1 and E means decrease in resistance R1. Hence the voltage drop across this section causes V E to decrease further and further ie . V E decreases when I E increase Thus UJT exhibits negative resistance characteristics

11. V-I Characteristic curve of UJT negative resistance region

12. Explanation of the curve The curve is drawn between I E (X-axis) and V E (Y-axis) The emitter voltage at which emitter current starts increasing is called the firing voltage (or triggering point) V1 When current I E exceed a particular value the voltage V E begins to increase with the current UJT has negative resistance region for specific operating voltage. Hence UJT can be used as a circuit element in an oscillator circuit Firing voltage V1 can be fixed as desired. So UJT is very useful in timing and control circuit

13. Problem The inter-base resistance of a UJT is 5k ohm. Its intrinsic stand-off ratio is 0.6. Find the values of R1 1 and R2 We know, R BB = R1 + R2 ie . 5 = R1 + R2 and η = R1/(R1 + R2) ie . η = 0.6 = R1/5 R1 = 0.6x5 = 3Kohm and R2 = 5 – R1 = 5 – 3 = 2kohm

14. MCQs 1 . A UJT has A) two pn junctions B) one pn junction C) three pn junctions D) none of the above 2 The emitter of a UJT is open. Then the resistance between its bases is A) High B) Low C) Extremely low D) zero

15. 3. UJT is also called ------- diode A) low resistance B) high resistance C) single-base D) double – based 4. UJT may be sued as A) an amplifier B) a saw tooth generator C) a rectifier D) none of the above 5. In a UJT p-type emitter is ----- doped A) lightly B) highly C) moderately D) not at all doped