Unijunction transistor

Unijunction transistor

1. UNIJUNCTION TRANSISTOR Dr.R.Hepzi Pramila Devamani , Assistant Professor of Physics, V.V.Vanniaperumal College for Women, Virudhunagar.

2. UJT UJT is a three terminal silicon diode. UJT has only one PN junction. UJT differs from ordinary diode in that it has three leads UJT differs from FET in that it has no ability to amplify. UJT has the ability to control a large ac power with a small signal. UJT exhibits a negative resistance characteristic which makes it useful as an oscillator.

3. CONSTRUCTION UJT consists of a lightly doped silicon bar with a heavily doped P type material alloyed to its one side for producing single P-N junction. As in figure, there are terminals, one emitter, E and two bases B1 and B2, at the top and bottom of the silicon bar. The emitter leg is drawn at an angle to the vertical and arrow points in the direction of conventional current when UJT is in the conduction state.

4. INTERBASE RESISTANCE Interbase resistance is the resistance between B 1 and B 2 . It is the total resistance of the silicon bar from one end to the other with emitter terminal open. R BB = R B2 + R B1 R B2 > R B1 R B1 = 60 % of R BB

5. INTRINSIC STAND-OFF RATIO When a battery of 30V is applied across B 2 B 1, there is a progressive fall of voltage over R BB provided E is open. The emitter acts as a voltage divider. With emitter open, I 1 = I 2 , the interbase current is given by ohm’s law. I = I 2 = V BB / R BB A part of V BB is dropped over R B2 and part on R B1 . V A is the voltage drop across R B1 . Using simple voltage divider relationship V A = V BB R B1 / R B1 + R B2 The voltage division factor is given by ⴄ, the intrinsic stand-off ratio ⴄ = R B1 / R B1 + R B2 V A = ⴄ V BB , ⴄ = 0.5 to 0.85

6. OPERATION Junction, When V BB is switched on, V A is developed and reverse- biases the junction. If V B is the barrier voltage of the P-N junction, then total reverse bias voltage is = V A + V B = ⴄ V BB + V B V B = 0.7 V. The emitter junction will not become forward-biased unless its applied voltage V B exceeds ( ⴄ V BB + V B). The value of V E is called peak-point voltage V p . When V e = V p emitter current I p starts to flow through R B1 to ground.

7. OPERATION The UJT is then said to have been fired or turned ON. Due to flow of IE through R B1 , number of charge carriers in R B1 is increased which reduces its resistance. As ⴄ depends on R B1 , its value is also decreased. When V E and I E , increases, R B1 decreases, ⴄ decreases and V A decreases. This decreases in V A causes more emitter current to flow which causes a further reduction in RB, ⴄ and V A .

8. OPERATION The process is regenerative. V A and V B quickly drop as IE increases. Since V E decrease when I E increases, the UJT possesses negative resistance. The terminals E and B 1 are active terminals whereas B 2 is the bias terminal i.e., it is meant only for applying external voltage across the UJT. UJT is triggered into conduction by applying a suitable positive pulse at its emitter. It can be brought back to OFF state by applying a negative trigger pulse.

9. APPLICATION The unique property of UJT is that it can be triggered by any one of its three terminals. Once triggered, the emitter current IE of the UJT increases regeneratively till it reaches a limiting value determined by the external power supply. Because of this particular behaviour, UJT is used in a variety of circuit applications such as Phase control switching Pulse generation Sine wave generator Sawtooth generator Timing Circuits Voltage or current regulated supplies.