Unijunction transistor ( UJT )
The UJT is three lead electronic semi-conductor device with only one junction . UJT is also known as unijunction transistor & that Acts only exclusive as electric controlled switch .we can say that the UJT is a triggered device. The UJT is made up by n-type semiconductor material and P-type semiconductor . the Base of UJT is made up by n-type semiconductor material & Emitter is made up by P-type semiconductor material . The Base is formed by lightly doped n- type material bar of silicon. The emitter is heavily doped by P-type material. In UJT only one junction & two terminal ( Base & Emitter ). We can define terminal of UJT B1 { Base 1 } & B2 { Base B2 } , E { Emitter }. We know that the UJT is work as electric controlled switch , in generally Base 1 { B1 } is grounded & positive supply is provided on Base2 { B2 } . The N - field is act as a voltage divider when value of Emitter current is zero.Thus some positive voltage remains at the confluence. If the value of the positive voltage given on the emitter is less than the current positive voltage on the base region, the transistor will remain on reverse bias.If the value of the emitter voltage exceeds the base area, the transistor will return to the forward bias. The resistance between B1 & B2 when the emitter is open circuit is called inter base resistance .the ujt is biased with a positive voltage between two bases .
●symble of ujt :-
●Peak point current :-
The peak point current is represent the minimum value of current for triggering device. It is inversely proportional to inter base voltage
VBB.
●Valley point current :-
The valley point current is the emitter current at the valley point. It increases with the increase in inter-base voltage VBB.
● ujt characteristics curve :-
●Applications of UJT :-
-A Fixed triggering Voltage.
-A very low value of triggering current.
-A high pulse current capability.
-A negative resistance characteristic.
-cut off & saturation region characteristics.
-A very low value of triggering current.
-A high pulse current capability.
-A negative resistance characteristic.
-cut off & saturation region characteristics.
●Cut off region:-
When the value of the emitter voltage (ve) is increased. So, initially the value of emitter current (Ie) does not increase. But when the emitter voltage (ve) is further increased, the value of the emitter current (ie) increases. Thus the emitter voltage reaches its maximum value. This maximum value point of the emitter voltage is called the peak point. In this case, UJT works in the cut off region.
We can say that UJT acts like a switch in the cut off region.
●Negetive resistance region :-
When the emitter voltage reaches Vp, the current starts to increase and the emitter
voltage starts to decrease. This is represented by negative slope of the
characteristics which is referred to as the negative resistance region, beyond the
valley point , VEB proportional to IE.
●Saturation region :-
After the negative resistance region which saw an increase in current comes the saturation region. This is the region where if the applied voltage to the emitter still increases, the current and voltage will rise.
● circuit diagram of ujt characteristics experiment:-
