UJT - Unijunction Transistor

Transistors are normally used for switching . For example ,BJT acts as switch but it has low efficiency and many faults.
Therefore UJT and SCR are more useful in electronic circuit for switching purpose.
In this article we learn about UJT which has high impedance at off condition and low impedance at on condition.
History
It was developed by shockley in 1949 from the alloyed germanium bar structure and that time it called as filamentary transistor .The first UJT is in form of bar and then it form in cubr structure at 1994.

UJT construction

1. It consist of lightly doped silicon bar and heavily
   doped p-type material.
2. P type material attached to n type firmly and produces a single P-N junction.
3. In UJT emitter is nearest to base2 than base1.

i.e emitter= b1>b2

Symbol

1. UJT has 3 terminals emitter (E),base1 and base2.
2. Emitter is formed by P-type material and base1 and base2 formed by N-type.
3. Emitter arrow indicates emitter current (IE) which decides wheater UJT is NPN or PNP.

Equivalent circuit of UJT
It is mainly consists of emitter diode which is connected between base1 and base2.
R1 and R2 are the base resistors.
Interbase resistance:  it is resistance between base1 and base2 .
rbb=b1+b2
The value of interbase resistance is in between 4.7 to 9.1 kilo ohm.
According to voltage divider network,
N= rb1/(rb1 + rb2)
Where, N is intrinsic stand-off ratio.
              Rb1 is resistance of base1.
              Rb2 is resistance of base2.
Here N is always less than 1 (n <1)
It is of the order of (0.5 to 0.95)

V-I characteristics

IE0= Reverse leakage current
Vp= peak point voltage
Iv= valley point current
Vv= valley point current
Ip= peak point current

There are mainly three regions in UJT structure cut-off region, Negative resistance region, saturation region.
1)Cut-off region:
The region between peak point voltage and before the zero (0) is known as cut off region. In this region UJT is normally off.
2)Saturation region:
The region beyond the Iv (valley point) is known as saturation region. In this region UJT is in ON condition and emitter voltage (VE) is increase with increase in (IE)emitter current.
3)Negative resistance region: 
The region between the peak point and valley point is known as Negative resistance region. In this region resistance decreases woth increase in (VE) emitter voltage.
UJT Advantages

1. It has low cost.
2. Negative resistance characteristics.
3. Very low value of triggering current.

UJT Disadvantages

1. Not stable

UJT Applications

1. Used in timing circuit.
2. As sawtooth generator.
3. In phase control circuit.

Notes:

• UJT has only one pn junction.
• It has an emitter and two bases b1 and b2
• rb1 and rb2 and interbase resistance.
• Intrinsic stand of ratio= rb1/(rb1+rb2)
• rb1 varies inversly with emitter current (Ie).
• Low impedance at on state.
• High impedance at off state.