UJT
The Uni-Junction Transistor is a three-terminal single-junction device. The switching voltage of the UJT can be easily varied.
The UJT is always operated as a switch in oscillators, timing circuits and in SCR/TRIAC trigger circuits.
UJT-Constructional Features:
q The UJT structure consists of a lightly doped n-type silicon bar provided with ohmic contacts on either side.
q The two end connections are called base B1 and base B2. A small heavily doped p-region is alloyed into one side of the bar. This p-region is the UJT emitter (E) that forms a p–n junction with the bar.
q Between base B1 and base B2, the resistance of the n-type bar called inter-base resistance (RB ) and is in the order of a few kilo ohm.
q This inter-base resistance can be broken up into two resistances—the resistance from B1 to the emitter is RB1 and the resistance from B2 to the emitter is RB 2.
q Since the emitter is closer to B2 the value of RB1is greater than RB2.
Total resistance is given by:
RB = RB1 + RB2
Equivalent circuit for UJT:
¢ The VBB source is generally fixed and provides a constant voltage from B2 to B1.
¢ The UJT is normally operated with both B2 and E positive biased relative to B1.
¢ B1 is always the UJT reference terminal and all voltages are measured relative to B1 . VEE is a variable voltage source.
UJT V–I characteristic curves:
ON State of the UJT Circuit:
o As VEE increases, the UJT stays in the OFF state until VE approaches the peak point value V P. As VE approaches VP the p–n junction becomes forward-biased and begins to conduct in the opposite direction.
o As a result IE becomes positive near the peak point P on the VE – IE curve. When VE exactly equals VP the emitter current equals IP
o At this point holes from the heavily doped emitter are injected into the n-type bar, especially into the B1 region. The bar, which is lightly doped, offers very little chance for these holes to recombine.
o The lower half of the bar becomes replete with additional current carriers (holes) and its resistance RB is drastically reduced; the decrease in BB1 causes Vx to drop.
o This drop, in turn, causes the diode to become more forward-biased and IE increases even further
OFF State of the UJT Circuit:
¢ When a voltage VBB is applied across the two base terminals B1 and B2, the potential of point p with respect to B1 is given by:
VP =[VBB/ (RB1 +RB2)]*RB1=η*RB1
¢ η is called the intrinsic stand off ratio with its typical value lying between 0.5 and 0.8.
¢ The VEE source is applied to the emitter which is the p-side.
¢ Thus, the emitter diode will be reverse-biased as long as VEE is less than Vx.
¢ This is OFF state and is shown on the VE – IE curve as being a very low current region.
¢ In the OFF state the UJT has a very high resistance between E and B1, and IE is usually a negligible reverse leakage current.
¢ With no IE, the drop across RE is zero and the emitter voltage equals the source voltage.
UJT Ratings:
¢ Maximum peak emitter current : This represents the maximum allowable value of a pulse of emitter current.
¢ Maximum reverse emitter voltage :This is the maxi mum reverse-bias that the emitter base junction B2 can tolerate before breakdown occurs.
¢ Maximum inter base voltage :This limit is caused by the maxi mum power that the n-type base bar can safely dissipate.
¢ Emitter leakage current :This is the emitter current which flows when VE is less than Vp and the UJT is in the OFF state.
Applications:
¢ The UJT is very popular today mainly due to its high switching speed.
¢ A few select applications of the UJT are as follows:
(i) It is used to trigger SCRs and TRIACs
(ii) It is used in non-sinusoidal oscillators
(iii) It is used in phase control and timing circuits
(iv) It is used in saw tooth generators
(v) It is used in oscillator circuit design
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