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
