Power electronics deals with controlling the power supplied to loads using semiconductor devices like UJTs, SCRs, and triacs. The UJT is a three-terminal semiconductor device with one PN junction between a P-type emitter and N-type base region. It exhibits negative resistance, where an increase in emitter current causes the emitter voltage to decrease. This property allows it to be used in timing and oscillator circuits.

1. Arunachalam .M

2. Power Electronics - Definition
 Power Electronics is the branch that deals with the
control of power
 In many applications it is required to control the power
fed to the load.
 The semiconductor devices used to fine control the
power are
 Uni-junction transistor (UJT)
 Silicon Controlled Rectifier (SCR)
 Diac
 Triac

3. Uni-junction Transistor (UJT)
 UJT is a three terminal (Emitter, Base1 and Base2) two layer
semiconductor switching device.
 It consists of n – type silicon bar doped with p-type
material at the middle.
 It has only one p-n junction Hence the name
Uni-junction transistor
 It exhibits negative resistance Characteristics
 Used for switching applications

4. UJT – Symbol& Construction
Construction

5. Construction of UJT
 The channel consists of lightly doped N type
 P type material is infused at the middle and doped heavily
 Thus it forms a single PN junction
 The terminals Base1 (B1) ,Base2 (B2) are taken from the N
channel through the ohmic contacts
 Emitter taken from P type material.
 Emitter terminal is closer to the B2

6. UJT - Circuit action (Fig.)
Normally B2 is positive with respect to B1

7. Equivalent circuit - UJT

8.  Normally B2 is positive with respect to B1
 Voltage VBB is applied between B1 and B2 with emitter (E)
open
 Voltage V1 between E and B1 produces reverse bias at the
pn junction and E is cut off
 Now the current through the N bar will produce a voltage
gradient
 Now the voltage between B1 and E is
V1 = η VBB
Here η = R1/ RBB = R1/(R1 + R2)
 η is the intrinsic stand-off ratio
 R1 - Resistance between B1 and Emitter junction
 R2- Resistance between Emitter junction and B2
 RBB- Resistance between B1 and B2 (Normally 4 to
10kΩ)

9.  Apply a positive voltage VE between the emitter (E) and
Base 1
 As long as VE is less than V1 the pn junction is reverse
biased and there is no emitter current (IE)
 When the input voltage VE becomes greater than V1
pn junction will be forward biased
 Now holes are injected into n region. These holes are
repelled by B2 and attracted by B1
 Now the conductivity of the n channel increases
causing large in increase in emitter current (IE)
 Now UJT is in ON state and (IE) is controlled by the
emitter power supply (VE)

10. Negative resistance
 Greater conductivity between B1 and E means decrease
in resistance R1.
 Hence the voltage drop across this section causes VE to
decrease further and further
 ie. VE decreases when IE increase
 Thus UJT exhibits negative resistance
characteristics

11. V-I Characteristic curve of UJT

12. Explanation of the curve
 The curve is drawn between IE (X-axis) and VE (Y-axis)
 The emitter voltage at which emitter current starts
increasing is called the firing voltage (or triggering
point) V1
 When current IE exceed a particular value the voltage
VE begins to increase with the current
 UJT has negative resistance region for specific
operating voltage. Hence UJT can be used as a circuit
element in an oscillator circuit
 Firing voltage V1 can be fixed as desired. So UJT is very
useful in timing and control circuit

13. Problem
 The inter-base resistance of a UJT is 5k ohm. Its intrinsic
stand-off ratio is 0.6. Find the values of R11 and R2
We know,
RBB = R1 + R2 ie. 5 = R1 + R2
and η = R1/(R1 + R2)
ie. η = 0.6 = R1/5
R1 = 0.6x5 = 3Kohm
and R2 = 5 – R1 = 5 – 3 = 2kohm

14. MCQs
1. A UJT has
A) two pn junctions B) one pn junction
C) three pn junctions D) none of the above
• 2The emitter of a UJT is open. Then the resistance
between its bases is
A) High B) Low
C) Extremely low D) zero

15.  3. UJT is also called ------- diode
A) low resistance B) high resistance
C) single-base D) double – based
• 4. UJT may be sued as
A) an amplifier B) a saw tooth generator
C) a rectifier D) none of the above
5. In a UJT p-type emitter is ----- doped
A) lightly B) highly
C) moderately D) not at all doped