UJT – Introduction:
The UniJunction Transistor is a three-terminal, two-layer, semiconductor device. The terminals are Emitter(E), Base-one(B1) and Base-two(B2). As it exhibits a negative resistance region, it is used as an oscillator and triggering device.
The symbol shows that Base-1 and Base-2 terminals at right angles to indicate that are non-rectifying ohmic contacts. But the emitter is represented by an arrow since it is a rectifying p-n junction. The arrow is pointing from the p-type emitter to the n-type base.
The RB1 and RB2 are the internal resistance from bases B1 and B2 to point A.
When a voltage VBB is applied across B1 and B2, voltage across point A and B1 is VAB1 = VBB * RB1/RB1 +RB2 = η.VBB
VAB1 = η.VBB
Where η = intrinsic stand-off ratio = RB1/RB1 +RB2 .
The UJT consists of an n-type silicon bar with two ohmic contacts for the base-1 and base-2 terminals. The Emitter terminal is a p-type aluminium wire which is alloyed onto the silicon bar to form a p-n junction.
- When VBB is applied emitter diode D is reverse biased due to voltage drop across RB1(ie, VAB1 = η.VBB).
- Therefore only a small leakage current will flow as shown in the device V-I characteristic.
- At this condition the emitter junction being reverse biased and the inter base resistance being in the region of 5-10 kΩ.
- Now when emitter voltage VE is applied between E and B1, the diode remains reverse biased till
VE < ηVBB + VD (VD = Voltage drop across diode)
- So emitter current is negative.
- When VE = ηVBB + VD, the E-B1 junction is forward biased and it begins to conduct. So IE is positive.
- This value of Emitter voltage which causes the internal diode to conduct (or) UJT to conduct is called Peak point voltage.
ie, VP = ηVBB + VD.
- When UJT turns-On, the current IE increases and voltage across the device (VE) decreases.
- The voltage decreases till it reaches the point called as Valley voltage(VV).
- After Valley voltage the device is in saturation region. When VE reaches VV, the device turns-off.
The datasheet of the device provides several parameters to design the UJT based circuits.
- The interbase resistance RBB is measured between the two base terminals with the emitter open, and its value varies slightly with applied voltage and with temperature.
- The UJT data sheets normally provides a curve showing the variation of RBB with temperature.
- The peak and valley point currents and voltages are also given in datasheets and these values define the negative resistance region.
- The valley point location is affected by the temperature and the value of the interbase voltage.
- The peak point location is depends on interbase voltage and intrinsic stand-off ratio.
- The intrinsic stand-off ratio is the very important parameter in the design of UJT circuits which is given in datasheet.
- Even though it varies from one UJT to another, it remains relatively constant for a device even with variations in supply voltage and temperature.
- The Emitter-Base2 leakage current, when this junction is reverse biased with Base-1 open, is also given in the datasheets.
- It is similar in value to that of leakage through a diode, and it affects the charging current of any capacitors used in timing circuits. So it needs to be taken into account in the circuit design.
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