UJT CHARACTERISTICS Electronic devices and circuits lab manual - Computer Programming

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## Wednesday, October 5, 2011

UJT CHARACTERISTICS
AIM: To observe the characteristics of UJT and to calculate the Intrinsic Stand-Off Ratio (η).

APPARATUS:
Regulated Power Supply (0-30V, 1A)      - 2Nos
UJT 2N2646
Resistors 10kΩ, 47Ω, 330Ω
Multimeters                                             - 2Nos
Connecting Wires

CIRCUIT DIAGRAM

THEORY:
A Unijunction Transistor (UJT) is an electronic semiconductor device that has only one junction. The UJT Unijunction Transistor (UJT) has three terminals an emitter (E) and two bases (B1 and B2). The base is formed by lightly doped n-type bar of silicon. Two ohmic contacts B1 and B2 are attached at its ends. The emitter is of p-type and it is heavily doped. The resistance between B1 and B2, when the emitter is open-circuit is called interbase resistance.The original unijunction transistor, or UJT, is a simple device that is essentially a bar of N type semiconductor material into which P type material has been diffused somewhere along its length. The 2N2646 is the most commonly used version of the UJT.

Circuit symbol
The UJT is biased with a positive voltage between the two bases. This causes a potential drop along the length of the device. When the emitter voltage is driven approximately one diode voltage above the voltage at the point where the P diffusion (emitter) is, current will begin to flow from the emitter into the base region. Because the base region is very lightly doped, the additional current (actually charges in the base region) causes (conductivity modulation) which reduces the resistance of the portion of the base between the emitter junction and the B2 terminal. This reduction in resistance means that the emitter junction is more forward biased, and so even more current is injected. Overall, the effect is a negative resistance at the emitter terminal. This is what makes the UJT useful, especially in simple oscillator circuits.When the emitter voltage reaches Vp, the current startsto increase and the emitter voltage starts to decrease.This is represented by negative slope of the characteristics which is reffered to as the negative resistance region,beyond the valleypoint ,RB1 reaches minimum value and this region,VEB propotional to IE.
PROCEDURE:
1.    Connection is made as per circuit diagram.
2.    Output voltage is fixed at a constant level and by varying input voltage corresponding emitter current values are noted down.
3.    This procedure is repeated for different values of output voltages.
4.    All the readings are tabulated and Intrinsic Stand-Off ratio is calculated using            η = (Vp-VD) / VBB
5.    A graph is plotted between VEE and IE for different values of VBE.
MODEL GRAPH:

OBSEVATIONS:
 VBB=1V VBB=2V VBB=3V VEB(V) IE(mA) VEB(V) IE(mA) VEB(V) IE(mA)
CALCULATIONS:
VP = ηVBB + VD
η = (VP-VD) / VBB­
η =  ( η1 +   η2 +    η3 ) / 3
RESULT: The characteristics of UJT are observed and the values of Intrinsic Stand-Off Ratio is calculated.