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What is UJT?

• A Unijunction transistor is a three terminal semiconductor device having only one p-n junction like diode but has three terminals.
• This device has a unique characteristics that when it is triggered, the emitter current increases regeneratively until is limited by emitter power supply.
• The unijunction transistor can be employed in a variety of applications like switching pulse generator, saw tooth generator etc. The basic construction of UJT and its symbol is shown in below figure.

CONSTRUCTION:

1. • It consists of an N type silicon bar with an electrical connection on each end and the leads to these connections are called base leads, Base 1 (B₁), Base 2 (B₂).
   • The small p-type region is doped at one side of the bar near to B₂ terminal and the lead taken from this p-type region is known as emitter.
   • Thus a p-n junction is formed between the emitter and base region. The emitter region is heavily doped, while the base region is lightly doped.
   • So resistivity of base material is very high. The resistance ratio is an important characteristic of UJT which is known as intrinsic stand-off ratio which is denoted by η which is given by the equation:

CHARACTERISTICS:

• The static emitter char­acteristic (a curve showing the relation between emitter voltage V_Eand emitter current I_E) of a UJT at a given inter base voltage V_BB is shown in figure.
• From figure it is noted that for emitter potentials to the left of peak point, emitter current I_Enever exceeds I_Eo . The current I_Eo corresponds very closely to the reverse leakage current I_Co of the conventional BJT. This region, as shown in the figure, is called the cut-off region.
• Once con­duction is established at V_E = V_Pthe emitter po­tential V_E starts decreasing with the increase in emitter current I_E. This Corresponds exactly with the decrease in resistance R_B for increasing cur­rent I_E. This device, therefore, has a negative resistance region which is stable enough to be used with a great deal of reliability in the areas of applications listed earlier. Eventually, the valley point reaches, and any further increase in emitter current I_E places the device in the saturation region, as shown in the figure. Three other important parameters for the UJT are I_P, V_V and I_V and are defined below:

1. Peak-Point Emitter Current. I_p. It is the emitter current at the peak point. It repre­sents the rnimrnum current that is required to trigger the device (UJT). It is inversely proportional to the interbase voltage V_BB.
2. Valley Point Voltage V_VThe valley point voltage is the emitter voltage at the valley point. The valley voltage increases with the increase in interbase voltage V_BB.
3. Valley Point Current I_VThe valley point current is the emitter current at the valley point. It increases with the increase in inter-base voltage V_BB.

OPERATION:

• 
• Here emitter diode is reverse biased by voltage drop across resistance R_B1 and its own barrier potential V_D. So total reverse bias is V_D + V_B = VD + η VBB (for silicon V_D = 0.7 Volts ).
• As long as applied emitter voltage is below the total reverse bias voltage across the diode, it remain reverse biased. And there is no emitter current. The value of emitter voltage which causes the diode conduction is called peak point voltage V_P. i.e. V_P = ηV_BB + V_D.
• Then the UJT is turned on and emitter current begins to flow. Under this condition the UJT is said to be fired or triggered or turned on. Thus as V_E along with I_E increases, R_B1, η and V_A This produce further decrease in R_B1, η and V_A. This process is regenerative.
• V_A as well as V_E decreases as I_E Due to this the UJT has negative resistance region in its VI characteristics. The curve between Emitter voltage V_E and emitter current I_E of a UJT at a given voltage V_BB between the bases this is known as emitter characteristics of UJT Initially in the cut off region as V_E increases from zero, slight leakage current flows from terminal B₂ to the emitter the current is due to the minority carriers in the reverse biased diode.
• Above a certain value of V_E forward I_E begins to flow, increasing until the peak voltage V_P and current I_p are reached at point P. After the peak point P an attempt to increase V_E is followed by sudden increases in emitter current I_E with decrease in V_E is a negative resistance portion of the curve.
• The negative portion of the curve lasts until the valley point V is reached with valley point voltage V_v. and valley point current I_v after the valley point the device is driven to saturation the difference V_p-V_v is a measure of a switching efficiency of UJT fall of V_BB

Special Features of UJT.
        The special features of a UJT are :

• A stable triggering voltage (V_P)— a fixed fraction of applied inter base voltage V_BB.

• A very low value of triggering current.
• A high pulse current capability.
• A negative resistance characteristic.
• Low cost.

ADVANTAGES OF UJT

• It is a Low cost device
• It has excellent characteristics
• It is a low-power absorbing device under normal operating conditions

APPLICATIONS OF UJT
It can be used as trigger device for SCR’s and TRIAC’s.

• To generate non sinusoidal oscillations.
• To generate saw tooth waveforms.
• In timing circuits it is used as switch