Unijunction Transistor

Unijunction Transistor :

The Uni junction Transistor or UJT for short, is another solid state three terminal device that can be used in gate pulse, timing circuits and trigger generator applications to switch and control either thyristors and triac’s for AC power control type applications.
Like diodes, uni +junction transistors are constructed from separate P-type and N-type semiconductor materials forming a single (hence its name Uni-Junction) PN- junction within the main conducting N-type channel of the device.

Although the Unijunction Transistor has the name of a transistor, its switching characteristics are very different from those of a conventional bipolar or field effect transistor as it can not be used to amplify a signal but instead is used as a ON-OFF switching transistor. UJT’s have unidirectional conductivity and negative impedance characteristics acting more like a variable voltage divider during breakdown.

Like N-channel FET’s, the UJT consists of a single solid piece of N-type semiconductor material forming the main current carrying channel with its two outer connections marked as Base 2 ( B₂ ) and Base 1 ( B₁ ). The third connection, confusingly marked as the Emitter ( E ) is located along the channel. The emitter terminal is represented by an arrow pointing from the P-type emitter to the N-type base.

 The Emitter junction is positioned along the channel so that it is closer to terminal B₂ than B₁. An arrow is used in the UJT symbol which points towards the base indicating that the Emitter terminal is positive and the silicon bar is negative material. Below shows the symbol, construction, and equivalent circuit of the UJT.

 Symbol and Construction :

The symbol for the uni junction transistor (N type) looks as shown in beloe fig. it is quite similar to that of the junction field effect transistor (JFET), except that it has a bent arrow representing the Emitter( E ) input. While similar in respect of their ohmic channels.

  The N-type channel basically consists of two resistors R_B2 and R_B1 in series with an equivalent (ideal) diode, D representing the p-n junction connected to their center point. This Emitter p-n junction is fixed in position along the ohmic channel during manufacture.

 Resistance R_B1 is given between the Emitter, E and terminal B₁, while resistance R_B2 is given between the Emitter, E and terminal B₂. As the physical position of the p-n junction is closer to terminal B₂ than B₁ the resistive value of R_B2 will be less than R_B1.

 These two series resistances produce a voltage divider network between the two base terminals of the unijunction transistor and since this channel stretches from B₂ to B₁, when a voltage is applied across the device, the potential at any point along the channel will be in proportion to its position between terminals B₂ and B₁. The level of the voltage gradient therefore depends upon the amount of supply voltage.

When used in a circuit, terminal B₁ is connected to ground and the Emitter serves as the input to the device. Suppose a voltage V_BB is applied across the UJT between B₂ and B₁ so that B₂ is biased positive relative to B₁.

 Operation of a UJT : 

This transistor operation starts by making the emitter supply voltage to zero, and its emitter diode is reverse biased with the intrinsic stand-off voltage. If VB is the voltage of the emitter diode, then the total reverse bias voltage is VA + VB = Ƞ VBB + VB. For silicon VB = 0.7 V, If VE gets slowly increases to the point where VE = Ƞ VBB, then IE will be reduced to zero. Therefore, on each side of the diode, equal voltages results no current flow through it, neither in reverse bias nor in forward bias.

 When the emitter supply voltage is increased rapidly, then the diode becomes forward-biased and exceeds the total reverse bias voltage (Ƞ VBB + VB). This emitter voltage value VE is called the peak-point voltage and is denoted by VP. When VE = VP, emitter current IE flows through the RB1 to the ground, that is, B1. This is the minimum current required for triggering the UJT. This is called the peak-point emitter current and is denoted by IP. Ip is inversely proportional to the Inter-base voltage, VBB.

 Now when the emitter diode starts conducting, charge carriers are injected into the RB region of the bar. As the resistance of a semiconductor material depends upon doping, the resistance of RB decreases due to additional charge carriers.

 Then the voltage drop across RB also decreases, with the decrease in resistance because the emitter diode is heavily forward biased. This in turn results in larger forward current, and as a result charge carriers are injected and it will cause the reduction in the resistance of the RB region. Thus, the emitter current goes on increasing until the emitter power supply is in limited range.

 VA decreases with the increase in emitter current, and UJT have the negative resistance characteristic. The base 2 is used for applying external voltage VBB across it. The terminals E and B1 are the active terminals. UJT usually gets triggered by applying a positive pulse to the emitter, and it can be turned off by applying a negative trigger pulse.

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