Unijunction Transistor (UJT)

UNIJUNCTION TRANSISTOR (UJT)

UJT CONSTRUCTION AND OPERATION

The UJT consists of a bar of lightly doped n-type silicon with a block of P-type material on one side (Fig. 1.54 (a)). The end terminals of the bar are identified as base 1 (B₁) and base 2 (B₂) and the P-type block is named the emitter (E).

Fig.1.54 (b) shows the UJT equivalent circuit. The resistance of the n-type silicon bar is represented as two resistors г_B1 from B₁ to point C, and r_B2 from B₂ to C as illustrated. The sum of r_B1 and r_B2 is called R_BB. The P type emitter forms a PN - junction with the n-type silicon bar, and this junction is shown as a diode (D₁) in the equivalent circuit.

With a voltage V_B1B2 applied, the voltage at the junction point C is

Note that V₁ is also the voltage at the cathode of the diode in the equivalent circuit with the emitter terminal open-circuited, the resistor current is 

If the terminal is grounded, the PN -junction is reverse biased and there is a small emitter reverse current (I_EO).

Now consider what happens when the emitter voltage (V_EB1) is slowly increased from zero. When V_EBI equals V₁, the emitter current is zero. Further increase in V_EBI forward-biases the PN-junctions and causes a forward current (I_E) to flow from the p -type emitter into the n-type silicon bar. When this occurs, charge carriers are injected into the r_B1 region. Since the resistance of the semiconductor material is dependent on doping, the additional charge carriers cause the resistance of the r_B1 region to decrease rapidly. The decrease in resistance reduces the voltage drop across r_B1 and so the pn junction in more heavily forward biased. This in turn results in a greater emitter current and more charge carriers that further reduce the resistance of the r_B1 region. (The process is termed as regenerative).

The input voltage is pulled down, and the emitter current (I_E) is increased to a limit deetrmined by the V_EBI Source resistance. The device remains in this on conditions until the emitter input is open-circuited or until I is reduced to very low level.

The circuit symbol for a UJT is shown in Fig. 1.54 (c). As always, the arowhead points in the conventional current direction for a forward-biased junction. In this case it points from the p-type emitter to n-type bar.

UJT CHARACTERISTICS

A plot of emitter voltage VEBI versus emitter current I_E gives the UJT emitter characteristics. Refer to the UJT terminal voltage and currents shown in Fig. 1.55 (a) and to the equivalent circuit Fig. 1.55 (b) Note that when V_B1B2 = 0, I_B2 = 0 and V₁ =  0

If V_EB1 is now increased from zero, the resultant plot of V_EB1 and I_E is simply the characteristics of a forward-biased diode with some series resistance. This is the characteristics for I_B2 = 0 in Fig. 1.55 (b).

When V_B1B2 is 20 V, the level of V₁ might be around 15 V₁ depending on the resistance of r_B1 and r_B2. With V_B1B2 = 20 V and V_EBI = 0, the emitter junction is reverse-biased and the emitter reverse current I_EO flows as shown at point 1 on the V_B1B2 = 20 V characteristic in Fig. 1.55 (b).

Increasing V_EBI until it equals V₁ gives I_E = 0. A further increase in V_EBI biases the emitter junction, and this gives the peak point on the characteristic. At the peak point, V_EBI is identified as the peak voltage (V_P) and I_e is termed the peak current (I_P).

Until the peak point V_P, the UJT is said to be operating in the cut off region of its characteristics. When V_EBI arrives at the peak voltage, charge carriers are injected from the emitter to decrease the resistance of r_B1.

The device enters the negative region, r_B1 falls rapidly to a saturation resistance (r_S), and V_EB1 falls to the valley voltage (V_V). I_E also increases to the valley current (I_V) at this time increase in I_E causes the device to enter the saturation region, where the sum of V_d and I_E X V_S.