Characteristics of a UJT

UJT Characteristics

The static emitter char­acteristic (a curve showing the relation between emitter voltage V_E and 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_E never 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_P the 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:

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.

Valley Point Voltage V_V The valley point voltage is the emitter voltage at the valley point. The valley voltage increases with the increase in interbase voltage V_BB.

Valley Point Current I_V The valley point current is the emitter current at the valley point. It increases with the increase in inter-base voltage V_BB.

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

1. A stable triggering voltage (V_P)— a fixed fraction of applied inter base voltage V_BB.
2. A very low value of triggering current.
3. A high pulse current capability.
4. A negative resistance characteristic.
5. Low cost.