The UJT characteristic is emitter voltage versus emitter current characteristic, as shown in the figure. There three operating regions in UJT
1.Cut off region: For emitter voltages less than VP (peak point voltage) the UJT is in the off state and magnitude of IE is not greater than IEO. The emitter current IEO corresponds very closely with the reverse leakage current ICO of a bipolar transistor. This region is known as the cut off region.
2. Negative Resistance region: As the emitter voltage increases and reaches VP = (ὴ VBB+VD), the UJT starts conducting. Then with increase in emitter IE the emitter voltage decreases as shown. The reduction in voltage across UJT is due to the drop in resistance RB1 with increase in the value of IE. This region of operation is known as a “Negative Resistance” region, which is stable enough to be used in various applications.
3. Saturation region: Eventually the “valley point” will be reached and further increase in IE will place the device into saturation and hence after valley point there is valley point.