Understanding UJT Operation and Negative Resistance Characteristics

UJT Emitter Voltage and Conduction

As the emitter supply voltage V_E is slowly increased, I_Eo eventually reduces to zero when V_E equals ηV_BB. At this point, with equal voltage levels on each side of the diode, no reverse or forward current flows. When the emitter supply voltage increases further, the diode becomes forward-biased as soon as it exceeds the total reverse bias voltage (ηV_BB + V_B).

Peak-Point Voltage and Current

The emitter voltage V_E at this threshold is known as the peak-point voltage (V_P). When V_E = V_P, the emitter current I_E begins to flow through R_B1 to ground (B₁). This represents the minimum current required to trigger the UJT, defined as the peak-point emitter current (I_P). Notably, I_P is inversely proportional to the interbase voltage, V_BB.

Negative Resistance Characteristics

When the emitter diode conducts, charge carriers are injected into the R_B region. Because semiconductor resistance depends on doping, the resistance of the R_B region decreases rapidly due to the influx of holes. This reduction in resistance causes the voltage drop across R_B to decrease, which further forward-biases the emitter diode. This creates a regenerative effect: larger forward current leads to more charge carriers, further reducing resistance. Consequently, the emitter current increases until limited by the power supply. Because V_A decreases as emitter current increases, the UJT exhibits a negative resistance characteristic.

Terminal Functions and Triggering

Base-2 (B₂) is used primarily to apply the external voltage V_BB, while terminals E and B₁ serve as the active terminals. A UJT is typically triggered into conduction via a positive pulse at the emitter and turned off using a negative trigger pulse.

Static Emitter Characteristics

The static emitter characteristic curve illustrates the relationship between V_E and I_E at a fixed V_BB.

Cut-off and Saturation Regions

• Cut-off Region: For emitter potentials to the left of the peak point, I_E never exceeds I_Eo, which corresponds closely to the reverse leakage current I_Co of a conventional BJT.
• Negative Resistance Region: Once conduction is established at V_E = V_P, the emitter potential V_E decreases as I_E increases, reflecting the drop in R_B resistance.
• Saturation Region: Eventually, the device reaches the valley point, after which any further increase in I_E places the device into the saturation region.