NDC

Negative differential conductance occurs in certain semiconductors and devices when an increase in applied voltage produces a decrease in current flow, the opposite of normal resistive behaviour. This counterintuitive phenomenon emerges in materials and structures where internal energy states or carrier dynamics create a region of negative resistance, measurable as a negative slope on the current-voltage characteristic curve.

NDC is most commonly encountered in tunnel diodes, resonant tunneling diodes (RTDs), and some Gunn diodes. In a tunnel diode, the effect arises from quantum tunneling through a heavily doped p-n junction. As forward bias increases, tunneling current initially rises, but then falls as direct thermionic emission takes over and the tunnel resonance condition moves out of alignment. This creates a distinct valley in the I-V curve between peak and valley currents, defining the negative conductance region.

The practical window for NDC operation is narrow. A typical silicon tunnel diode might exhibit negative conductance over a voltage swing of only 0.1 to 0.3 volts, with peak currents in the milliampere range and valley currents substantially lower. RTDs push operation into the microwave domain, with much sharper peaks and steeper negative slopes, making them valuable for high-frequency oscillators and switching circuits where conventional transistors struggle.

Stability and application constraints

Devices operating in the NDC region are inherently unstable in constant-voltage circuits, since any perturbation that increases current further reduces voltage drop across the device, destabilizing the operating point. A series resistor must be included to provide negative feedback and stabilize the bias point. This series resistance also limits the maximum negative conductance magnitude that can be exploited without oscillation.

NDC finds use in voltage-controlled oscillators, amplifiers at millimetre-wave frequencies, and as a building block for quantum cascade structures in solid-state physics. The effect has declined in mainstream power electronics as conventional semiconductor technology matured, but remains relevant in specialised applications where the sharp I-V nonlinearity and fast response time offer advantages over conventional active devices.