NDR

Negative differential resistance occurs in certain electrical devices when an increase in applied voltage actually causes the current flowing through them to decrease, rather than increase as Ohm's law would predict for ordinary conductors. This counterintuitive behavior appears in the I-V characteristic curve as a region where the slope is negative, meaning the device resists more strongly as you push harder on it electrically.

Tunnel diodes and Esaki diodes are the classic examples in semiconductor physics. They exploit quantum tunneling effects in heavily doped p-n junctions to create this effect. Resonant tunneling diodes (RTDs) extend the principle using heterostructure barriers and can show pronounced NDR regions. Gunn diodes used in microwave oscillators also exhibit NDR, though through a different mechanism involving transferred electron effects in gallium arsenide. Each device type has its own voltage range where the effect dominates.

The practical value of NDR lies in oscillation and switching. Because the device naturally wants to move away from the NDR region when disturbed, circuits built around it tend to oscillate spontaneously at frequencies determined by the load line and circuit inductance. Tunnel diodes have been used for decades in oscillators operating from low frequency to microwave bands. The speed is limited mainly by the junction capacitance and resistance, not by carrier transit time, which makes them valuable where semiconductor switching speed matters.

Where NDR creates problems

In power electronics and grid-scale applications, NDR-like behaviors can appear unintentionally in certain operating regimes and cause instability. Inverters and power converters may exhibit regions in their control characteristics that resemble NDR, leading to subsynchronous resonance or voltage collapse if the network impedance interacts poorly with the control loops. Engineers designing large renewable energy systems must check for these interactions in their stability studies.

The name "negative differential resistance" directly describes the mathematics: if you compute the differential resistance as dV/dI (the slope of the voltage-current curve), you get a negative number in the problematic region. This distinguishes it sharply from static resistance, which is V/I and always positive. The term has remained standard since the 1950s because it precisely captures the physical phenomenon without invoking device physics or applications.