Other meanings of Negative resistance
Electronics
Negative resistance is an electrical phenomenon in which an increase in voltage across a device leads to a decrease in current through it (or vice versa) over a portion of its operating range. This contravenes Ohm's law, which describes positive resistance where current and voltage change in the same direction. Negative resistance is not a source of energy but a region of differential resistance where the incremental resistance dV/dI is negative. It is exploited in oscillators, amplifiers, and switching circuits.1
Negative resistance arises from distinct physical mechanisms in different devices. In the tunnel diode, heavily doped p-n junctions allow quantum tunneling; as forward bias increases, tunneling current rises to a peak then falls, creating a region of negative differential resistance.2 The Gunn diode relies on the Gunn effect in GaAs: under a high electric field, electrons transfer to a low-mobility valley, causing current to drop as voltage rises.3 IMPATT diodes use impact ionization and transit-time delay to produce negative resistance at microwave frequencies. Gas-discharge tubes (e.g., neon lamps) exhibit negative resistance in the glow-to-arc transition due to increased ionization. In all cases, the device's internal physics provides a local decrease in current with increasing voltage, but the overall I-V curve remains passive (no net power generation).
The primary use of negative resistance is in oscillators. A negative resistance device connected to a resonant circuit can cancel the circuit's positive losses, sustaining oscillations at the resonant frequency. Tunnel-diode oscillators operate up to 100 GHz, while Gunn-diode oscillators are common in radar and microwave sources.2 Amplifiers employ negative resistance to achieve gain, particularly in reflection-type amplifiers. In switching circuits, the negative resistance region enables bistable operation (e.g., in Schmitt triggers) and relaxation oscillators used in timer circuits. Less common applications include frequency multipliers and active filters where the negative impedance can cancel parasitic reactances.4 In superconducting electronics, Josephson junctions exhibit negative resistance at certain bias points, used in rapid single-flux-quantum logic.
The concept of negative resistance was first noted by Edwin H. Armstrong in 1915 while working on regenerative receivers; he used a triode's negative resistance to create oscillators.1 In the 1960s, Leo Esaki's discovery of the tunnel diode earned him a Nobel Prize, and his I-V curve became a textbook example of negative resistance.2 A lesser-known edge case is the memristor: when driven with alternating voltage, its pinched hysteresis loop can include regions of negative differential resistance under certain conditions, though this is still debated.5 In biological systems, the squid giant axon's voltage-clamp measurements show a region of negative conductance during the action potential, critical for nerve impulse propagation.6 Negative resistance also appears in thyristors during the switching transition, and in avalanche photodiodes under high bias.
In small-signal analysis, a negative resistance device is modeled as a negative resistor with impedance Z = R + jX where R < 0. The differential resistance is r = dV/dI < 0. For a tunnel diode, the I-V curve near the peak is approximated by a cubic polynomial, giving a region of negative slope. Stability criteria require that the magnitude of the negative resistance be less than the positive resistance of the external circuit to avoid unwanted oscillations. In oscillator design, the condition for sustained oscillation is Rdevice + Rload ≤ 0. Negative resistance can also be synthesized using operational amplifiers in circuits like the negative impedance converter (NIC), which uses positive feedback to emulate a negative resistor over a certain frequency range.4
Early observations of negative resistance date to the arc lamp in the 19th century, where increasing current caused a voltage drop (negative differential resistance). In 1918, William Eccles and Frank Jordan used negative resistance in a vacuum-tube flip-flop. The transferred-electron effect (Gunn effect) was discovered by John B. Gunn in 1963, enabling solid-state microwave oscillators.3 The IMPATT diode was proposed by William Shockley and Robert L. Johnston in the 1950s, becoming a key source for millimeter-wave power. Negative resistance remains a fundamental concept in semiconductor physics and circuit design, with ongoing research in quantum-well devices and graphene-based transistors that exhibit negative differential conductance at terahertz frequencies.7
Negative resistance is a differential property; it does not violate energy conservation because the device dissipates net power.
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