Negative differential conductivity

Negative differential conductivity (NDC) is a nonlinear electrical phenomenon. If J is not a monotonic function of E at some values of E, the conductivity will be negative: $\sigma=\frac{dJ}{dE}<0$and negative differential conductivity is experienced; that is, in some electric field, as the current density increases, the electric field strength is decreased. In practice, one observes that the voltage drop across the "device under test" decreases when the current is increased.

Negative differential conductivity has been observed experimentally in some semiconductors (tunnel diodes, thyristors, and like structures). As well, non-equibrium plasma shows this behaviour. Even a simple gas-filled tube from a fluorescent lamp does.

The physical mechanisms that result in such effect are different, but they can be understood generally under processes such as impact ionisation.

Types

Different shapes of the J versus E function exist, which according to the shape, S, N, and Z-type materials can be categorised. In the S-type or S-shaped negative differential conductivity (SNDC) in a range of electric field, the current density is a multi-valued function of the electric field but the electric field is a single-valued function of current density and can be calculated withouth ambiguity from the value of the current density. A current filaments— an inhomogeneity in the current density distribution lateral to the direction of the current flow (orthogonal to the current density vector)—is common in devices showing current-type negative differential conductivity, especially of S-types. A Thyristor is a good example of an SNDC device.

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