Question

What is the physical reason behind the negative differential resistance (NDR)?

Answer 1

Negative resistance is a property of some electrical circuit

Answer 2

As you know, there is an effect called negative differential resistance in some electric devices and materials, but I could not find any article or book or any other reference to talk about the physical reason of this effect. I mean, what happens for electrons to show such a behavior? (NDR: decrease in current by increasing voltage.)