English: Current-voltage characteristics or IV curves showing two types of negative differential resistance.

The left graph shows voltage controlled negative resistance (VCNR) also called N type. In this type the voltage is a multivalued function of current, but the current is a single-valued function of voltage. The graph is shaped vaguely like the letter N, with a voltage region with negative resistance between two positive resistance regions. Electronic devices with this type of negative resistance include the tunnel diode, Gunn diode, and vacuum tubes operating in the "dynatron" mode.

The right graph shows current controlled negative resistance (CCNR) also called S type. In this type the voltage is a multivalued function, while the current is a single-valued function. The graph is shaped vaguely like the letter S, with a region of current with negative resistance between to regions with positive resistance. Devices with this type of negative resistance include the IMPATT diode, unijunction transistor, SCR, and gas-discharge tubes like neon lamps and fluorescent tubes.