The Ridley–Watkins–Hilsum Theory (RWH) explains the mechanism by which differential negative resistance is developed in a bulk solid state device when a voltage is applied to the terminals of the sample.[1]
Such effects had been observed in the laboratory by J. B. Gunn in 1962,[2] and were thus named the "Gunn Effect", but it was Kroemer who suggested that J. B. Gunn's observations were in complete agreement with the RWH in 1964.[3]
In essence, RWH mechanism is the transfer of conduction electrons in a semiconductor from a high mobility valley to lower-mobility, higher-energy satellite valleys. This phenomenon can only be observed in materials that have such energy band structures.
Normally, in a conductor, increasing electric field causes higher charge carrier (usually electron) speeds and results in higher current consistent with Ohm's Law. In a multi-valley semiconductor, though, higher energy may push the carriers into a higher energy state where they actually have higher effective mass and thus slow down. In effect, carrier velocities and current drop as the voltage is increased. While this transfer occurs, the material exhibits a decrease in current – that is, a negative differential resistance (NDR). Normal voltage-current relation resumes once the bulk of the carriers are kicked into the higher energy-mass valley.
Of the type of semiconducting materials satisfying these conditions, gallium arsenide (GaAs) is the most widely understood and used. However RWH mechanisms can also be observed in indium phosphide (InP), cadmium telluride (CdTe), zinc selenide (ZnSe) and indium arsenide (InAs) under hydrostatic or uniaxial pressure.