Ridley - Watkins – Hilsum (RWH) Theory:

• RWH proposed this theory to explain the phenomenon of –ve differential resistance (NDR) in certain bulk materials.
• NDR devices are classifieds into two groups

• The electric field is multi-valued in the case of voltage controlled NDRs and it is electric current that is multi-valued in the case of current controlled NDRs.
• The initially homogeneous semiconductor becomes heterogeneous to achieve stability.
• It result in ‘High Field Domain’ in voltage controleed NDRs and ‘?High Currenbt filaments’ in current NDRs as shown below in the figure.

Theory of Two Valley Model:

• It explains the effect of generation of negative resistance in certain types of bulk semiconductor materials.
• In the conduction band of n- type GaAs, a high m,obility lower valley is separated from a low mobility upper valley by an energy difference of 0.36eV.
• The effective mass and mobility of the electrons in lower valley are 0.068 and 8000cm2/Vs whereas these quantities are 1.2 and 180 cm2/Vs in the upper valley of the n – type GaAs as shown below.

Under the equilibrium conditions the electron densities in both the valleys remain same when the applied field is lower than the field corresponds to the energies of the electrons in the lower valley, then no transfer of electrons takes place from one to other valley. The mobility of the carriers is positive.

When the applied filed is higher than the field corresponding to the energies of the electron in the lower valley and lower than the field corresponding to the energies in the upper valley then transfer of electrons takes place from high mobility lower to low mobility upper valley.

This mobility of electrons or carrier becomes negative, When the applied field is higher than the field corresponding to the energies of the electrons in the higher valley. No transfer of electrons takes place because by that time all the electrons of the lower valley must have been transferred to the upper valley and the mobility is positive.