Unified treatment of small-signal space-charge dynamics in bulk-effect devices

Space-charge dynamics in semiconductors with negative differential mobility are discussed in a small-signal approximation taking account of the role of carrier diffusion and are shown to exhibit fundamentally four different properties. These differences originate from two conditions, one being the direction of propagation of the space-charge wave and the other being whether the space-charge wave is characterized by frequency-independent velocity (convective instability) or frequency-independent wavelength (absolute instability). These behaviors of space-charge waves have a great influence on the performance of bulk-effect devices and give rise to extremely different characteristics. When the space-charge wave is characterized by frequency-independent velocity, two-terminal impedance of bulk-effect devices shows the conventional transit-time effect. On the contrary, the property of space-charge waves characterized by frequency-independent wavelength gives rise to the static negative resistance in two-terminal impedance.