The physics of space charge instabilities and temporal chaos in extrinsic photoconductors

This paper reviews experimental and theoretical work carried out on space charge instabilities and temporal chaotic behavior in cooled extrinsic p-type Germanium photoconductors. Measured dc current-voltage (I–V) characteristics of these devices are strongly nonlinear for moderate electric fields 0.1 V/cm due to field dependence of the rates of free hole capture and impurity impact ionization. Below the threshold field for impurity breakdown, Ge samples behave like damped nonlinear oscillators, exhibiting characteristic chaotic response when driven by a time-periodic voltage. Above impurity breakdown, we observe voltage-controlled negative differential resistance (NDR) in the I–V curves accompanied by spontaneous current oscillations due to moving space charge domains with velocities 10³ to 10⁴ cm/s. Measurements are well explained by a simple rate equation model in which negative differential behavior in the impact ionization rate plays a crucial role. Related work on semiconductor chaos and possible future directions for research are also mentioned.