In a one-dimensional, homogeneously doped CdS crystal the following conditions can be identified under which stationary high-field domains occur (Boer 2012a):
(1) The reduction of the electron density with the field must be over-linear.
(2) The electron density at the electrode boundary must be low enough to fall into the over-linear decrease of conductivity (Schottky barrier condition).
(3) The range between the first and second singular point must span a region in the forth quadrant of direction (this is not the case for the Gunn effect, hence here stationary domain can not exist [18, 41-43]).
(4) All external parameters must remain stationary.
Under the following additional conditions, anode-adjacent high-field domains must occur:
(a) The continuation of the drift current curve intersects the electron neutrality curve a second time at higher fields.
(b) The region between the second and the third singular point spans a region of the second quadrant of the field of direction.
Fig. 37.14 Double trace oscilloscope picture of current (j) and Franz-Keldysh optical absorption (a), the latter obtained through a pinhole focused on a fraction of the primary domain and measured with a photomultiplier (Boer and Ward 1967a)