As indicated previously, the field of the domain is uniquely determined by the boundary electron concentration at the cathode. For changing the domain field one needs to change this boundary concentration. This can be done with a quasi-cathode
Fig. 37.6 Virtual cathode produced by a shadow band across the CdS crystal, creating a high-field domain adjacent to it Boer et al.
When the shadow is positioned a few Debye lengths beyond the cathode, it can vary the boundary density n* at the edge of the shadow in a wide range below the bulk electron density n10. This is achieved by reducing the light intensity in the shadow or by adding some quenching light.
With the virtual cathode one can produce sequentially high-fleld domains with different n* (Boer and Voss 1968c) and consequently with different domain fields that can be obtained in each case similar to the procedure shown in Fig. 37.3. Consequently one obtains also the field-quenched electron density (that is equal to n* since the domain-field extends homogeneously from the virtual cathode) from the drift current in each of the domains. The resulting n1(F) is shown in Fig. 37.7 (Hadley et al. 1972), that is obtained in the same crystal.
When with increasing bias the expanding high-field domain finally fills the entire crystal between both electrodes, an anode-adjacent high-field domain is created that is discussed below.