Double Extraction of Uniformly Generated Electron-Hole Pairs from Insulators with Noninjecting Contacts

A simple physical model has been used to derive an approximate theory of the double extraction of uniformly generated electron-hole pairs from a photoconductor layer with noninjecting contacts. The theory predicts 4 regimes of current versus applied voltage behavior: (1) At low voltage, I∝V. (2) At higher voltage, a transition region between I∝V and I∝V1/2. (3) At still higher voltage, I∝V1/2. (4) At very high voltage (above a saturation value), I=constant. The model relates the extent of these regimes to the physical parameters of the system, viz., μpτp (mobility × lifetime product for holes), μnτn (mobility × lifetime product for electrons), and the layer thickness. Good agreement is obtained between the theory and measurements on lead oxide vidicons. The theory also provides some insight into the nature of the transient phenomena ``red fade'' and ``after image'' sometimes observed in the operation of lead oxide vidicons. At high light excitation levels, space-charge-limited currents are expected. In this case, two regimes of current versus applied voltage behavior can be predicted: (1) Below a saturation value of voltage, I∝V1/2. (2) Above a saturation value of voltage, I=constant. The reasons for the one-half power current-voltage relationships are distinctly different in the μτ-limited and the space-charge-limited cases. In addition, the dependence of current on light intensity in the space-charge-limited case is a three-quarter power relation whereas in the μτ-limited case the dependence is a linear one. Also, the saturation voltage varies as the one-half power of the light intensity in the space-charge-limited case and is independent of the light intensity in the μτ-limited case.