  While space-charge-limited current measurements are often used to
characterize charge-transport in relatively intrinsic, low-mobility
semiconductors, it is currently difficult to characterize lightly or heavily
doped semiconductors with this method. By combining the theories describing
ohmic and space-charge-limited conduction, we derive a general analytical
approach to extract the charge-carrier density, the conduction-band edge and
the drift components of the current density-voltage curves of a single-carrier
device when the semiconductor is either undoped, lightly doped or heavily
doped. The presented model covers the entire voltage range, i.e., both the
low-voltage regime and the Mott-Gurney regime. We demonstrate that there is an
upper limit to how doped a device must be before the current density-voltage
curves are significantly affected, and we show that the background
charge-carrier density must be considered to accurately model the drift
component in the low-voltage regime, regardless of whether the device is doped
or not. We expect that the final analytical expressions presented herein to be
directly useful to experimentalists studying charge transport in novel
materials and devices.
