  We have investigated the electrical conduction processes in as-grown and
thermally cycled ZnO single crystal as well as as-grown ZnO polycrystalline
films over the wide temperature range 20--500 K. In the case of ZnO single
crystal between 110 and 500 K, two types of thermal activation conduction
processes are observed. This is explained in terms of the existence of both
shallow donors and intermediately deep donors which are consecutively excited
to the conduction band as the temperature increases. By measuring the
resistivity $\rho(T)$ of a given single crystal after repeated thermal cycling
in vacuum, we demonstrate that oxygen vacancies play an important role in
governing the shallow donor concentrations but leave the activation energy
($\simeq27\pm$2 meV) largely intact. In the case of polycrystalline films, two
types of thermal activation conduction processes are also observed between
$\sim$150 and 500 K. Below $\sim$150 K, we found an additional conduction
process due to the nearest-neighbor-hopping conduction mechanism which takes
place in the shallow impurity band. As the temperatures further decreases below
$\sim$80 K, a crossover to the Mott variable-range-hopping conduction process
is observed. Taken together with our previous measurements on $\rho (T)$ of ZnO
polycrystalline films in the temperature range 2--100 K [Y. L. Huang {\it et
al.}, J. Appl. Phys. \textbf{107}, 063715 (2010)], this work establishes a
quite complete picture of the overall electrical conduction mechanisms in the
ZnO material from liquid-helium temperatures up to 500 K.
