where
e is electron charge,
IS1,
IS2 are pre-exponential factors,
E1,
E2 are the characteristic energies for low and medium bias levels respectively. In general, for a p-n junction diode, the
I-
V characteristics can be expressed by
I =
ISexp(
eV/
nKT–1), where
n is ideality factor,
K is Boltzmann constant and
T is temperature. When the value of
n lies between 1 and 2, the current flow is mainly diffusion recombination current; while if
n is larger than 2, tunneling mechanism becomes dominant. If we assume the characteristic energies
E1 =
n1KT for low bias and
E2 =
n2KT for medium bias regions, then these energies represent the tunneling transparency of the energy barrier at the junction interface [
8]. So, it is clear that the characteristic energies are directly proportional to both ideality factor (
n) and temperature (
T). Since
n is increasing with lowering of temperature it indicates that the effect of tunneling is becoming more important. But the characteristic energy being a function of both
n and
T, is only weakly dependent on temperature, as
n decreases with increase of
T and vice versa. This weak dependences of
E1 and
E2 on temperature indicate that the dominant transport mechanism is associated with tunneling of carriers rather than thermal diffusion. Similar conclusion for different devices has been arrived at by other workers [
10] also. The values of the characteristic energies
E1,
E2 and ideality factors
n1,
n2 as obtained from our data are listed in Table 1.