The ideality factor of a diode is a measure of how closely the diode follows the ideal diode equation. The derivation of the simple diode equation uses certain assumptions about the cell. In practice, there are second order effects so that the diode does not follow the simple diode equation and the ideality factor provides a way of describing them.
The ideal diode equation assumes that all the recombination occurs via band to band or recombination via traps in the bulk areas from the device (i.e. not in the junction). Using that assumption the derivation produces the ideal diode equation below and the ideality factor, n, is equal to one.
However recombination does occur in other ways and in other areas of the device. These recombinations produce ideality factors that deviate from the ideal. Deriving the ideal diode equation by considering the number of carriers the need to come together during the process produces the results in the table below.
|SRH, band to band (low level injection)
|Recombination limited by minority carrier.
|SRH, band to band (high level injection)
|Recombination limited by both carrier types.
|Two majority and one minority carriers required for recombination.
|Depletion region (junction)
|two carriers limit recombination.