The transmission probability can be derived from Schrödinger's equation

For a rectangular potential barrier with width *d* and height *E*_{0} it
is (see Appendix C.3 for a derivation)

For a typical strength parameter
of 30 the
transmission probability is shown in Fig. 2.4.
The strength parameter allows a classification of tunnel barriers according
to their opaqueness.

A strength parameter of 1 is typical of the barriers present in most ohmic contacts to semiconductors, a value of 5 is typical for the ionization of an atom by an electric field and a value of 30 is characteristic for the -decay from a radioactive nucleus [47].