Standard Fowler-Nordheim (FN) theory, as derived from the work of Murphy and Good, involves the "simple JWKB approximation". This approximation provides that the tunnelling probability D for an electron approaching a tunnelling barrier described by a "quasi-classical potential energy" V(x) is approximated as D ≈ exp{-G}, where the "JWKB exponent" G is defined by G = g{sub}e ∫ V{sup}(1/2)(x) dx, where g{sub}e is an universal constant of value 10.24634 eV{sup}(-1/2) nm{sup}(-1) and the integral is taken over the region between the classical turning points, where V(x) is positive.
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