For an electron approaching a given barrier from the inside, the probability of escape (or " transmission coefficient " or "penetration coefficient") is a function of h and F, and is denoted by D (h, F). The primary aim of tunneling theory is to calculate D (h, F). For physically realistic barrier models, such as the Schottky–Nordheim barrier, the Schrödinger equation cannot be solved exactly in any simple way. The following so-called "semi-classical" approach can be used. A parameter G (h, F) can be defined by the JWKB (Jeffreys-Wentzel-Kramers-Brillouin) integral: