Function File: mu = osc_mobility_poolefrenkel (u0, de0, Teff, beta, gamma, vmax)

Poole-Frenkel mobility model:

mupf = u0 * exp (- (de0/(K_b *Teff)) + sqrt(abs(E)) * ((beta/(Kb T_eff))- gamma)

mulim = vmax/abs(E)

mu = 1/((1/mupf)+(1/mulim))