The maximum electron current per unit area provided by an emitter is given by (where J = current density, T = absolute temperature in K, ɸ = work function in keV, k = Boltzmann’s constant & A=emission constant)

• A. J = AT exp(-ɸ/kT)
• B. J = AT (-ɸ/kT)
• C. J = AT2 exp(-ɸ/kT)
• D. J = exp(-Aɸ/kT2)