The beam energy is chosen in such a way that the electron beam diameter is minimized and the beam is focused in the optimum way. Therefore the choice of this parameter depends on the used measurement equipment. Typical values are on the order of 3 to 5 keV.
From the simulations presented in the previous section it is obvious that there is an upper limit for the number of secondary electron-hole pairs determined by the potential difference across the junction becoming too small. The number of secondary electron-hole pairs is proportional to the product of the electron beam energy and electron beam current. With the electron beam energy determined by the optimum focus the beam current can be varied. The upper limit of the beam current is reached when the potential difference across the pn-junction becomes too small and the location where the second derivative of the AVC potential equals zero shifts too far from the metallurgical junction. The upper limit of the beam current shows a strong dependence on the doping concentrations and whether the lower doped side of the junction is n- or p-doped.
On the other hand the beam current cannot be made arbitrarily small. There have to be a sufficiently many secondary electrons of which the energy spectrum can be measured. When the number of emitted secondary electrons is too small, then the signal-to-noise ratio will not be sufficient for reliable measurements. A second factor for the lower limit of the beam current of course is determined by the effects of surface states and the surface dipole layer. The number and the type of surface states depends on the sample preparation. For fixed energy of the primary electrons the minimum beam current has to be large enough to saturate the the surface states [10].
For appropriately chosen beam currents and energies the AVC method can be used for pn-junction delineation without the need of inverse modeling. For the characterization of the net doping inverse modeling [16] is necessary for all beam currents.