Influence of the dopant density profile on minority-carrier current in shallow,heavily doped emitters of silicon bipolar devices

Experimental determination of the dependence of recombination current in p⁺ and n⁺ regions on the dopant profile for shallow emitters of ion-implanted silicon solar cells is described. The results are analyzed by extending a previous analytical model for the transport of minority carriers in heavily doped regions. The extension accounts for an effective electric field, defined by heavy-doping effects at the surface, and suggests that the energy-gap narrowing for p⁺ silicon is slightly smaller than that for n⁺ silicon and/or that minority-carrier diffusivities are substantially lower than the majority-carrier ones at comparable dopant densities. The very high dopant densities achieved with the ion implantation/laser annealing technique provide an in situ surface passivation that supresses surface recombination and minimizes the emitter recombination current.