Electrical behaviors of the nickel contacts on p+ and n+ channels under high current density were investigated. Silicide line was found to form in p+-Si channel and initiated from the cathode contact. On the other hand, no silicide line formation in n+-Si channel was observed. Network structures were observed in both Co and Ni samples. The depth of silicide formation was found to extend to the junction depth. The silicide lines were only observed in Ni and Cu/p+-Si samples, but not in Ti and Co samples. The diffusivities of metals at high temperature determine the line formation. A model of the silicide line formation is proposed. The relations between the silicide length and the contact size, the applied current and the method of the applied current are discussed. For Ni (or Co) contacts on p+-Si, the preferred failure at the negative contacts is attributed to the electron-hole recombination. For Ni (or Co) contacts on n+-Si, failure at the positive contacts is controlled by a wear-out mechanism due to electromigration-assisted Ni (or Co) diffusion away from the Si.