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The interactions between electrons and holes are known to alter the energy levels in semiconductors. At high carrier densities, these interactions produce extended states that can be described by a carrier-induced band gap narrowing. Below the Mott density, this description is no longer valid and electron-hole interactions produce localized excitonic states. Excitons in Si have been thoroughly studied at low temperatures but they are usually ignored in semiconductor device operation. We have performed an analysis of the thermodynamics of excitons in Si below the Mott transition and find that the presence of excitons is expected to be significant at certain carrier densities, especially at 77 K. The electrical properties of semiconductors containing excitons are described and contrasted with the situation above the Mott transition where the conventional rigid band gap narrowing of extended states is valid. There are two key results. First, excitons mimic a rigid band gap narrowing in that they lead to an increase in the carrier density at a given voltage level. This occurs because as electrons occupy the excitonic states, the total electron density increases without increasing the density of electrons in the extended conduction band states. Second, excitons affect device transport and the result is different from the rigid band gap narrowing case. Since excitons can be mobile, they can contribute to diffusion. Because they are neutral, however, they cannot contribute to drift currents. In the extreme limit that all the carriers exist as excitons, there will be a finite ambipolar diffusion constant, but the conductivity mobility will drop to zero. Such an outcome is not possible within the framework of conventional device modeling. The necessary modifications are discussed.

Very high efficiencies, from 25 % up to 28 %, have been demonstrated under concentration with silicon solar cells having interdigitated contacts on the backside. However, only laboratory cells of small dimension have reached very high efficiencies. We present, in this paper, an analysis of the series resistance of different metallization schemes. The need for developping a multilevel metallization technology for backside contact concentrator solar cells of large area is demonstrated. We propose also a new design for the metallization of backside contact cells that presents a series resistance independent of the cell size. The particular features required for such a multilevel interconnection are studied and a process using anodic oxidation of aluminum is presented. Backside contact silicon solar cells of 0.64 cm2 have been processed in this technology resulting in 26.2 % efficiencies at 10 W/cmZ (100 suns AM1.5, 25.5"C). Subsequent runs with a simplified process and a new cell design have given 27.3 % efficiency cells. These results are the highest efficiencies reported to date for solar cells of this area. The cells have been soldered on alumina mounts and results of thermal cycling are given.

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The effect of electron-hole scattering on transport in semiconductors has been considered from first principles. We conclude that the conventional equations for electron and hole currents are theoretically incorrect when electron-hole scattering is present. From thermodynamic considerations, we introduce the more general equations. A Boltzmann transport calculation including electron-hole scattering has been performed for Si. The key result is that the impact of electron-hole scattering depends primarily on the relative velocities of electrons and holes and therefore shows up differently in different device situations. Electron-hole scattering has the largest effect during conduction in high-level injection. This is because electrons and holes have net drift velocities in opposite directions. In contrast, electron-hole collisions have almost no effect on ambipolar diffusion because the carriers are moving in the same direction. Intermediate cases are also covered by this treatment. In low injection, electron-hole scattering serves to reduce the effective minority-carrier mobility. The degree of the reduction depends on whether the majority carriers are nearly static, as in diffusion situations, or whether they are flowing by drift, as in the Haynes–Shockley experiment.