In this thesis the theoretical and experimental concentration and temperature dependent band gap narrowing in uncompensated n-type silicon is studied. Electron-electron and electron-impurity interaction energies are used to calculate the theoretical band gap narrowing in the plasmon-pole approximation. These reveal an increase of 14 meV in the band gap narrowing at 300 K for a donor concentration of 3.10(^19) cm(^-3) above the zero temperature value of 95 meV. For higher concentrations the degeneracy deepens and the zero and finite temperature band gap narrowing curves converge. Localized states in the band gap resulting from local fluctuations in the electron-impurity interaction, a result of the random position of the impurities, are also considered. When the analysis includes the effect on the host band of the electron-impurity interactions calculated above the resulting density of states in the band tail of uncompensated silicon is found to be ten times smaller than is usually imagined. Using published values for the minority carrier mobility both the band gap narrowing and the minority carrier lifetime are experimentally determined in the buried n-type layer of an Integrated Injection Logic transistor. The transport factor in the base of a parasitic pnp transistor formed by the p-type substrate, buried layer and p-type Integrated Injection Logic transistors base region is calculated by monitoring the substrate current density and minority carrier injection into the buried layer. A range of temperatures from 200 K to 400 K are used to determine the temperature dependence of the minority carrier mobility in the buried layer (T(^0)). A band gap narrowing of (100 ± 15) meV) and minority carrier lifetime of (30 ± 10) ns are measured for the buried layer (2.4.10(^19) cm(^-3)).