Negative Temperature Coefficient of Resistance in Bismuth I.
Measurements of the electrical resistance have been made on bismuth 1 between 15 and 35 kilobars at temperatures between 77.4 degrees and 120 degrees K. Above about 150 degrees K, the temperature coefficient of resistance is positive, as in a metal; below 150 degrees K, the coefficient becomes negative, as is characteristic of semiconductors. On the basis that bismuth is a semiconductor, the energy gap, calculated by the exponential resistance formula, is 0.006 ev at 15 kb with a steady rise to 0.018 ev at 35 kb. At higher pressures, bismuth I is transformed into a metallic modification with the normal temperature dependence of the resistance. The energy gap in bismuth I is not visible at room temperature because thermal excitation populates the conduction band and metallic behavior is the result. From available evidence the observed behavior is due to an energy gap rather than to a decrease in carrier mobility.
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