Hole nonequilibrium fraction

The free-volume concept dates back to the Clausius [1880] equation of state. The need for postulating the presence of occupied and free space in a material has been imposed by the fluid behavior. Only recently has positron annihilation lifetime spectroscopy (PALS see Chapters 10 to 12) provided direct evidence of free-volume presence. Chapter 6 traces the evolution of equations of state up to derivation of the configurational hole-cell theory [Simha and Somcynsky, 1969 Somcynsky and Simha, 1971], in which the lattice hole fraction, h, a measure of the free-volume content, is given explicitly. Extracted from the pressure-volume-temperature PVT) data, the dependence, h = h T, P), has been used successfully for the interpretation of a plethora of physical phenomena under thermodynamic equilibria as well as in nonequilibrium dynamic systems. 

Increasing the minority carrier lifetime is very important in photonic devices. In a heavily doped material, the majority carriers far out number the minority carriers under equilibrium conditions because of the law of mass action that requires the np product to be constant. However, absorption of photons produces equal numbers of electron-hole pairs. Under this nonequilibrium situation, the majority carriers are increased by only a small fraction whereas, the minority carriers are increased by a very large fraction. Since there are a very large number of majority carriers to annihilate them as the system returns to equilibrium, it is necessary for the minority carrier lifetime to be long enough for them to reach an electrode in order to be counted. By using a nipi structure to capture and separate the electrons and holes, highly efficient photoconductive devices can be fabricated. 

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