It is well established experimentally that the transient response of extrinsic photoconductors often consists of multiple components, with a slow component whose characteristic time can far exceed either the free carrier lifetime or dielectric relaxation time. However, various models that developed over a period of 30 years have failed to adequately explain the experimental results. We present a comprehensive model for the transient response to changes in photoexcitation of a low temperature extrinsic semiconductor of finite length. The model includes both drift and diffusion effects and, in contrast to earlier work, can be applied through a range of low to high fields, where both carrier diffusion and/or field-dependent sweep-out may occur. Changes in carrier concentration and electric field are calculated for a common operating regime where sweep-out is dominant and displayed in three-dimensional plots illustrating spatial and temporal variations. The slow transient response depends on generation rate, electric field and intercontact length but is independent of mobility and lifetime. Analytical models are presented for the limits where either carrier diffusion or carrier sweep-out is clearly dominant. The results offer insight into the role of contacts and space charge for transport in insulators and low temperature semiconductors.
Kinetics of physical aging in archetypic 45S5 bioactive silicate glass composition with different types of phase separation are studied in situ below the glass transition temperature (T g ). The qualitative nature of aging is found to be almost independent of the structural differences on the micrometer scale. A well-expressed step-like behavior in the enthalpy recovery kinetics is observed for aging temperatures T a ∼ 0.90T g and T a ∼ 0.85T g , which, however, disappears when the aging occurs at T a ∼ 0.95T g . The overall kinetics are described by a stretched-exponential function with stretching exponent close to 3/7 at T a ∼ 0.95T g , and 1/3 when the aging temperature drops to ∼0.90T g and below. The values correlate well with the predictions of Phillips' diffusion-to-traps and percolating fractals models. Appearance of step-like behavior at larger departure from T g is attributed to the hierarchical scheme of approaching equilibrium based on an alignment-shrinkage mechanism of physical aging proposed earlier for chalcogenide glasses.
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