This paper analyzes the small signal ac impedance of electron diffusion and recombination in a spatially restricted situation with application in systems such as porous TiO2 nanostructured photoelectrodes and intrinsically conducting polymers. It is shown that the diffusion−recombination model with the main types of boundary conditions assumes a finite set of possible behaviors in the frequency domain, which are classified according to relevant physical parameters. There are four possible cases:  (i) the impedance of finite diffusion with reflecting boundary, (ii) the impedance of finite diffusion with absorbing boundary, (iii) the impedance of diffusion-reaction in semiinfinite space or Gerischer impedance, and (iv) the impedance that combines Warburg response at high frequency and a reaction arc at low frequency. The generality of the approach is discussed in terms of the spatial distribution of the electrochemical potential or quasi-Fermi level and also in terms of the transmission line representation....

Theory of the Impedance of Electron Diffusion and Recombination in a Thin Layer

An inconsistency between commonly used values of the silicon intrinsic carrier concentration, the effective densities of states in the conduction and valence bands, and the silicon band gap is resolved by critically assessing the relevant literature. As a result of this assessment, experimentally based values for the valence‐band ‘‘densities‐of‐states’’ effective mass are determined in the 300–500 K range and are shown to be in good agreement with recent theoretical calculations. At 300 K, experimentally based values of 3.1×1019 cm−3 for the valence‐band effective densities of states and 1.08×1010 cm−3 for the intrinsic carrier concentration are determined. Although in good agreement with theoretical calculations, these are significantly higher and lower, respectively, than commonly used values in the past. These results have important implications in the calculation of other silicon material and device parameters.

Intrinsic concentration, effective densities of states, and effective mass in silicon

Classical and quantum mechanics of the damped harmonic oscillator

A new electrooptic effect in certain classes of nematic liquid crystals is presented. The effect has been termed "dynamic scattering" because scattering centers are produced in the transparent, anisotropic medium due to the disruptive effects of ions in transit. The ions can be produced by field assisted dissociation of neutral molecules and/or Schottky emission processes. The rise times of 1 to 5 ms and decay times of less than 30 ms, together with dc operating voltages in the 10 to 100 V range, make dynamic scattering seem attractive for such applications as alphanumeric indicators, and do not preclude its use in line-at-a-time matrix addressed, real-time displays. Reflective contrast ratios of better than 15 to 1 with efficiencies of 45 percent of the standard white have been demonstrated.

Dynamic scattering: A new electrooptic effect in certain classes of nematic liquid crystals

A method is presented whereby short minority carrier diffusion lengths in semiconductors may be determined by measuring the variation of surface photovoltage as a function of optical absorption coefficient. The method does not depend upon the specific form of the relationship between the surface photovoltage and the density of the excess minority carriers injected at the bulk edge of the surface space charge region. Only capacitive contacts to the sample are needed for the measurement. The method has been used to determine the minority carrier diffusion length in both n and p type gallium arsenide.

C. Kittel

Papers

Solid state physical electronics