Table of contents

A number of methods has been devised in the past few years for measuring the lifetime of minority carriers in semiconductors. With germanium all these methods give results consistent with each other. When another semiconductor, namely silicon, is studied quite large differences in the results from different methods are found. These discrepancies are described and their origin discussed. It is concluded that an uncritical application of well-established germanium technology to any other semiconductor may lead to erroneous conclusions.

It is proposed that the constriction of the positive column into a wall-independent filament which sets in with increasing density comes about as a consequence of concentrational effects accompanying electron energy loss. As a first approximation this energy loss has been treated as a damping field acting on the electron motions only. In helium, using either M-B or Smit distributions, the constriction is predicted to lie in a range of densities in which it is indeed known to occur. A semi-empirical calculation of the critical values is made for other gases as well, with reasonable agreement with the limited data available.

Measurements have been made on the starting potential, current, and rate of growth of current of electrodeless discharges in hydrogen. The gas was contained in cylindrical glass or quartz vessels placed between plane parallel electrodes. The wavelength was varied from 50m to 6×10⁶m (frequency range 6 Mc/s to 50c/s), and the gas pressure from 1 to 76 mm Hg.

At short wavelengths the starting potential and current are low, and the discharge develops slowly. At a critical `cut off' wavelength the starting potential rises abruptly and then remains almost constant to the longest wavelengths. At wavelengths just greater than cut-off the current is large, grows rapidly and flows continuously: at all longer wavelengths a smaller current flows in a brief pulse which occurs near each peak of the applied field, and is of constant height and shape.

The growth of the pulses is thought to be due to ionization in the gas, and electron emission from the walls of the vessel by photons. A pulse ceases when wall and space charges sufficiently reduce the field in the gas.

Pulses occur so regularly that electrons must be left over from one pulse to enable the next to start. They are loosely bound on the `anode' wall at the end of one pulse and easily pulled off when the field reverses. This emission occurs in quite small fields (<1000 v cm^-1), and is distinct from normal field emission.

This idea has been tested using alternating square wave fields of very long period, and by removing the wall charges.

In view of earlier contradictory published results on the photoconductivity of crystalline calcium tungstate, this effect has been examined using artificial single crystals and an intense ultra-violet source. Photoconductivity has been demonstrated, with a space charge build up of long relaxation time. This latter is probably linked with the long duration phosphorescence previously reported in this material. The evidence produced indicates that calcium tungstate is an intrinsic semiconductor with an activation energy of 2 1 eV. The photoconductive properties are in approximate agreement with the assumption of an exponential trap distribution if a variation of capture cross section with temperature is postulated.

Two new simplified electrostatic systems for the correction of third-order spherical aberration in electron lenses are described and related to earlier systems of Seeliger and Burfoot. Numerical examples are given in which the voltages and positioning tolerances of the various electrodes are calculated The effects of scaling and change of electrode shape are discussed.

The application of Stockbarger's technique to the growth of PbF₂ crystals is described. The melting point of PbF₂ was found to be 822°C±2°C, and owing to its fairly high vapour pressure at this temperature, the crystals were grown in an atmosphere of oxygen-free nitrogen at a pressure of 2 to 10 mm of mercury. The crystal structure was found to be of the fluorite type, and the lattice constant 5.942±0.001 Å at 18°C. Refractive indices are given for various wavelengths in the visible spectrum. The transmission limits, defined as the wavelength at which a specimen of thickness 1 cm absorbs 50% of the incident radiation, are 11.6μ and 2800 Å.

A general solution for the diffraction of non-monochromatic waves by slits and gratings is derived from Kirchhoff's solution. The results of measurements agree well with calculations in the range where Kirchhoff's solutions can be accepted.