Semiconductor Parameters Research Articles

Semiconductor Counters: I. Theory

A general theory is given of the modes of operation and performance limits of semiconductor conduction counters. The magnitude and shape of the current pulses in an external circuit are derived for a number of different sets of circuit and semiconductor parameters. The various factors limiting energy resolution are discussed in turn, the effects of thermal and current noise being compared with the linewidth due to incomplete charge collection. For radiation absorbed at random in the volume of a counter, such as occurs with gamma rays, incomplete charge collection is likely to be the principal limitation on resolution.

Dependence of the Free-Carrier Faraday Ellipticity in Semiconductors on Scattering Mechanisms

The theory of the Faraday ellipticity in semiconductors is developed, via the Boltzmann transport equation, under the assumption of an isotropic energy-dependent time of relaxation $\ensuremath{\tau}$. Equations relating ellipticity to semiconductor parameters are derived for various ranges of the collision, cyclotron, and applied frequencies. It is observed that, besides its dependence on the value of the scattering parameter, Faraday ellipticity is rather sensitive to the type of scattering mechanism as such, and to the distribution function. Some specific experiments are suggested in the ranges where ellipticity appears particularly promising as a tool for investigating these aspects of the scattering process. Numerical examples, calculated for thermal and ionized impurity scattering in nondegenerate carrier systems, are contrasted with the results of the constant-$\ensuremath{\tau}$ approximation, showing the inadequacy of the latter approach. Finally, the effect of spheroidal surfaces of constant energy on Faraday ellipticity is briefly discussed.

Low-Noise Parametric Amplifier

The theory of the parametric amplifier has been reformulated to permit the prediction of noise temperature and pump power in terms of the physical parameters of the nonlinear element and circuit. The results of this analysis are supported by careful experiments at S-band using a gold-bonded germanium semiconductor diode. They can be summarized as follows. 1) Performance of a parametric amplifier using semiconductor diodes with respect to noise temperature and pump power can be accurately predicted, once the diode and circuit parameters have been measured. 2) It is shown that the ultimate limitation on noise temperature depends simply on the product of the diode cutoff frequency and the normalized capacitance swing. 3) The derived figure of merit for the diode can serve as a basis for optimum design of the semiconductor parameters.

Silicon Junctions as Particle Spectrometers

Both Au-Si surface barriers and silicon p-n junctions operate satisfactorily as particle spectrometers. The use of high resistivity material gives a depletion region wide enough to stop high-energy protons, α particles, and heavier ions. A resolution of 16 kev (width at half-maximum) for 6.04 mev α particles is obtained with a junction depletion layer sufficient to stop 10 mev α particles. With the depletion region very close to the surface, ß particles in the range from 10 kev to several hundred kev are detectable. The pulse amplitude distribution from Co <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">57</sup> indicates the presence of the two well-known lines at 115 and 129 kev. An attempt has been made to correlate the observed performance with the semiconductor parameters and the diode static characteristics.

The use of semiconductor devices in p.a.m. system demodulation

The paper discusses the design and engineering details of two types of demodulator equipment which will operate from a p.a.m. system, and could be used, for example, in the field of electronic telephone exchanges. Apart from normal circuit-elements the equipments contain only semiconductor diodes and transistors. The equipments described will provide electrical characteristics conforming to different levels of performance such as might be required for local or trunk routing. They are both designed to operate on a 100-channel system and will provide gain of the signal information of about 40 dB.The effects of variations in device parameters and environmental conditions on electrical performance are considered in some detail, and the results achieved in practice are discussed in relation to the standards of performance likely to be required. Special attention has been paid to the stability of the semiconductor parameters with life, and the effect of this on the long-term performance of the circuits. A preliminary investigation to determine life trends of the type of transistor specified has been undertaken, and the results obtained are presented and discussed.

The Electrical Conductivity and Thermoelectric Power of Bismuth Telluride

The electrical conductivity and thermoelectric power of the semiconductor Bi2Te3 have been measured between 150°K and 300°K. n-type and p-type samples with a wide range of carrier concentration have been included. Most samples have shown extrinsic conduction and have been partially degenerate over at least part of the temperature range, so it has been necessary to use Fermi-Dirac statistics in interpreting the results. A few samples have exhibited mixed and intrinsic conduction at the higher temperatures. It has been possible to determine the variation of carrier mobility with temperature and to estimate the energy dependence of the relaxation time, as well as a number of the semiconductor parameters.

Noise, Time-Constant, and Hall Studies on Lead Sulfide Photoconductive Films

Measurements of the noise in seven lead sulfide photoconductive films over the frequency range 20-16 000 cps show that the noise consists mainly of a $\frac{1}{f}$ component at frequencies below 100 cps, a generation-recombination component between 100 and 10 000 cps and a Nyquist component at higher frequencies. The data are analyzed to give the magnitude, ${N}_{s0}(\mathrm{G}\ensuremath{-}\mathrm{R})$, and the time constant, ${\ensuremath{\tau}}_{n}$, of the generation-recombination noise. The signal time constant, ${\ensuremath{\tau}}_{s}$, is found by measuring the frequency dependence of the photoconductive response. The product of the majority carrier density and the film thickness, $\mathrm{pd}$, is found from measurement of the Hall voltage. Representative values are ${N}_{s0}(\mathrm{G}\ensuremath{-}\mathrm{R})=4.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ rms noise $\frac{\mathrm{volts}}{[(\mathrm{dc}\mathrm{bias}\mathrm{volt}\ifmmode\times\else\texttimes\fi{}{(\ensuremath{\Delta}f)}^{\frac{1}{2}}]}$, ${\ensuremath{\tau}}_{n}=230$ \ensuremath{\mu}sec, ${\ensuremath{\tau}}_{s}=280$ microseconds, and $pd=3.0\ifmmode\times\else\texttimes\fi{}{10}^{12}/{\mathrm{cm}}^{2}$.From ${\ensuremath{\tau}}_{s}$ and $\mathrm{pd}$, a theoretical value of ${N}_{s0}(\mathrm{G}\ensuremath{-}\mathrm{R})$ is calculated and found to be in good agreement with the experimental value. From this and the reasonable agreement between ${\ensuremath{\tau}}_{s}$ and ${\ensuremath{\tau}}_{n}$ it is concluded that these measurements furnish a verification of the theory of photoconductivity in semiconductor films recently published by one of us. Calculation shows that the generation-recombination noise is mainly due to lattice processes; less than 1% is due to radiation fluctuations. It is also shown that noise can be used in place of Hall measurements to evaluate certain semiconductor parameters.

Theory of Photoconductivity in Semiconductor Films

Photoconductive films of the lead salt family are composed of a system of crystallites separated by intercrystalline barriers. The crystallites are lead salts while the intercrystalline barriers are an oxide of lead or of the lead salt. Space charge regions are present at the surface of the crystallites. The lifetime of hole-electron pairs is determined in part by surface states while the resistivity is strongly affected by intercrystalline barriers.We analyze a model which incorporates the above characteristics of a photoconductive film, except for the space charge effects. It is assumed that the primary photoeffect is absorption of light and production of hole-electron pairs in the crystallites. The change in conductivity results from a change in majority carrier density in the crystallites, and from reduction of intercrystalline barrier potentials.Equations are developed for the response to radiation, for the noise, and for the limit of sensitivity of the detector. These expressions contain familiar semiconductor parameters, and a new parameter which characterizes the relative importance of the change in carrier density as compared to the change in barrier potential. No attempt is made to calculate the parameters, but measurements necessary for their evaluation are briefly discussed. This permits a prediction of numerical values for responsivity, noise, and sensitivity which can be compared with experiment.

Theoretical Considerations Governing the Choice of the Optimum Semiconductor for Photovoltaic Solar Energy Conversion

The theory of the photovoltaic effect is used to predict the characteristics of a semiconductor which would operate with an optimum efficiency as a photovoltaic solar energy converter. The existence of such an optimum material results from the interaction between the optical properties of the semiconductor which determine what fraction of the solar spectrum is utilized and its electrical properties which determine the maximum efficiency of conversion into electricity. Considerable attention is devoted to the effect of the forbidden energy gap (EG) of the semiconductor. It is shown that atmospheric absorption causes a shift in the solar spectrum which changes the value of the optimum forbidden energy gap between the limits 1.2 ev&amp;lt;EG &amp;lt;1.6 ev. Furthermore, plausible departures of the diode reverse saturation current (I0) from the parametric dependence predicted by Shockley are considered, and it is shown that such departures reduce the advantage of the optimum material over others in the range 1.1 ev&amp;lt;EG&amp;lt;2.0 ev. The relation between EG and the load impedance for maximum power transfer from the solar converter is discussed. Finally, I0 is computed from the published values of the semiconductor parameters of three intermetallic compounds, i.e., InP, GaAs, and CdTe, and it is shown that the efficiencies predicted for these materials are greater than those predicted for other materials which have been proposed, i.e., Si, CdS, Se, and AlSb.