Showing papers in "Solid-state Electronics in 1967"

Ohmic contacts for GaAs devices

Abstract: Contact alloys were developed for use on a wide variety of GaAs devices such as high temperature transistors and Gunn oscillators. The alloys are composed of silver, indium and germanium for n -type GaAs and of silver, indium and zinc for p -type GaAs. Fabrication steps that require temperatures of up to 770°K for already contacted devices can be performed. GaAs transistors can be operated over a range from 20 to 770°K using Ag-In-Ge contacts for emitter and collector and Ag-In-Zn contacts for the base. Gunn oscillators have been built for the frequency range between 13 and 26 GHz with efficiencies as high as 3 percent at 15.8 GHz and as high as 1 percent at 25 GHz in continuous wave operation. A simple technique was developed to evaluate the specific contact resistance on thin epitaxial layers. Specific contact resistance is well below 10 −4 ω-cm 2 on 0.1 ω-cm or lower resistivity p - or n -type GaAs. The highest value was 1 × 10 −3 ω-cm 2 measured on 0.6–2.6 ω-cm n -type GaAs.

Effective mass and intrinsic concentration in silicon

Abstract: Experimental observations bearing on density-of-states effective masses and on the intrinsic concentration in silicon are reviewed and correlated. These indicate effective masses to be temperature and energy dependent. The valence band structure as determined by Kane is used to calculate the temperature and donor density dependence of hole effective mass. A first order approximation to the explicit temperature variation of both hole and electron effective masses is made using the measured temperature dependence of the energy gap. When these temperature-dependent effective masses are substituted into the theoretical expression for intrinsic concentration the agreement with reported measurements of ni is within the limits of error. Density-of-states effective masses at 300°K are found to be m e ∗ = 1·18 and m h ∗ = 0·81 in contrast to the generally used 4·2°K values of m e ∗ = 1·06 and m h ∗ = 0·59 .

Thermal conductivity of silicon, germanium, III–V compounds and III–V alloys

Abstract: The thermal conductivities as a function of temperature for silicon, germanium, gallium arsenide, indium phosphide, indium arsenide, indium antimonide, gallium phosphide, aluminum antimonide and gallium antimonide are presented. Also included are the thermal conductivities of the mixed III–V compounds: indium arsenide-phosphide, gallium-indium arsenide and gallium arsenide-phosphide. These data are derived from the publications listed in the bibliography and represent the author's selection of the “most probable” values. A brief phenomenological discussion of the mechanisms involved in thermal conduction is presented.

Metal-semiconductor contacts for GaAs bulk effect devices

Abstract: An alloy of Au, Ge and Ni has been shown to produce uniform linear electrical contacts to GaAs over a resistivity range useful for bulk effect devices. Batch fabrication of devices with contacts of controlled size and shape is readily accomplished by evaporating the contact material through masks onto the semiconductor surface. An eutectic alloy of Au and Ge also produces a linear contact but tends to produce nonuniform films. The preparation of the contact materials, surface treatment of the GaAs, and device fabrication procedures are described.

Calculation of avalanche breakdown voltages of silicon p-n junctions

Abstract: An empirical formula for the ionization coefficient given by α = CEg (C and g constants, E electric field) and which is considered as a common effective value for both holes and electrons yields tractable expressions for the breakdown voltages of abrupt and graded silicon pn junctions. Numerical values compare favourably with breakdown voltages obtained from expressions which, though derived from α values of a more theoretical lineage are considerably more involved and not so readily incorporated in comprehensive device design calculations.

Theoretical analysis of the series resistance of a solar cell

Abstract: An equivalent resistance circuit of a solar cell which takes into account all the sources of linear resistance in a solar cell is developed. Equations which will determine the resistance of the diffused layer of a solar cell are also developed assuming a constant gradient across the diffused layer. The equations for the total series resistance of an n dimensional solar cell are evolved where n represents the number of grids in the cell. The equations have been utilized in conjunction with experimentally determined values of the component resistances to predict the total series resistance of several cell types. Very good correlation was obtained between the experimentally measured total series resistance and the theoretically predicted total series resistance of these cells.

Tunneling in metal-oxide-silicon structures

Abstract: Measurements of d.c. and a.c. tunneling characteristics are performed on Si-SiO 2 -Cr/Au structures using p ++ , p , and n ++ type silicon and various SiO 2 layers less than 50 A thick. For p ++ type samples, strong influences by the silicon band structure and the interface-state densities are observed in the I-V characteristics. The tunneling currents are found to increase rapidly for an applied negative voltage on the metal exceeding 1.1 V corresponding to the silicon band gap. These currents decrease by about one to two orders of magnitude by changing from dry to steam-grown and hydrogen-annealed oxide layers of the samples. For n ++ type samples, no strong influence of the interface states is observed. Schottky emission characteristics are also observed on samples with relatively lower dopings and thinner oxide films such that the Fermi level of the metal is virtually pinned to the semiconductor surface. These results are qualitatively explained using simple energy band models including surface states at the semiconductor-oxide interface. From the admittance measurements, an equivalent circuit for MOS tunneling structures is constructed.

Stabilization of MOS devices

Abstract: It is shown that there are three basic instabilities present in MOS devices; ( a ) ion drift in the gate insulator, ( b ) temperature dependent deep trapping at the insulator-silicon interface and ( c ) generation of fast interface states under positive Bias Temperature (BT) stress. The sources of these instabilities and techniques suitable for eliminating them will be discussed.

Comparison of classical approximations to free carrier absorption in semiconductors

Abstract: Because of the recent interest in infrared techniques for the measurement of semiconducting material properties, a theoretical model of free carrier absorption is needed. This paper presents a comparison of some classical approximations to free carrier absorption using the minimum in reflectivity associated with plasma resonance as a criteria. Four approximations together with some numerical integrations are compared with old and new experimental data for n - and p -type silicon, n -type germanium and n -type gallium arsenide.

Influence of surface conditions on silicon planar transistor current gain

Abstract: A detailed and systematic experimental investigation of surface effects on hFE has been carried out in especially designed and fabricated transistors. A metal electrode over the oxide protecting the E-B junction has been incorporated and used to modify surface potentials in that region in a controlled manner. In diffused base transistors, surface changes affect the base current, IB, while no change in collector current IC is observed, leading to variations in hFE IC/IB. For base surface doping < 3 × 1018 atoms/cm3, a minimum hFE occurs when the base surface reaches a near intrinsic condition. hFE recovers partially or fully when the surface is inverted depending on the area of inversion and bulk lifetime near the surface. This type of variation in hFE is shown to be due to surface recombination dependence on surface potential and is significant only at low current levels of device operation. Surface recombination through a single level generation-recombination center has been analyzed using Shockley-Read-Hall statistics and compared with the experimental results and is shown that the main features can be explained satisfactorily with this simple analysis. When surface doping exceeds 4 × 1018 atoms/cm3, tunnel current conduction occurs upon surface inversion between the inversion layer and adjoining bulk. This causes severe degradation in hFE both at low and high current levels of operation even for small areas of inversion. When the emitter surface is depleted and inverted, hFE decreases monotonically. The tunnel current in the inversion region is shown to be a significant cause of such decrease in hFE. In lightly doped epitaxially grown base transistors, both IC and IB increase when the base surface is inverted and a case is presented where hFE actually improves during such change.

Current-voltage characteristics and capacitance of isotype heterojunctions

Abstract: Heterojunctions were made by alloying n-type Ge on n-type Si. A number of these junctions showed current saturation for both polarities of voltage. Current-voltage characteristics at various temperatures and capacitance dependence both on frequency and on bias voltage were investigated for the latter junctions. An analysis is given for the electrical behaviour of these junctions using the model of two Schottky diodes back-to-back as proposed by Oldham and Milnes. Interesting relations are found which can be used in general for the analysis of n-n or p-p heterojunctions (‘isotype heterojunctions’) with current saturation for both polarities of voltage. An anomalous behaviour of the capacitance follows directly from the analysis. Application of the analysis to the Ge-Si n-n junctions shows that the energy position of the Fermi level at the interface differs much for various junctions, indicating that the energy levels of the interface states are not uniquely defined by the pair of semiconductor materials. The values of the discontinuity in the conduction band ΔEc derived from different kinds of measurements are in good agreement. ΔEc has the constant value 0·15 ± 0·01 eV for all junctions independent of the concentrations of the n-type dopings of the starting materials and independent of the preparation procedure of the samples. This excludes the presence of a (variable) dipole at the junction interface. In the absence of a dipole layer ΔEc equals the difference in electron affinity between Ge and Si. The present work thus leads to a directly determined value of the affinity difference of 0·15 ± 0·01 eV. The results of the mathematical analysis may also be applied to evaluate the difference in barrier height which arises when two rectifying contacts of different metals are found on one piece of semiconductor material.

The preparation and C-V characteristics of SiSi3N4 and SiSiO2Si3N4 structures

Abstract: Amorphous silicon nitride films have been deposited on silicon substrates, with main faces of {111} and {100} orientations, at 800–1100°C using the nitridation of silane with ammonia. In many cases, the substrate surfaces were etched in situ with mixtures of hydrogen-hydrogen chloride, hydrogen-water, or hydrogen-oxygen prior to the deposition of silicon nitride. In several instances, epitaxial silicon of good structural perfection was deposited on silicon substrates before the deposition process. Irrespective of the substrate surface treatments and other deposition conditions, the charge distribution of AlSiSi 3 N 4 structures was found to be unstable under large voltage bias at room temperature. This charge instability appears to be due to tunneling and trapping of carriers in the vicinity of the silicon-silicon nitride interface, and is concluded to be a basic property of the SiSi 3 N 4 structure prepared by the chemical deposition technique. The use of a thin silicon dioxide interlayer between the silicon nitride and the silicon has been found to suppress the charge instability in MNS structures; MNOS structures are stable over wide ranges of electrical and thermal stress.

Tunneling in MIS structures—I. Theory

Abstract: Quantum mechanical tunneling through insulating barriers has received considerable attention in recent years, especially in metal-insulator-metal systems and p − n junctions. It is the purpose of this paper to discuss tunneling in the MIS contact. In this paper we define the d.c. currents which flow and then calculate the voltage distribution in the contact. It is found that as long as the semiconductor spacecharge region is of the order of the insulator thickness (∼ 50 A) considerable voltage drops across the semiconductor. Using this, a voltage region of low conductance is shown to exist, over which the metal Fermi level is opposite the forbidden gap of the semiconductor. This region is shown to be greater than the semiconductor energy gap. In addition, the a.c. frequency dependent currents which flow in an MIS contact due to interface states are calculated. It is shown that interface states can contribute to the a.c. conductance via two mechanisms; time lag in trapping and recombination of carriers in the semiconductor bands and tunneling via interface states. Both of these mechanisms are discussed and the conditions under which one or the other make the major contribution to the a.c. conductance is analyzed.

Epitaxial growth of aluminum nitride

Abstract: The epitaxial growth of aluminum nitride on the basal planes of hexagonal silicon carbide and aluminum nitride substrates has been studied using the ammonolysis of aluminum trichloride in a gas flow system. Adherent and continuous films were obtained under a wide range of experimental conditions, and the substrate temperature is the most influential factor determining the crystalline perfection of the deposit. In the temperature range 1200–1250°C, single crystal aluminum nitride films epitaxial with respect to the substrates, as thick as 25 μ, have been deposited reproducibly by using reactant mixtures with AlCl 3 /NH 3 molar ratios in the range of 5 × 10 −4 to 5 × 10 −3 . The use of mercury and hydrogen selenide as dopants has produced, respectively, p - and n -type aluminum nitride films.

X-ray measurement of elastic strain and lattice constant of diffused silicon

Abstract: The diffusion of boron or phosphorus into silicon causes a lattice contraction when these impurities are substitutionally present on lattice sites. We have measured this contraction in thin diffused layers using an X-ray double crystal spectrometer. Strains as small as 2 × 10−4 are readily detected. This corresponds to lattice constant shrinkage less than 1 × 10−3 A. We find lattice contraction coefficients β = 2.3 × 10−24cm3/atom for boron and β = 7.2 × 10−25cm3/atom for phosphorus. The former is considerably smaller than the presently accepted value for boron, and both values are close to those originally determined by Pearson and Bardeen.

Characteristics of junction field effect devices with small channel length-to-width ratios

Abstract: The characteristics of junction field effect devices with small channel length-to-width ratios are discussed For such devices, the junctions deviate significantly from the parallel planar junction case normally considered in the analysis of field effect devices The spreading of the junction depletion region beyond the ends of the gate junctions becomes important A model is developed for calculating the shape of the junction depletion region including circular gate geometries near the ends of the channel The model is essentially an extension of Shockley's original model to nonparallel gate junctions Numerical results for several channel length-to-width ratios show slight deviations from Shockley's model in the terminal I–V characteristics for small length-to-width ratios but reduce to the results of Shockley's model for large length-to-width ratios A significant feature of the model which differs from previous models is the absence of complete channel pinch-off in the current saturation region The effect of carrier velocity saturation near the drain end of the channel is also discussed For devices with small values of channel length-to-width ratio, carrier velocity saturation greatly reduces the saturation value of drain current The importance of velocity saturation is found to depend upon a single parameter which is a function of the material constants and device dimensions

Statistical considerations in MOSFET calculations

Abstract: The use of statistics in the calculation of the performance of MOS field-effect devices is considered. Since MOSFET's frequently operate with degenerate free-carrier concentrations at their surfaces, the proper formulation of the dependence of source-drain conductance in terms of Femri-Dirac statistics is discussed. Exact calculations are compared with results based on approximations that employ Maxwell-Boltzmann statistics. Computer solutions for both the accurate and the approximate statistical formulations are given. The results are interpreted in terms of present technologies for silicon MOS structures and deposited CdS thin-film transistors. Inequalities are derived which permit an evaluation of the accuracy of Maxwell-Boltzmann statistics for calculation of source-drain conductance for an unspecified semiconductor. It is shown that this approximate procedure suffices for practical devices.

Optical thickness measurement of SiO2Si3N4 films on silicon

Abstract: The thickness of SiO 2 and Si 3 N 4 films grown or deposited on silicon substrates is determined by an interference technique, using a spectrophotometer in the u.v. and visible wavelength range. The accuracy is estimated at ±50 A . The precision is ±0.3 percent. Empirically calibrated fringe charts are presented for SiO 2 and Si 3 N 4 avoiding errors due to dispersion and ‘net’ phase shift. The refractive index and the net phase shift for both dielectric materials is measured as a function of wavelength using the empirical calibration. The refractive index of the SiO 2 films was found to be about 2 percent higher than published values for fused silica. The refractive index of the Si 3 N 4 was 1.97 at λ = 546 mμ . It is shown that the thickness of double layers of nitride on oxide or oxide on nitride can be determined if the thickness of one layer is known. A color chart for Si 3 N 4 is presented.

Recombination in silicon p−π−n diodes

Abstract: An account is given of the recombination of excess carriers in silicon over seven orders of magnitude of injected carrier density, and the Shockley-Read theory verified throughout the same range. The technique used is to fill the base region of an n+−p−p+ diode with excess carriers by pulsing the device, and then observe the open circuit voltage transient across the structure. The lifetime is then inversely proportional to dV dt , and the carrier concentration is a simple function of the voltage. The observed decay curve is predicted theoretically and a value for the uncompensated impurity density obtained. The variation of lifetime with temperature is examined and the nature of the recombination centres discussed. This method, which has not previously been used on silicon, is compared with the junction recovery technique and is found to have many advantages.

The capacitance of p-n junctions☆

Abstract: The capacitance of a p - n junction is unlike the capacitance between parallel metal plates in that the charge carriers in the p - n junction are distributed throughout the device rather than fixed at well-separated distances. An analysis of both the displacement current and the two charge-carrier currents in the transition region leads to a rigorous solution for the differential capacitance of a p - n junction. It is shown that the capacitance of the p - n junction is given by the integral of the differential concentration of either holes or electrons, as hypothesized by Shockley. Analysis of the capacitance is obtained by numerical integration using the Automatic Taylor Series method. The results of this analysis are as follows. The p - n junction capacitance is nearly a square-root function of the voltage, in agreement with the Schottky theory. However, the capacitance measurements cannot be used to determine the junction barrier height.

Electron beam induced potential contrast on unbiased planar transistors

Abstract: Planar transistors have been examined in a scanning electron microscope and surface potential contrast, induced by the inspecting electron beam, has been observed on unbiased transistors. Both planar transistors with and without an oxide coating have been studied and the effect of beam voltage on contrast formation has been investigated. The technique may be useful for the inspection of integrated circuits before interconnection.

Theory of lateral transistors

Abstract: Theory of the lateral transistor current gain and frequency response is developed and is shown to compare favorably with measured values. The current gain, β0, varies greatly and ⨍T < 50 MHz with a 3 db/octave drop off; resulting in a flat gain available only in the low frequency range. A modified structure is discussed to improve β0 and ⨍T such that they will be comparable to normal planar transistors.

Drift phenomena in CdSe thin film FET's

Abstract: Cadmium selenide TFT's operated at fixed gate and drain bias show a slow decay of the drain current. The time dependence of this decay was found to be logarithmic. Below room temperature the rate of decrease of the number of charge carriers in the surface-channel depends only weakly on both temperature and gate bias. These results can be described with a model in which tunnelling of electrons from the channel to traps in the oxide forming the gate insulation is assumed. As time proceeds the traps are filled by this process to an ever-increasing depth. Somewhat above room temperature a strong drift of a different nature occurs. In contrast to the low temperature drift, which can be suppressed by suitable treatment, this type of drift could not be removed. It presents a serious drawback to practical applications of the CdSe TFT. In several respects the TFT's used in this investigation differ from the conventional type. Their construction is briefly discussed.

Hole-electron product of pn junctions

Abstract: Assertions that the hole-electron product may be larger than ni2 exp (qV/kT) under high injection conditions are found in the old and recent literature. This paper shows that the np product must always be ⩽ ni2 exp(qV/kT). Plots of numerical results for electrostatic potential, quasi-fermi levels and carrier concentration for an asymmetrical np junction under low and high injection conditions are given.

Synthesis of complex electronic functions by solid state bulk effects

Abstract: A new way of realizing complex electronic functions which is not based on an interconnected assembly of passive and active components is proposed. The factors determining the interaction of moving high field domains with inhomogeneities in the drift path are analysed. It is shown how complex electronic functions (both digital and analogue) may be derived from the transit of high field domains through suitably processed drift paths in semiconductors exhibiting departure from ohmic behaviour for hot electrons. The results of experiments illustrating various means of writing and reading the information from integrated circuits using domain originated functions are given. The application to optical scanning and visible display is indicated.

Delay of the stimulated emission in GaAs laser diodes near room temperature

Abstract: Specific experiments were conducted to investigate the validity of various models designed to explain the long time delay between the application of a current pulse and the generation of coherent oscillations in certain GaAs junction lasers near room temperature. A particular model is developed which leads to good quantitative agreement with a number of different experiments performed in the temperature range between 200 and 300°K. It is found that all facets of the delay can be explained by assuming that a moderate number of optical absorbing centers (> 10 16 cm -3 ) causes optical losses which are sufficient to prevent the diode from lasing until these centers are filled by electrons injected into the p -side of the junction. It is postulated that the relatively long filling time of these traps is caused by a coulomb barrier such as is displayed by double acceptors in II–VI compounds.

Single crystal InSb thin films by electron beam re-crystallization

Abstract: Single crystals in excess of 1 mm 2 in area have been grown in evaporated InSb thin films by electron beam microzone melting The electrical and galvanomagnetic properties have been studied as a function of temperature between 300 and 90°K In 3·6 μ films electron mobilities of 61,000 cm 2 /V-sec at room temperature rising to 71,000 cm 2 /V-sec at 85°K have been measured Both the values and temperature dependence of mobility are comparable with those reported for single crystal bulk material of similar net donor concentration of 4 × 10 16 / cm 3 This is in contrast with the anomalous mobility temperature dependence previously reported for thin films of InSb A thickness dependence of mobility has been found in the range 1 to 5·8 μ; however magneto-resistance measurements suggest that the lower mobilities in the thinner films could be due to the inhomogeneity of these samples

Space charge and oscillation effects in gold-doped silicon p-‘i’-n diodes☆

Abstract: Properties of Au-doped Si p-‘i’-n diodes in the region of the I–V characteristics preceding the usual double injection negative resistance have been examined. At lowest applied voltages the I–V characteristic obeys Ohm's law. With the voltage increased to the point where the time required for electrons to cross the ‘i’ region becomes comparable to the electron lifetime, the I–V characteristic in many cases deviates from Ohm's law to a cubic dependence of the current on the applied voltage. This region is described by a modification of a known theory of electron-hole space-charge-limited emission. As the applied voltage is increased further, a point is reached in the I–V characteristic where the current both deviates from a cubic dependence on applied voltage and develops an oscillatory component which can vary in frequency from about 5 to greater than 100 MHz. These oscillations occur in a positive resistance region of the I–V characteristic.

Solubility and diffusion coefficient of sodium and potassium in silicon

Abstract: For introducing alkali metals—sodium and potassium—into silicon two techniques were used: diffusion from a very thin layer of alkali metal into the silicon surface, and introduction of sodium or potassium into silicon crystals during the electrolysis of molten alkali halogenides. The temperature dependence of the solubility of both elements was followed up to 1200°C with sodium and up to 1100°C with potassium. The content was determined by means of flame photometry so as to include into the determination that portion, too, which might have been precipitated during cooling of the sample. The solubility of sodium in silicon attains a maximum of 9 × 1018 cm−3 and that of potassium 7 × 1018 cm−3 in the temperature range 1100–1200°C. The temperature dependence of the diffusion coefficients of sodium and potassium in silicon has been determined. The general relations are D=1 ⋅65X10 3 exp[–16,700/RT] for sodium and D=1 ⋅1X10 3 exp[–17,500/RT] for potassium.