Showing papers in "Solid-state Electronics in 1975"

A review of the theory and technology for ohmic contacts to group III–V compound semiconductors

Abstract: The technology for ohmic contacts to group III–V compound semiconductors is reviewed in this paper. The basic principles of current transport in metal-semiconductor (Schottky barrier) contacts are presented first. The modes of current transport considered are thermionic emission over the barrier, and tunneling through the barrier due to thermionic-field or field emission. Special attention is devoted to the parameters of temperature and doping concentration which determine the dominant mode of conduction. As the primary mode of conduction changes from thermionic emission dominated to tunneling dominated, the current-voltage behavior of the contact changes from rectifying to ohmic in character. The experimental techniques for fabricating ohmic contacts to III–V compound semiconductors are then described. Contact problems as they pertain to specific device applications are considered. Finally, present difficulties with contacts to mixed III–V crystals are discussed.

Threshold energy effect on avalanche breakdown voltage in semiconductor junctions

Abstract: The band bending for avalanche breakdown in semiconductor junctions and its temperature dependence are predicted taking account of threshold energy effects on the ionization process in semiconductors. Where experimental results exist, the theoretical predictions and experimental results are in excellent agreement. In the high electric field region inclusion of both bulk and boundary threshold energy effects is essential. The predictions were based on exact solutions in the nonlocalized ionization coefficient formulation developed by Okuto and Crowell who showed that ionization coefficients as usually understood are functions of both electric field and position in a device. Predictions for abrupt and p - i - n junctions in Ge, Si, GaAs and GaP are presented.

Thermal resistance of heat sinks with temperature-dependent conductivity

Abstract: Kirchhoff's transformation is summarised in a form appropriate for semiconductor-device heat sinks and then illustrated with a brief application to the thermal resistance of a GaAs laser. Under the most common semiconductor boundary conditions his transformation immediately converts the steady-state linear temperature rise (based on a temperature-independent conductivity σ 0 ) of a uniform heat sink of any shape into the nonlinear rise [based on a temperature-dependent conductivity σ ( T ) of any functional form].

Metallurgical and electrical properties of alloyed Ni/AuGe films on n-type GaAs

Abstract: An experimental study of the alloying characteristics of a composite thin-film structure which is often used as an ohmic contact to GaAs is presented. A AuGe layer of eutectic composition covered by a thin-film of Ni and deposited on n-type epitaxial GaAs is investigated in order to better understand the relationship between the alloying behavior and the electrical properties of the contact. The barrier energy ϕBn and the specific contact resistance of the Ni/AuGe/GaAs system is measured for a wide range of alloy temperatures and times. The metallurgical properties of the Ni/AuGe/GaAs system are obtained with Auger electron spectroscopy and scanning electron microscopy. Auger spectroscopy combined with in situ sputter etching is used to determine depth-composition profiles for all constituents of both as-deposited and alloyed Ni/AuGe/GaAs contacts. In samples heat-treated below the AuGe eutectic temperature, Ni is found to move rapidly through the intervening AuGe layer to collect at the GaAs interface, and the effective value of ϕBn rises to the value characteristic of Ni/GaAs Schottky diodes. For heat-treatment above the AuGe eutectic temperature, ohmic contact behavior is observed, and uniform alloyed contact surfaces are found to result from the presence of Ni at the GaAs interface. Ga outdiffusion and surface accumulation resulting from GaAs dissociation occurs for all alloy temperatures and times. The Ga outdiffusion appears to be characterized by a very low activation energy.

The effect of local non-uniformities on thermal switching and high field behaviour of structures with chalcogenide glasses

Abstract: A refinement of the theory of thermal switching is presented which takes into account local defects (pre-existing or built-in during the forming process). The contribution of these local defects in reducing the filament temperature is calculated. When high-field non-ohmic effects are also included in the theory, some switching phenomena can be explained, which were not considered as thermal, and in which the average dissipation is no longer a significant factor. It is also shown that the presence of local defects may have significantly altered the available information concerning very steep high field behaviours in chalcogenide glasses.

Influence of the floating substrate potential on the characteristics of ESFI MOS transistors

Abstract: A model is proposed to explain the anomalous current-voltage characteristics of ESFI MOS transistors. Due to the floating state the substrate potential of the ESFI transistor is increasing with increasing majority carrier current flowing through the substrate to source. In the region of multiplication by avalanche that effect will get quite pronounced. The change of substrate potential yields a change of the threshold voltage hereby increasing the drain current and resulting a bend in the I D (U D ) curves. The assumptions of the model have been justified by additional experiments as with illumination of light or increased temperatures. Based on the physical model a computer program was developed to simulate the I D (U D ) characteristics of ESFI MOS transistors of the enhancement type, resulting good agreement between measured and simulated characteristics.

A simplified model for graded-gap heterojunctions

Abstract: A simplified method for calculating the energy band profiles of graded-gap heterojunctions, based on the generalized model of Oldham and Milnes, is presented. The profiles are derived by superposing an energy band grading function and the electrostatic potential in the heterojunction. The latter is obtained by using the depletion layer approximation as for conventional p-n homojunctions. The energy band profiles of hypothetical p(GaAs)-n(Al0·4Ga0·6As) heterojunctions are calculated using the simplified method. For small grading layer widths, the results are in good agreement with the generalized model. The barrier lowering factor η as a function of the graded layer width l is calculated for such heterojunctions. It is found, for acceptor and donor densities of 1018 and 1016 cm−3 respectively, that the barrier height is reduced from 0·47 eV to zero as l increases from zero (abrupt case) to ≈300 A. The applications of these analyses to practical heterojunctions are discussed.

Determination of the semiconductor doping profile right up to its surface using the MIS capacitor

Abstract: A new method has been developed by which the doping profile of the semiconductor can be obtained right up to its surface. Contrary to the well known dC/dV-method, which is valid only in the depletion range, we have considered the majority carriers in deriving relations for doping density as well as distance from the semiconductor surface, in terms of the space charge capacitance and its derivative with respect to the surface potential. This method can be realized experimentally on any MIS structure. Measurements on MOS capacitors with steam grown oxides showed a drooping profile towards the SiSiO2 interface for boron doped and a rising one for phosphorus doped silicon. Post-oxidation annealing in dry nitrogen at 1200°C produced a uniform profile for phosphorus doped silicon.

Frequency dependence of the photo-EMF of strongly inverted Ge and Si MIS structures—I. Theory

Abstract: The dependence of the real and imaginary photo-EMF components on frequency of the incident light modulation has been calculated for Ge and Si MIS structure in the inversion regime at the interface. It is shown that physical parameters of a semiconductor, such as doping levels in the bulk and near the interface, the rate of surface recombination proceeding at the backside of the sample, diffusion coefficient of minority carriers and light absorption coefficient can be derived from the experimental results.

Determination of the density and the relaxation time of silicon-metal interfacial states

Abstract: In this article a method is described for the determination of the density and the relaxation time of the Si-metal interfacial states from measurements of the admittance of a forward biased Schottky diode.

Electrical characteristics and threshold switching in amorphous semiconductors

Abstract: The relative role of thermal and electronic effects on the threshold switching mechanism in the amorphous semiconductor Ge 17 Te 79 Sb 2 S 2 , has been investigated, using virgin sandwich type devices. Evidence is presented in support of a critical field bulk switching mechanism, in which secondary heating effects play a role only because of the variation of the critical switching field with temperature. Switching delay time measurements are interpreted in terms of adiabatic and steady state heating, for times shorter and longer, respectively, than the thermal time constant of the device. An isothermal switching field is calculated by correcting the measured switching field for the estimated change in temperature of the device prior to switching. Switching voltage measurements were made with pulse widths from 1 nsec. to d.c. In this range the isothermal switching field is independent of the pulse width. The d.c. switching field is shown to be a function of the thermal constraints of the device structure. In contrast, the isothermal switching field is a fundamental material parameter. It is established, from short pulse (1·5 nsec) measurements of the current voltage characteristic that the critical switching field marks the onset of a previously unreported high field conduction region. This characteristic, in the vicinity of the switching field, is similar to that of a reverse biased diode in the vicinity of its breakdown voltage.

Die rekombination in thyristoren und gleichrichtern aus silizium: Ihr einfluss auf die durchlasskennlinie und das freiwerdezeitverhalten

Abstract: The influence of the recombination properties of the different silicon layers in power rectifiers and thyristors on the forward characteristic and the dynamic behaviour is discussed quantitatively, taking into account Auger-recombination. This leads to more exact dimensioning guidelines for the development of technical devices. Measurement methods that permit an independent determination of the recombination properties of the individual silicon layers are discussed and measurement results are given. For the recombination parameters h of the highly doped boundary regions we obtained values between 1 · 10 −14 and 6 · 10 −14 cm 4 sec −1 fairly uniformly, independent of the method of preparation (alloyed or diffused). This is in accord with h -values that were found with differently prepared emitters of transistors for communication purposes. The possibilities to improve the specification combination of power devices that could be expected as a result of a reduction of these recombination parameters h are quantitatively discussed.

A theoretical and experimental study of recombination in silicon p−n junctions

Abstract: This paper describes the results of a numerical solution of the one-dimensional transport equations as applied to p−n junctions. Generation-recombination is included in the model, and the method of solution is based on that of De Mari. The modifications of Choo are included to allow the solution of problems involving heavy recombination. Theoretical current-voltage characteristics are presented for both uniform and non-uniform distributions of recombination centres, and a comparison is made with experimental results on carbon implanted, silicon, p−n diodes. The dependence of the I V characteristics on activation energy, capture cross sections, and density of recombination centres is described. Good agreement between theory and experiment is obtained, and the results suggest that the I V characteristics can be described by the sum of two independant components of current. The first component is the generation-recombination current with an exp (eV/mkT) dependence, and the second is the diffusion current with an exp (eV/kT) dependance.

Planar InSb photodiodes fabricated by Be and Mg ion implantation

Abstract: Planar p−n junction photodiodes in InSb have been made by ion implantation of Be and Mg acceptors. Both ions were incorporated with a doping efficiency of about 50 per cent and each produced excellent photodiodes. At 77°K, 20-mil-dia. diodes have typical zero-bias resistances of 6 Mμ. At the wavelength peak of 5·3 μm, quantum efficiencies of 60–70 per cent and detectivities with a 77°K background of 2 × 10 12 cm Hz W were measured. Field plate guard rings were used to adjust the surface potential at the diode perimeters for optimum performance.

Determination of Si-metal work function differences by MOS capacitance technique

Abstract: Very accurate and reliable values of the difference between silicon electron affinity, χSi, and the metal work function, ΦM, have been obtained for seven different metals by the MOS flat-band capacitance technique. Different MOS structures with the same interface charge density and the same metal but varying oxide thickness were manufactured on the same wafer by employing a small temperature gradient during steam oxidation in an r.f. induction heated vertical furnace. The flat-band voltage, VFB, vs oxide thickness, tox, graphs obtained are very good straight lines. Use of both p- and n-Si in case of the metals Au and Cr produced two values of the silicon bandgap EG. The value of EG obtained in case of Au differed from the established value of 1·11 eV at room temperature by −0·04 eV only, and by 0·07 eV in case of Cr. This gives an indication of the experimental accuracy of the ΦHS = ΦM − χSi values. Φ MS ∗ obtained by this technique is 0·73 eV for Ag. −0·11 eV for Al, 0·82 eV for Au, −0·06 eV for Cr, 0·63 eV for Cu, −1·05 eV for Mg, and −0·82 eV for Sn. The total inaccuracy is limited to −0.06 +0.03 eV .

Avalanche injection into the oxide in silicon gate-controlled devices—I theory

Abstract: The avalanche injection into the oxide region of silicon gate-controlled devices is analysed in some detail, in terms of the physical theory of hot electrons in silicon as developed by Bartelink, Moll and Meyer. Numerically computed universal plots are given for the calculation of the hot-carrier injection ratio (avalanche-induced gate current over junction current) at given maximum interface electric field at breakdown. Features such as barrier reflexion due to transverse momentum of electrons, non-normal orientation of the electric field and Schottky barrier lowering have been incorporated in the analysis. Values of the hot-carrier injection ratio in positive-gate biased p+−n diodes (electron injection) calculated in this theory range between 4·3 × 10−4 and 4·7 × 10−3 for maximum interface electric fields at breakdown between 1·0 × 106 V/cm and 1·4 × 106 V/cm, if the fitting parameters of the above quoted physical theory are used.

Semiconductor profiling using an optical probe

Abstract: The use of a small spot of light for the detection of semiconductor inhomogeneities, is described. The method employed involves a scanned measurement of the surface photovoltage generated in the surface space charge region of the semiconductor. It is shown that scanned surface photovoltage signals may be used to detect defects in Si using either MOS, Schottky barrier or electrolyte contacts to sense the signal. Furthermore, from a spectral measurement it is possible to monitor the variation of carrier lifetime across the area of the sample. It has proven possible using the technique to detect both gross defects in the Si as well as to measure, on a point by point basis, the reduction in carrier lifetime due to them.

Boron diffusion in polycrystalline silicon layers

Abstract: Boron was diffused into the poly Si layers of 5·5 μm thickness with boron nitride and diborane as boron sources. The diffusion depth was found to be proportional to the square root of diffusion time. The boron diffusion in the poly Si layers can be expressed in the well known complementary error function. The diffusion coefficient of boron in the poly Si layers is larger when diborane is used than when boron nitride is used as a boron source and is 10–50 times larger than that in the single crystal silicon substrates in the experimental range. The diffusion coefficients of boron at 1050°C in the single crystal silicon substrates with diborane as a boron source in the poly Si layers with boron nitride and diborane as boron sources are 8·80 × 10 −14 , 1·17 × 10 −12 and 1·95 × 10 −12 cm 2 /sec, respectively. The activation energies of the diffusion coefficients of boron in the above each case are 3·42, 2·39 and 2·51 eV, respectively.

Boron and phosphorus diffusion through an SiO2 layer from a doped polycrystalline Si source under various drive-in ambients

Abstract: The diffusion of boron and phosphorus into thermally grown SiO2 from a doped polycrystalline Si source has been found to be dependent on drive-in ambients and to be especially fast in an H2 ambient. Diffusion coefficients have been calculated by the two boundary diffusion model. It was found that the diffusion coefficient of boron in SiO2 is about two orders of magnitude larger than that of phosphorus. This fact is very important for Si gate processes. For P-channel Si gate transistors, boron diffused in the gate SiO2 film from the doped polycrystalline Si gate electrode causes new types of instabilities in transistor characteristics.

Measurements on depletion-mode field effect transistors and buried channel MOS capacitors for the characterization of bulk transfer charge-coupled devices

Abstract: The properties of bulk transfer charge-coupled devices (BCCD's) may be characterized from measurements obtained using MOS capacitors and field effect transistors. Models are presented for the MOS capacitor and field effect transistor for the case where a shallow doped layer of polarity opposite to that of the substrate is incorporated between the oxide and the substrate. These models explain the observed frequency dependence of the capacitance-voltage ( C - V ) characteristics of these devices. Techniques are presented for determining the impurity profile of the buried layer from the low frequency C - V measurements made on MOS transistors. The majority carrier mobilities in the buried layer and at the surface are measured for the BCCD's and compared to the surface minority carrier mobility measured for the surface channel CCD's. Generation lifetimes at the surface, in the buried layer and in the underlying substrate are determined from capacitance-time (pulse bias C - t ) measurements and leakage current measurements of the MOS capacitors and transistors. Methods are demonstrated whereby the depth from the oxide interface of the potential minimum (depth of the buried channel) and its potential can be determined as a function of the various applied biases.

Electrical and cathodoluminescence measurements on ion implanted donor layers in GaAs

Abstract: GaAs has been doped by the ion implantation of silicon, sulphur, selenium and tin. After annealing at 700°C, the layers were n-type in all cases but with the heavier ions, selenium and tin, it was necessary to implant above room temperature. Van der Pauw measurements showed that for all the impurities the surface concentration of free electrons as a function of ion dose reached a maximum of approximately 1013 electron/cm2 with an average Hall mobility of 2000 cm2/V sec. The spatial distributions of active donors were obtained from both differential Hall measurements and capacitance measurements on reverse biased Schottky barriers. The maximum carrier density measured was 1018/cm3 at the peak of the distribution of tin ions implanted at 200°C. With selenium and tin implants the concentration and mobility of free electrons and the depth of the donor distribution were dose dependent. The cathodoluminescence spectra from implanted layers were dominated by broad low energy bands due to recombination at defects. A VGa-Si complex was thought to be responsible for one of the most intense bands at 1·18 eV. The results indicate that under certain conditions both defects and impurities migrate into the substrate.

A unified approach to the base widening mechanisms in bipolar transistors

Abstract: In this paper a unified study of the base-widening mechanisms of bipolar transistors is presented. In the first part, the hypothesis which forms the basis of the analysis is formulated from the qualitative description which is given of the distribution of the electric field in the collector in terms of the bias conditions. This hypothesis is as follows: only when the electric field attains values such that the carriers move at their scattering-limited velocity is there a high injection into the collecting zone. This enables us to establish analytical expressions which characterise fully the base widening phenomena whatever the bias conditions. The approach presented here is justified a posteriori by a comparison between our results and those obtained from a numerical simulation of the behaviour of the collector-base junction.

Energy levels and concentrations for platinum in silicon

Abstract: The energy levels and electrically active concentrations of platinum in silicon have been measured by Hall techniques Analysis shows platinum to have two electrically active sites The usual site N Pt I (assumed to be substitutional) predominates (>80%) and has two levels, a donor at E v +0·28 eV, degeneracy γ I + =2 in p -type material and an acceptor at E c −0·20 eV, γ I − = 1 6 , in n -type material However a second platinum site exists, and is present to a concentration N Pt II of about 10 per cent with an acceptor level at E v + 0·42 eV (γ II = 1 8 ) The physical nature of this Pt associated site is unknown Neutron activation analysis has been used to determine total atomic platinum concentrations for diffusions from 800 to 1250°C These results, in conjunction with Hall measurements, show the electrical activity to be very high Previous studies on platinum are reviewed and compared to the result of this work

Shallow impurity states in semiconductors: Absorption cross-sections, excitation rates, and capture cross-sections☆

Abstract: Shallow impurity state wave functions are derived for both a coulomb and δ-function binding potential within the effective mass formalism of Kohn. These wave functions, along with the Bloch functions of a perfect crystal are then used to obtain analytic expressions for photon spectral absorption cross-sections, thermal photon excitation rates, and carrier radiative capture cross-sections into the shallow centers. The calculated absorption cross-sections are in satisfactory agreement with both magnitude and spectral shape of experimentally measured cross-sections. The absorption cross-sections are such as to make the centers very sensitive to thermal photon excitation over a wide range of temperatures or center ionization energies. Finally, it is shown that radiative carrier trapping is normally negligible for shallow centers.

An introduction to the theory of photovoltaic cells

Abstract: This paper seeks to furnish an elementary, but in some ways novel, introduction to the theoretical physical principles underlying solar cell operation. By simple concepts and mathematics this is achieved by first explaining the operation of p - n junctions and then focussing attention on ideas such as lifetimes, load matching and efficiencies.

Lead telluride-lead tin telluride heterojunction diode array

Abstract: PbTePb 0·80 Sn 0·20 Te heterojunction diode arrays have been fabricated from layers grown by the liquid phase epitaxy technique on Pb 0·80 Sn 0·20 Te substrates. The quality of grown surfaces was found to be dependent upon crystal orientation and substrate surface treatment. Constitutional supercooling has not been identified as a contributing factor. From the capacitance data, an inversion layer is believed to exist at the PbTePbSnTe interface. The diodes exhibit external quantum efficiencies ranging from 15 to 35 per cent without an anti-reflection coating. Junction zero bias impedances vary from 3 to 7 kΩ for a diode are of 6 × 10 −4 cm 2 , yielding R 0 A products from 1·8 to 4·2 Ω-cm 2 . The average D λ p ∗ at λ p = 10 μ m for an 18-element array is 2·3 × 10 10 cm Hz 1 2 /W for 180°FOV a 300°K background.