Showing papers in "Solid-state Electronics in 1979"

Absorption coefficient of silicon for solar cell calculations

Abstract: The optical absorption coefficient is an important parameter in calculating the performance characteristics of solar cells. For silicon solar cells it is desirable to know the absorption coefficient over the range of 1.1–4.0 eV and over a wide range of temperature, particularly when evaluating the concentration type systems. An analytical (empirical) expression has been developed for this purpose. We have interpreted the available experimental data in terms of three bands of silicon. With our fit, the experimental data can be explained to within an accuracy of 20% and its validity extends from 1.1 to 4.0 eV and over the temperature range of 20–500°K.

Surface and interface depletion corrections to free carrier-density determinations by hall measurements

Abstract: Errors in the determination of (ND-NA) for semiconductor epitaxial layers by the Hall method can result if corrections for carrier depletion are omitted in the calculations. Simple practical procedures are discussed to correct for carrier depletion that occurs in epitaxial layers at their free surfaces, and their interfaces with semi-insulating substrates. Theoretical estimates of carrier depletion in GaAs indicate that depletion regions can extend several microns into high purity epitaxial layers, and can cause (ND-NA) to be considerably underestimated. Experimental evidence is presented in support of the theory.

Electrical characteristics of GaAs MIS Schottky diodes

Abstract: The current-voltage (I-V) and capacitance-voltage (C-V) characteristics of GaAs metal-insulator-semiconductor (MIS) Schottky barrier diodes are investigated over a wide temperature range and compared with MS diodes. The effects of the insulating layer on barrier height and carrier transport are delineated by an activation energy analysis. Excess currents observed at low forward and reverse bias have also been analyzed and their cause identified. A capacitance anomaly consistently noticed in MIS Schottky barriers is resolved by stipulating a non-uniform interfacial layer, and a self-consistent model of the GaAs MIS Schottky barrier is developed by analyzing I-V and C-V data of both MIS and MS diodes.

A better approach to the evaluation of the series resistance of solar cells

Abstract: Series resistance is an important parameter in solar cell design and fabrication. Methods reported in literature for its determination are not suitable for routine use. We describe a simple method for obtaining the series resistance. Universal current-voltage characteristics of the solar cells are also given to illustrate the effects of series and shunt resistance. We also offer an explanation for the often observed drop in efficiency when small area cells are scaled upward. Cells less than 0.25 cm2 represent the intrinsic potential of any given structure or diode but do not reflect series resistance effects that must be eliminated with larger area cells and the practical problems of griding to allow for current collection.

A long-channel MOSFET model

Abstract: The model describes correctly the drain current and the small signal parameters in all regions of operation, including the subthreshold regime and the saturation regime. The model contains as an approximation the charge-sheet model proposed b Brews (see ibid., vol.21, p.345, 1978). Mobility variations along the channel, resulting from the normal and lateral electric fields, can be taken into account.

Low-frequency noise in Schottky barrier diodes

Abstract: The low-frequency excess noise in Schottky barrier diodes has been investigated. In the ideal case where the saturation current is completely determined by thermionic emission of electrons, no 1/ƒ noise will be produced in the barrier. The presence of trap states in the depletion region can lead to generation-recombination noise. At sufficient high forward currents 1/ƒ noise can be generated in the series resistance of the Schottky diode. Deviations from the ideal diode, for example as a result of edge effects, produce 1/ƒ noise and increase at the same time the ideality factor. It is empirically found that the 1/ƒ noise level decreases very rapidly if the ideality factor tends to unity.

Carrier transport across heterojunction interfaces

Abstract: A general theory is presented to describe the carrier transport across heterojunction interfaces. In matching the boundary conditions at the interface, the conservation of total energy and perpendicular momentum is assumed and the difference of effective masses on two sides of the junction is taken into account. The quantum mechanical transmission coefficient is calculated by a combined numerical and WKB method. Application of the present model to an AlxGa1−xAsGaAs N-n heterojunction is performed and it gives rise to rectifying characteristics together with non-saturated reverse current. Comparison with the classical thermionic emission model is made to show the significance of tunneling and effect of quantum mechanical reflection.

Solid phase formation in Au: Ge/Ni, Ag/In/Ge, In/Au: Ge GaAs ohmic contact systems

Abstract: Solid phase epitaxy formation in Au: Ge/Ni, Ag/In/Ge, and In/Au:Ge contacts to GaAs has been identified utilizing micro-spot Auger spectroscopy and selected area electron channeling. It is shown that the lateral extent of solid phase formation directly controls the value of the specific contact resistivity. Solid phase growth occurs as a result of dissolution of GaAs by the contact constituents in the vicinity of the eutectic temperature. Solid phase growth also results in regions free of oxide layers and contaminants and hence a lower contact resistivity.

Temperature dependent threshold behavior of depletion mode MOSFETs. Characterization and simulation

Abstract: During the study of depletion mode MOSFET behavior at low temperatures, unusual changes in the threshold characteristics of the devices were observed First, the effectiveness of the donor implantation in producing a negative threshold voltage shift was significantly reduced At the same time the substrate sensitivity was found to be substantially reduced A third observation was the existence of an unusual structure in the subthreshold region of the device at low temperatures Computer simulation is used to explore these observations and to demonstrate that they are caused by impurity freezeout as temperature is reduced The computer simulation program, usable over the temperature range 50–350 K, is discussed, and a threshold definition suitable for numerical analysis of devices with arbitrary channel structures is developed

Dissociation of GaAs and Ga0.7Al0.3As during alloying of gold contact films

Abstract: Although gold and gold-based alloys are frequently used in making ohmic contacts to Ga x Al 1− x As, the temperature- and time-dependence of the reactions which take place during the alloying procedure have not been extensively described previously. Using an evolved gas analysis (EGA)[11] with mass spectrometer, we have directly detected As which has passed through the thin gold contact film during alloying. With this technique, we have studied the dissociation of GaAs and Ga 0.7 Al 0.3 As during alloying as a function of processing temperature, time, and gold thickness. The total quantity of As released after prolonged annealing increases linearly with gold thickness, consistent with the idea that gold becomes “saturated” with Ga with respect to solid GaAs. An attempt to correlate the As evolved with the Au-Ga phase diagram is given. The amount of arsenic evolved and the rates of evolution were the same for gold on clean GaAs and GaAlAs. Changes in the surface preparation procedure prior to gold deposition had no effect on the As evolved from the Au/GaAs contact. On the Au/GaAlAs contact, poor or no predeposition cleaning lead to reduced As evolution.

Theory of switching in p-n-insulator (tunnel)-metal devices: Part I: Punchthrough mode

Abstract: The four-layered structure ( M - I (leaky)- n - p + ) is found to exhibit a current-controlled negative resistance region in its I-V characteristics. In this paper, a quantitative physical model of the device in the punch-through mode is presented. The negative resistance behaviour is due to a positive feedback mechanism between the tunnel MIS and the n - p + junction parts of the device. The effect of the device parameters on its I-V characteristics is studied.

Electrical behavior of an NPN GaAlAs/GaAs heterojunction transistor

Abstract: This paper deals with a theoretical and experimental study of GaAlAs/GaAs heterojunction transistors fabricated by liquid phase epitaxy. Starting from the general transport equations in a structure of variable composition, the expressions for the injection efficiency and for the transfer characteristic (IC, VBE) are first established. Using reasonable simplifying assumptions, allowed by the physical characteristics of the base and emitter regions, the major role played by the zone including the conduction band spike is clearly shown. In particular, it appears that the presence of such a spike entails a substantial decrease of the injection efficiency and also modifies, over a wide bias range, the (IC, VBE) characteristic which then follows a non ideal law. The experimental behaviour of devices fabricated by liquid phase epitaxy are described. The measurements were carried out at temperatures ranging from 77 to 316°K. The results obtained are compared with those predicted by the theoretical model.

Titanium in silicon as a deep level impurity

Abstract: Titanium inserted in silicon by diffusion or during Czochralski ingot growth is electrically active to a concentration level of about 4 x 10 to the 14th per cu cm. It is reported that Hall measurements after diffusion show conversion of lightly doped p-type Si to n-type due to a Ti donor level at E sub c -0.22 eV. In addition, in DLTS measurements of n(+)p structures this level shows as an electron (minority carrier) trap at E sub c -0.26 eV with an electron capture cross section of about 3 x 10 to the -15th per sq cm at 300 K. Finally, a Ti electrically active concentration of about 1.35 x 10 to the 13th per cu cm in p type Si results in a minority carrier (electron) lifetime of 50 nsec at 300 K.

The interface between Hg1−xCdxTe and its native oxide

Abstract: The results of a study of the electrical properties of the interface between Hg 1− x Cd x Te with x = 0.21 and its native oxide at 77°K are presented. The native oxide is formed by anodic oxidation and results in an interface with reproducible properties. The surface charge, the surface mobility and the effective lifetime are obtained from galvanomagnetic measurements and are related to the semiconductor bulk parameters, the oxide thickness and the annealing conditions. The surface state charge and the metal-semiconductors work function difference are obtained from the shift of the flat band voltage of metal-oxide-semiconductor (MOS) capacitor characteristics. The interface between Hg 1− x Cd x Te and its native anodic oxide is characterized by a density of fast surface states of the order of 5 × 10 11 cm −2 (eV −1 ) near the middle of the bandgap. The density of states increases towards the band edges to the order of 10 13 cm −2 (eV −1 ). The measured flat band voltage is approximately −0.5 V for an oxide thickness of 500 A and for an n -type semiconductor with an electron carrier concentration in the range 1–3 × 10 15 cm −3 at 77°K. The fixed oxide surface state charge is positive for both p -type and n -type semiconductors and is of the order of 6 × 10 11 charges per cm −2 . The surface properties, the significance and the reproducibility of the results are evaluated.

Some considerations in the formulation of IC yield statistics

Abstract: Some published modifications of the Poisson distribution for describing IC yield are critiqued. It is shown that it is incorrect to obtain an average yield for a non-uniform defect population by integrating, either in the geometrical space or in the density space, the Poisson distribution with some assumed density distribution functions. The correct way, and happily also the simplest way, is to average the yields of regionally partitioned subpopulations in a discrete manner. The simple Poisson distribution would become rigorously correct when the size of an imaginary IC increases to one quarter of a wafer, regardless of the non-uniformity in defect density. It is also shown that both cases of clustering of defects, one due to interaction among defects themselves, and the other due to wafer regional preference, result in increased yield for a given defect density in a wafer. On the other hand when there are interactions between defects and IC active area elements, or when defects themselves have physical dimensions, there would be a decreased yield for a given defect density, and a non-zero intercept in the plot of the logarithm of yield vs the active device area.

The effects of two-dimensional charge sharing on the above-threshold characteristics of short-channel IGFETS

Abstract: In a short-channel Insulated Gate Field Effect Transistor (IGFET), a significant fraction of the electric field lines associated with the depleted region under the gate are terminated on the source and drain junctions. In this situation the two-dimensional sharing of the depleted substrate charge between the source, drain and gate terminals, has a dramatic effect on the device behaviour. A model is presented to describe the above-threshold characteristics of short-channel IGFETS when they are affected by the proximity of the source and drain junctions. The analytical forms allow a continuous description of the drain current from subthreshold to above threshold conduction. The model takes into account the fact that the device may be turned on by the drain voltage rather than by the gate voltage; in addition, it describes naturally the enhanced drain conductance commonly encountered in short-channel devices. The description includes both the linear and saturation regions over the complete range of drain and substrate voltages and for gate voltages below the value where channel-drain junction interactions become important or velocity saturation sets in. The model therefore provides an analytical description for a short-channel IGFET in the voltage regime where high-field effects in the channel do not significantly effect the current flow. The results indicate that the dominant effects for this region of operation in a short-channel device may be represented by the use of a drain-voltage and geometry-dependent threshold voltage. In the saturation region, the effects of the threshold variations are reflected in the parameter VSAT, the saturation voltage. The principle features of the model are verified by a detailed comparison with short-channel devices.

Recombination in the end regions of pin diodes

Abstract: The recombination currents in the end regions of pin diodes have been investigated by means of an IR absorption technique. The diodes had graded doping profiles in the end regions, and a wide base (374 μm). It has been found experimentally that over a wide range of currents the recombination is proportional to the square of the injected carrier density at the junctions, which justifies the use of constant h parameters to characterise the recombination. Models of Shockley-Read recombination centres in the end regions are described, which give good agreement with experimental results.

Computer analysis of punch-through in MOSFETs

Abstract: A two-dimensional numerical analysis is made for MOSFETs having short channel lengths. The short channel MOSFET is especially characterized by the existence of the punch-through current which cannot be explained by the one-dimensional MOSFET models. The two-dimensional analysis makes clear the following facts relating about the punch-through mechanism. The punch-through is a condition in which the depletion layers of the source and the drain connect mutually at the deep region in the substrate even in equilibrium. The punch-through current is injected through the saddle point of the intrinsic potential into the drain region by the electric field from the drain, at the low gate voltages.

A proposal for a new approach to heterojunction theory

Abstract: The Anderson theory of heterojunctions is reviewed with particular reference to the derivation of the affinity rule. We consider in detail the fundamental assumptions on which the Anderson model is base, analyzing arguments which either confirm or dispute these underlying ideas. An alternative model is then proposed, and again we discuss the arguments for and against. Our conclusion is that further work must be done both theoretically and experimentally before a definite determination can be made as to the more appropriate of these two theories.

Theory of nonequilibrium properties of MIS capacitors including charge exchange of interface states with both bands

Abstract: An improved model of the nonequilibrium behaviour of MIS varactors is presented taking into account the charge exchange of interface states with both bands. C–V curves were calculated for donor and acceptor type interface states of typical distribution in energy. By means of the proposed model the bulk minority carrier generation time constant can be measured and related with semiconductor bulk properties. This allows one to distinguish between interface states acting with the conduction band and those interacting with the valence band. A possible experimental proof of the presented model as well as its limitations are discussed in detail.

Analysis of the Greek cross, a Van der Pauw structure with finite contacts

Abstract: A theory is given for the Greek cross, a special Van der Pauw structure with contacts of finite lengths. The potential problem has been solved by means of conformal transformations. Approximate formulae have also been derived.

Impedance characteristics of double-hetero structure laser diodes

Abstract: The effects of the onset of lasing on the I-V characteristics, the impedance characteristics and the light modulation characteristics of laser diodes have been discussed by introducing the rate equations which involve carrier diffusion process in the active layer interacting with the radiation field intensity The static I-V characteristics exhibit a kink at lasing threshold current, reflecting the decrease of effective lifetime of carriers Effective carrier lifetimes decrease with increasing light intensity, which results in a steep attenuation of injected carrier density in the active region The impedance and light modulation characteristics are obtained in the small signal approximation The small signal light modulation characteristics depend strongly on whether the laser diode is excited by the constant current modulation or the constant voltage modulation The impedance is changed drastically by the onset of lasing and exhibits a resonance which coincides exactly with the optical modulation resonance frequency

Theory of metal-oxide-semiconductor solar cells

Abstract: Theoretical calculations are presented for a metal-oxide-semiconductor solar cell by considering various physical processes involved in the transport of the carriers across the oxide layer and their recombination at the surface states. Results obtained here are quite general and incorporate various possibilities considered by earlier workers. Physical conditions under which the results of earlier theories would be consistent with the present one are pointed out. Numerical results are given for a typical n-type silicon base MOS solar cell.

Nature of 1f frequency fluctuations in quartz crystal resonators

Abstract: The frequency fluctations measured in quartz crystal resonators of quality factor Q are proportional to Q −4.3 . The quantum approach to 1 f noise predicts fundamental fluctuations of the cross sections of elementary dissipative processes. Starting therefore from fluctuations of the total dissipative coefficient, a Q −4 -law is derived. Consequently, the noise is caused by dissipation fluctuations, rather than fluctuations in the density, or the real part of the Young modulus of the crystal.

Conductance noise investigations on symmetrical planar resistors with finite contacts

Abstract: Calculations and experimental results are presented of the voltage fluctuations across a pair of sensor electrodes on a planar resistor A constant current is passed through another pair of driver electrodes Three types of geometry are considered which are all invariant for rotations of 90° Areas of low and high contribution to the voltage fluctuations are calculated assuming homogeneous conductivity fluctuations The calculation method rests on the sensitivity theorem in electrical network theory Calculations are in agreement with experimental results

A computer-aided analysis of one-dimensional thermal transients in n-p-n power transistors

Abstract: An algorithm is presented which permits the one-dimensional simulation of thermal transients in n - p - n transistors. Band distortion effects, temperature dependent mobilities and thermal conductivity are incorporated into the model. Accurate expressions are derived for the heat generated at any point within the semiconductor and the finite difference forms of the current continuity equations. Results showing the internal device behaviour under various bias conditions are presented.

Power gain and noise of InP and GaAs insulated gate microwave FETs

Abstract: LPE GaAs and InP n-channel depletion mode insulated gate field effect transistors (MISFETs) having 4 μm gate lengths have been fabricated employing pyrolytic SixOyNz, pyrolytic SiO2 and an anodic dielectric for gate insulation. The microwave power gain, noise figure, maximum output power and power-added efficiency were measured and compared to those parameters measured on GaAs Schottky barrier gate devices of identical geometry. The results show that, at least at the microwave frequencies measured, power gain and noise are essentially the same in the GaAs Schottky gate FET and anodic MISFET devices while the maximum output power of a typical InP MISFET was greater than that of a representative GaAs Schottky device.

Bevel cross sectioning of ultra-thin (∼ 100 Å) III–V semiconductor layers

Abstract: A method is described for extending the use of bevel cross sectioning of semiconductor layers from the usual limits of 500–1000 to 80–100 A. The modified bevel cross sectioning described is used to view the 11 layers (6 GaAs and 5 AlGaAs, each ∼ 120 A) and the 7 layers (4 GaAs and 3 AlGaAs, each ∼ 80 A) of two multiple quantum-well AlxGa1−xAsGaAs (laser) heterostructures grown by metalorganic chemical vapor deposition (MO-CVD).

An investigation of dark current and photocurrent superposition in photovoltaic devices

Abstract: In the design and analysis of photovoltai cells, a principle of superposition of light and dark currents is usually assumed to apply. This principle states that the current flowing in an illuminated device subject to a bias V is given by the superposition of the short circuit photocurrent and the current that would flow at bias V in the dark. Using a straightforward modification of Shockley's theory of the pn junction diode, the validity of the superposition principle is established here for Si and GaAs homojunction cells exposed to one sun illumination. The argument commences by demonstrating that the quasi-fermi potentials are essentially constant across the depletion region for a device exposed to one sun illumination and biased at a reasonable operating point. Proceeding in this way, it is found that superposition applies even when recombination and photogeneration in the depletion region contribute substantially to both the dark current and the photocurrent. The theoretical argument is confirmed by direct numerical solution of the basic semiconductor equations.

One-dimensional device model of the npn bipolar transistor including heavy doping effects under fermi statistics

Abstract: Carrier transport equations extended to Fermi statistics will be given with characteristic parameters, including heavy doping effect. These equations will be solved numerically by introducing a new method for calculating the parameters. Calculated results will accurately reproduce the significant decrease in transistor current gain with a high impurity concentration at emitter-base junction.