Showing papers on "Heterojunction published in 1972"

Mobility gaps: a mechanism for band gaps in melanins.

Abstract: The semiconductor behavior of melanins is reviewed and compared with quantum mechanical models of conduction in amorphous solids. The available data are consistent with extensions of Mott's basic model for amorphous semiconductors, whereas they are inconsistent with crystalline semiconductor models. An investigation of the specific conduction mechanisms operative in melanins in terms of the amorphous model should reveal important aspects of the band structure.

High‐efficiency Ga1−xAlxAs–GaAs solar cells

Abstract: Heterojunction solar cells consisting of pGa1−xAlxAs–pGaAs–n GaAs are grown by liquid‐phase epitaxy and exhibit power conversion efficiencies of over 16% (corrected for contact area) measured in sunlight for air mass 1 at sea level, while efficiencies of 19–20% are obtained for an air mass value of 2 or more. The improved efficiencies compared to conventional homojunction (Si and GaAs) cells are attributed to the reduction of series resistance and the reduction of surface recombination losses resulting from the presence of the heavily doped Ga1−xAlxAs layer. Open‐circuit voltages of 0.98–1.0 V and short‐circuit currents of 18–21 mA/cm2 (corrected for contact area) are observed for a solar input intensity of 98.3 mW/cm2.

GaAsGaAlAs heterojunction transistor for high frequency operation

Abstract: A bipolar transistor structure is proposed having application for either high frequency operation or integration with certain types of light emitting devices. The structure involves liquid phase epitaxially grown layers of GaAs for the collector and base regions, and of Ga1−xAlxAs for the heterojunction emitter. The high frequency potential of this device results primarily from the high electron mobility in GaAs and the ability to heavily dope the base region with slowly diffusing acceptors. The Ga1−xAlxAs emitter region provides a favorable injection efficiency and, because it is etched preferentially relative to GaAs, access to the base layer for making contact. Transistor action with d.c. common emitter current gains of 25 have been thus for observed. Calculations of the high speed capability of this transistor are presented.

The graded‐gap Alx Ga1 − x As–GaAs heterojunction

Abstract: The electrical properties of n‐n (AlGa)As–GaAs heterojunctions and the photoresponse of n‐p and p‐n (AlGa)As–GaAs heterojunctions, all grown by liquid‐epitaxy techniques, are explained in terms of a graded‐gap model of the heterojunction interface. This model, in which the grading occurs over a few hundred angstroms now makes possible a consistent explanation of the operation of double‐heterostructure lasers in contrast to the abrupt model of the heterojunction which predicts a series resistance due to reverse biasing of the n‐n heterojunction. Most n‐n heterojunctions measured were Ohmic with the percentage of non‐Ohmic (rectification ratio ∼ 5 : 1) junctions decreasing with increasing growth temperature. The grading and its statistical nature are explained in terms of the initial nonequilibrium growth condition in liquid‐phase epitaxy when the (AlGa)As melt is placed in contact with a GaAs solid rather than the (AlGa)As solid alloy determined from the equilibrium phase diagram. The measured photorespons...

Large‐Optical‐Cavity (AlGa) As–GaAs Heterojunction Laser Diode: Threshold and Efficiency

Abstract: Detailed operating properties and first‐order theory of the large‐optical‐cavity (LOC) laser diode are described, including threshold current density, efficiency, mirror damage under pulsed operation, and cw operation at room temperature. This new structure differs from the single‐heterojunction and conventional double‐heterojunction laser in that the optical‐cavity thickness can be made very large and is varied independently of the pair recombination region which is always less than a diffusion length thick. Large optical cavities are possible with high differential quantum efficiencies at room temperature because the radiation propagates predominantly in low‐loss n‐type material, allowing the design of lasers for specific applications. The problem of catastrophic degradation in the wide‐cavity devices is reduced because of the lower optical flux density. The threshold current density depends on the width of the mode‐guiding region, increasing with increasing thickness of that region. Power conversion efficiency values of 22% have been achieved at room temperature with narrow‐cavity lasers.

Efficient GaAs–AlxGa1−xAs Double‐Heterostructure Light Modulators

Abstract: Properly designed GaAs–AlxGa1−xAs double heterostructures produce strong optical waveguides The propagation constants of the waveguide modes can be readily modulated by the linear electro‐optic effect Measurements at a wavelength λ = 1153 μm have yielded a phase modulation of 180° with ‐10 V applied bias to a device only 1 mm long The power necessary to phase modulate light at λ ≈ 1 μm by 1 rad is of the order of 01 mW per 1‐MHz band‐width The power dissipation is very strongly dependent on wavelength At present, the high‐frequency modulation is limited by the series resistance and capacitance of the device The highest cutoff frequency determined thus far, ≈ 4 GHz, is considerably lower than that calculated based on the geometry and material properties

Photocapacitance Effects at a Cu2S–CdS Heterojunction

Abstract: The influence of trapped charge on the photovoltaic properties of an efficient Cu2S-CdS single-crystal heterojunction has been studied by a photocapacitance technique. For the nonheat-treated cell, a persistent increment in capacitance as high as 70% of the dark capacitance remained after illumination of the junction by band-gap light at 100 K. The additional capacitance is due to holes trapped in deep levels in the CdS depletion region near the interface. The trapped holes enhanced the 100 K photocurrent spectrum uniformly by a factor of 3 before heat treatment. After a 200 C heat treatment in air, the maximum trapped charge at 100 K enhanced the photocurrent by two orders of magnitude. The results are interpreted in terms of the tunneling through a conduction band spike of electrons photoexcited in the Cu2S.

Theoretical analysis of heterojunction phototransistors

Abstract: The optical gain and spectral response of heterojunction phototransistors with wide-gap emitters have been examined theoretically in idealized cases. The optical gain is found to be closely related to the current gain in the common-emitter configuration β of a heterojunction transistor. Because of Kroemer's factor, the injection efficiency of the emitter junction is very high, resulting in a high β or a large optical gain. However, β or the optical gain is limited by the base transport efficiency when the injection efficiency is extremely high. From the analyses, the optical gains and spectral responses of an n.GaAs-p. Ge-n.Ge heterojunction phototransistor and n-p-n homojunction phototransistors of GaAs and of Ge are numerically computed. It becomes evident that the heterojunction phototransistor has a higher optical gain and a wider spectral response ( \sim5 \times 10^{5} at wavelengths ranging from 0.9 to 1.5 µm) than either of the homojunction phototransistors.

Dependence of Threshold Current Density and Efficiency on Fabry‐Perot Cavity Parameters: Single Heterojunction (AlGa)As–GaAs Laser Diodes

Abstract: A detailed study was made of the dependence of the threshold current density and the external differential quantum efficiency on the cavity end loss in single‐heterojunction laser diodes. To eliminate possible errors introduced by studying lasers of varying lengths, this study relied on changes of the facet reflectivity by SiO coatings of appropriate thickness. The results are consistent with a linear relationship between the threshold current density and the gain coefficient at threshold. The values of the gain factor, absorption coefficient, and internal quantum efficiency at 300 and 77°K are in excellent agreement with some of the previous studies. However, for effective end losses greater than ∼50 cm−1 the differential quantum efficiency shows an anomalous effect by decreasing with increasing end loss instead of increasing as predicted by simple theory. This effect occurs at 77°K as well as 300°K and is tentatively attributed to internal trapping of the stimulated emission.

Tuneable infrared photocathode

Abstract: A tuneable field assisted photocathode structured as a three layer double heterojunction device with a low work function cesium oxide coating on the electron emitting surface. An internal field assistance bias aids the flow of electrons from a narrow bandgap region, where they are photo-generated, to the wider bandgap negative electron affinity surface region for vacuum emission.

Lasing Transitions in p+‐n‐n+ (AlGa) As–Ga As Heterojunction Lasers

Abstract: A study of the lasing and spontaneous emission from p+‐n‐n+ heterojunction lasers with lightly doped n‐type active regions shows that the stimulated emission is not due to a simple band‐to‐band recombination process. It is further shown that the 300 K gain coefficient dependence on the junction current differs greatly from that predicted on the basis of band‐to‐band lasing involving a parabolic density‐of‐states distribution.

Electrical properties of CdSPbS heterojunctions

Abstract: Photovoltaic effects of CdSPbS heterojunction were measured for visible and near-i.r. regions. A marked photovoltaic response was observed extending to the wavelength of 3·3 μ and found to be fairly uniform in quantum efficiency. The detectivity was determined to be 3·4 × 10−11 W8 for NEP (noise equivalent power) and 1·5 × 1010 cm ( Hz ) 1 2 / W for D (detectivity) at 900 Hz. Electrical properties of the heterojunctions such as junction capacitance and current-voltage characteristics were measured and the energy band scheme of the heterojunction was constructed. The experimental values of barrier height obtained from different methods were in good agreement and they gave the value of approximately 0·3 eV. From the results the photovoltaic effect in the i.r. region was considered to be due to the photoemission of electrons from PbS to CdS.

The effect of double heterojunction waveguide parameters on the far field emission patterns of lasers

Abstract: A model of double-heterostructure lasers is developed which gives the far-field pattern of the laser in terms of such waveguide parameters as the active region width and the dielectric constants of the layers forming the waveguide. For symmetrical waveguides an explicit expression is given. Experimental results are presented which show a close agreement between the theory and the measured far-field patterns. By adding an extra passive layer to the conventional double heterostructure laser a structure is produced in which the measured half-power width of the far-field distribution is reduced from 55° to 32°. It is shown in this case that the model is useful for estimating the near-field pattern from the measured far-field pattern and hence determining the unknown waveguide parameters.

Experimental studies of injection lasers: spontaneous spectrum at room temperature

Abstract: The current dependence of the spontaneous and coherent intensities and differential intensities from GaAs injection lasers operated at 300 K is reported. A compensated material with single heterojunction and a low‐doped double heterojunction unit were studied. While the spontaneous intensities rise smoothly with current at all power levels, the differential intensities show structure associated with onset of lasing. At threshold there is a sharp cusp, indicating a change in the internal dynamics. The data are found to be consistent with 100% radiative recombination but do not exclude nonradiative transitions. New features are negative differential intensity of lasing; a logarithmic increase of Fermi level with current in the lasing region; and marked differences of the spreading of the lasing spectrum and of the current saturation of the two lasers.

ZnTe-InAs Heterojunctions Prepared by Liquid-Phase Epitaxy

Abstract: ZnTe-InAs heterojunctions are prepared by the liquid-phase epitaxial growth technique. InAs layers are grown from the In solutions on ZnTe substrates. The study on the optical transmittance reveals that InAs-ZnTe mixed-crystal layer exists at the boundary. The ZnTe-InAs heterojunction diodes show SCLC characteristics at forward bias. It is suggested that in the ZnTe substrates, high-resistivity layers are formed by the diffusion of In from the InAs layers. The forward I–V characteristics can be explained by the hole space-charge limited flow in the high-resistivity layers. By using Lampert theory, it is concluded that this layer has a trap density of about 1×1014cm-3 and the trap level is located about 0.38 eV above the valence band edge.

Spontaneously emitting hetero-structure junction diodes

Abstract: A light emitting heterostructure diode includes a multilayered structure having a common conductivity type heterojunction and a p-n junction separated therefrom by a distance less than the diffusion length of minority carriers, thereby defining an intermediate region bounded by said junctions. In a single heterostructure (SH) diode there is one such heterojunction separating narrow and wide band gap regions of the same conductivity type and the p- n junction is a p-n homojunction formed in one instance by the diffusion of impurities into the narrow band gap region. When provided with an appropriate resonator, a confinement effect produced by an energy step (at the heterojunction) in the conduction band permits the SH diode to lase at higher temperatures and lower thresholds than heretofore possible, radiative electron-hole recombination occurring between the conduction and valence bands. In a double heterostructure (DH) the diode is provided with a second heterojunction positioned on the side of the p-n junction remote from the other heterojunction, or positioned coincident with the p-n junction, thereby defining an intermediate region between the pair of heterojunctions. When provided with an appropriate resonator the DH diode exhibits lower thresholds at higher temperatures than even the aforementioned SH diode. In both diodes additional improvment in the threshold occurs if the diode is provided with deep impurity levels or deep band tails. Without a resonator, both the SH and DH diodes function as electroluminescent diodes with radiation being emitted from the intermediate region through the wide band gap region, thereby advantageously resulting in lower absorption losses and higher efficiency. Dome-like configurations of the wide band gap region of this diode are also disclosed.

Uniform Layer Hetero-Junction Targets for Television Camera Tubes

Abstract: Publisher Summary This chapter investigates the uniform-layer heterojunction targets for television camera tubes. In this experimental study, holes generated by optical absorption in a thin n-type silicon wafer diffuse to a surface on which a 1-μm-thick layer of antimony trisulfide has been deposited by evaporation. This layer is scanned by a low-energy electron beam, as it is in a vidicon, and biased by connecting the silicon wafer through a load resistor to a source a few volts to tens of volts more positive than the electron-gun cathode. A typical spectral-response curve for a target structure is shown. The experimental data shows that heterojunction structures formed by evaporation of amorphous Sb 2 S 3 on to an n-type silicon wafer can function as a camera tube target, having low dark current, a nearly linear transfer characteristic, and the spectral response of silicon. On the negative side, the best collection efficiency measured so far is only 1 to 2% as compared to the 60 to 70% measured for a diode target, and considerable variability in characteristics between targets underlines the need for a far better understanding of the physics of the interface.

Electroreflection spectra with low modulation intensity

Abstract: Describes an apparatus and a heterojunction that can be used under 2.6 eV, for any temperature; these were used for measurements of electroreflection by a surface barrier. Delta R/R of 10-6 was measured with a voltage modulation of 0.2 mV. Quantitative measurements showed that there were few surface states and that the radial electric field was homogeneous. A magneto-electroreflection spectrum of Ge shows an example of use at low temperature.

Improvements in or relating to injection lasers

Abstract: 1,273,284 Semi-conductor lasers STANDARD TELEPHONES & CABLES Ltd 13 Oct, 1970, No 48610/70 Headings H1C and H1K In a heterostructure injection laser, either the heterojunction or the p-n junction is non- planar so as to restrict the injection current to a limited p-n junction region The heterojunction is formed between n-type GaAlAs and GaAs, and the p-n junction is formed by selective change of conductivity to p-type by diffusion of zinc In Fig 1 an injection laser is formed by etching a channel in a piece of GaAlAs growing sufficient GaAs to fill the channel and so provide a filament 10, polishing to remove the GaAs not in the channel, and growing a further layer of GaAlAs into which is diffused zinc to the appropriate depth to form a planar p-n junction intersecting the GaAs Alternative single heterostructure lasers are shown in Figs 2a and 2b In Fig 2a the heterojunction between GaAs and GaAlAs layers is planar, and an intersecting non-planar p-n junction is formed by notching the GaAlAs layer at 2D before diffusing with zinc The heterojunction shown in Fig 2b is made non- planar by providing a central ridge in the GaAs layer before the layer of GaAlAs is grown This layer has its upper surface polished to remove a central ridge formed by the growth process, and zinc is then diffused into the planar polished surface to form a planar p-n junction intersecting the non-planar heterojunction The arrangements shown in Figs 2a and 2b may be modified to provide double heterostructure lasers, Figs 3a and 3b, which show a non-planar p-n junction and non-planar heterojunctions respectively In Fig 3c a GaAlAs-GaAs-GaAlAs planar heterojunction structure is grown which has a step formed in the upper surface before the diffusion of zinc Consequentially a non- planar p-n junction is formed which extends through both heterojunctions Other constructions are described, Figs 3d and 3e, in which the zinc doping extends wholly across one heterojunction and partially across the other In each of the arrangements the p-n junction has a narrow strip portion located in a region bounded by a heterojunction and having a low band gap so that injection current is concentrated in this region

Electrical properties of Te-Si heterojunctions†

Abstract: The electrical properties of Te(p)-Si(n) hetorodiodes are investigated at room and liquid air temperatures. The heterojunotion is formed between an evaporated Te film and an n-type Si single crystal substrate. The forward and reverse characteristics are explained on the basis of an energy band diagram similar to Ge—GaAs and a tunnelling mechanism. The capacitativo eut-off voltage and current cut-off voltage, at room temperature, are found to be about 0·7 v. Preliminary indications are that such a heterojunction is sensitive to I.E. radiation.

Field Effect of GaAs Photoluminescence in the GaP-GaAs and Ga 0.3 Al 0.7 As-GaAs Heterojunctions

Abstract: GaAs was excited with a He-Ne laser through a GaP window of the GaP-GaAs heterojunctions. The photoluminescence intensity depends on the applied voltages. Photoexcited carriers in the depletion layer which extends into n-type GaAs do not contribute to the photoluminescence, so that the photoluminescence is quenched when the diodes are reverse biased. The same phenomenon was observed for the Ga0.3Al0.7 As-GaAs heterojunctions at a relatively low excitation level.

Electroluminescent semiconductor devices

Abstract: AN ELECTROLUMINESCENT SEMICONDUCTOR DEVICE COMPRISING, A FIRST SEMOCONDUCTOR BODY PORTION OF A FIRST CONDUCTIVITY TYPE COMPRISING WIDE ENERGY BAND GAP, LUMINESCENT MATERIAL, A SECOND SEMICONDUCTOR BODY PORTION COMPRISING NARROW ENERBY BAND GAP MATERIAL ADJACENT THE FIRST PORTION AND FORMING WITH SAID FIRST PORTION AN ABRUPT HETEROJUNCTION, AND TRANSISTOR MEANS LOCATED IN THE SECOND PORTION FOR PRODUCING AND HEATING CHARGE CARRIERS OF AN OPPOSITE SECOND CONDUCTIVITY TYPE IN THE SECOND PORTION, SO THAT HOT SAID CHARGE CARRIERS ARE INJECTED FROM THE SECOND PORTION INTO THE FIRST PORTION TO PRODUCE LUMINESCENCE THEREIN AND FURTHER SUCH TRASISTOR MEANS FOR EXTRACTING FROM SAID FIRST PORTION THOSE OF THE HEATED SAID CHARGE CARRIERS OF SAID SECOND CONDUCTIVITY TYPE NOT PERMANENTLY INJECTED INTO SAID FIRST PORTION.