Emitter Region Research Articles

Tuning the room temperature nonlinearI−Vcharacteristics of a single-electron silicon quantum dot transistor by split gates: A simple model

We propose an experiment that potentially allows a single-electron silicon quantum dot transistor to operate at room temperature. The emitter and collector of the device consist of silicon quantum wires and the base contains a single silicon dot buried in silicon dioxide. We suggest that split gates are added to the usual experimental situation, to provide additional and variable confinement perpendicular to the transport direction in the emitter and collector regions. The current-voltage curve is calculated using the Bardeen transfer Hamiltonian method. The potential defined by the gates is approximated to a harmonic form. We predict the nonlinear structure in the current-voltage curve, will survive to room temperature for systems with an emitter and collector with dimensions of the order 20-40 nm, and where the harmonic potentials have subband level spacing of the order 4-8.5 meV. Furthermore, we predict that the peak positions and peak to valley ratios in the current-voltage curve can be tuned by changing the split gate voltage.

Coupling between lateral modes in a vertical resonant tunneling structure

We present experimental results and theoretical calculations of the vertical electron transport through a laterally constricted resonant tunneling transistor. Current–voltage measurements at 4.2 K show numerous current peaks that exhibit a complex dependence on the applied gate voltage. A scattering-matrix approach combined with the Landauer formalism was used to perform quantum mechanical calculations of the electron transport through a quantum dot structure with laterally confined emitter and collector regions. The simulations qualitatively reproduce the measured data, suggesting a strong coupling between the lateral modes in the quantum dot and the collector.

A contactless photoconductance technique to evaluate the quantum efficiency of solar cell emitters

A new technique, the spectral response of the steady-state photoconductance, is proposed for solar cell characterisation in research and development. The emphasis of this paper is on the evaluation of the carrier collection efficiency of the emitter region based on a simple, two-wavelength approach. We show that in addition to the well-established determination of the wafer recombination properties that results from a long-wavelength photoconductance measurement, detailed emitter quantum-efficiency information can be obtained by performing a second measurement with short-wavelength light. The method is experimentally demonstrated with silicon solar cell precursors having emitters with markedly different levels of surface and bulk recombination losses. The main advantages of the spectral photoconductance technique are that it is fast, contactless, and can be used immediately after junction formation before metallisation; these properties make it very appropriate for routine monitoring of the emitter region, including in-line process control.

Correlation of Photoreflectance Spectra with Performance of GaInP/GaAs Heterojunction Bipolar Transistors

We have measured the photoreflectance (PR) spectra at 300 K and dc current gain for GaInP/GaAs heterojunction bipolar transistors (HBTs) grown by metalorganic chemical vapor deposition with different growth conditions. From the observed Franz–Keldysh oscillations, we have evaluated the built-in dc electric fields and associated band gaps in the GaInP emitter and GaAs collector regions. The observed increase in current gain with the low GaInP band gap is in agreement with band alignment between partially ordered GaInP and GaAs, but the observed current gain is not strongly dependent on electric field. For samples with comparable GaInP band gaps we have thereby detected a limiting factor, base bulk recombination, governing the current gain of HBTs from Gummel plots.

Polarized-photoreflectance characterization of an InGaP/InGaAsN/GaAs NpN double-heterojunction bipolar transistor structure

We have characterized an InGaP/InGaAsN/GaAs NpN double-heterojunction bipolar transistor structure using polarized photoreflectance (PR) spectroscopy. The ordering parameter of the InGaP is deduced from the polarization {[110] and [11̄0]} dependence of the PR signals from the emitter region. The ordering related piezoelectric field is also found to influence the electric field, as evaluated from observed Franz–Keldysh oscillations, in the InGaP emitter region. The field in the emitter region is found to be about 25 kV/cm smaller than the theoretical value that does not take into account the possible ordering induced screening effect, while the field in the collector region agrees well with the theoretical value. In addition, the InGaAsN band gap is also determined by analyzing the PR spectrum of the base region.

Planar 4H- and 6H-SiC p–n diodes fabricated by selective diffusion of boron

Graphite film was used as a protective and selective mask for realizing diffusion of boron in SiC. Secondary ion mass spectroscopy was employed to identify the diffusion profile in SiC. No significant difference between diffusion profiles in 4H-SiC and 6H-SiC was found. Planar p–n diodes with local p-type emitter regions were fabricated in 4H-SiC and 6H-SiC based on this process. The current density versus voltage ( J–V) curves of the formed diodes exhibited good rectification characteristics. The 4H-SiC p–n diodes had much lower forward voltage drops and much less temperature dependence in comparison with 6H-SiC p–n diodes.

Quantized Resonant-Tunneling Phenomena of AlGaAs/GaAs/InGaAs Heterojunction Bipolar Transistors

The quantized resonant-tunneling behaviors of heterojunction bipolar transistors (HBTs) with an n-AlGaAs/n-GaAs/n--InGaAs heterostructure emitter and an ultra thin p+-GaAs base layer are demonstrated by theoretical and experimental analysis. In these devices, a thin n--InGaAs pseudomorphic quantum well (QW) in the emitter region is formed between the n-GaAs emitter and the ultra thin p+-GaAs base layer. By solving the Poisson equation, the design of the n-GaAs emitter layer is discussed. A transfer-matrix method is developed to describe the quantum mechanism of miniband structures, in which electrons tunnel resonantly from the depleted emitter side to the collector side through the InGaAs QW and ultra thin p+-GaAs base layers., A device with high current gain, low offset voltage, and a pronounced N-shaped negative-differential-resistance (NDR) phenomenon at room temperature is observed experimentally.

The Effect of Device Parameters on the Emitter Transit Time of a PET

Polysilicon Emitter Transistors (PETs) which are now being widely used in high speed bipolar circuits because of their extremely low emitter transit time τE, have broken-up interfacial oxide layer due to annealing process which they undergo. The effect of oxide layer break-up at the mono-poly interface of the emitter region of a PET on τE is studied here. The effect of doping profile, peak doping concentration, thickness of interfacial oxide layer on τE is studied also. Exponential doping distribution alongwith concentration dependent bandgap narrowing effect and concentration dependent diffusion constant in the monosilicon emitter region are considered.

Gain limitations of scaled InP/InGaAs heterojunction bipolar transistors

We investigate a current gain degradation mechanism in self-aligned InP/InGaAs heterojunction bipolar transistors. We show that surface level pinning at the emitter sidewall gives rise to a peripheral emitter injection current into the base which increases the base current due to electrons recombining at the base contacts. The surface charge at the extrinsic base surface causes the formation of a conducting channel which further enhances the electron flow from the emitter to the base contacts. Two-dimensional numerical simulations of the emitter region combined with Monte-Carlo simulations of the injection process at the abrupt base-emitter heterojunction are in good agreement with the measurements. This effect will be especially severe for submicron emitter widths where the emitter perimeter to emitter area ratio is large.

Heterojunction wavelength-tailorable far-infrared photodetectors with response out to 70 μm

Results are presented on the performance of a heterojunction interfacial workfunction internal photoemission (HEIWIP) wavelength-tailorable detector. The detection mechanism is based on free-carrier absorption in the heavily doped emitter regions and internal emission across a workfunction barrier caused by the band gap offset at the heterojunction. The HEIWIP detectors have the high responsivity of free-carrier absorption detectors and the low dark current of quantum well infrared photodector type detectors. For a 70±2 cutoff wavelength detector, a responsivity of 11 A/W and a D*=1×1013 cmHz/W with a photocurrent efficiency of 24% was observed at 20 μm. From the 300 K background photocurrent, the background limited performance (BLIP) temperature for this HEIWIP detector was estimated to be 15 K. This HEIWIP detector provides an exciting approach to far-infrared detection.

Analysis of anomalous current–voltage characteristics of silicon solar cells

The current–voltage characteristics of solar cells, under illumination and in the dark, represent a very important tool for characterizing the performance of the solar cell. The PC-1D computer program has been used to analyze the deviation of the dark current–voltage characteristics of p–n junction silicon solar cells from the ideal two-diode model behavior of the cell, namely the appearance of “humps” in the I– V characteristics. The effects of the surface recombination velocity, the minority-carrier lifetimes in the base — and emitter regions of the solar cell, as well as the temperature dependence of the I– V characteristics have been modeled using PC-1D. It is shown that the “humps” in the I– V characteristics arise as a result of recombination within the space-charge region of the solar cell, occurring when conditions for recombination are different from the simple assumptions of the Sah–Noyce–Shockley theory.

Photoreflectance characterization of an AlInAs/GaInAs heterojunction bipolar transistor structure with a chirped superlattice

We have measured the photoreflectance spectrum at 300 K from an AlInAs/GaInAs (lattice matched to InP) heterojunction bipolar transistor structure with a chirped superlattice (ChSl) grown by molecular-beam epitaxy. From the observed Franz–Keldysh oscillations we have evaluated the built-in dc electric fields and associated doping levels in the n-GaInAs collector and n-AlInAs emitter regions. The oscillatory signal originating from the ChSl is caused both by the uniform quasielectric and nonuniform space-charge fields in this region.

Variation in emitter diffusion depth by TiSi/sub 2/ formation on polysilicon emitters of Si bipolar transistors

The emitter diffusion depth of wider emitters was found to be shallower than that of narrower emitters for transistors with a TiSi/sub 2/-formed in-situ phosphorus doped polysilicon (IDP) emitter. This dependence of emitter diffusion depth on emitter width W/sub E/ resulted in a large dependence of h/sub FE/ on W/sub E/. We found that the emitter diffusion in emitters with TiSi/sub 2/ formation was shallower than that in emitters without TiSi/sub 2/ formation. We conclude that the dependence of emitter diffusion depth on W/sub E/ is due to variation in phosphorus diffusivity caused by the TiSi/sub 2/ formation. We found by X-ray diffraction measurements that TiSi/sub 2/ formation reduced the crystal lattice volume of the IDP emitter layers by 0.2-0.30%. We attribute this reduction to a reduction in packing density due to a supersaturation of vacancies caused by the TiSi/sub 2/ formation. We believe that the vacancy supersaturation decreases the density of interstitials in the emitter regions, which decreases the emitter diffusion depth, and that the dependence of emitter diffusion depth on W/sub E/ is due to variations in the density of vacancies depending on W/sub E/.

Sulfur Deposition in Asia: Seasonal Behavior and Contributions from Various Energy Sectors

Sulfur transport and deposition in Asia, on an annual andseasonal basis, is analyzed using the ATMOS model. Calculationsare performed for two complete years (1990 and 1995). Deposition amounts in excess of 0.5 g S m-2 yr-1 are estimated for large regions in Asia, with values as high as 10 g S m-2 yr-1 in southeastern China. Annual averaged SO2 concentrations in excess of 20 μg SO2 m-3 are calculated for many urban and suburban areas ofeastern China and S. Korea, with an average of 5 μg SO2 m-3 over most of the emitter regions. Sulfur deposition by major source categories is also studied. Southeast Asia (Indonesia, Malaysia, Philippines, Singapore)receives ∼25% of its sulfur deposition from shipping activities. Sulfur deposition from bio-fuel burning is significant for most of the underdeveloped regions in Asia. Volcanoes are a major source of sulfur emissions in the PacificOcean, Papua New Guinea, Philippines and Southern Japan. Sulfur deposition is shown to vary significantly throughout the year.The monsoons are found to be the largest factor controlling sulfur transport and deposition in the Indian sub-continent andSoutheast Asia. India receives over 35% of its total depositionduring the summer months. In East Asia, sulfur deposition isestimated to be 10% higher during summer and fall than winterand spring. Model results are compared with observations from a number of monitoring networks in Asia and are found to be generally consistent with the limited observations.

Doping of 6H–SiC by selective diffusion of boron

Experimental evidence of selective boron doping of 6H–SiC via diffusion is given. Selective diffusion has been realized at 1800–2100 °C using graphite film as a protecting mask. Cathodoluminescence measurements as well as an anodic oxidation technique have been employed to identify the local doped regions. In addition, a diffused planar p–n diode based on the local p-type emitter region was fabricated. The ideality factor extracted from current–voltage characteristics was 1.97, which indicates that the Shockley–Hall–Read recombination is the dominant mechanism in the conduction region. The value of breakdown voltage for this diode measured at room temperature was a little greater than 800 V, and a leakage current density of 5.7×10−6 A/cm2 at 800 V was achieved.

Co-optimisation of the emitter region and the metal grid of silicon solar cells

The procedure for the simultaneous optimisation of the dopant density profile and the front metal grid of silicon solar cells is illustrated with representative cases of laboratory and commercial devices. Contour plots for the saturation current density and the photo-generated current density of phosphorus doped emitter regions of silicon solar cells are calculated as a function of emitter thickness and dopant concentration, for the particular case of Gaussian dopant profiles. To expose the competing factors of surface and bulk emitter recombination, grid shading and series resistance, the other regions of the device are assumed ideal, that is, loss-less. The calculated contour plots of the conversion efficiency indicate that relatively thick emitters are optimum if surface passivation is available, whereas thin, heavily doped emitters are preferable in the absence of surface passivation. Copyright © 2000 John Wiley & Sons, Ltd.

Application of an improved band-gap narrowing model to the numerical simulation of recombination properties of phosphorus-doped silicon emitters

The commonly used band-gap narrowing (BGN) models for crystalline silicon do not describe heavily doped emitters with desirable precision. One of the reasons for this is that the applied BGN models were empirically derived from measurements assuming Boltzmann statistics. We apply a new BGN model derived by Schenk from quantum mechanical principles and demonstrate that carrier degeneracy and the new BGN model both substantially affect the electron–hole product within the emitter region. Simulated saturation current densities of heavily phosphorus-doped emitters, calculated with the new BGN model, are lower than results obtained with the widely used empirical BGN model of del Alamo.

Monolithic bidirectional switch.: I. Device concept

A new device structure is introduced which facilitates the production of monolithic bidirectional switches. Astonishingly enough, the diverse demands on the characteristics can be completely realized by intrinsic silicon exhibiting the only doped zones to form ohmic contacts. Emitters are formed by accumulation layers which are caused by a suited potential on MOS-gate electrodes. This allows to invert the type of charge carriers as is required for bidirectional operation. The emitter regions are proposed as tiny mesas of 0.3 μm width and 1.0 μm height covering about 6% of both the surfaces. Thus, the structure appears rather simple, but its production requires fine line resolution and processing on both the surfaces of the wafer.

Comparative analysis of minority carrier transport in npn bipolar transistors with Si, Si1−xGex, and Si1−yCy base layers

Here we present a comparative analysis of vertical minority carrier transport in Si, Si0.925Ge0.075, Si0.998C0.002, and Si0.99C0.01 base layers of bipolar transistors. We show that a conventional transit time analysis for extracting the minority carrier mobilities fails for doping profiles containing a low doped emitter region. The contribution of locally compensated charge storage, called neutral charge storage, in the emitter-base depletion region must not be neglected. To overcome drawbacks of the simple transit time analysis, we use 2D device simulations to obtain an improved understanding of the measured high-frequency parameters. Taking into account the real doping profiles and device structures, and using a calibrated parameter set for strained SiGe, the simulation results for the Si, Si1−xGex, and Si1−yCy (y≤0.2%) base layer transistors reproduce very well the measured transit times (assuming the Si data for the electron mobility μn) in the heteroepiaxial base layers. In the case of higher carbon concentration (y=1%), the electron mobility is reduced by a factor of two.

Characterization of AlxGa1−xAs/GaAs heterojunction bipolar transistor structures using cross-sectional scanning force microscopy

We have characterized base-layer width and dopant distributions on cleaved cross-sections of AlxGa1−xAs/GaAs heterojunction bipolar transistor (HBT) structures using a variation of electrostatic force microscopy. The contrast observed is sensitive to the local dopant concentration through variations in the depletion layer depth extending into the sample surface, and enables delineation of individual device regions within the epitaxial layer structure with nanoscale spatial resolution. In two epitaxially grown HBT structures, one with 50 nm base width and the other with 120 nm base width, we are able to delineate clearly the emitter, base, collector, and subcollector regions, and to distinguish regions within the collector differing in dopant concentration by a factor of two. We have also distinguished clearly between the base widths in these samples and have precisely measured the difference to be 63±3 nm, in excellent agreement with the nominal difference of 70±7 nm.