Showing papers on "Depletion region published in 1990"

Vectorial electron injection into transparent semiconductor membranes and electric field effects on the dynamics of light-induced charge separation

Abstract: Transparent titanium dioxide membranes (thickness 2.7 {mu}m) were prepared by sintering of 8-nm colloidal anatase particles on a conducting glass support. The dynamics of charge recombination following electron injection from the excited state of RuL{sub 3} (L = 2,2{prime}-bipyridine-4,4{prime}-dicarboxylic acid) into the conduction band of the semiconductor were examined under potentiostatic control of the electric field within the space charge layer of the membrane. Biasing the Fermi level of the TiO{sub 2} positive of the flat-band potential sharply reduced the recombination rate, a 1,000-fold decrease being associated with a potential change of only 300 mV. Photoelectrochemical experiments performed with the same RuL{sub 3}-loaded membrane in NaI-containing water show the onset of anodic photocurrent to occur in the same potential domain. Forward biasing of the membrane potential impairs photosensitized charge injection turning on the photoluminescence of the adsorbed sensitizer.

Semiconductor device and a method of manufacturing the same

Abstract: Provided is a manufacturing method of a semiconductor device which comprises forming, all over the surface of a substrate below the channel region of a MISFET, a p type impurity layer having a first peak in impurity concentration distribution and another p type impurity layer having a second peak in impurity concentration distribution, each layer having a function of preventing punch-through. Compared with a device having a punch through stopper layer of a pocket structure, the device of the present invention is suppressed in fluctuations in the threshold voltage. Moreover, with a relative increase in the controllable width of a depletion layer, a sub-threshold swing becomes small, thereby making it possible to prevent lowering of the threshold voltage and to improve a switching rate of the MISFET.

Transport mechanisms in ZnO/CdS/CuInSe2 solar cells

Abstract: The transport mechanisms in ZnO/CdS/CuInSe2 solar cells prepared by ARCO (now Siemens) Solar Inc. have been analyzed by measurements of current versus voltage at different temperatures in the dark, short‐circuit current versus open‐circuit voltage at different temperatures in the light, spectral response of quantum efficiency, and junction capacitance. In the dark, recombination in the depletion region and/or thermally assisted tunneling are the dominant transport mechanisms. The observation of a smaller open‐circuit voltage than would be predicted from the dark transport parameters is the result of a small change in the transport parameters under illumination, probably without a change in transport mechanism.

Maximum low-temperature mobility of two-dimensional electrons in heterojunctions with a thick spacer layer.

Abstract: The calculation of the mobility of the two-dimensional electron gas limited by the remote ion scattering is presented which takes into account the correlation in the spatial distribution of the charged impurities caused by the Coulomb interaction. The correlation function of the charge is characterized by the freezing temperature ${\mathit{T}}_{0}$. The low-temperature distribution of the charge is supposed to be a snapshot of the equilibrium distribution corresponding to this temperature. At low enough ${\mathit{T}}_{0}$ this distribution corresponds to the ground state of the system consisting of the charged and neutral impurities. The effect of the correlation is shown to be very essential at low ${\mathit{T}}_{0}$ and at large spacer thickness. The numerical calculations are performed for modulation-doped ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs heterostructures. Input parameters are as follows: spacer width (s), temperature (${\mathit{T}}_{0}$), density of the channel electrons (${\mathit{N}}_{\mathit{s}}$), and density of the charge in the depletion layer (${\mathit{N}}_{\mathrm{depl}}$).

Photovoltaic effects in photoemission studies of Schottky barrier formation

Abstract: It is shown here that much of the recent photoelectron spectroscopy literature describing the onset of pinning in adsorbate-semiconductor systems at low temperature must be reinterpreted in light of surface photovoltaic effects. Two sources of surface charging are discussed, both of which are strongly enhanced at low temperature. The surface photovoltage resulting from separation of electron-hole pairs by the electric field in the depletion region is usually the dominant source of surface potential shifts, and causes flattening of the semiconductor bands. In addition, surface charging due to photoemission into the vacuum may reverse bias a p-type diode at low temperatures, causing increase band bending.

Modes of operation and radiation sensitivity of ultrathin SOI transistors

Abstract: Improved short-channel behavior, reduced subthreshold slopes, and mobility enhancements previously observed in NMOS transistors made in thin, fully depleted silicon-on-insulator (SOI) films are discussed. These results were obtained with the back interface held in depletion during operation. It is shown from basic principles of device operation that the observed performance improvements are sensitive to the applied substrate voltage. In addition, the exposure of the back interface to the surface depletion region in these devices makes the transistor performance sensitive to radiation-induced charging effects at the back interface. The anticipated effects of radiation on threshold voltage, subthreshold slope, and mobility in ultrathin, fully depleted SOI transistors are discussed, and an estimate is made of the expected radiation sensitivity of these parameters for a typical ultrathin SOI technology. >

Anomalous effects on the current‐voltage characteristics of p‐channel metal‐oxide‐semiconductor transistors in the temperature range 4.2–50 K

Abstract: The current‐voltage characteristics of p‐channel MOSFETs (metal‐oxide‐semiconductor field‐effect transistors) in the temperature range 4.2‐50 K have been investigated in detail. A peak is observed at the beginning of the drain‐current–drain‐voltage (Id, Vd) curves for low Vg values, which is not observed in the case of n‐channel MOSFETs and is found to be a very slow transient effect. This time‐dependent peak is observed for temperatures below ∼25 K and for channel lengths greater than ∼3 μm. In this temperature range a strong hysteresis effect is also observed at low Vg voltages. The above behavior will be explained in terms of a very slow depletion layer formation in the drain region, introduced by the large time constant of the field‐assisted thermal emission of carriers in n‐type bulk semiconductors at temperatures below ∼20 K. Another effect, analogous to the ‘‘kink effect’’ observed in n‐channel MOSFETs at temperatures below 16 K, is also observed for low Vg and Vd values. This effect is observed wi...

Evidence for the existence of a negatively charged hydrogen species in plasma‐treated n‐type Si

Abstract: We demonstrate the drift of a donor‐passivating hydrogen species under the action of the electric field in the depletion region of a reverse‐biased Au/n‐Si Schottky diode hydrogenated by exposure to a low‐frequency discharge. The redistribution is explained by the unidirectional drift of a negatively charged passivating species and is confirmed by secondary‐ion mass spectrometry profiling in deuterated diodes. The results are consistent with the presence of an acceptor level for hydrogen in n‐type Si, and are analogous to the situation in p‐type Si where drift experiments reveal the existence of positively charged hydrogen donor species.

Thin-film ZnO as micromechanical actuator at low frequencies

Abstract: A new model is proposed for the low-frequency piezoelectric activity of ZnO films grown on CVD SiO2. In this MOS structure, with ZnO as the semiconductor, a depletion layer is induced by means of a d.c. bias voltage. Using standard semiconductor theory, an expression is derived relating the electric field in this depletion layer with the driving a.c. and d.c. voltages. Due to the built-in charge at the ZnO-SiO2 interface, a depletion layer exists, even when no d.c. bias is applied. We measured the vibration amplitude at resonance of the tip of a silicon cantilever, upon which the MOS structure was deposited, as function of a.c. and d.c. voltages. The results show good agreement with calculated curves. Therefore, it can be concluded that thin-film ZnO can be used as a piezoelectric actuator for micromechanical devices working at low frequencies.

A Gummel-Poon model for abrupt and graded heterojunction bipolar transistors (HBTs)

Abstract: A Gummel-Poon model for abrupt and graded GaAlAs/GaAs/GaAs heterojunction bipolar transistors (HBTs) is developed. The effect of carrier recombination at the emitter-base heterojunction, space charge region (SCR) width modulation effect, and base-widening effect at large collector currents have been considered. Results from this model are compared with numerical results, experimental results, and results from the most recent analytical models. The results show that the common-emitter current gain behavior in the low collector current region can be predicted more accurately by this model, and that interface and surface recombination affect the current gain more dominantly than the other recombination processes. Dependence of cutoff frequency on collector current obtained from the present model agrees well with the experimental results. This model can also predict both current gain and cutoff frequency falloffs at large collector current. This model can be easily implemented in the SPICE program.

Injection and drift of a positively charged hydrogen species in p‐type GaAs

Abstract: Transport of the acceptor‐passivating hydrogen species in p‐type GaAs has been observed in reverse bias annealed Al Schottky diode samples. The motion of the positively charged hydrogen across the depletion region of these diodes is confirmed both by changes in the electrically active acceptor profiles with time, and by direct measurement of the migration using secondary‐ion mass spectrometry on deuterated samples. Acceptor passivation is unstable under minority‐carrier injection by illumination at 25 °C. Hydrogen injection into p‐type GaAs during boiling in water or etching in H2SO4:H2O2:H2O has also been demonstrated.

Interpretation of thin-film polycrystalline solar cell capacitance

Abstract: Variation of thin-film polycrystalline solar cell capacitance with frequency and voltage is used to determine (1) carrier density, and hence Fermi-level position, in the more lightly doped semiconductor, (2) width of the intrinsic layer often found adjacent to a polycrystalline heterojunction, and (3) density of extraneous states at the Fermi level in the depletion region. For CuInSe/sub 2/ cells from three laboratories, the values extracted show a relatively modest dependence on temperature and light intensity. Reliable interpretation of the capacitance measurements requires continual monitoring of actual junction voltage, the real to imaginary impedance ratio, and the degree of voltage sweep hysteresis. >

Interfacial Segregation in Perovskites: IV, Internal Boundary Layer Devices

Abstract: A proposed model for interfacial segregation in perovskites, with induced heterogeneous defect distributions, is extended here to account for the formation of internal boundary layer devices, such as positive temperature coefficient of resistance (PTCR) thermistors and internal boundary layer capacitors (IBLC). Boundary layer effects in doped BaTiO3 are attributed to factors which contribute to the formation of highly resistive boundary layers by a segregation-induced shift in donor incorporation and/or acceptor segregation, and the inhibiting action of segregated donors on boundary mobility and grain growth. The distribution of space charges, formed by electron transfer from conductive grains to resistive boundary layers, leads to the formation of impedance barriers in the grain-boundary vicinity. Depending on the grain size, and on relative size and spatial distribution of the space charge layer and the resistive layer, a transition from semiconducting properties to insulating properties may take place. This model accounts for the observed PTCR and IBLC phenomena.

An analytical model for 1/f noise in polycrystalline silicon thin films

Abstract: Noise calculations based on Hooge's empirical mobility fluctuation model are presented for mobility fluctuations occurring in the quasi-neutral and the depletion-barrier regions of low to moderately doped polycrystalline silicon resistors. Comparing the theoretical predictions with the experimental results, it is concluded from the bias dependence and the magnitude of the noise density that the 1/f noise in polysilicon is depletion-region dominant. The limiting role of grain boundaries, the noise correlation between depletion regions on both sides of a grain boundary, and a noise source weight function are taken into account. The Hooge parameter found from the model and data is 1.45*10/sup -3/. >

Scanning tunneling spectroscopy on cleaved silicon pn junctions

Abstract: Scanning tunneling microscope (STM) I–V experiments in air on freshly cleaved Si pn junctions, are presented. It is shown that the interaction between the tip and the sample may cause the whole structure to behave as a solid‐state device. With the tip in the vicinity of the pn junction the junctions depletion layer may pinch off the tip current, in much the same way as the increasing gate bias of a junction field‐effect transistor reduces the channel current. Under other bias conditions, the STM tip may locally forward bias the pn junction causing a sharp increase in the tip current. Using this mode of operation the pn junction was localized to within 50 nm. A dependence of the tip‐sample I–V characteristic on the local impurity concentration of the sample was also observed. A simple qualitative model consisting of a diode, which represents the tip–Si junction, in series with the resistance of the underlying material, explains these measurements.

Photoluminescence at a semiconductor-electrolyte contact around and beyond the flat-band potential

Abstract: Photoluminescence measurements have been performed to study CdSe-, CdS-, and GaP-electrolyte contacts under cathodic bias. A strong increase of the luminescence intensity has been found in those cases where an accumulation layer is formed at the interface. This potential dependence is interpreted in terms of a quenching of surface recombination by the repulsive field in the space charge region which impedes the minority carriers to reach the surface. A theoretical description based on the Stevenson-Keyes approach for recombination is derived by introducing an exponential potential dependence of the surface recombination rate into the calculations of the luminescence generation in the bulk of the semiconductor.

Photoreflectance and the electric fields in a GaAs depletion region

Abstract: We present results which may resolve the recently reported discrepancy between the experimental and the calculated electric fields in the depletion region of undoped GaAs. The photoreflectance theory reportedly underestimates the electric field by nearly a factor of 2. We have found that changes in photoreflectance with laser pump penetration reveal the full character of the electric field over the entire depletion zone. It is often assumed that the built‐in surface potential produces a uniform electric field throughout a thin (100 nm) undoped layer of GaAs grown on top of a heavily doped energy pinning underlayer. Instead, it appears that the heavily doped underlayer provides a potential step at the interface. The step is separated from the surface depletion zone by a region of low electric field which is characteristic of the low fields found in thick, undoped GaAs with (2–4)×1014/cm3 of unintentional impurities.

Process for manufacturing a high-voltage withstanding planar p-n junction

Abstract: Starting from a planar pn junction (12a, 12b) which isolates a semiconductor region (11a, 11b) of a first conduction type from a semiconductor body (1) of a second conduction type, the peripheral region (11b) of the semiconductor region having a lateral doping gradient over its entire width, which exceeds the penetration depth of the semiconductor region, a part of the peripheral region (11b) is removed in an etching step. The etching depth (17) is so dimensioned that the surface breakdown voltage of the p-n junction (12a, 12b) is adjusted to specified value.

Mechanism and device-to-device variation of leakage current in polysilicon thin film transistors

Abstract: An experimental study of mechanisms responsible for off-stage leakage currents in n- and p-channel poly-TFTs (thin-film transistors) is presented Unlike turn-on characteristics, leakage currents are not symmetrical with respect to source-drain voltage polarity The conduction mechanism in the off-state is consistent with the model of thermionic-field emission near the drain The spatial and energetic distribution of trap-states in the drain depletion region affects the tunneling probability and causes device-to-device variation and the asymmetry in leakage current The temperature dependence of the leakage current arises from the thermally activated carrier population at the energy state of the maximum tunneling probability Cumulative leakage currents follow a Poisson-type log-normal distribution which can be significantly affected by processing control Less than 10-fA/ mu m TFT width in leakage current and more than nine orders of magnitude on/off current ratio at 10-V drain bias were achieved in n-channel poly-TFTs >

Confirmation of the temperature-dependent photovoltaic effect on Fermi-level measurements by photoemission spectroscopy

Abstract: Soft-x-ray photoemission spectroscopy (SXPS) measurements of Fermi-level positions within the band gap of clean, GaAs(100) surfaces demonstrate that photovoltaic charging produces a major shift in the apparent band bending at low (50--100 K) temperatures. These measurements confirm recent predictions of Hecht [M. H. Hecht, Phys. Rev. B 41, 7918 (1990)], based on restoring currents limited by carrier transport through the depletion region, highlight the effects of photovoltaic charging for clean (versus metallized) semiconductor surfaces, and justify Hecht's claims for a reassessment of many previous low-temperature photoemission studies of Fermi-level movement.

Self-aligned SiGe-base heterojunction bipolar transistor by selective epitaxy emitter window (SEEW) technology

Abstract: In the device a SiGe epitaxial base is integrated in a structure which uses in situ doped epitaxial lateral overgrowth for the formation of the emitter window and the extrinsic base contact. Nearly ideal I-V characteristics have been achieved for a base width of 60 nm with an intrinsic base resistance of 4.6 k Omega / Square Operator and for emitter widths down to 0.4 mu m. A DC collector current enhancement factor of 3.1 was obtained relative to a Si homojunction transistor with a 1.25 times higher intrinsic base resistance. The breakdown voltage BV/sub CBO/ is identical for both Si and SiGe devices, even though the collector-base depletion region is partly overlapped with the reduced-bandgap SiGe strained layer. The lower BV/sub CEO/, measured for the SiGe-base transistor, is due to the higher current gain. Based on these results the fabrication of high-speed bipolar circuits that take advantage of SiGe-base bandgap engineering seems possible using selective epitaxy emitter window (SEEW) technology. >

Lucky-hole injection induced by band-to-band tunneling leakage in stacked gate transistors

Abstract: The band-to-band tunneling leakage characteristics of thin oxide MOSFETs are investigated by utilizing stacked-gate structure FETs The behavior of threshold voltage variations under drain stress with open-circuit source is analyzed in detail A simple lucky hole injection model is proposed to explain the behavior Program-disturb phenomena in EPROM cells were well described by the model The depletion layer along the channel formed by the applied drain voltage gives energy to cold holes created by band-to-band tunneling Such lucky holes are injected easily and reach the gate electrode >

Photovoltaic properties of iodine‐doped magnesium tetraphenylporphyrin sandwich cells. Optimization of doping, and dark electrical properties

Abstract: By fabricating, doping, and testing sandwich cells of the structure Al/Al2O3 /magnesium tetraphenylporphyrin (MgTPP)/Au under high vacuum, without contact with the atmosphere, the effect of iodine doping on the dark and photovoltaic response has been determined. The properties were found to be extremely sensitive to small variations in iodine concentration, and a method of doping was developed to give highly reproducible devices with optimum photovoltaic efficiency for this system. The dopeddevice behaves as a metal‐insulator‐semiconductor structure in which a depletion layer of thickness ∼100 nm and carrier concentration ∼1016 cm3 is formed in the porphyrin near the Al‐Al2O3/MgTPP interface. From analysis of the time dependence of the electrical properties after doping, a value of 1×10−15 cm2/s is obtained for the diffusion coefficient of iodine in the MgTPP film. Rectification of the dark characteristics is observed only for optimized devices, with a rectification ratio of 240 obtained for a cell having a 200‐nm‐thick porphyrin layer. Current transport at moderate field strengths appears to be space‐charge limited in the presence of a discrete trapping level in the porphyrin, enhanced at higher fields by Poole–Frenkel emission in the oxide layer.

Bipolar device in which carrier lifetime is controlled

Abstract: A bipolar type semiconductor device includes a first semiconductor region (101) of a first conductivity type, a second semiconductor region (102) of a second conductivity type formed on the main surface of the first semiconductor region, a third semiconductor region (103) of the second conductivity type formed on the main surface of the second semiconductor region, a fourth semiconductor region (104) of the first conductivity type formed in at least part of the main surface area of the third semiconductor region (103), and a fifth semiconductor region (105) of the second conductivity type formed in at least part of the main surface area of the fourth semiconductor region (104). A low carrier-lifetime layer (110) having a peak of the number of recombination centers of carriers is formed in the second semiconductor region (102). The low carrier-lifetime layer (110) is formed by irradiating high-energy particles such as protons, particles, heavy hydrogen, or hydrogen molecule ions from the rear surface side of the first semiconductor region (101). In a case where the second semiconductor region (102) is not formed, the low carrier-lifetime layer (110) is formed in that part of the third semiconductor region (103) which lies near the junction between the first semiconductor region (101) and the first semiconductor region (103).

Semiconductor input protection device

Abstract: A semiconductor input protection device is disclosed which comprises a well type punch-through transistor consisting of a pair of parallel-opposed well layers through intermediation of a field oxide film, one of which is connected to an input terminal and the other to a reference potential. The device further comprises an impurity diffusion layer resistance with an end thereof connected to the input terminal. The lower limit distance between the opposed sides of the well layers to each other and the channel stopper is set to be smaller than that between the channel stopper and the input terminal-side well layer in the area where the latter and the impurity diffusion layer resistance intersect. The two lower limit distances depend on punch-through voltage, the width of the depletion layer in the well layer at applied punch-through voltage, and the junction disruptive strength of the well layer.

A model for the non-linear base-collector depletion layer charge and its influence on intermodulation distortion in bipolar transistors

Abstract: A model of the collector-base depletion layer is worked out. Doping profiles of base and epitaxial layer to substrate transition are approximated by exponential functions. The depletion approximation is used. The model provides relations between transistor doping profiles and collector-base capacitance as a function of bias voltage and collector current density. Higher order derivatives of the capacitance with respect to bias voltage and collector current are calculated and their influence on third order harmonic distortion is shown. Relations between transistor doping profiles and third order distortion are evaluated. Since the model is essentially one-dimensional it is only valid at low and intermediate collector current densities.