Thermal Generation Rate Research Articles

Photoconductivity gain over 10 at a large electric field in wide gap a-Si:H

Wide band gap amorphous silicon films and p–i–n diodes were prepared by a chemical annealing method. Films had dark conductivities consistent with the large band gaps, low impurity levels, intrinsic conduction, and corresponding low thermal generation rates. Primary photo current measurements of these films was consistent with an electron μτ of 10−8 cm2/V. To provide blocking at large electric fields, p and n-layers were optimized . Since it is known that at electric fields greater than 106 V/cm other amorphous materials such as a-Se exhibit avalanche multiplication, the wide band gap amorphous silicon p–i–n diodes were used to probe the prospect of avalanche multiplication in the amorphous silicon system.

Comparison of the performance of quantum well and conventional bulk infrared photodetectors

Investigations of the performance of quantum well infrared photodetectors (QWIPs) as compared o other types of semiconductor infrared detectors are presented. Two types of QWIPs are considered: GaAs/AlGaAs intersubband quantum well photoconductors and type II staggered InAs/InGaSb photodiodes. In comparative studies the HgCdTe photoconductors and photodiodes, PbSnTe photodiodes, Schottky barrier photoemissive detectors and doped silicon detectors are considered. It is assumed that the performance of HgCdTe and PbSnTe photodiodes is due to thermal generation governed by the Auger mechanism in the base regions. Investigations of the fundamental physical limitations of HgCdTe photodiodes indicate better performance of this type of detectors in comparison with GaAs/AlGaAs QWIPs operated in the range 35 to 77 K. In practice however, in temperature range below 50 K the performance of the HgCdTe photodiodes is determined by trap-assited tunneling. As a result, advantage of GaAs/AlGaAs QWIPs increases in the wider spectral range (below 14 μm) and temperature below 50 K. This observation plus the maturity of GaAs/AlGaAs technology and its radiation hard characteristics promise that QWIPs technology can produce high quality focal plane arrays (FPAs) for space applications. The operating temperature for HgCdTe detectors is higher than for other types of photondetectors. HgCdTe detectors with background limited performance operate with thermoelectric coolers in the medium wavelength range, instead the long wavelength detectors operate at ≈ 100 K. HgCdTe is characterized by high absorption coefficient and quantum efficiency and relatively low thermal generation rate compared to extrinsic detectors, silicide Schottky barriers and GaAs/AlGaAs QWIPs. However, the cooling requirments for GaAs/AlGaAs QWIPs with cutoff wavelengths below 10 μm are less stringent in comparison with extrinsic detectors and Schottky barrier devices. The theoretically predicted performance of long wavelength InAs/GaInSb photodiodes are comparable with HgCdTe photodiodes. The high performance of InAs/InGaSb detectors is due to suppression of band-to-band Auger recombination rate.

Ultimate performance of infrared photodetectors and figure of merit of detector material

A general model describing the ultimate performance of infrared photodetectors arising from fundamental properties of a semiconductor material is presented. The model is applicable to a wide variety of infrared photodetectors and materials. It was shown that the ultimate performance of any type of infrared photodetectors is proportional to (α/G) 1 2 , where α is the absorption coefficient and G is the thermal generation rate. Therefore, α/ G is a figure of merit which can be used to compare various materials for infrared photodetectors. The α/ G factor and ultimate detectivity has been calculated for (Hg,Cd)Te photodetectors operating in the 2–20 μm range at temperatures of 200–300 K. It was shown that thicker active regions are required for longer wavelength devices. Ways of achieving the ultimate performance in practice are discussed. The theoretical predictions have been compared with the experimental performance of near room temperature photodetectors.

Dynamic hydro-thermal economic dispatch with water availability constraints and variable water head

Operation of a system having both hydro and thermal plants is quite complex as hydro plants have negligible operating cost, but are required to operate under constraints of water available for hydro generation in a given period of time. The problem thus belongs to the realm of dynamic optimization. The earliest work on optimal load flow for hydro-thermal systems was probably due to Ramamoorthy [1]. Head variation was neglected and nonlinear programming techniques were employed. In this paper, the problem of minimizing the operating cost of a hydro-thermal system can be viewed as one of minimizing the fuel cost of thermal plants under the constraint of water availability (storage and inflow) and variable water head for hydro generation over a given period of operation. Optimization will be carried out with thermal power generation and water discharge rate as control variables, with transmission losses accounted for by the exact network model, namely the load flow equations. The problem is solved here using nonlinear programming technique in conjunction with successive approximation method. The method of solution has been accelerated to reduce the computational time burden using the first order gradient method. Because of the large-scale nature of the problem, special attention is paid to finding convergence characteristics and realistic initial guess estimates for the algorithm.

Comment on ‘‘Temperature limits on infrared detectivities of InAs/InxGa1−xSb superlattices and bulk Hg1−xCdxTe’’ [J. Appl. Phys. 74, 4774 (1993)

Three papers published recently [P. M. Young, C. H. Grein, H. Ehrenreich, and R. H. Miles, J. Appl. Phys. 74, 4774 (1993); G. M. Williams, J. Appl. Phys. 77, 4153 (1995); C. H. Grein, M. E. Flatte, H. Ehrenreich, and R. H. Miles, J. Appl. Phys. 77, 4156 (1995)] concerning the comparison of theoretically predicted performance of HgCdTe photodiodes with InAs/InGaSb superlattice photovoltaic detectors are obscure with respect to optimal selection of HgCdTe photodiode structures. Both Comments [G. M. Williams, J. Appl. Phys. 77, 4153 (1995); C. H. Grein, M. E. Flatte, H. Ehrenreich, and R. H. Miles, J. Appl. Phys. 77, 4156 (1995)] have not noticed the important results of papers published by Humpreys [Infrared Phys. 23, 171 (1983); Infrared Phys. 26, 337 (1986)], who critically reexamined the role of a radiative mechanism in the detection of infrared radiation. To explain our point of view on competition between InAs/InGaSb SLs and ‘‘bulk’’ HgCdTe detectors, we present a generalized model of an infrared photodetector and derive the figure merit of any material for infrared photodetector as the ratio of absorption coefficient to the thermal generation rate. This determines the detectivity of infrared photodetectors. Using that model for the most common n+-p and p+-n long wavelength HgCdTe photodiodes, it is shown that the highest performance can be obtained with a low doping of the base photodiode regions. This means that the previously assumed (Young et al.) highly doped HgCdTe photodiode structure with a thick base region is far from optimal. Our calculations carried out for optimal device structures indicate that the ultimate detectivities of long wavelength HgCdTe photodiodes operating at 77 K are higher than those for InAs/InGaSb SLs.

96/00377 Modelling and verification of the PWR burnable poison designs by ELCOS code system

The heat generation within a shield can be a primary consideration in its design, especially when the shield is for high power reactor in which rather large temperature increases can be expected. The heating up of the shield itself can be utilized via a suitable flow of water on the outside of the said shield or, in the case of thermal shields, using the nuclear reactor coolant leading to energy saving in nuclear power plants. The thermal energy generation rate inside the shielding materials is calculated using the build up factor method for multi-energy photons emitted from disc geometry radiation source and a comparison has been made in order to estimate the percentage of energy captured, and therefore saved, inside the shielding layers. In this paper the use of build up factor is suggested for thermal energy estimation in shielding materials such as Al, Fe and Pb and comparison with the results produced from the application of MCNP-4A computer code. This is a continuation of the work done by Bakos [Bakos, G.C., 1995. Benchmark data for g-rays emitted by an Na-24 source penetrating stratified shielding slabs. IEEE Trans. Nucl. Sci. 42 (2), 61–65].

Generation of Electron Traps at High Field in Silicon Oxide Films

Temperature dependences of the high-field electron trapping in a SiO2 thin film for temperature ranging from 100 to 423 K are investigated. It is found that in the investigated temperature range, when the temperature decreases the effective surface density of the electron traps in the film decreases; the energy levels of the effective electron traps at high field concentrate at very narrow energy range. The thermal generation rate is found to be 1.283 × 1010 / cm2·K and its activation energy is 0.192 eV. Based on these results, a model for the electron traps generated at high field in thin oxide is proposed.

Noise performance of bound-to-miniband transition III-V quantum-well infrared photodetectors

Dark current noise measurements between 10 and 105 Hz were carried out at T=77 K on three different types of III-V quantum-well infrared photodetectors designed for 8–12 μm IR detection. These devices have superlattice barriers leading to miniband transport in the extended conduction band. For frequencies between 102 and 104 Hz, noise plateau levels stemming from the trapping and detrapping of electrons in the quantum-well bound states are observed. From the measured noise data the low-bias electron diffusion length, the bias-dependent noise gain, the electron trapping probability, and the thermal electron generation rate are calculated.

A vertically integrated bipolar storage cell in 6H silicon carbide for nonvolatile memory applications

A vertically integrated one transistor memory cell, in which an n-p-n bipolar access transistor is merged with a p-n-p storage capacitor, based on the wide-bandgap semiconductor silicon carbide (SiC), results in a greatly reduced thermal generation rate. Extrapolation of charge recovery data obtained at elevated temperatures suggests a room temperature recovery time of over 10/sup 6/ years.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Unexpected increase in the thermal generation rate of bulk GaAs due to electron-beam metallization

It is demonstrated that the thermal generation rate in the bulk depletion regions of GaAs p-n junctions of samples metallized in e-beam evaporators can be as much as three-orders-of-magnitude greater than those metallized in thermal evaporators. This degradation is unexpected because the low-energy radiation present in such e-beam evaporation systems is unable to cause the atomic displacement responsible for defect creation in GaAs. Results derived from samples metallized in the e-beam evaporator indicate, however, that low-energy electrons are somehow involved in the formation of the defects responsible for the increased generation rate. Experiments performed in controlled-radiation environments have demonstrated clearly that simple radiation damage either from electrons or from X-rays is insufficient to degrade these GaAs devices. The simple diffusion of impurity ions in the absence of a high fluence of a low-energy electrons is also insufficient. Therefore, the experimental results suggest a damage mechanism in which the presence of both low-energy electrons and impurity ions is necessary to cause the increase in generation rates observed in bulk GaAs. >

Increased thermal generation rate in GaAs due to electron-beam metallization

Leakage currents due to thermal generation in a reverse-biased p-n junction can be accurately monitored by measuring the capacitance recovery transient of a p-n-p structure. Using this technique, it has been demonstrated that the thermal generation in the bulk depletion region of GaAs p-n junctions grown by molecular beam epitaxy can be as much as three orders of magnitude greater for samples metallized in electron-beam evaporators as compared to thermal evaporators. The increase in thermal generation rate is shown to be dependent upon the device area exposed during the evaporation, the type of metal initially evaporated onto the sample, the growth conditions during molecular beam epitaxy, and the depth of the p-n junction from the semiconductor surface.

Temperature dependence of photoconductivity in buckminsterfullerene films

We report the temperature dependence from 330 to 230 K of the photoconductivity measured in sublimed films of buckminsterfullerene, specifically, C60/70, up to 3 μm thick. In contrast to the temperature dependence of the dark conductivity, where reversible structure, associated with the onset of a phase transition to orientational ordering below 250 K, is observed, the photoconductivity shows an essentially monotonic decrease to lower temperatures. This provides additional evidence that the thermal generation rate increases as the C60/70 becomes orientationally ordered. It also suggests that the increased order below the transition temperature does not lead to significantly increased carrier mobilities.

Conductivity and light-induced absorption in BaTiO3

A charge transport model including deep and shallow traps explains both the nonlinear relation between photoconductivity and light intensity and the light-induced absorption in BaTiO3. A correlation between measurements of photoconductivity and light-induced absorption as a function of temperature yields parameters for the shallow center, among them thermal activation energy and generation rate.

Interface properties of plasma-enhanced chemical vapor deposited SiOxNy/n-GaAs metal-insulator-semiconductor system

Interface properties of plasma-enhanced chemical vapor deposited SiOxNy/n-GaAs metal-insulator-semiconductor systems are investigated by capacitance-voltage and deep level transient spectroscopy measurements. At room temperature, these metal-insulator-semiconductor devices exhibit deep depletion mode operation which is characteristic of the low thermal generation rate for minority carriers in GaAs and the large number of traps located at or near the interface. For accumulation mode bias voltage, the small signal capacitance does not approach the SiOxNy capacitance and exhibits anomalous frequency dispersion behavior. A simple method is used to correlate the capacitance-voltage and deep level transient spectroscopy data. An interface-state band model best fits the experimental results.

Electrophotographic Properties of n+-a-Si:H/a-Si:H/a-Sic:H Heterostructures

ABSTRACTFor negative charging electrophotographic applications a three layer heterostructure (Al/n+-a-Si:H/a-Si:H/a-SiC:H) has been investigated. To some extent a separation of the function of charge storage by a high band gap amorphous silicon carbon alloy and of that of photogeneration of carriers and of sufficient photosensitivity in an intrinsic amorphous silicon film has been achieved. Due to a low injection of electrons from the surface into the a-SiC:H and a small thermal generation rate of carriers within the a- Si:H film, the dark decay of the negative surface potential is dominated by space charge limited currents of holes. Thus a considerable difference in surface potential for negative and positive charging occurs, which is related to the difference in electron and hole mobility in a-Si:H. Since there is no remarkable barrier at the heterointerface a-Si:H/a-SiC:H an excellent injection of photogenerated holes into the a-SiC:H can be provided.

Thermal Energy Generation of LiAl / FeS Cells

Two independent methods were employed to determine the rate of thermal energy generation in 200 A‐hr lithium‐aluminum/iron sulfide cells. The first is an indirect method using thermodynamic calculations based on precise measurements of the cell potential as a function of temperature and state of charge, combined with measurements of the overpotential during discharge. The second is the direct measurement of the cell heat generation using a new high temperature battery calorimeter. The results obtained by these two methods are in excellent agreement. The rate of heat generation is 40–50W per cell during the last 30% of discharge at 100A. Under certain conditions the cell reaction, initially exothermic, becomes endothermic and then exothermic again during discharge. This unusual behavior is caused by the formation of intermediate J‐phase and X‐phase compounds during the course of discharge.

Bipolar model for traveling-wave induced nonequilibrium double-layer streaming in insulating liquids

A model is proposed for the time evolution of ion concentrations in a thin charge layer near an interface between an insulating solid and a semi-insulating bipolar liquid, subjected to an applied electric field. Given the specific case of a traveling-wave applied field of peak magnitude E0 and planar geometry, solutions for the charge density and electric field in the layer are used to calculate the time-average stress moment and, hence, the pumping velocity of the liquid. Analytic solutions, valid in the regimes of small applied field magnitude (E0LD/Vt≪1, where L0 and Vt are the Debye length and thermal voltage, respectively) or frequency (ωε/σ≪1, ω the angular frequency and ε and σ the permittivity and equilibrium conductivity, respectively), predict charge layers with a characteristic dimension of the Debye length and fluid pumping in the direction of propagation of the traveling wave (forward). Numerical solutions, in the regime of large magnitude fields with ωε/σ∼1, predict either forward or backward pumping, as well as a charge layer with thickness on the order of a migration length (L = 2πbE0/ω, where b is the ion mobility). Parameters such as ion mobility, thermal generation rate, and level of ionization in the liquid are important in determining the rate (and even direction) of the pumping.

The steady-state optical response of the homojunction triangular barrier photodiode

The n-type homojunction triangular barrier photodiode (TBP) is shown to have an extremely high optical gain of several thousands. This high gain at low light levels is shown to result from the low thermal generation rate of holes in the active region of the TBP, indicating a small concentration of deep traps in the material. A detailed analysis of the dependence of responsivity on applied voltage bias, incident optical power level, and physical device parameters has been made. This analysis indicates that the bandwidth of the TBP can be increased by either increasing the applied bias or by significantly increasing the intensity of the incident light.

On the rate theory for thermal soliton generation

The use of absolute rate theory is suggested for a simple description of the dynamics of thermally generated solitons in the underdamped regime. Rate theory is reformulated to describe thermal transitions between continuous states of nonlinear wave equations emphasizing the soliton or pseudoparticle interpretation. The final rate expression, which applies also to metastable states, is compared with that for the decay of unstable vacua in quantum field theory. As an example, the thermal generation rate of distant kink pairs on the Sine Gordon chain is calculated exactly and also within the semiclassical approximation.

Inversion and accumulation layer formation at elevated temperatures in n-type GaAs-anodic oxide MIS devices

At room temperature n-type GaAs (n-GaAs)-anodic oxide MIS devices typically exhibit some form of deep depletion mode operation which is characteristic of a low thermal generation rate for minority carriers in the semiconductor. In addition, for accumulation mode bias voltages the small signal capacitance often does not approach the oxide capacitance and instead exhibits a dispersion in which this capacitance varies with the frequency of the a.c. signal and the sweep rate of the d.c. bias voltage. In this paper, capacitance-voltage measurements taken at elevated temperatures with varying d.c. bias voltage sweep rates are reported for n-GaAs-anodic oxide MIS devices. The data indicate that a stable inversion layer is formed even at high sweep rates, and no accumulation mode capacitance dispersion is observed for a capacitance-voltage measurement frequency of up to 1 MHz at temperatures above room temperature. At high temperatures and slow sweep rates, considerable mobile ion motion in the oxide is observed. A discussion of this observation based on recently developed theories for oxide growth is presented.