An improved technique and experimental results for the extractionof electron and hole mobilities in MOS accumulation layers

For the first time, experimental results are presented for electron and hole mobilities in the electron and hole accumulation layers of a MOSFET for a wide range of doping concentrations. Also presented is an improved methodology that has been developed in order to enable more accurate extraction of the accumulation layer mobility. The measured accumulation layer mobility for both electrons and holes is observed to follow a universal behavior at high transverse electric fields, similar to that observed for minority carriers in MOS inversion layers. At low to moderate transverse fields, the effective carrier mobility values are greater than the bulk mobility values for the highest doping levels. This is due to screening by accumulated carriers of the ionized impurity scattering by accumulated carriers, which dominates at higher doping concentrations. For lower doping levels, surface phonon scattering is dominant at low to moderate transverse fields so that the carrier mobility is below the bulk mobility value     

Dependence of charge carrier mobility of 4,4′,4″-tris(N-3-methylphenyl-N-phenylamino)triphenylamine on doping concentration of tetrafluoro-tetracyano-quinodimethane

Electrical transport of pure and tetrafluoro-tetracyano-quinodimethane doped 4,4′,4″-tris(N-3-methylphenyl-N-phenylamino)triphenylamine (m-MTDATA) films have been studied at various temperatures and doping concentrations. Pure films show space charge limited conduction with field and temperature dependent mobility. The J-V characteristics of doped m-MTDATA were ohmic at low voltages due to thermally released carriers from dopant states. At higher voltages the current density increases nonlinearly due to field dependent mobility and carrier concentration thereby filling of tail states of HOMO of the host. The conductivity of doped films were analysed using the Unified Gaussian Disorder Model (UGDM). The carrier concentration obtained from the fitting show a non-linear dependence on doping concentration which may be due to a combined effect of thermally activated carrier generation and increased carrier mobility.     

Minority-charge-carrier mobility at low injection level in semiconductors

From the kinetic equations, the distribution functions for majority and minority charge carriers are obtained at a low injection level. For describing the electron-hole collisions, the Landau collision integral is used. The carrier scattering at ionized or neutral impurity and at acoustic phonons is taken into account. The majority-carrier distribution function is presented in the analytical form. The minority-carrier mobility is calculated and analyzed, and the features of its behavior at low temperatures are revealed. It follows from the developed theory that the hole mobility in an n-type material increases with doping and neutral-impurity concentration. This effect is attributed to mutual charge-carrier collisions and different effective masses of different-sign carriers.     

Defect Modulation Doping

The doping of semiconductor materials is a fundamental part of modern technology, but the classical approaches have in many cases reached their limits both in regard to achievable charge carrier density as well as mobility. Modulation doping, a mechanism that exploits the energy band alignment at an interface between two materials to induce free charge carriers in one of them, is shown to circumvent the mobility restriction. Due to an alignment of doping limits by intrinsic defects, however, the carrier density limit cannot be lifted using this approach. Here, a novel doping strategy using defects in a wide bandgap material to dope the surface of a second semiconductor layer of dissimilar nature is presented. It is shown that by depositing an insulator on a semiconductor material, the conductivity of the layer stack can be increased by 7 orders of magnitude, without the necessity of high‐temperature processes or epitaxial growth. This approach has the potential to circumvent limits to both carrier mobility and density, opening up new possibilities in semiconductor device fabrication, particularly for the emerging field of oxide thin film electronics.     

A comprehensive model for inversion layer hole mobility forsimulation of submicrometer MOSFET's

A comprehensive model of effective (average) mobility and local-field mobility for holes in MOSFET inversion layers is presented. The semiempirical equation for effective mobility, coupled with the new local-field mobility model, permits accurate two-dimensional simulation of source-to-drain current in MOSFETs. The model accounts for the dependence of mobility on transverse and longitudinal electric fields, channel doping concentration, fixed interface charge density, and temperature. It accounts not only for the scattering by fixed interface charges, and bulk and surface acoustic phonons, but it also correctly describes screened Coulomb scattering at low effective transverse fields (near threshold) and surface roughness scattering at high effective transverse fields. The model is therefore applicable over a much wider range of conditions compared to earlier reported inversion layer hole mobility models while maintaining a physically based character     

Measurement of hole mobility in heavily doped n-type silicon

Accurate measurements of the mobility (and diffusion coefficient) of minority-carrier holes in Si:P with doping in the 10¹⁹cm^-3range have been done. The technique employed the measurement of diffusion length by means of lateral bipolar transistors of varied base widths, and the measurement of minority-carrier lifetime on the same wafers from the time decay of luminescence radiation after excitation with a short laser pulse. Minority-carrier hole mobility is found to be about a factor of two higher than the mobility of holes as majority carriers in p-type Si of identical doping levels.     

A new approach to verify and derive a transverse-field-dependentmobility model for electrons in MOS inversion layers

A modeling approach is described that extracts the functional dependence of carrier mobility on local transverse and longitudinal fields, channel doping, fixed interface charge, and temperature in MOS inversion and accumulation layers directly from the experimentally measured effective (or average) mobility. This approach does not require a priori detailed knowledge of the experimental variation of mobility within the inversion or accumulation layer, and it can be used to evaluate the validity of other models described in the literature. Also, an improved transverse-field dependent mobility model is presented for electrons in MOS inversion layers that was developed using this new modeling approach. This model has been implemented in the PISCES 2-D device simulation program. Comparisons of the calculated versus measured data show excellent agreement for I_D-V_G and I_D-V_D curves for devices with L_eff=0.5 to 1.2 μm     

应变Si_(1-x)Ge_x材料掺杂浓度的表征技术

在分析研究Si1 -xGex 材料多子迁移率模型的基础上,建立了Si1 -xGex 材料电阻率与其Ge组分、掺杂浓度及温度关系的曲线谱图.同时,通过对半导体材料掺杂浓度各种表征技术的分析和实验研究,提出了采用四探针法表征Si1 -xGex 材料掺杂浓度的技术.此表征技术与Si材料掺杂浓度的在线检测技术兼容,且更加简捷.此表征技术的可行性通过实验及对Si1 -xGex 材料样品掺杂浓度的理化分析得到了验证     

应变Si1-xGex材料掺杂浓度的表征技术

在分析研究Sil-xGex材料多子迁移率模型的基础上，建立了Sil-xGex材料电阻率与其Ge组分、掺杂浓度及温度关系的曲线谱图．同时，通过对半导体材料掺杂浓度各种表征技术的分析和实验研究，提出了采用四探针法表征Sil-xGex材料掺杂浓度的技术．此表征技术与Si材料掺杂浓度的在线检测技术兼容，且更加简捷．此表征技术的可行性通过实验及对Sil-xGex材料样品掺杂浓度的理化分析得到了验证．     

Effect of Doping and Excitation Wavelength on Charge Carrier Dynamics in Hematite by Time‐Resolved Microwave and Terahertz Photoconductivity

The charge carrier dynamics of epitaxial hematite films is studied by time‐resolved microwave (TRMC) and time‐resolved terahertz conductivity (TRTC). After excitation with above bandgap illumination, the TRTC signal decays within 3 ps, consistent with previous reports of charge carrier localization times in hematite. The TRMC measurements probe charge carrier dynamics at longer timescales, exhibiting biexponential decay with characteristic time constants of ≈20–50 ns and 1–2 μs. From the change in photoconductance, the effective carrier mobility is extracted, defined as the product of the charge carrier mobility and photogeneration yield, of differently doped (undoped, Ti, Sn, Zn) hematite films for excitation wavelengths of 355 and 532 nm. It is shown that, unlike in conventional semiconductors, donor doping of hematite dramatically increases the effective mobility of the photogenerated carriers. Furthermore, it is shown that all hematite films possess higher effective mobility for 355 nm excitation than for 532 nm excitation, although the time dependence of the photoconductance decay, or charge carrier lifetime, remains the same. These results provide an explanation for the wavelength dependent photoelectrochemical behavior of hematite photoelectrodes and suggest that an increase in photogeneration yield or charge carrier mobility is responsible for the improved performance at higher excitation energies.     

Velocity-field characteristics of electrons in doped GaAs

We report the first comprehensive measurements of the velocity-electric field charac-teristics of electrons in doped GaAs with carrier concentrations between 1 × 10¹⁵ and 1.5 × 10¹⁸ cm^s-3 at both 77 and 300 K. These measurements from 0 to 9000 V/cm make use of a 35 GHz technique in order to avoid charge domain formation which would render a DC measurement impossible in regions of negative differential resistance. We observe peak velocities at 300 K of between 0.9 × 10⁷ and 2.2 × 10⁷ cm/s in this doping range. Higher doping results in lower peak velocities which occur at higher electric fields. At 77 K, the trend is the same as at 300 K with peak velocities of between 1.1 × 10⁷ and 3.5 × 10⁷ cm/s. In addition, these measurements demonstrate anincrease in electron differential mobility in GaAs with increasing electric field. This effect occurs in moderately doped GaAs because the warm electrons which have been heated by the electric field scatter less efficiently with the ionized impurities than do the colder elec-trons. The increasing differential mobility occurs at electric fields well below the critical field for the onset of negative differential mobility and is more pronounced at 77 K than at 300 K. These results are significant both for the understanding of hot carrier trans-port in doped semiconductors as well as for understanding and modelling GaAs MESFETs.     

Tunable Dopants with Intrinsic Counterion Separation Reveal the Effects of Electron Affinity on Dopant Intercalation and Free Carrier Production in Sequentially Doped Conjugated Polymer Films

Carrier mobility in doped conjugated polymers is limited by Coulomb interactions with dopant counterions. This complicates studying the effect of the dopant's oxidation potential on carrier generation because different dopants have different Coulomb interactions with polarons on the polymer backbone. Here, dodecaborane (DDB)‐based dopants are used, which electrostatically shield counterions from carriers and have tunable redox potentials at constant size and shape. DDB dopants produce mobile carriers due to spatial separation of the counterion, and those with greater energetic offsets produce more carriers. Neutron reflectometry indicates that dopant infiltration into conjugated polymer films is redox‐potential‐driven. Remarkably, X‐ray scattering shows that despite their large 2‐nm size, DDBs intercalate into the crystalline polymer lamellae like small molecules, indicating that this is the preferred location for dopants of any size. These findings elucidate why doping conjugated polymers usually produces integer, rather than partial charge transfer: dopant counterions effectively intercalate into the lamellae, far from the polarons on the polymer backbone. Finally, it is shown that the IR spectrum provides a simple way to determine polaron mobility. Overall, higher oxidation potentials lead to higher doping efficiencies, with values reaching 100% for driving forces sufficient to dope poorly crystalline regions of the film.     

Dielectric–Semiconductor Interface Limits Charge Carrier Motion at Elevated Temperatures and Large Carrier Densities in a High‐Mobility Organic Semiconductor

The fundamental nature of charge transport in highly ordered organic semiconductors is under constant debate. At cryogenic temperatures, effects within the semiconductor such as traps or the interaction of charge carriers with the insulating substrate (dipolar disorder or Fröhlich polarons) are known to limit carrier motion. In comparison, at elevated temperatures, where charge carrier mobility often also decreases as function of temperature, phonon scattering or dynamic disorder are frequently discussed mechanisms, but the exact microscopic cause that limits carrier motion is debated. Here, the mobility in the temperature range between 200 and 420 K as function of carrier density is explored in highly ordered perylene‐diimide from 3 to 9 nm thin films. It is observed that above room temperature increasing the gate electric field or decreasing the semiconducting film thickness leads to a suppression of the charge carrier mobility. Via X‐ray diffraction measurements at various temperatures and electric fields, changes of the thin film structure are excluded as cause for the observed mobility decrease. The experimental findings point toward scattering sites or traps at the semiconductor–dielectric interface, or in the dielectric as limiting factor for carrier mobility, whose role is usually neglected at elevated temperatures.     

Chlorine-doped CdS thin films from CdCl<Subscript>2</Subscript>-mixed CdS powder

The CdS:Cl thin films have been prepared using thermally evaporated, CdCl₂-mixed CdS powder at 200°C substrate temperature. The percentage of CdCl₂ in the mixture varied from 0% to 0.20%. The electrical properties and the grain size of the deposited films were investigated. The results show that light doping, resistivity, carrier concentration, and mobility follow Seto’s model for polycrystalline material. However, with heavy doping, these properties undergo a saturation trend. The saturation behavior can be understood in terms of the rapid formation of the A-center complexes in the films. The deposited films were annealed at 250°C and 300°C. The resistivity of pure and lightly doped CdS films increased with annealing temperature, whereas carrier concentration and mobility in these films decreased. However, for the higher doping concentrations, the resistivity decreased, whereas carrier concentration and mobility showed improvement. These changes in electrical properties of the deposited films with annealing and doping concentration are attributed to a reduction in the lattice defect sites in CdS upon annealing. The experimental results are interpreted in terms of a modified version of Seto’s model for polycrystalline materials.     

Filamentation and beam propagation in broad-area semiconductorlasers

The spatio-temporal dynamics of broad-area semiconductor lasers is studied theoretically by numerically solving space-dependent coupled partial differential equations for the (complex) optical fields, the interband polarization and the density of charge carriers. The formation and longitudinal propagation of unstable transverse optical filamentary structures is analyzed in a model configuration for typical double-heterostructure broad-area lasers. Spectral and spatial holeburning in the carrier distributions is observed as a consequence of selective density depletion by the transverse laser modes. The transverse holeburning leads to complex spatio-temporal patterns     

An improved electron and hole mobility model for general purposedevice simulation

A new, comprehensive, physically-based, semiempirical, local model for transverse-field dependent electron and hole mobility in MOS transistors is presented. In order to accurately predict the measured relationship between the effective mobility and effective electric field over a wide range of substrate doping and bias, we account for the dependence of surface roughness limited mobility on the inversion charge density, in addition to including the effect of coulomb screening of impurities by charge carriers in the bulk mobility term. The result is a single mobility model applicable throughout a generalized device structure that gives good agreement with measured mobility data and measured MOS I-V characteristics over a wide range of substrate doping, channel length, transverse electric field, substrate bias, and temperature     

Some characteristics of highly N-doped VPE grown GaAs epilayers

The doping behaviour of S and Se in the VPE growth of GaAs at 760 and 660°C is studied by carrier concentra-tion, mobility, photo and cathodoluminescence measure-ments. The carrier concentration increases linearly with the partial pressure of S or H₂Se up to a solubi-lity limit, the highest value of which is obtained with Se at about 10¹⁹cm^-3. The mobility for Se-doped layers is higher than for S-doped ones when n > 4.10¹⁷cm^-3, and a mobility decrease is observed for both at dopings exceeding the solubility limit. Postgrowth annealing at the growth temperature of highly doped samples decreases their carrier concentration to values corresponding to the bulk solubility limit. The decrease of the room temperature luminescence intensity at high doping levels in tentatively interpreted as due to precipitate for-mation. Finally, the linear dependence on the dopant partial pressure of the impurity incorporation as well as the observed annealing behaviour are interpreted by an incorporation mechanism controlled by the surface states.     

Spectral response limitation mechanisms of a shallow junction n+-p photodiode

Modulated monochromatic visible light at various wavelengths was used to generate photocurrent in a silicon n⁺-p diffused diode. A numerical model which includes electric field, heavy doping bandgap reduction, and doping level mobility dependence was used with fitting techniques to determine the carrier lifetime in the n⁺-region at each wavelength. From these measurements it was concluded that the physical mechanisms involved in limiting the spectral response of the n⁺-p photodiode at short wavelengths (0.42 µm) is due to heavy recombination of photogenerated carriers in the n⁺-region. The latter is caused by the heavy doping which results in a fraction of a nanosecond minority carrier lifetime and a retarding or reduced electric field in the n⁺-region. Surface recombination velocity has little influence on this loss mechanism.     

Comparison of the saturated current in normal and inverted modulation-doped In0.53Ga0.47As/InP structures

We measured the saturated current in both normal (doping layer on top) and inverted (doping layer on bottom) structures of modulation-doped In0.53Ga0.47As/InP. In the inverted structure, we find a saturated current consistent with the low-field carrier density and the bulk electron saturated velocity; but for the normal structure the saturated current is significantly lower than expected. Measurements indicate that this low saturation current is due to a loss of carriers at high fields.     

Nonlinear interaction of acoustic surface waves in epitaxial gallium arsenide

A technique is described for obtaining the nonlinear interaction between two surface waves propagating in a piezoelectric semiconductor. The coupling between the waves is associated with the transverse electric fields of the surface wave and the carriers in the semiconductor. The effective carrier concentration for the interaction has been altered by forming a surface-depletion region beneath a metal?semiconductor contact. This scheme has been used to produce the convolution of two oppositely travelling acoustic waves in epitaxial gallium arsenide, and it has been found that a strong nonlinearity can be obtained at small acoustic-power levels.