Self-consistent model of minority-carrier lifetime, diffusionlength, and mobility

A self-consistent approach is proposed for extracting the minority-carrier mobility from fits to experimental data for lifetime and diffusion length and then comparing the extracted mobility to experimental mobility data. A value for electron and hole lifetime is extracted using a doping-dependent Shockley-Read-Hall mechanism with an Auger process. The hole lifetime is used to extract a minority carrier hole mobility that is consistent with the reported measurements of the hole diffusion length. The good agreement between extracted and experimental mobilities justifies incorporating the results into numerical device and circuit CAD tools     

Minority-carrier diffusion coefficients and mobilities in silicon

A new method for accurate measurement of minority-carrier diffusion coefficients in silicon is described. The method is based on a direct measurement of the minority-carrier transit time through a narrow region of the p-n junction diode. The minority-carrier mobility is obtained from the diffusion coefficient using the Einstein relation. The method is demonstrated on low-doped n- and -p-type Si (dopings ∼10¹⁵cm^-3) and is compared with the literature data for the majority-carrier mobilities. The results show that in low-doped Si the electron minority- and -majority-carrier mobilities are comparable, but the hole minority-carrier mobility is significantly higher (∼30 percent) than the corresponding majority-carrier value. The results confirm earlier data of Dziewior and Silber.     

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.     

On the universality of inversion layer mobility in Si MOSFET's:Part I-effects of substrate impurity concentration

This paper reports the studies of the inversion layer mobility in n- and p-channel Si MOSFET's with a wide range of substrate impurity concentrations (10¹⁵ to 10¹⁸ cm^-3). The validity and limitations of the universal relationship between the inversion layer mobility and the effective normal field (E_eff) are examined. It is found that the universality of both the electron and hole mobilities does hold up to 10¹⁸ cm ^-3. The E_eff dependences of the universal curves are observed to differ between electrons and holes, particularly at lower temperatures. This result means a different influence of surface roughness scattering on the electron and hole transports. On substrates with higher impurity concentrations, the electron and hole mobilities significantly deviate from the universal curves at lower surface carrier concentrations because of Coulomb scattering by the substrate impurity. Also, the deviation caused by the charged centers at the Si/SiO₂ interface is observed in the mobility of MOSFET's degraded by Fowler-Nordheim electron injection     

Impact of Materials versus Geometric Parameters on the Contact Resistance in Organic Thin‐Film Transistors

The contact resistance is known to severely hamper the performance of organic thin‐film transistors, especially when dealing with large injection barriers, high mobility organic semiconductors, or short channel lengths. Here, the relative significance of how it is affected by materials‐parameters (mobility and interfacial level‐offsets) and geometric factors (bottom‐contact vs top‐contact geometries) is assessed. This is done using drift‐diffusion‐based simulations on idealized device structures aiming at a characterization of the “intrinsic” situation in the absence of traps, differences in the film morphology, or metal‐atoms diffusing into the organic semiconductor. It is found that, in contrast to common wisdom, in such a situation the top‐contact devices do not always outperform the bottom‐contact ones. In fact, the observed ratio between the contact resistances of the two device structures changes by up to two orders of magnitude depending on the assumed materials parameters. The contact resistance is also shown to be strongly dependent on the hole mobility in the organic semiconductor and influenced by the chosen point of operation of the device.     

The role of intercarrier scattering in excited silicon

We have used laser and electron-beam excitation to investigate the influence of intercarrier scattering upon several properties of high-purity silicon, viz. the drift and Hall mobilities, the ambiopolar diffusion coefficient, the optical absorption cross section (at 10.6 μm) and Verdet coefficient (at 1.06 μm), covering the carrier density range n = 10¹⁵ ? 4 · 10¹⁸ cm^?3. The mobility data obtained are in accordance with theoretical models that take both lattice scattering and electron-hole (e-h) scattering into account as the main scattering mechanisms. In the region above n ≈ 10¹⁷ cm^?3 the additional effects of electron-electron and hole-hole scattering must be considered as well. For the other parameters studied, however, lattice scattering alone is found to be quite dominant. Finally, the Caughey-Thomas formula for the mobilities in doped silicon has been modified to be applicable to the present case of a light-induced e-h plasma.     

Drift hole mobility in strained and unstrained doped Si1-xGex alloys

Results of the drift hole mobility in strained and unstrained SiGe alloys are reported for Ge fractions varying from 0 to 30% and doping levels of 10¹⁵-10¹⁹ cm^-3. The mobilities are calculated taking into account acoustic, optical, alloy, and ionized-impurity scattering. The mobilities are then compared with experimental results for a boron doping concentration of 2×10¹⁹ cm^-3. Good agreement between experimental and theoretical values is obtained. The results show an increase in the mobility relative to that of silicon     

Test structure design for the evaluation of carrier-carrierscattering effect on hole and electron mobilities

In this paper we present a new three-terminal test structure that enables one to simply evaluate the reduction in the hole and electron mobilities due to the carrier-carrier scattering effect. The use of a three-terminal device is necessary to remove the limitations of other measurement methods and to separately obtain μ_n and μ _p. An investigation of the role of the geometrical parameters of the test structure highlights its suitability and flexibility for the mobility extraction procedure. Numerical simulation is used to carefully design the test structure and to verify the accuracy of the measurement method. Experimental results obtained from n-type silicon regions are presented and compared to major analytical models     

The influence of deposition conditions and alloying on the electronic properties of amorphous selenium

Electronic properties of a-Se as a function of the source (boat) temperature and as a function of As (up to 0.7%) and Cl (up to 40 wt ppm) concentrations have been experimentally studied by carrying out conventional and interrupted field time-of-flight (IFTOF) transient photoconductivity measurements that provide accurate determinations of the drift mobility and the deep trapping time (lifetime). No variation in electron and hole lifetimes and mobilities for pure a-Se was observed with the source temperature, that is, no dependence was observed either on the deposition rate or on the vapor composition. The addition of As reduces the hole lifetime but does not change the hole mobility. At the same time, As addition increases the electron lifetime while reducing the electron mobility. The electron range μτ, however, increases with the As content, which means that the overall concentration of deep electron traps must be substantially reduced by the addition of As. Cl addition in the ppm range increases the hole lifetime but reduces the electron lifetime. The drift mobility of both carriers remains the same. We interpret the results in terms of a shallow-trap-controlled charge transport in which deep traps are due to potential under-and overcoordinated charged defects that can exist in the structure.     

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.     

Origin of high fill factor in polymer solar cells from semiconducting polymer with moderate charge carrier mobility

The fill factor of polymer bulk heterojunction solar cells (PSCs), which is mainly governed by the processes of charge carrier generation, recombination, transport and extraction, and the competition between them in the device, is one of the most important parameters that determine the power conversion efficiency of the device. We show that the fill factor of PSCs based on thieno[3,4-b]-thiophene/benzodithiophene (PTB7):[6,6]-phenyl C₇₁-butyric acid methylester (PC₇₁BM) blend that only have moderate carrier mobilities for hole and electron transport, can be enhanced to 76% by reducing the thickness of the photoactive layer. A drift–diffusion simulation study showed that reduced charge recombination loss is mainly responsible for the improvement of FF, as a result of manipulating spatial distribution of charge carrier in the photoactive layer. Furthermore, the reduction of the active layer thickness also leads to enhanced built-in electric field across the active layer, therefore can facilitate efficient charge carrier transport and extraction. Finally, the dependence of FF on charge carrier mobility and transport balance is also investigated theoretically, revealing that an ultrahigh FF of 80–82% is feasible if the charge mobility is high enough (∼10⁻³–10⁻¹ cm²/V s).     

Thin-film strained-SOI CMOS Devices-physical mechanisms for reduction of carrier mobility

We have examined physical mechanisms responsible for the reduction in both electron and hole mobility in strained-silicon-on-insulator (SOI) CMOS devices with thin strained-Si layers. A slight decrease in the electron mobility with thinning strained-Si layers is attributable to the quantum-mechanical confinement effect of the inversion layer electrons, originating in the conduction band offset of the strained-Si layers. Also, the diffusion of Ge atoms into the SiO/sub 2//strained-Si interface is found to generate interface states near the valence band edge, leading to the reduction in hole mobility in the lower E/sub eff/ region through Coulomb scattering. Moreover, the decrease in hole mobility enhancement in both thin and thick strained-Si structures at the higher electric field is caused by the reduction of the energy splitting between the heavy and the light hole bands, with an increase in the electric field. Based on considerations of these factors affecting the mobility reduction, the strained-Si thickness and the Ge content have been designed to realize high-speed strained-SOI CMOS under the 90-nm technology and beyond.     

Theoretical comparative studies of charge mobilities for molecular materials: Pet versus bnpery

Charge mobility is the most important issue for organic semiconductors. We calculate the electron and hole mobilities for prototypical polycyclic hydrocarbon molecules, perylothiophene (pet) and benzo(g,h,i)-perylene (bnpery) using Marcus electron transfer theory coupled with a diabatic model and a homogeneous diffusion assumption to obtain the charge mobility. The first-principles DFT calculations show that the hole mobility is about an order of magnitude higher than the electron mobility in pet. However, we find that for bnpery, the electron and hole transports are balanced, namely, very close in mobility, indicating the possible application in light-emitting field-effect transistor. The crystal packing effects on the frontier orbital coupling are found to be essential to understand such differences in transport behaviors.     

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     

Lattice mobility of holes in strained and unstrained Si1-xGex alloys

Results of the lattice drift mobility in strained and unstrained SiGe alloys are reported for Ge fractions, 0.0⩽x⩽1.0. The mobilities are calculated using acoustic, optical, and alloy scattering mechanisms. Due to the strain-induced symmetry reduction in the band structure of Si_1-xGe_x, the mobility is found to be a tensor with two distinct components parallel and perpendicular to the growth plane. Assuming that the scattering mechanisms are independent of the strain, the strained mobility increases exponentially with increasing Ge content, for x=0.3     

Analysis of negative differential resistance in theI-V characteristics of shorted-anode LIGBT's

The physical mechanism responsible for the negative differential resistance (NDR) in the current-voltage characteristics of the shorted anode lateral insulated gate bipolar transistor (SA-LIGBT) is explained through two-dimensional numerical simulation. The NDR regime is an inherent feature of all SA-LIGBTs, and results from the two different conduction mechanisms responsible for current flow in the device. These conduction mechanisms are minority-carrier injection and majority-carrier flow. Since both the anode geometry and the doping profile control the onset and the degree of minority-carrier injection, the effect these parameters have on the NDR is investigated. A simple lumped-element equivalent model of the SA-LIGBT allows qualitative predictions to be made on how changes in the device geometry and doping profiles influence the NDR regime. It is shown that conductivity modulation is a necessary but not sufficient condition for the occurrence of negative resistance in SA-LIGBT devices. Also required is a large voltage drop in the high-resistivity drift region before conductivity modulation is initiated. This causes small changes in the anode current level, greatly decreasing the total resistance across the drift region     

Explaining the dependences of the hole and electron mobilities inSi inversion layers

In this work the hole surface roughness-limited mobility in Si MOSFETs is investigated. Based on full-band Monte Carlo simulations and on an equivalent relaxation-time numerical model, we show that the differences between hole and electron experimental mobilities are not due to any peculiar physics of the valence band. They can be instead accounted for by a suitable shape of the power spectrum describing the surface roughness. The new power spectrum explains the experimental dependences of both electron and hole mobilities using the same surface roughness parameters. Finally the spatial features of the roughness described by the new spectrum are discussed and compared to those represented by Gaussian and exponential auto-covariance functions     

Hall and drift mobilities in molecular beam epitaxial grown GaAs

A series of nominally undoped and Si-doped GaAs samples have been grown by molecular beam epitaxy (MBE) with Hall concentrations ranging from 10¹⁵ to 10¹⁹ cm⁻³ and mobilities measured at 77 and 300K by Hall-van der Pauw methods. Drift mobilities were calculated using the variational principle method and Hall scattering factors obtained from a relaxation-time approximation to permit cross-correlation of experimental data with drift or Hall mobilities and actual or Hall electron concentrations. At 77K, both high purity and heavily doped samples are well represented by either drift or Hall values since piezoelectric acoustic phonon scattering and strongly screened ionized impurity scattering hold the Hall factor close to unity in the respective regimes. Between n≊10¹⁵ and 10¹⁷ cm⁻³, where lightly screened ionized impority scattering predominates, Hall mobility overestimates drift mobility by up to 50 percent and Hall concentration similarly underestimates n. At 300K, polar optical phonons limit mobility and a Hall factor up to 1.4 is found in the lowest doped material, falling close to unity above about 10¹⁶ cm⁻³. Our calculation also agrees remarkably well with the Hall mobility of the highest purity MBE grown sample reported to date.     

Dember effect in fast photoconductive circuit elements

The observed independence of mobility on surface conditions in InP:Fe fast photoconductivity devices is explained by inclusion of the Dember electric field resulting from unequal electron and hole mobilities. From the calculated electron-concentration distribution as a function of the distance normal to the device surface, we determine that the mean position of the electron concentration in InP lies more than one standard deviation below the surface for laser excitation of 780 nm and a high-injection ambipolar surface recombination velocity of 10⁶cm/s. Results are presented for InP:Fe as a function of surface recombination velocity, and related results are presented for Si and for a material with equal hole and electron mobilities.