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     

Hole and electron mobilities in heavily doped silicon: comparison of theory and experiment

Most device models for npn or pnp transistors assume that hole (electron) mobilities in n-type and p-type silicon are equal. Partial-wave phase shift calculations for the contributions of carrier-dopant ion scattering to the carrier mobilities lead to unequal minority hole (electron) and majority hole (electron) mobilities at the same doping density. These calculations are valid over the doping range of 2 x 10¹⁹ to 8 x 10¹⁹ cm⁻³ in n-type and p-type silicon and contain the assumptions that the holes and electrons move in isotropic parabolic energy bands and are scattered by the screened Coulomb potentials of the dopant ions. When the effects of carrier-acoustic phonon and carrier-carrier scatterings are included, these calculations agree to within the spread of experimental value for the majority mobilities reported in the literature. This agreement is a substantial improvement by factors of 2–4 over the results of earlier theories such as first order Born and nondegenerate theories. The results of this work, particularly the inequality of minority and majority carrier mobilities, have implications for the modeling of both bipolar and field effect transistors.     

MOSFET Characteristics in low-temperature plasma-enhanced chemical vapor deposited epitaxial silicon

The fabrication of high-quality MOSFET's using low-temperature (750-800°C) Plasma-Enhanced Chemical Vapor Deposited (PECVD) epitaxial silicon is reported here for the first time. The fabricated devices include PMOS transistors with hole channel mobilities of 213 cm²/V.s (versus 218 cm²/V.s in bulk silicon controls) and NMOS transistors with electron channel mobilities of 520 cm²/V.s (versus 560 cm²/V.s in bulk silicon controls), and with an on-current to off-current ratio of 10⁷. These results indicate that epitaxial silicon films deposited by the PECVD technique are of high quality, even though the epitaxial deposition temperature was only 750-800°C.     

Electrical characterization of epitaxial silicon deposited at low temperatures by plasma-enhanced chemical vapor deposition

The initial results of an investigation of the electrical properties of epitaxial silicon films deposited at low temperatures (i.e., 750°C) by the Plasma-Enhanced Chemical Vapor Deposition (PECVD) technique are presented. The major results indicate that (1) the electron and hole drift mobilities in the epitaxial layers are the same as in bulk silicon for carrier concentrations between 10¹⁷cm^-3and 10¹⁹cm^-3, and (2) high quality p-n junction diodes with no sign of soft breakdown and an ideality factor of 1.10 are obtained at an epitaxial deposition temperature of only 750°C. These p-n diodes represent the first ever reported IC devices fabricated in PECVD epitaxial films.     

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     

Device characterization on monocrystalline silicon grown over SiO2by the ELO (epitaxial lateral overgrowth) process

MOS and lateral bipolar transistors have been fabricated on epitaxial silicon layers which have been laterally overgrown over SiO2. These device characteristics were than compared to those measured on devices fabricated on homoepitaxial silicon and bulk silicon. The measurements indicate essentially identical MOS device characteristics for all three materials with a typical hole field effect mobility of about 180 cm²/vs. Lifetime measurements using pulsed C-V techniques showed essentially the same values for ELO material and homoepitaxial material with the ELO value being about 20 µS for 10¹⁵cm^-3doping level. These lifetime values correlate will with diode and bipolar transistor measurements.     

Electron velocity overshoot at room and liquid nitrogentemperatures in silicon inversion layers

Effective electron velocities in silicon MOSFETs exceeding the bulk saturation values of 10⁷ cm/s at room temperature and 1.3×10⁷ cm/s at liquid-nitrogen temperature are inferred. This conclusion suggests that electron velocity overshoot occurs over a large portion of the device channel length. To infer this phenomenon, submicrometer-channel-length Si MOSFETs with lightly doped inversion layers were fabricated. These devices have low field mobility of 450 cm²/V-s and showed only slight short-channel effects. Effective carrier velocities are calculated from the saturated transconductance g_m at V_DS=1.5 V after correction for parasitic resistances of source and drain     

Carrier effective mobilities in germanium MOSFET inversion layer investigated by Monte Carlo simulation

This paper presents the Monte Carlo studies of inversion mobility in Ge MOSFETs covering a wide range of bulk-impurity concentrations (10¹⁴ cm⁻³–10¹⁷ cm⁻³), and substrate bias (0–10 V). Carrier mobilities in Ge MOSFETs have obviously increased compared with those in Si MOSFETs. At low effective field, both electron and hole mobilities have increased over 100%; while at high effective field the increase is reduced due to the effect of surface roughness. Similar to Si MOSFETs, the carrier effective mobilities in Ge MOSFETs also have a universal behavior. The universality of both electron and hole mobilities holds up to a bulk-impurity concentration of 10¹⁷ cm⁻³. On substrates with higher bulk-impurity concentrations, the carrier effective mobilities significantly deviate from the universal curves under low effective field because of Coulomb scattering by the bulk impurity.     

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.     

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     

Effect of zinc impurity on silicon solar-cell efficiency

Zinc is a major residue impurity in the preparation of solar-grade silicon material by the zinc vapor reduction of silicon tetrachloride. This paper projects that in order to get a 17-percent AM1 cell efficiency for the Block IV module of the Low-Cost Solar Array Project,¹the concentration of the zinc recombination centers in the base region of silicon solar cells must be less than 4 × 10¹¹Zn/cm³in the p-base n+/p/p+ cell and 7 × 10¹¹Zn/cm³in the n-Base p+/n/n+ cell for a base dopant impurity concentration of 5 × 10¹⁴atoms/cm³. If the base dopant impurity concentration is increased by a factor of 10 to 5 × 10¹⁵atoms/cm³, then the maximum allowable zinc concentration is increased by a factor of about two for a 17-percent AM1 efficiency. The thermal equilibrium electron and hole recombination and generation rates at the double-acceptor zinc centers are obtained from previous high-field measurements as well as new measurements at zero field described in this paper. These rates are used in the exact dc-circuit model to compute the projections.     

Improved Ge Surface Passivation With Ultrathin SiOX Enabling High-Mobility Surface Channel pMOSFETs Featuring a HfSiO/WN Gate Stack

To realize high-mobility surface channel pMOSFETs on Ge, a 1.6-nm-thick SiO_X passivation layer between the bulk Ge substrate and HfSiO gate dielectric was introduced. This approach provides a simple alternative to epitaxial Si deposition followed by selective oxidation and leads to one of the highest peak hole mobilities reported for unstrained surface channel pMOSFETs on Ge: 332 cm² middotV^-1middots^-1 at 0.05 MV/cm-a 2times enhancement over the universal Si/SiO₂ mobility. The devices show well-behaved output and transfer characteristics, an equivalent oxide thickness of 1.85 nm and an I_ON/I_OFF ratio of 3times10³ without detectable fast transient charging. The high hole mobility of these devices is attributed to adequate passivation of the Ge surface     

The effect of channel boron implants on electron mobility inNMOSFET's

The Hall mobilities and Hall concentrations of channel electrons in boron-implanted NMOSFETs were measured at 77 and 300 K. At both temperatures, the mobilities were found to decrease with increasing implantation dose (10¹¹-10¹² cm^-2) only for electron concentrations <2×10¹² cm^-2, the effect being more pronounced at 77 K. It is suggested that the mobility degradation is mainly due to impurity scattering     

Reliability of MINP compared to MIS, SIS, and N/P silicon solar cells under 1.0-MeV electron and environmental effects

The effects of 1.0-MeV electron radiation are compared for MIS, SIS, N/P, and MINP silicon solar cells. MIS, SIS, and N/P silicon solar cells are comparable in performance except that SIS cells degraded faster due to use of n-type Si substrates. MINP cells exhibited superior performance in that efficiency degraded 9 percent at a fluence of 1 × 10¹⁵e^-/cm²and 32 percent at a fluence of 1 × 10¹⁶e^-/cm²compared to 29 percent and 49 percent, respectively, for N/P cells. MINP cells utilize an SiO2insulator layer over a thin N-region, and a low work function metal contact. This design gives a high ultraviolet response and low surface recombination velocity which maintains high efficiency since most of the radiation loss occurs in the infrared region due to bulk damage effects.     

Electron mobility in n-channel depletion-type MOS transistors

It is found that surface electron mobility can exceed electron mobility in bulk. Field-effect mobilities of n-channel depletion-type MOS transistors were measured, comparing transistorG_{m} - V_{g}and diodeC - V_{g}characteristics. Electron mobility in a highly doped n-type channel layer exceeds that in bulk at around the onset of surface accumulation. This can be explained by reduction in ionized impurity scattering under accumulation conditions. In accumulation conditions, excessive electrons screen the Coulombic field from ionized impurities. This is confirmed by theoretical calculation based on the Maxwell-Boltzmann distribution of electrons and electron-concentration-dependent electron mobility.     

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.     

Theory of materials for thermoelectric and thermomagnetic devices

The relation between the thermoelectric figure of merit Z and the basic properties of the material such as carrier mobilities, band structure, thermal conductivity and minority carrier lifetime is discussed. For isotropic materials with parabolic bands it is shown that the figure of merit based on the Seebeck coefficient will increase when a magnetic field is applied, if acoustic-mode scattering is predominant. If optical-mode scattering is predominant, the figure of merit will increase if ZT>0.77. Materials for devices based upon the Nernst or Ettingshausen effects will have large figures of merit only if 1) (m^*)^3/2µ/κL is large, 2) the energy gap is less than kT, and 3) the electron and hole mobilities are similar. Materials requirements for Nernst- and Seebeck-type devices are compared.     

Vertical n-p-n bipolar transistors fabricated on buried oxide SOI

Vertical n-p-n bipolar transistors have been fabricated in silicon-on-insulator (SOI) films prepared by buried oxide implantation. Electrical device characteristics are shown to be comparable to those obtained on devices fabricated in bulk silicon, indicating no significant degradation owing to the buried oxide layer. Dielectric isolation in excess of 10¹¹Ω.cm and µ 3 × 10⁶V/cm is measured.     

High-field transport in InGaAs/InAlAs modulation-doped heterostructures

The velocity-field and mobility-field characteristics of normal and inverted InGaAs/InAlAs modulation-doped heterostructures grown by molecular-beam epitaxy have been measured at 300 and 77 K. Veloczities of 3.0 × 10⁷and 1.7 × 10⁷cm/s have been measured in the normal and inverted structures, respectively, at 77 K. Current instabilities are observed at the corresponding field values. Hall mobilities decrease With field beyond 500 V/cm, principally due to phonon scattering. The mobilities in normal and inverted heterostructures attain Similar values at fields higher than 1 kV/cm, irrespective of the low-field values.     

Quantum transport of p-type GaAs/(AlGa)As heterostructures grown on non-(100) substrates by molecular beam epitaxy

We report on a series of Be-doped GaAs/(AlGa)As two-dimensional hole gas (2DHG) structures grown on (110), (111)B, (211)B and (311)B oriented substrates and compare their properties with high mobility samples grown on (311)A using Si doping. The samples were prepared and grown under the same conditions in order to render them comparable. They are found to have mobilities which are strongly anisotropic within the plane. All the samples show strong low-field positive magnetoresistance with resistance increases of up to 30% at magnetic fields of only 0.1 T. The presence of this feature on all the different planes shows that it does not depend upon the details of the band structure. It is identified with the lifting of the degeneracy of the spin sub-bands by the asymmetrical potential giving rise to a classical two-band magnetoresistance.

The modulation-doped GaAs/(AlGa)As heterostructures grown on the (311)A GaAs surface using silicon as the acceptor produced 2DHGs with low-temperature hole mobility exceeding 1.2 × 10⁶ cm² V⁻¹ s⁻¹ with carrier concentrations as low as 0.8 × 10¹¹ cm⁻². This hole mobility is the highest ever observed at such low densities by any growth technique. These 2DHG samples show for the first time the persistent photoconductivity effect. This effect is normally absent in 2DHG systems. An analysis of the number density and temperature dependence of the mobility leads us to conclude that the mobility is limited by phonon scattering above 4 K and interface scattering at lower temperatures.