Influence of the interface-state density on the electron mobility in silicon inversion layers

The electron inversion-layer mobility in a metal oxide semiconductor field effect transistor, as a function of the transverse electric field, has been studied in the temperature range 13–300K for different interface-state densities. Experimental data are in excellent agreement with a simple semi-empirical model. However, the term attributed by other authors to phonon scattering depends on the interface-state density, even at high temperatures, and becomes negative at low temperatures. These facts are shown to be a consequence of the dependence of coulomb scattering on the transverse electric field.     

Surface charges and surface potential in silicon surface inversion layers

The effect of interface charges on the channel conductance in MOS transistors has been investigated. It has been found that, by measuring the conductance as a function of temperature, it is possible to determine both the "fixed" interface charge which is independent of the surface potential and the charge trapped in surface states whose occupancy is a function of the surface potential. The characteristics of the two charge components are discussed. It appears that neither a continuous nor a delta-function energy distribution alone is adequate to describe the observed surface-state density.     

Velocity of surface carriers in inversion layers on silicon

Velocity-field curves for surface free-carriers in silicon are determined from measurements on resistivegate IGFETs. The measurements were performed on n-channel devices fabricated on both (100) and (111) substrates and on p-channel devices fabricated on (100) substrates. The channel length of the devices is ～8 μm and the impurity concentration of the substrates is ～ 10¹⁵ cm^?3. The dependence of velocity on the field strength along the channel is found to be well approximated by an empirical relationship involving three parameters: low-field mobility μ₀, a critical field $\text{E}{}_{\mathrm{cy}}$ signalling the onset of velocity saturation, and a parameter α that determines the curvature between the constant-mobility and constant-velocity branches of the curve. The curve-fitting parameters are given in tabular form for the two n-channel and one p-channel systems studied. The dependence of the velocity-field curves on temperatures in the range 100–350K is also reported.     

Observation of velocity overshoot in silicon inversion layers

Employing a test structure, velocity overshoot in silicon inversion layers is observed at room temperature. For channel lengths longer than 0.3 μm, the velocity/field relation follows the well-known behavior with no channel length dependence. The first indication of velocity overshoot is seen at a channel length of 0.22 μm, while at L=0.12 μm, drift velocities up to 35% larger than the long-channel value are measured     

Charge-sheet model for silicon carbide inversion layers

The charge-sheet model for metal-oxide-semiconductor (MOS) inversion layers is extended to silicon carbide. The generalized model is based on an analytical solution of the Poisson equation for the case of incomplete ionization of dopant impurities and incorporating Fermi-Dirac statistics. The results are compared with the conventional charge-sheet model which assumes complete impurity ionization and nondegenerate statistics. It is found that, at room temperature and for gate voltages in weak and moderate inversion, the present model predicts higher inversion-layer charge density at a given gate voltage. However, the relationship between the inversion charge and the surface Fermi potential is essentially independent of the degree of impurity ionization. In strong inversion or at temperatures above ~600 K, the differences between the two models are small. A formula is given for the threshold voltage as a function of the impurity ionization energy. The effects of several different interface state energy distributions on inversion charge are investigated. It is found that a slowly-varying interface-state density has an effect on threshold voltage of a MOSFET similar to that of a fixed oxide charge, while an interface-state density that increases at least exponentially with energy has the effect of lowering the field-effect mobility and transconductance     

Relationship between empirical and theoretical mobility models insilicon inversion layers

The theoretical and empirical expressions most commonly used for modeling the variation of the low field surface mobility of MOSFETs are discussed. It is shown that both approaches may be reconciled, and a new physical definition of the parameters of the empirical model is presented, In particular, we propose an analytical formula of the factor &thetas;₂ characterizing the quadratic dependence of the reciprocal mobility on the inversion charge. Both the formula and experiment agree and show that &thetas;₂ does not only depend on the surface roughness scattering term, but also on phonon scattering     

Electron transport properties of quantized silicon carbide inversion layers

Electron transport properties in SiC quantized inversion layers have been studied by means of a Monte Carlo procedure. It has been observed that the contribution of polar-optical phonon scattering produces a significant influence of the effective-electric field on the high longitudinal field transport regime, this being the main difference of SiC with respect to standard Si inversion layers. The energy- and momentum-relaxation times have been calculated and the results suggest that electron velocity overshoot effects are less important than in Si metal-oxide semiconductor field effect transistors. The electron mobility is not very different from their silicon counterparts, but the saturation velocity is higher.     

Electron mobility in the inversion layers of fully depleted SOI films

The dependences of the electron mobility μ_eff in the inversion layers of fully depleted double–gate silicon-on-insulator (SOI) metal–oxide–semiconductor (MOS) transistors on the density N _e of induced charge carriers and temperature T are investigated at different states of the SOI film (inversion–accumulation) from the side of one of the gates. It is shown that at a high density of induced charge carriers of N _e > 6 × 10¹² cm^–2 the μeff(T) dependences allow the components of mobility μ_eff that are related to scattering at surface phonons and from the film/insulator surface roughness to be distinguished. The μ_eff(N _e ) dependences can be approximated by the power functions μ_eff(N _e) ∝ N _e ^?n . The exponents n in the dependences and the dominant mechanisms of scattering of electrons induced near the interface between the SOI film and buried oxide are determined for different N _e ranges and film states from the surface side.     

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     

Electron and hole mobilities in inversion layers on thermally oxidized silicon surfaces

Extensive measurements of electron and hole mobilities in inversion layers on thermally oxidized silicon surfaces were performed using the field effect conductance technique. It was found that both electron and hole mobilities are practically constant and approximately equal to one half of their respective bulk values up to a surface field of about 1.5 × 10⁵volts/cm, corresponding to about 10¹²electronic charges/cm²induced in the silicon. At higher fields the inversion layer mobilities begin to decrease slightly. The temperature dependence of inversion layer mobilities follows a T^-1.5rule at the upper range of the interval -196 to 200°C, indicating a scattering mechanism similar to lattice scattering. This observation is further supported by the lack of a significant effect of an order-of-magnitude variation in the bulk impurity concentration (10¹⁵- 10¹⁶cm³) on the inversion layer mobilities. No significant effect of structural and geometrical parameters (such as channel length and shape, oxide type and thickness, and surface charge density) was found on the inversion layer mobilities.     

Investigations of interface-state density in SiMOS structures

Admittance measurements of n-type MOS capacitors indicate a large density of surface states near the conduction band and a small hillock or peak of surface state density near the midgap of the silicon energy band gap. The conductance peak related to the surface states at midgap was found to be sensitive to temperature changes of the system. These surface states we associate with the dangling bonds of Si at the interface. The time constant and hence the capture cross-section of these surface states were found to be different than those at other positions in the Si band gap.     

Fixed charge density and inversion layer mobility of thin gate oxides

Thin gate-oxide MOSFET's with different oxide thickness (100-400 Å) are fabricated. Improved measurement techniques are utilized and carefully controlled experiments are planned to minimize the usual errors encountered in thin-oxide measurements. It is found that the fixed charge density increases and inversion layer mobility decreases as the oxide thickness is reduced for the as-grown samples, whereas no similar dependence is observed for the samples with oxides etched down to the same thickness. The mobility degradation for the thin as-grown samples can be explained by the effects of coulomb and surface roughness scattering.     

Transfer ratio of Langmuir-Blodgett films as an indicator of the single-crystal silicon surface modified by polyionic layers

Multilayer nanoscale Langmuir-Blodgett films based on diphilic β-cyclodextrins with various numbers of alkyl chains were grown on single-crystal silicon substrates both unmodified and modified by cationic and anionic polyelectrolyte layers using the polyion complex technique. Ratios of β-cyclodextrin monolayer transfer on substrates were calculated and analyzed. It was shown that this ratio depends on the sign of the polyelectrolyte adhesive layer charge, as well as on the number of alkyl chains in the β-cyclodextrin molecule and the number of deposited monolayers. The observed phenomena characterizing indicator properties of β-cyclodextrin monolayers were interpreted.     

Correlation between inversion layer mobility and surface roughnessmeasured by AFM

The correlation between inversion layer mobility of MOSFET's and surface micro-roughness of the channel has been studied using split CV measurements and AFM analysis. The mobility at high normal field decreases with increasing the surface roughness over a wide range of roughness from 0.3 nm to 4.3 nm (RMS). The trend is the same even for very thin gate oxides down to 3 nm. Careful AFM measurements are used to show that the gate oxide thickness doesn't affect the surface roughness, supporting the independence of mobility on the gate oxide thickness     

Harmonic generation due to hot electrons in surface inversion layers

Harmonic generation due to hot electron nonlinearity in Si-inversion layers at 77 K in the presence of a large high frequency signal is studied on a drifted Maxwellian model. The third harmonic content increases with increase in field amplitude but decreases to negligible values for frequencies of about 1000 GHz.     

Measurement of the high-field drift velocity of electrons in inversion layers on silicon

A new technique is described for the study of high-field transport along semiconductor interfaces. The technique involves observation of the time-of-flight of a packet of carriers across a region of uniform tangential electric field at the semiconductor surface. We use the technique to measure the drift velocity of electrons along the Si-SiO2interface for tangential fields in the range 2.5-4.0 kV/cm. Drift velocities as high as 8.5 × 10⁶cm/s are observed; these values are almost 40% higher than previously reported in the literature.     

Negative differential resistance in (100) n-channel silicon inversion layers

A voltage controlled negative differential resistance (NDR) in n-channel MOSFETs of (100) orientation is investigated for lattice temperatures between 1.5 and 100 K in transverse magnetic fields up to 7 Tesla. By variation of the length of the pulses and by investigating samples of different geometry it is shown, that the NDR is not caused by lattice heating. Position and magnitude of the NDR depend substantially on the magnetic field. The observed effects can be explained by cooling of the hot carriers by the magnetic field.     

Determination of interface-state parameters in a MOS capacitor by DLTS

A modification of the DLTS (Deep Level Transient Spectroscopy) technique for interface-state measurement is described in which the surface potential is used to determine the energy of the interface states contributing to the emission signal. This technique allows an accurate and unambiguous determination of interface-state energies and cross sections. Expressions are determined for interface-state emission as a function of surface potential. Measurements of interface-state density and majority-carrier cross sections as functions of energy for n- and p-type MOS samples are presented.