Noise In MOS Transistors Research Articles

A unified model for the flicker noise in metal-oxide-semiconductor field-effect transistors

A unified flicker noise model which incorporates both the number fluctuation and the correlated surface mobility fluctuation mechanism is discussed. The latter is attributed to the Coulombic scattering effect of the fluctuating oxide charge. The model has a functional form resembling that of the number fluctuation theory, but at certain bias conditions it may reduce to a form compatible with Hooge's empirical expression. The model can unify the noise data reported in the literature, without making any ad hoc assumption on the noise generation mechanism. Specifically, the model can predict the right magnitude and bias dependence of the empirical Hooge parameter. Simulated flicker noise characteristics obtained with a circuit-simulation-oriented flicker noise model based on the new formulation were compared with experimental noise data. Excellent agreement between the calculations and measurement was observed in both the linear and saturation regions for MOS transistors fabricated by different technologies. The work shows that the flicker noise in MOS transistors can be completely explained by the trap charge fluctuation mechanism, which produces mobile carrier number fluctuation and correlated surface mobility fluctuation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Flicker noise in MOS transistors operated at room and liquid helium temperatures

Flicker noise in MOS transistors operated in the linear and non linear regions is studied at room and liquid helium temperatures. A theoretical analysis of the drain current noise is conducted within the mobility and carrier number fluctuation models. A close correlation between the drain current noise and the transconductance squared dependencies with gate and drain voltages is found experimentally both at room and liquid helium temperatures. It is concluded that the carrier number fluctuation model is suitable for the interpretation of our data at room and liquid helium temperatures in ohmic and non ohmic regimes. Peculiarities of the drain current noise in the kink effect region are also discussed.

Temperature dependence of low-frequency noise in n-channel MOS transistors

Monitoring of the time sequence and frequency spectrum of a random noise in the MOS transistor reveals that the low-frequency noise in this device is due to the superposition of individual capture or emission events at the silicon-oxide interface or in the bulk of the gate-oxide. In addition, the noise level of the MOS device is found to fluctuate greatly with temperature in the range of 77–375 K. In a noise amplitude versus temperature plot, valleys and peaks are observed at certain temperatures, and their locations seem to convey some special information. Moreover, the spectral slope is found to possess the same temperature dependence as the amplitude, which implies that these two quantities could be governed by the same physical mechanisms. On the other hand, result also suggests that the temperature dependence of the noise is strongly governed by the gate-oxide preparation techniques.

Influence of dynamical interactions between density and mobility of carriers in the channel on [formula omitted] noise of MOS transistors below saturation—I. Mechanisms

The influence of drain voltage V D , gate voltage V G and of the chip geometry on 1 ƒ noise of MOS transistors below saturation in strong inversion is analyzed, with the assumption that the carrier mobility along the channel is non-constant. Taking into consideration the effect of dynamical interactions between the carrier density and mobility it is shown, in part I of this paper, that the frequency spectrum of mean-square fluctuations in conductivity dS σ(ƒ) increases vs V D , in relation to the case when the mobility along the channel μ( y) is assumed as constant. This, in particular, concerns the smaller MOS transistors operating near the region of saturation.

Influence of dynamical interactions between density and mobility of carriers in the channel on [formula omitted] noise of MOS transistors below saturation—II. Implications

In part II of this paper it is shown that when an MOS transistor operates below saturation the dynamical interactions between density and mobility of carriers influence the equivalent input noise voltage level due to density fluctuations S VGp (ƒ) . S VG p increases vs V D in relation to the case when the mobility along the channel is assumed as constant.

A simple derivation of Reimbold's drain current spectrum formula for flicker noise in MOSFETs

A simple derivation of Reimbold's drain current spectrum formula is obtained from the usual input voltage spectral density for flicker noise in MOS transistors. The demonstration relies on a general expression of the MOSFET transconductance. Complete equivalence between approaches based on drain current or input voltage noise spectral densities is emphasized.

Correlation between most 1/ f noise and CCD transfer inefficiency

The 1/ f noise in MOS transistors has been investigated and is shown to correlate with charge transfer inefficiency experiments on surface-channel CCDs. Both independent phenomena can be quantitatively explained by the same interface state model. The oxide trap density turns out to vary by more than a factor 10. The 1/ f noise is compared with McWhorter's number fluctuation model and with the mobility fluctuation model. The oxide trap density is calculated from the charge transfer inefficiency in surface CCDs. Both the quantitative agreement between oxide trap density and 1/ f noise and the observed dependence of 1/ f noise on gate voltage here give strong arguments in favour of the McWhorter model. The investigated MOS transistors fall into a category that cannot be explained by the present mobility fluctuation model.

White noise of MOS transistors operating in weak inversion

A theory is given for the thermal noise of MOS transistors in the weak inversion regime. For MOS transistors with low surface-state density, the relation for the thermal noise in saturation is shown to be the same as a shot noise relation. The theory is compared with measurements and the origin of the white noise in MOST's operating in weak inversion is discussed.

Equivalent input spectrum and drain current spectrum for 1/ƒ noise in short channel MOS transistors

Flicker noise in MOS transistors can be evaluated by measuring the spectrum S I D of the drain current fluctuation or the spectrum Sve of an equivalent gate fluctuation. We show here that experimental variations of S I D Sve are in good agreement with g m 2 by considering a model of the transconductance g m which takes into account the variations of the channel carriers mobility with the surface electric field. The model agrees with the experimental results obtained on short channel MOS transistors which exhibit large variations of mobility with the gate voltage. The validity of physical interpretations of noise data on MOS transistors is examined.

Model for 1/ f; noise in MOS transistors biased in the linear region

1/ f; Noise calculations and experiments are presented for conductance fluctuations in inversion layers. The layers are biased in the ohmic region at very low drain-source voltages. The model makes use of an experimental fact that competing scattering mechanisms other than lattice scattering lead to a reduction of 1/ f; noise, but does not consider trapping of charge carriers in surface states as the source of 1/ f; noise. The free charge carrier distribution and a mobility profile play an important part in the model. The model describes the measured results well. A reduction of the effective mobility with increasing gate voltage is accompanied by a strong reduction of the noise.

Effects of the substrate on surface state noise in silicon MOS FET's

The effects of the substrate resistivity on surface-state noise in MOS FET's are investigated. Particular consideration is given to the tunneling model introduced by McWhorter and discussed by Christensson et al. as a possible source of 1 f noise in MOS transistors. The noise calculations are extended to pinch-off using a one-dimensional analysis for saturated operation of the device. It has been found that the variation of the noise with bias is small for traps at any energy level in the forbidden gap. In addition, the noise contributed by traps at different energies (with the exception of traps at midgap) does not vary significantly, if the same bias conditions are assumed in all cases. Traps near midgap do not contribute appreciably, even at pinch-off. It is found that although the number of carriers available for trapping depends strongly on the substrate resistivity, the overall effect on the noise is small at pinch-off, over the range of impurity concentrations used in the manufacture of commercial devices.

Two components of 1/ƒ noise in mos transistors

Measurements of 1/ƒ noise in some p-channel MOS transistors are reported. In these samples the noise can be separated into two components with different dependences on gate bias and temperature. Using the tunnelling model of current noise, similar behaviour is predicted assuming that two distinct energy-distribution of oxide traps are present, one uniform over the band-gap and one concentrated near the valence band.

Characterization of low 1/f noise in MOS transistors

By careful processing MOS transistors have been fabricated with a low value of the interface states density (2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> eV). Consequently the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/f</tex> noise in these devices is low and in the same order of magnitude as for junction FETs. The experimental values of the equivalent noise voltage and the equivalent noise current are compared to an expression derived from straight physical arguments. From the comparison it is concluded that the noise equivalent voltage in saturated operation is proportional to the effective gate voltage, the interface state density, and inversely proportional to the gate input capacitance. Moreover, it is concluded that a proper heat treatment not only reduces the number of states but also removes the near bandedge peaks, which usually appear in the trap distribution function.

Surface state related [formula omitted] noise in MOS transistors

The theory of surface state effects on 1 ƒ noise in p− n junctions is extended to explain low-frequency excess noise in MOS transistors. Experimental results are critically compared to theory. The results show that low-frequency excess noise in MOS transistors is due to the fluctuation of charge density in the conduction channel caused by the surface potential fluctuation. The fluctuation of the surface potential is introduced by the random charge occupancy of surface states. The low-frequency excess noise in MOS transistors is found to be proportional to the surface state density and the square of the transconductance of the device, and inversely proportional to the gate area and the square of the unit area gate-insulator capacitance. It is also shown that the surface state density when the surface is strongly inverted can be obtained from noise measurements. Finally it is shown that by proper heat treatment it is possible to reduce the low-frequency excess noise of MOS transistors.

Geometrical dependences of the low-frequency generation-recombination noise in mos transistors

A comparison between theory and experiments for the dependences of generation-recombination noise on the gate-oxide thickness and the channel length of MOS transistors is presented. The results show good agreements between theory and experiments.

Theory and experiments of low-frequency generation-recombination noise in MOS transistors

A theoretical model for the generation-recombination (g-r) noise in MOS transistors is presented. This model takes into account the charge induced on all electrodes by the charge fluctuation of the impurity center in the depletion region. The model gives a finite equivalent gate noise resistance at saturation. Gold-doped and no-gold control devices were fabricated to verify the theory experimentally. The drain-voltage dependence of the g-r noise, which is shown to be distinctly different from the 1/f noise and thermal noise, is used to check the theory. Good agreement between theory and experiment is obtained.

Low frequency noise in MOS transistors—I Theory

The low frequency noise voltage spectrum for a MOS transistor is calculated under the assumption that a time constant dispersion, giving a 1/ƒ- spectrum , is caused by tunneling of carriers at the silicon-silicon oxide interface to traps located inside the oxide. The magnitude of the noise from different trap distributions is calculated. The largest noise contribution is shown to come from traps near the quasi Fermi level of the current carriers in the channel. The influence of temperature and operating point on the noise is examined. The theoretically predicted noise magnitude agrees with the experimental results (presented in Part II). The theory presented describes qualitatively the shape of the low frequency noise spectrum.

Low frequency noise in MOS transistors—II Experiments

Noise measurements on a number of commercial MOS-transistors have been performed. The low frequency noise spectrum and the dependence of the noise on the temperature and the operating point of the transistor have been examined for frequencies between 30 Hz and 100 kHz and for temperatures between 77° and 373°K. It is found that the noise dependence on temperature is different for p-channel and n-channel MOST's. In a p-channel MOST the magnitude of the noise increases with temperature, while the magnitude of the noise decreases in a n-channel MOST. The experimentally found noise behaviour is compared with the theoretically predicted behaviour in Part I of this work.

Surface states and 1/f noise in MOS transistors

Surface-state density in n-channel MOS transistors operating in the inversion mode has been determined from the channel conductance and related to 1/ <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</tex> noise in these devices. It has been found that the noise is proportional to the surface-state density at the Fermi level. The surface orientation and temperature affect the noise output only indirectly, through their influence on the surface-state density and the position of the Fermi level at the surface.