D. Rigaud

1/f noise in MOS devices, mobility or number fluctuations?

Recent experimental studies on 1/f noise in MOS transistors are reviewed. Arguments are given for the two schools of thought on the origin of 1/f noise. The consequences of models based on carrier-number /spl Delta/N or mobility fluctuations /spl Delta//spl mu/ on the device geometry and on the bias dependence of the 1/f noise are discussed. Circuit-simulation-oriented equations for the 1/f noise are discussed. The effects of scaling down on the 1/f noise is studied in the ohmic region as well as in saturation. In the ohmic region the contribution of the series resistance often can be ignored. However, in saturation the noise of the gate-voltage-dependent series resistance on the drain side plays a role in lightly doped drain LDD mini-MOSTs. Surface and bulk p-channel devices are compared and the differences between n-and p-MOSTs often observed is discussed. The relation between degradation effects by hot carriers or by /spl gamma/-irradiation on the one hand and the 1/f noise on the other is considered in terms of a /spl Delta/N or /spl Delta//spl mu/. Experimental results suggest that 1/f noise in n-MOSTs is dominated by /spl Delta/N while in p-MOSTs the noise is due to /spl Delta//spl mu/. >

1/f noise in MODFETs at low drain bias

The 1/f noise in normally-on MODFETs biased at low drain voltages is investigated. The experimentally observed relative noise in the drain current S/sub I//I/sup 2/ versus the effective gate voltage V/sub G/=V/sub GS/-V/sub off/ shows three regions which are explained. The observed dependencies are S/sub I//I/sup 2/ varies as V/sub G//sup m/ with the exponents m=-1, -3, 0 with increasing values of V/sub G/. The model explains m=-1 as the region where the resistance and the 1/f noise stem from the 2-D electron gas under the gate electrode; the region with m=0 at large V/sub G/ or V/sub GS/ equivalent to 0 is due to the dominant contribution of the series resistance. In the region at intermediate V/sub G/, m=-3, the 1/f noise stems from the channel under the gate electrode, and the drain-source resistance is already dominated by the series resistance. >

1/f noise investigations in small channel length amorphous silicon thin film transistors

Conduction and low-frequency noise are analyzed in hydrogenated amorphous thin film transistors with small channel length. From current–voltage characteristics a set of conduction parameters is extracted pointing out parasitic resistances in series with the active channel. The low-frequency noise behavior is studied by means of the small equivalent circuit of the device. Intrinsic channel noise is separated from access resistance noise. Channel noise variations versus device biases agree with Hooge’s theory (carrier mobility fluctuations) but the noise levels are greater than in crystalline metal-oxide-semiconductor transistors. For high drain current 1/f noise in access series resistances prevails and becomes the main noise source. So, the results show the important part taken by these resistances in conduction and noise. Some comments for the design of thin film transistors are given.

1/f noise modeling in long channel amorphous silicon thin film transistors

1/f noise investigations in thin film transistors with long channel and thin thickness of amorphous silicon film are presented. It is found that the noise behavior follows the mobility fluctuation model in ohmic and saturation regimes, whereas in the subthreshold conduction, a quadratic law versus the drain current is observed. The noise modeling is proposed taking into account the equations usually utilized for crystalline silicon metal–oxide–semiconductor field-effect transistors according to Hooge’s theory. Moreover, the Berkeley Short-Channel Insulated Gate Field-Effect Transistor Model is adapted to predict the noise levels. Two noise parameters have been extracted: The first is used in the subthreshold region, whereas we show that the second, directly related to Hooge’s parameter, is adequate to describe alone the noise in normal conduction up to the saturation.

1/f Noise in amorphous silicon thin film transistors: effect of scaling down

Abstract 1/f noise is investigated in thin film transistors as a function of gate geometry and film thickness. Two noise sources are present associated with the intrinsic channel and the access resistances. Intrinsic channel noise agrees with Hooges theory and the value of αH (≈4×10−3) is independent of the device geometry. In order to characterize the noise in the access resistances a model is proposed. It describes accurately the later noise source versus gate width and film thickness. It is shown that only one parameter is necessary to model the experimental data. Finally the contribution of each noise source to the total channel noise is modelled, showing the important part due to the thickness of the a-Si:H film.

Coherence between gate- and drain-current fluctuations in MESFET's and MODFET's biased in the ohmic region

The gate and drain current fluctuations and their coherence have been investigated on MESFETs from NEC and RTC and on MODFETs from NEC, Fujitsu, and Sony. In the frequency range of 10 Hz to 100 kHz the observed spectra show mainly 1/f and generation-recombination noise. Some devices show a complete absence of coherence in that frequency range. Other devices show a coherence as high as 0.55 at low frequencies and a drop at higher frequencies. Some devices show a very low coherence level which increased above a characteristic frequency. The proposed model explains the experimentally observed trends and their physical meaning. The coherence measurement can be used as an additional diagnostic tool for MESFET and MODFET reliability studies. >

Gate current 1/f noise in GaAs MESFET's

Gate current 1/f has been investigated on commercial GaAs MESFETs. It is found that devices with slight differences in drain current noise can have quite a different type of I/sub g/ versus V/sub ds/ and V/sub gs/ and even greater differences in the gate current noise. The 1/f part in the spectrum of the gate current is a better diagnostic tool for characterizing the quality of the junction than the drain current noise. A model that is based on a Schottky barrier shunted by edge currents is proposed to explain the experimental results. >

CONDUCTION AND 1/F NOISE ANALYSIS IN AMORPHOUS SILICON THIN-FILM TRANSISTORS

Conduction and low‐frequency noise are analyzed in the channel of hydrogenated amorphous silicon (a‐Si:H) thin‐film transistors. 1/f noise expressions are proposed starting from a simple conduction model describing drain current in the ohmic range. Carrier fluctuations (ΔN model) and mobility fluctuations (Δμ model) are investigated. For long‐channel transistors the conduction is quite similar to crystalline metal–oxide–semiconductor field‐effect transistors but involving low mobility values. The 1/f noise behavior is analyzed by mobility fluctuations as predicted by Hooge’s theory. For small channel transistors a crowding effect appears and access series resistances affect the conduction. The excess noise is then mainly controlled by these resistances when large gate voltages VGS are applied.

1/f noise in metal–oxide–semiconductor transistors biased in weak inversion

An approach to model 1/f noise in the weak inversion range of metal–oxide–semiconductor transistors (MOST) is proposed, based on Hooge’s theory (mobility fluctuation model). Starting from conduction equations in the subthreshold regime, a method to evaluate the total number of carriers under the gate is presented and allows us to deduce the Hooge parameter αH. This model is applied to p-channel MOSTs. With the proposed model, the value of αH obtained in weak inversion is quite similar to this extracted in strong inversion allowing a unique description of the 1/f noise.

Correlation measurement of carrier multiplication noise sources in MOS transistors at low frequencies

Low frequency techniques of cross spectrum and correlation coefficient measurements are presented and applied to investigate drain and substrate noises of MOS transistors when multiplication occurs in the channel. For low substrate currents it is shown that the experimental data agree with theoretical expectations: the correlation coefficient is maximum at low frequencies and reaches its minimum value which is only dependent of the multiplication factor in the upper frequency range. For high substrate currents uncorrelated 1/f noise appears in bulk conductance. >