M. Valenza

Impact of Random Telegraph Signal in CMOS Image Sensors for Low-Light Levels

This paper presents the investigation of fluctuating pixels resulting from the random telegraph signal (R.T.S) of the source-follower transistor. The work takes into account the impact of the source-follower noise power spectral density (P.S.D) dispersion through correlated double sampling (C.D.S) readout circuit used in CMOS active pixel image sensors. The results allow the determination of the output r.m.s noise versus R.T.S noise characteristics. The distinctiveness of the observed flickering pixels is discussed in detail and the proposed mechanisms behind the phenomena are viewed in light of the collected data. We show that R.T.S noise main parameters dispersion of the source-follower transistor is a major factor influencing the circuit output r.m.s noise value distribution in a pixel array. Results are compared with experimental data

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.

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. >

Response of Correlated Double Sampling CMOS Imager Circuit to Random Telegraph Signal Noise

This paper presents analytical and experimental noise of a correlated double sampling (CDS) readout circuit used in CMOS active pixel image sensors. The work takes into account low frequency noise, mainly random telegraph signal (RTS) noise, in modern MOS transistors with very small geometries. The impact of the source-follower transistor noise power spectral density through CDS is studied. The results allow the determination of the output rms noise versus random telegraph signal noise characteristics. We show that R.T.S. noise of the source-follower is a major factor influencing the circuit output rms noise. Theoretical results are compared with experimental data

Impact of scaling down on 1/f noise in MOSFETs

An overview of the theoretical 1/f noise models is given. Analytical expressions showing the device geometry and bias dependence of 1/f noise in all conduction regime are summarized. Recent experimental studies on 1/f noise in MOS transistors are presented with special emphasis for PMOS from a 90 nm CMOS technology. Gate and drain noise sources are investigated. It is shown that in subthreshold regime drain current noise agrees with carrier number fluctuation model whereas in strong inversion the evolutions can be described by mobility fluctuation model. Gate current noise shows 1/f and white noise. White noise is very close to shot noise, and we have a quadratic variation of 1/f noise with gate current. Coherence measurements show that the increase of drain noise at high gate biases can be attributed to tunneling effects. Input-referred gate noise and the volume trap density can be used as figure of merit. Discrepancies with the ITRS roadmap are discussed.

1/f noise measurements in n-channel MOSFETs processed in 0.25 μm technology: Extraction of BSIM3v3 noise parameters

Abstract Low frequency noise has been studied from the weak to strong inversion regime in n-channel MOS transistors. The 1/ f current noise power spectrum density S ID is measured as a function of the drain current and gate voltage with the gate length as a parameter. Analysis of the noise characteristics shows that the channel noise agrees with the mobility fluctuation model and can be predicted in the linear and saturation region using the α H parameter only. Finally, the three parameters NOIA, NOIB and NOIC used in the BSIM3v3 noise model are extracted. Some discrepancies of the noise simulation with the experimental data are observed.