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CMOS Noise Modeling

CMOS Noise Modeling. For RF applications. Presented by: Sameh Assem Ibrahim. What is noise ?. Some unwanted fluctuations that, when added to a signal, reduces its information content. Classified into internal or external noise.

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CMOS Noise Modeling

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  1. CMOS Noise Modeling For RF applications Presented by: Sameh Assem Ibrahim

  2. What is noise ? • Some unwanted fluctuations that, when added to a signal, reduces its information content. • Classified into internal or external noise. • Internal noise: Generated by components within the systems such as resisters and transistors. • External noise: Generated by sources outside the system such as atmosphere and man-made. • Noise defines the lowest limit of the signal that can be detected.

  3. Noise Notations • Noise is random in nature. It is represented by a time varying random variable X(t). • The mean value • The variance • PSDF: Power Spectral Density Function : the time averaged noise power over a one Hertz bandwidth at any given f. • PSDFs of a DC current and voltage are represented by SI(f) and SV(f) • Noise voltage generator • Noise current generator

  4. Types of Noise in SC Devices1. Thermal Noise • M.B. Johnson was the first to report careful measurement of Thermal noise in 1927. • White noise. (Independent on frequency) • The most fundamental and important noise in electronic devices. • Physical explanation: Random travelling of electrons due to collisions with the lattice of the semiconductor. • SV(f)=4kTR • SI(f)=4kT/R

  5. Types of Noise in SC Devices2. Shot Noise • First discovered and explained by W. Schottky in 1918. • White noise. (Independent on frequency) • Exists whenever a DC current flows and electrons cross potential barriers. • Physical explanation: Fluctuation of the emission rate of carriers. • SI(f)=2qI

  6. Types of Noise in SC Devices3. Flicker Noise • 1/fn noise with n close to unity. (Pink noise) 1. The carrier number fluctuation model (Δn) proposed by McWorther: Flicker noise is attributed to the trapping and de-trapping processes of the charges in the oxide traps at the Si-SiO2 interface. 2. The mobility fluctuation model (Δμ) proposed by Hooge: Flicker noise results from bulk mobility fluctuation on the basis of empirical hypothesis. 3. The unified model by Hung : Δn model at high bias and Δμ model at low bias

  7. Types of Noise in SC Devices4. Burst Noise • It is also called popcorn noise, generation-recombination noise or random telegraph signal. • The noise switches between two or more discrete values at random times. • The origins of burst noise remain still uncertain.

  8. Types of Noise in SC Devices Flicker Noise Corner

  9. Induced gate noise Terminal resistance thermal noise Thermal and flicker noise in the channel Substrate resistance thermal noise Noise in a MOS Transistor

  10. RF Noise Model of a MOSFET

  11. Channel Thermal Noise Modeling 1 • At HF, the dominant noise contribution comes from the channel thermal noise. • Results from the random thermal motion of carriers in the channel. • At zero VDS, the channel can be treated as a homogeneous resistor. • This gives SId(f)=4.k.T.gdo where k is Boltzman constant T is absolute temperature gdo is channel conductance at zero drain-source voltage

  12. Channel Thermal Noise Modeling 2 • When VDS is not zero: • Neglecting the dependency of VT on xgives: SId(f)=4.k.T.gdo in linear region SId(f)=4.k.T.(2/3).gm in saturation region Where gm is the MOSFET transconductance

  13. Channel Thermal Noise Modeling 3 • If the dependency of VT on x is considered, we can write: Where is a bias dependent factor. • For long channel devices: for linear region for saturation region • For short channel devices, velocity saturation and hot electrons effects must be considered. This gives a higher Where is for the long channel device is the relaxation time

  14. For NLEV = 3 : Linear region Saturation region is a model parameter Channel Thermal Noise ModelsHSPICE Model • A parameter NLEV used to select different equations for calculating channel thermal noise. • For NLEV < 3 : Used for both saturation and linear regions At VDS=0 Sid(f) = 0  Velocity saturation is not considered  It is not adequate for short channel devices

  15. For NOIMOD = 2,4 : Channel Thermal Noise ModelsBSIM3v3 Model • The standard MOSFET model for industry use. • A parameter NOIMOD used to select different equations for calculating channel thermal noise. • For NOIMOD = 1,3 : It solved the problem at VDS=0 but underestimates noise in linear region  Velocity saturation is not considered  It is not adequate for short channel devices

  16. Channel Thermal Noise ModelsBSIM4 Model • A parameter TNOIMOD used to select different equations for calculating channel thermal noise. • For TNOIMOD = 1 (charge based, same as BSIM3v3)

  17. Channel Thermal Noise ModelsBSIM4 Model • For TNOIMOD = 1 (Holistic Model): • All the short-channel effects and velocitysaturation effect incorporated in the IV model are automatically included.

  18. Induced Gate Noise Modeling • At HF, noise current in the channel generates noise in the gate through the gate-channel capacitance CoxWL. • It is correlated to the channel thermal noise. • Still under research and difficult to model. It is not very critical for frequencies much smaller than fTof the devices. • Modeled in BSIM4 noise model. • Can be approximated as :

  19. Flicker Noise Modeling 1 • Noise at LF in a MOSFET is dominated by flicker noise. • Cannot be neglected in some RF circuits such as mixers, oscillators or frequency dividers. • These circuits upconvert LF to HF and thus deteriorates phase noise and SNR. • Modeling using the mobility fluctuation model: • Described by the Hooge empirical equation: Where is called the Hooge 1/f noise parameter. N is the total number of free carriers in the device. • Experiments give a value of about 2*10-3 for

  20. Flicker Noise Modeling 2 • Experiments shows that only photon scattering gives 1/f noise. • If other scattering mechanisms exist: • The Hooge equation is only valid for homogeneous devices. • It is suitable for usage in the linear region only. • In the saturation region, Hooge equation is used for small sections and then integration over the channel is done.

  21. Flicker Noise Modeling 3 • Modeling using the number fluctuation model: • Statistics of trapping and de-trapping of free carries in oxide traps yields a fluctuation in the number of trapped carriers • This causes fluctuations in the channel free carriers. • Fluctuations in the channel current thus results. • The final result will be

  22. Flicker Noise Modeling 4

  23. Flicker Noise Modeling 4 • Modeling using the unified model: • It suggests that the capture and emission of carriers by the interface traps cause fluctuation in both the carrier number and the mobility. • The most attractive model available today in circuit simulators. • Still all the three models have limitations and cannot explain all the details of experimental data.

  24. Flicker Noise ModelsHSPICE Model • For NLEV = 0 : • For NLEV = 1 : • For NLEV = 2,3 : Where KF and AF are fitting parameters

  25. Flicker Noise ModelsBSIM3v3 Model • For NOIMOD = 1,4 : (SPICE2 flicker noise model) • For NOIMOD = 2,3 : (Unified flicker noise model) If VGS > VT + 0.1:

  26. Flicker Noise ModelsBSIM3v3 Model Otherwise And Slimit the calculated flicker noise for VGS = VT + 0.1

  27. Flicker Noise ModelsBSIM4 Model • FNOIMOD = 0 is equivalent to the BSIM3v3 SPICE2 flicker noise model. • FNOIMOD = 1 is the same unified flicker noise model used in BSIM3V3 but with many improvements. For instance, it is now smooth over all the bias regions and considers the bulk charge effect.

  28. Flicker Noise in Switched MOSFET circuits • Flicker Noise estimates based on standard models can be quite inaccurate especially for switched circuits. • Switched circuits are widely used in RF applications. • Examples are switched capacitor networks, modulators, demodulators and frequency converters. • Measured 1/f noise in these circuits is much lower than estimates. • The discrepancy results from the very high probability that the traps are empty when the transistor is OFF. • A model was developed to account for this discrepancy. This model is called the nonstationary model. [5]

  29. References • M.J.Deen, T.A. Fjeldly , “CMOS RF Modeling, caharcterization and applications”, World Scientific, 2002. • W.S. Yang, “Microelectronic Devices”, Mc-Graw Hill, 1988. • Gray, et al., “Analysis and design of analog integrated circuits”, Tohn Wiley & Sons, 2001. • Nikhef, “Noise sources in MOSFET transistors” ,JDS, January 1999. • H. Tian, A. El Gamal, “Analysis of 1/f noise in switched MOSFET circuits”, April 2001. • B. Rajendran, “Review of noise in semiconductor devices and modeling of noise in surrounding gate MOSFET”, December 2001. • K. H. Lundberg, “Noise sources in bulk CMOS” , 2002 • C. H. Chen, “Noise characterization and modeling of MOSFETS for RF IC Applications”, A thesis for PhD. Degree, September 2002. • BSIM3v3 Manual, UC Berkeley, 1996. • BSIM4 Manual, UC Berkeley, 2001

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