A. S. Roy

Compact modeling of thermal noise in the MOS transistor

Although some of the recently proposed compact models for thermal noise in MOS transistors exhibit a good match with experimental data, we believe most of the existing compact models suffer from incorrect physical assumptions or modeling (e.g., absence of carrier heating, incorrect modeling of velocity saturation effect, wrong modeling of diffusivity, etc.). This brief presents a new, completely analytical thermal noise model based on consistent physical assumptions.

Noise modeling methodologies in the presence of mobility degradation and their equivalence

For compact modeling of the noise in devices, one of the following three methods is usually applied: 1) An equivalent circuit based approach, 2) the classical Langevin or Klaassen-Prins approach, or 3) the impedance field method. It is well known that for long-channel MOST (where mobility degradation due to a lateral field is absent), all three methods obtain the same result. But it is still not recognized how these methodologies need to be changed when the mobility starts to depend on the electric field. In this work we demonstrate how these methodologies can be adapted to incorporate mobility degradation and show that for any arbitrary mobility model /spl mu/(E) all the methods yield the same expressions for induced gate and drain noise current, which demonstrates the equivalence of the methods. We also present, for the first time, a general expression of induced gate noise which is valid for any mobility model (an expression of the drain current noise was already presented in our previous work) and some very general expressions of noise parameters that can be used for noise modeling with any kind of mobility model.

Analytical Modeling of Large-Signal Cyclo-Stationary Low-Frequency Noise With Arbitrary Periodic Input

Low-frequency (LF) noise under cyclo-stationary excitation has received considerable attention recently. Until now, only semiempirical models for cyclo-stationary random telegraph signal (RTS) and flicker noise have been reported in the literature. In this paper, we report, starting from basics, the analytical nonstationary extension of the standard LF noise models. This analytical model is valid for any kind of periodic excitation. After a rigorous analysis, by introducing some suitable approximations, we identify the main reason behind nonstationary behavior. Our analysis clearly shows that, for almost all practical purpose, an averaged time constant and an averaged trap density can model the cyclo-stationarity of RTS and flicker noise, respectively.

Source–Drain Partitioning in MOSFET

The Ward-Dutton (WD) partitioning scheme [IEEE Trans. Electron Devices, vol. ED-27, p. 1571, Aug. 1980] is used extensively to develop transient and high-frequency advanced compact models for MOSFET devices. Recently, it has been shown that WD partitioning fails for field-dependent mobility [IEEE Electron Device Lett., vol. 27, p. 674, Aug 2006] or for laterally asymmetrical doping [IEDM Tech. Dig., p.751, Dec. 2004], [IEEE Trans. Electron Devices, vol. 53, p. 270, Feb. 2006]. This paper is aimed at presenting a generalization of the partitioning concept. We show that, although it is not possible to guaranty a partitioning scheme for general operation of the MOSFET, it is possible to show the existence of a partitioning scheme for small-signal operation of the device accounting for both field-dependent mobility and lateral asymmetry. Application of this new concept in capacitance evaluation of lateral asymmetric MOSFET considerably simplifies the existing capacitance-evaluation methodology and accounts for any arbitrary field-dependent mobility. It also gives a mean to physically understand the unusual behavior delta Cdg in lateral asymmetric MOSFET. We also show how failure of the WD partitioning can affect the present compact-modeling Methodologies.

Noise Modeling in Lateral Nonuniform MOSFET

In this paper, we present an analytical noise modeling methodology for lateral nonuniform MOSFET. We demonstrate that the noise properties of lateral nonuniform MOSFETs are considerably different from the prediction obtained with the conventional Klaassen-Prins (KP)-based methods which, at low gate voltages, depending on the doping profile can overestimate the thermal noise by 2-3 orders of magnitude. We show that the presence of lateral nonuniformity makes the vector impedance field (the quantity responsible for noise propagation) position and bias dependent. This insight clearly explains the observed discrepancy and shows that the bias dependence of the important noise parameters cannot be predicted by conventional KP-based methods.

Partitioning Scheme in Lateral Asymmetric MOST

Lateral asymmetric MOSFET, which has longitudinal doping variation in the channel, is the core of high voltage MOSFET. Recently it has been recognized that capacitance property of this kind of device is fundamentally different from conventional MOST because Ward-Dutton (WD) charge partitioning is not applicable to this kind of devices (Aarts, 2006). In this work we show the existence of a partitioning scheme for small-signal operation of the device. We also provide physical explanations of unusual behavior of Cdg in lateral asymmetric MOST. The proposed theory is validated by extensive numerical and device simulation

Noise Modeling in Lateral Asymmetric MOSFET

In this work the authors present, for the first time, an analytical noise modeling methodology in presence of lateral asymmetry. The authors also show that noise properties of lateral asymmetric (LA) MOSFETs are considerably different from the prediction of conventional Klaassen-Prins (KP) based methods and at low gate voltages; they can overestimate the noise by 2-3 orders of magnitude

Impact of Lateral Asymmetry of MOSFETs on the Gate–Drain Noise Correlation

Recent studies of Lim et al. have shown that channel engineering can effectively be used to globally improve the device noise performance. This happens because the doping profile can strongly impact the correlation between the drain and induced gate noise. In this brief, we will use the theory developed in the recent works of Roy et al. to provide a physical understanding and insight on the behavior of the gate-drain correlation coefficient, which will be very useful for understanding the mechanism by which the doping profile impacts the RF noise performance of a MOSFET.

Compact Modeling of Anomalous High-Frequency Behavior of MOSFET's Small-Signal NQS Parameters in Presence of Velocity Saturation

In this work we present a physics based compact small-signal nonquasi static (NQS) model for MOST including velocity saturation and demonstrate that this can substantially change the behavior of the gate transadmittance (y/sub dg/) in saturation at high frequency. The magnitude of y/sub d/g starts to increase (instead of decreasing) after a certain frequency and its real part also starts to increase in negative direction instead of becoming zero. In addition, even for a long channel MOST, the weak inversion charge present at the drain can also effect the y/sub dg/ in a similar way.

A comprehensive study of thermal noise in the MOS transistor

This paper revisits the fundamental theory of thermal noise in the MOS transistor. It has been recognized quite early that carrier velocity saturation and eventually also carrier heating degrades the thermal noise performance of short-channel MOS devices. This degradation is evaluated in terms of the delta thermal noise parameter defined initially by van der Ziel as the ratio between the thermal noise conductance at the drain and the channel conductance at VDS=0. For long-channel devices this factor is equal to 2/3. Today, there is still a controversy about what the value of this factor actually is for short-channel devices. Some authors measured a significant degradation of up to 7, attributing it mainly to carrier heating. Some other measured values that where always smaller than 2 on several devices over several technologies and pretend that there is no need of carrier heating to explain this moderate degradation, assuming that velocity saturation only can explain it. More recently, some other authors attribute this degradation to the effect of channel-length modulation. Based on a truly physical charge-based model, this paper tries to clarify the contribution of these different effects on δ. It also highlights the fact that for circuit designers, the real important parameter is not so much the δ factor but rather the ratio of the thermal noise to the transconductance at the same bias point defined as the γ thermal noise excess factor.