W. Redman-White

Self-heating effects in SOI MOSFETs and their measurement by small signal conductance techniques

Self-heating is an important issue for SOI CMOS, and hence, so is its characterization and modeling. This paper sets out how the critical parameters for modeling, i.e., thermal resistance and thermal time-constants, may be obtained using purely electrical measurements on standard MOS devices. A summary of the circuit level issues is presented, and the physical effects contributing to thermally related MOSFET behavior are discussed. A new thermal extraction technique is presented, based on an analytically derived expression for the electro-thermal drain conductance in saturation. Uniquely, standard MOSFET structures can be used, eliminating errors due to additional heat flow through special layouts. The conductance technique is tested experimentally and results are shown to be in excellent agreement with thermal resistance values obtained from noise thermometry and gate resistance measurements using identical devices. It is demonstrated that the conductance technique can be used confidently over a wide range of bias conditions, with both fully and partially depleted devices.

High-linearity continuous-time filter in 5-V VLSI CMOS

High-linearity self-tuning continuous-time filters, fabricated in a standard 1.6- mu m 5-V CMOS process, are presented. Frequency control is achieved using switchable arrays of highly linear double-polysilicon capacitors in an active RC filter structure, resulting in tunable filters with very low signal distortion. One filter, a Tow-Thomas biquad, exhibits dynamic range and signal linearity of typically 91 dB. Another smaller implementation, a Sallen and Key filter, attains >or=76 dB. Cutoff frequency response is maintained to an accuracy of around +or-5%. >

A high bandwidth constant g/sub m/ and slew-rate rail-to-rail CMOS input circuit and its application to analog cells for low voltage VLSI systems

A new rail-to-rail CMOS input architecture is presented that delivers behavior nearly independent of the common-mode level in terms of both transconductance and slewing characteristics. Feedforward is used to achieve high common-mode bandwidth, and operation does not rely on analytic square law characteristics, making the technique applicable to deep submicron technologies. From the basis of a transconductor design, an asynchronous comparator and a video bandwidth op amp are also developed, providing a family of general purpose analog circuit functions which may be used in high (and low) bandwidth mixed-signal systems. Benefits for the system designer are that the need for rigorous control of common-mode levels is avoided and input signal swings right across the power supply range can be easily handled. A further benefit is that having very consistent performance, the circuits can be easily described in VHDL (or other behavioral language) to allow simulation of large mixed-signal systems. The circuits presented may be easily adapted for a range of requirements. Results are presented for representative transconductor, op amp, and comparator designs fabricated in a 0.5 /spl mu/m 3.3 V digital CMOS process.

Integrated continuous-time balanced filters for 16-b DSP interfaces

A fully-balanced integrated continuous-time leapfrog filter, with signal linearity and dynamic range compatible with 16-bit digital audio processing, is presented. The filter employs high performance balanced op-amps and linear resistors; switchable arrays of linear capacitors controlled by a digital code perform the response tuning function. A new area efficient automatic on-chip calibrator capable of programming the response of the leapfrog filter is described. All circuitry was fabricated in a standard CMOS process. ¿94dB THD and dynamic range of 95dB are achieved with a 5V supply.

Impact of self-heating and thermal coupling on analog circuits in SOI CMOS

This paper examines the influence of the static and dynamic electrothermal behavior of silicon-on-insulator (SOI) CMOS transistors on a range of primitive analog circuit cells. In addition to the more well-known self-heating close-range thermal coupling effects are also examined. Particular emphasis is given to the impact of these effects on drain current mismatch due to localized temperature differences. Dynamic electrothermal behavior in the time and frequency domains is also considered, measurements and analyses are presented for a simple amplifier stage, current mirrors, a current output D/A converter, and ring oscillators fabricated in a 0.7-/spl mu/m SOI CMOS process. It is shown that circuits which rely strongly on matching, such as the current mirrors or D/A converter, are significantly affected by self-heating and thermal coupling. Anomalies due to self-heating are also clearly visible in the small-signal characteristics of the amplifier stage. Self-heating effects are less significant for fast switching circuits. The paper demonstrates how circuit-level simulations can be used to predict undesirable nonisothermal operating conditions during the design stage.

Higher Order Noise-Shaping Filters for High-Performance Micromachined Accelerometers

Micromachined inertial sensors that have been incorporated in sigma-delta force-feedback loops have been proven to improve linearity, dynamic range, and bandwidth, and also provide a direct digital output. Previous work mainly focused on using the sensing element only to form a second-order single-loop sigma-delta modulator (SigmaDeltaM); however, the advantages of higher order single-loop electromechanical SigmaDeltaM have not been fully explored. High-performance inertial sensors require higher signal-to-quantization noise ratio (SQNR). This paper presents topologies for higher order single-loop electromechanical SigmaDeltaM with optimal stable coefficients that lead to a better SQNR. The topologies have good immunity to fabrication tolerances, which was verified by Monte Carlo analysis. The topologies are applicable not only to accelerometers but also to other inertial sensors such as gyroscopes.

The effect of body contact series resistance on SOI CMOS amplifier stages

This paper examines some implications for analogue design of using body ties as a solution to the problem of floating body effects in partially-depleted (PD) SOI technologies. Measurements on H-gate body-tied structures in a 0.7-/spl mu/m SOI process indicate body-tie series resistances increasing into the M/spl Omega/ region. Both circuit simulation and measurement results reveal a delayed but sharper kink effect as this resistance increases. The consequences of this effect are shown in the context of a simple amplifier configuration, resulting in severe bias-dependent degradation in the small signal gain characteristics as the body-tie resistance enters the M/spl Omega/ region. It is deduced that imperfectly body tied devices may be worse for analogue design than using no body-tie at all.

A small-signal model for the frequency-dependent drain admittance in floating-substrate MOSFET's

The frequency-dependent drain admittance of silicon-on-sapphire (SOS) MOSFETs is examined from the perspective of the circuit designer. Measurements of small-signal drain characteristics as a function of frequency, bias conditions, and device geometry, which have major implications for analog circuit design, are presented. These are explained in terms of a small-signal circuit model. Physical explanations for the observations are given and the poles and zeros of the model identified to assist designers carrying out hand calculations with easily manipulated expressions. Frequency-dependent thermal effects are discussed. It is shown that similar effects can be expected in other SOI technologies. >

Automated design of switched-current filters

This paper describes the automated design and synthesis of switched-current (SI) filters using SCADS, a flexible CAD system integrated in a major VLSI design suite. With this system, the nonspecialist can produce high performance analog filters suitable for mixed signal CMOS ICs fabricated using only standard digital processes. To achieve high levels of performance on silicon, filter designs are realized using an enhanced differential circuit technique (S/sup 2/I) in its integrators and sample-and-hold cells. The design system is described in terms of the embedded circuits, its integrated tool set, the filter design flow and the engineering procedures for ensuring reliable circuit operation. Examples of high performance video frequency filters are presented, each generated automatically by SCADS within one day. Fabricated in a 0.8 /spl mu/m standard CMOS process, they demonstrate state-of-the-art performance.

Micromachined Vibratory Gyroscopes Controlled by a High-Order Bandpass Sigma-Delta Modulator

This work reports on the design of novel closed-loop control systems for the sense mode of a vibratory-rate gyroscope based on a high-order sigma-delta modulator (SigmaDeltaM). A low-pass and two distinctive bandpass topologies are derived, and their advantages discussed. So far, most closed-loop force-feedback control systems for these sensors were based on low-pass SigmaDeltaMs. Usually, the sensing element of a vibratory gyroscope is designed with a high quality factor Q to increase the sensitivity and, hence, can be treated as a mechanical resonator. Furthermore, the output characteristic of vibratory rate gyroscopes is narrowband amplitude-modulated signal. Therefore, a bandpass SigmaDeltaM is a more appropriate control strategy for a vibratory gyroscope than a low-pass SigmaDeltaM. Using a high-order bandpass SigmaDeltaM, the control system can adopt a much lower sampling frequency compared with a low-pass SigmaDeltaM while achieving a similar noise floor for a given oversampling ratio (OSR). In addition, a control system based on a high-order bandpass SigmaDeltaM is superior as it not only greatly shapes the quantization noise, but also alleviates tonal behavior, as is often seen in low-order SigmaDeltaM control systems, and has good immunities to fabrication tolerances and parameter mismatch. These properties are investigated in this study at system level