Analog Circuit Design in Nanoscale CMOS Technologies

As complementary metal-oxide-semiconductor (CMOS) technologies are scaled down into the nanometer range, a number of major nonidealities must be addressed and overcome to achieve a successful analog and physical circuit design. The nature of these nonidealities has been well reported in the technical literature. They include hot carrier injection and time-dependent dielectric breakdown effects limiting supply voltage, stress and lithographic effects limiting matching accuracy, electromigration effects limiting conductor lifetime, leakage and mobility effects limiting device performance, and chip power dissipation limits driving individual circuits to be more energy-efficient. The lack of analog design and simulation tools available to address these problems has become the focus of a significant effort with the electronic design automation industry. Postlayout simulation tools are not useful during the design phase, while technology computer-aided design physical simulation tools are slow and not in common use by analog circuit designers. In the nanoscale era of analog CMOS design, an understanding of the physical factors affecting circuit reliability and performance, as well as methods of mitigating or overcoming them, is becoming increasingly important. The first part of the paper presents factors affecting device matching, including those relating to single devices as well as local and long-distance matching effects. Several reliability effects are discussed, including physical design limitations projected for future downscaling. In some cases, it may be helpful to exceed foundry-specified drain-source voltage limits by a few hundred millivolts. Models are presented for achieving this, which include the dependence on the shape of the output waveform. The condition
Vsb
> 0 is required for cascode circuit configurations. The role of other terminal voltages is discussed, as
Vsb
> 0 increases both hot and cold carrier damage effects in highly scaled devices. The second part of the paper focuses on trends in device characteristics and how they influence the design of nanoscale analog CMOS circuits. A number of circuit design techniques employed to address the major nonidealities of nanoscale CMOS technologies are discussed. Examples include techniques for establishing on-chip accurate and temperature-insensitive bias currents, digital calibration of analog circuits, and the design of regulator and high-voltage circuits. Achieving high energy efficiency in ICs capable of accommodating 10
⁹
devices is becoming critically important. This paper also presents a survey of the evolution of figure of merit for analog-to-digital converters.