CMOS VLSI Design and Circuit Simulation Tasks

September 11, 2020 Cadence PCB Solutions

Key Takeaways

  • CMOS VLSI design is broken into two steps: circuit block design and physical design.

  • Circuit block design involves connecting transistors into logic blocks, which are then integrated into a larger integrated circuit.

  • Simulation tools are needed to extract the electrical characteristics of your circuit blocks for VLSI.

Silicon wafer CMOS VLSI design

CMOS VLSI design is the first step in creating a silicon wafer with dozens of ICs.

CMOS (complementary metal-oxide-semiconductor) VLSI (very-large-scale integration) design has enabled massive scaling in a variety of semiconductor devices. Combining the CMOS process with VLSI has helped push packages to smaller levels while keeping costs reasonable. While not as dense and small as FinFET technology, CMOS still has a major role to play in earlier technology nodes, which are not expected to be phased out anytime soon. If you’re designing a new integrated circuit, SoC, or other component, it will most likely need to be created using CMOS VLSI design.

Something like 99% of digital, analog, and RF integrated circuits are manufactured using CMOS processes. As newer designs continue the trend of integration and revised CMOS architectures are being designed for tasks like embedded AI, circuit simulation tasks in CMOS circuit blocks will remain critical. Here are some of the important circuit simulation tasks you’ll need to perform before integrating circuit blocks.

Getting Started with CMOS VLSI Design

CMOS VLSI design is like a modular approach to creating ICs. Small circuit blocks are connected into larger circuit blocks which are then connected at the system level to create a complete integrated circuit. These smaller circuit blocks can be analog, digital, or mixed-signal circuits.

The main challenge in CMOS VLSI design is twofold:

  1. Designing and simulating small circuits and larger circuit blocks.

  2. Linking circuit blocks together into a larger system.

The first of these tasks requires some basic front-end circuit design and simulation tasks to create individual circuits; some examples are shown below. These smaller circuits then need to be combined into circuit blocks that provide specific functions. Once you’ve finished creating and simulating some larger circuit blocks, you’ll have to move on to creating a layout, which integrates multiple circuit blocks into an entire system.

Digital CMOS Circuit Blocks

The foundational digital circuit block in CMOS VLSI design is the CMOS inverter--a simple circuit combining a PMOS and NMOS transistor:

CMOS inverter circuit schematic

CMOS inverter circuit as part of CMOS VLSI design.

This fundamental circuit is basically a NOT gate. MOSFET transistors can be combined in other ways to produce any other fundamental logic gates, which can then be combined to produce larger logic circuits. Depending on how these gates are combined, different functions can be implemented on the chip. Integration also needs to happen at the physical level, where different inputs and outputs need to be connected together in physical circuit design. The example below shows how a CMOS inverter can be physically integrated into a larger circuit block.

CMOS inverter circuit physical design

Physical design of a CMOS inverter circuit.

Analog CMOS Circuit Blocks

Analog building blocks can also be built from CMOS circuitry, and many modern products are based on proven decades-old circuit designs. If a CMOS IC includes an operational amplifier or integrated power regulator, it is probably built on a CMOS circuit that was first designed decades earlier. Today, newer designs are making greater use of custom CMOS circuitry for analog signal processing at the chip level, rather than converting the signal to a digital signal and performing digital arithmetic.

Whether you’re reusing older circuit blocks or designing new architectures based on CMOS processes, you’ll take the same approach shown above for digital CMOS circuit blocks, both for front-end circuit design and physical design. The various analog circuit blocks in CMOS VLSI get connected into a larger system that eventually gets fabricated from a semiconductor wafer. Many devices that appear to be all-digital are actually mixed-signal systems and will integrate some analog circuit blocks into the design.

What to Analyze in CMOS VLSI Design

There is plenty to analyze in CMOS VLSI design, both before and after you create a physical layout. Simulations should be performed at 3 levels to understand electrical behavior and each requires a different set of simulations:

  • Small-scale circuit simulations. Tasks like transient analysis, operating point analysis, and frequency sweeps (for linear components) will give you the quantitative behavior of your circuits in the time and frequency domains. For nonlinear components, operating point analysis is especially important for designing a circuit to operate in the linear regime. 

  • Large-scale circuit simulations. Once larger circuit blocks are created, you’ll need to run logic simulations (for digital circuits) or a range of analog simulations. These larger-scale circuit simulations will show how a larger circuit block produces outputs for a given set of inputs. This is an important part of circuit block verification before integrating multiple blocks into an entire system.

  • System-level simulations. Results from block-level simulations can be used in higher-level system simulations to show how groups of circuit blocks in CMOS VLSI design will function. This type of simulation takes specialized modeling tools that reduce circuit blocks into functional blocks, which are then used in graphical simulations. Many programs for system-level modeling take this approach and provide a graphical representation of the system’s function (see below).

 A hand pointing to a CMOS VLSI design integrated circuit system-level simulation on a computer screen

System-level modeling for a complex integrated circuit.

In a circuit simulation, you’ll need to use SPICE models for your components to perform necessary simulations, especially for MOSFETs used in CMOS inverters. These component models are phenomenological; they are not intended to simulate COTS components. Instead, these component models are meant to describe the electrical behavior of different components in your circuits, which will eventually be simulated as an entire system.

Before you get to the level where you need system-level simulations in CMOS VLSI design, you need tools to design and analyze individual circuit blocks for your next system. The front-end design features from Cadence and the powerful PSpice Simulator package give you everything you need to create analog, digital, and mixed-signal CMOS circuit blocks. You can then import your designs into IC layout tools to create real circuit blocks, and you can simulate an entire layout with system-level analysis tools.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.

 

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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