Numerous semiconductor devices have seen significant scalability due to CMOS (complementary metal-oxide-semiconductor) VLSI design. Packages have been made smaller while maintaining affordable pricing as a result of CMOS technology combined with VLSI. Although CMOS technology is not as compact and dense as FinFET, it still plays a significant role at older technology nodes and is not anticipated to be phased out very soon. A new integrated circuit, SoC, or other components that are being created will almost certainly need to be produced utilising CMOS VLSI design.
Digital, analog and RF Integrated Circuits are produced utilising CMOS methods in close to 99% of all cases. Circuit simulation activities in CMOS circuit blocks will continue to be crucial as newer designs follow the trend of integration and updated CMOS architectures are developed for projects like embedded AI.
This blog is a comprehensive guide on everything you must know about CMOS VLSI design.
Getting Started with CMOS Design
Designing CMOS VLSI is comparable to building integrated circuits in a modular manner. A full integrated circuit is produced by connecting smaller circuit blocks to bigger circuit blocks, which are subsequently coupled at the system level. These more compact circuitry units may be analog, digital, or mixed-signal. The fundamental challenge in CMOS VLSI design comes in two forms. Designing and modeling both smaller circuits and bigger circuit blocks and connecting circuit blocks to form a larger system are the two forms. To create individual circuits, the first of these tasks necessitates relatively straightforward front-end circuit design and simulation work. Subsequently, these smaller circuits must be assembled into circuit blocks that carry out specific responsibilities. Creating a layout, which combines numerous circuit blocks into a system as a whole, comes next once you’ve done designing and modeling a few bigger circuit blocks. The CMOS inverter, a simple circuit combining a PMOS and NMOS transistor, is the fundamental digital circuit block in CMOS VLSI design. A NOT gate is essentially what this fundamental circuit is. Other combinations of MOSFET transistors can result in any other basic logic gates, which can then be coupled to create bigger logic circuits. On the chip, a variety of functionalities may be achieved depending on how these gates are coupled. Additionally, integration is required at the physical level, where many inputs and outputs must be coupled in physical circuit architecture.
What to Look for When Designing CMOS VLSI
In CMOS VLSI design, there is a lot to consider both before and after creating a physical layout. To comprehend electrical behaviour, simulations need to be run at three separate levels, each requiring a unique set of simulations. The three stages are system-level simulations, large-scale circuit simulations, and small-scale circuit simulations.
Small-scale circuit simulations:
Transient analysis, operating point analysis, and frequency sweeps (for linear components) are examples of activities that can be used to quantify the behaviour of the circuits in the time and frequency domains. Operating point analysis is particularly crucial when constructing a circuit to function in the linear regime for nonlinear components.
Large-scale circuit simulations:
After creating larger circuit blocks, one will need to do several analog simulations or logic simulations (for digital circuits). The outputs of a larger circuit block for a certain set of inputs will be demonstrated by these circuit simulations at a greater size. Before integrating several circuit blocks into a system as a whole, a crucial step in circuit block verification.
System-level simulations:
Results from block-level simulations can be utilised in higher-level system simulations to demonstrate the behaviour of groupings of circuit blocks in CMOS VLSI designs. This kind of simulation uses specialist modeling software to break down circuit blocks into functional units that may be represented graphically. This method is used by many system-level modeling applications, which also depict the function of the system graphically.y programs for system-level modeling take this approach and provide a graphical representation of the system’s function.
You must employ SPICE models for your components in a circuit simulation to run the appropriate simulations, particularly for MOSFETs used in CMOS inverters. These component models are phenomenological rather than Commercial off-the-shelf (COTS) component simulators. The electrical behaviour of the many components in your circuits, which will ultimately be emulated as a whole system, is intended to be described by these component models instead. For CMOS VLSI design, you need tools to design and evaluate individual circuit blocks before you reach the point where you need system-level simulations. After that, you may integrate your designs into IC layout tools to make actual circuit blocks, and you can use system-level analysis tools to simulate a full layout.
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