previously conceived and captured in a procedural description, imitating an expert's decision. An example are cell generators, such as PCells. Coping with variability A challenge most critical to analog IC design involves the variability of the individual devices built on the semiconductor chip. Unlike board-level circuit design which permits the designer to select devices that have each been tested and binned according to value, the device values on an IC can vary widely which are uncontrollable by the designer. For example, some IC resistors can vary ±20% and β of an integrated BJT can vary from 20 to 100. In the latest CMOS processes, β of vertical PNP transistors can even go below 1. To add to the design challenge, device properties often vary between each processed semiconductor wafer. Device properties can even vary significantly across each individual IC due to doping gradients. The underlying cause of this variability is that many semiconductor devices are highly sensitive to uncontrollable random variances in the process. Slight changes to the amount of diffusion time, uneven doping levels, etc. can have large effects on device properties. Some design techniques used to reduce the effects of the device variation are: Using the ratios of resistors, which do match closely, rather than absolute resistor value. Using devices with matched geometrical shapes so they have matched variations. Making devices large so that statistical variations becomes an insignificant fraction of the overall device property. Segmenting large devices, such as resistors, into parts and interweaving them to cancel variations. Using common centroid device layout to cancel variations in devices which must match closely (such as the transistor differential pair of an op amp). Vendors The three largest companies selling electronic design automation tools are Synopsys, Cadence, and Mentor Graphics. See also Electronic circuit design Electronic design automation Power network design (IC) Processor design IEEE Transactions on Computer-Aided Design of Integrated Circuits and Systems Multi-project wafer service Standard cell References Further reading Electronic Design Automation For Integrated Circuits Handbook, by Lavagno, Martin, and Scheffer, A survey of the field of electronic design automation, one of the main enablers of modern IC design. Integrated circuits Electronic design Electronic engineering