The circuit elements used in this process begin with fundamental building blocks such as transistors, resistors, capacitors, and wires. These elements are combined to form more complex functions such as logic gates or precision amplifiers, which are then combined to form more complex functions such as adders and multipliers. This process continues to build on itself, resulting in the availability of increasingly complex circuit building blocks.
Circuit design utilizes discrete, pre-manufactured elements to form the circuit. In the case of IC design, there is an important difference. Here, the circuit elements are made from miniaturized components that are implemented on a silicon substrate using a process called photolithography. The photolithography process creates various geometric shapes on the silicon substrate where the electrical properties of the region defined by that shape are altered. Basic circuit elements are created when these regions are combined and superimposed over each other.
So, IC design consists of two distinct processes. First, circuit elements are assembled to perform the objective function. Next, the various geometric shapes that implement those circuit elements must be assembled and interconnected on the silicon substrate. The first process is typically called logic, or circuit, design and the second process is called physical design. Based on the type of signal being processed by the IC, a digital or analog methodology is used. In the case of an analog/mixed-signal, or AMS, design, both methodologies are used. In addition, how the various circuit elements fit the requirements of the design is also relevant. When circuit elements must be modified to achieve the requirements of the design, a full-custom design methodology is used.