Integrated circuit design usually takes "module" as the design unit. For example, for a multi bit full adder, its secondary module is a one bit adder, and the adder is composed of the and gate and not gate modules of the next level. The and gate and not gate can finally be decomposed into CMOS devices of lower abstraction level.

From the abstract level, digital integrated circuit design can be top-down, that is, first define the functional modules at the high logic level of the system, define the sub modules according to the requirements of the top-level modules, and then continue to decompose layer by layer; The design can also be bottom-up, that is, first design specific modules respectively, and then use these bottom modules to realize the upper modules like building blocks, and finally reach a high level. In many designs, top-down and bottom-up design methodologies are mixed. System level designers plan the overall architecture and divide sub modules, while bottom-level circuit designers design and optimize individual modules layer by layer. After that, the designers of the two directions meet at a certain abstraction level in the middle to complete the whole design. For different design requirements, engineers can choose to use semi customized design methods, such as using programmable logic devices (field programmable logic gate array, etc.) or special integrated circuits based on standard cell library to realize hardware circuits; Full custom design can also be used to control the transistor layout to all details of the system structure. Full custom design this design method requires designers to use the layout editor to complete layout design, parameter extraction and unit characterization, and then use these self-designed units to complete the construction of the circuit. Generally, full custom design is to maximize and optimize circuit performance. If there is a lack of a required unit in the standard unit library, it is also necessary to adopt the method of full custom design to complete the required unit design. However, this design usually takes a long time.