An application-specific integrated circuit (ASIC) is an integrated circuit (IC) customised for a particular use, rather than intended for general-purpose use. For example, a chip designed solely to run a cell phone is an ASIC.
As feature sizes have shrunk and design tools improved over the years, the maximum complexity (and hence functionality) possible in an ASIC has grown from 5,000 gates to over 100 million. Modern ASICs often include entire 32-bit processors, memory blocks including ROM, RAM, EEPROM, Flash and other large building blocks. Such an ASIC is often termed a SoC (System-on-a-chip). Designers of digital ASICs use a hardware description language (HDL), such as Verilog or VHDL, to describe the functionality of ASICs.
The general term application specific integrated circuit includes FPGAs, but most designers use ASIC only for non field programmable devices (e.g. standard cell or sea of gates) and differ between ASIC and FPGAs.

The initial ASICs used gate array technology. Ferranti produced perhaps the first gate-array, the ULA (Uncommitted Logic Array), around 1980. Customization occurred by varying the metal interconnect mask. ULAs had complexities of up to a few thousand gates. Later versions became more generalized, with different base dies customised by both metal and polysilicon layers. Some base dies include RAM elements.

Essentially there are three levels of ASIC that can be used :

Gate Array

This type of ASIC is the least customisable. Here the silicon layers are standard but the metallization layers allowing the interconnections between different areas on the chip are customisable. This type of ASIC is ideal where a large number of standard functions are required which can be connected in a particular manner to meet the given requirement.

Standard cell

For this type of ASIC, the mask is a custom design, but the silicon is made up from library components. This gives a high degree of flexibility, provided that standard functions are able to meet the requirements.

Full custom design

This type of ASIC is the most flexible because it involves the design of the ASIC down to transistor level. The ASIC layout can be tailored to the exact requirements of the circuit. While it gives the highest degree of flexibility, the costs are very much higher and it takes much longer to develop. The risks are also higher as the whole design is untested and not built up from library elements that have been used before.

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