Application Specific Integrated Circuits (ASIC)

Application Specific Integrated Circuits (ASIC)

Introduction.

1.1    Construction of ICs.

     ICs are made on a thin (a few hundred microns thick), circular silicon wafer, with each Wafer  Holding hundreds of die (sometimes people use dies or dice for the plural of die). The transistors and wiring are made from many layers (usually between 10 and 15 distinct layers) built on top of one another. Each successive mask layer has a pattern that is
defined using a mask similar to a glass photographic slide. The first half-dozen or so layers define the transistors. The last half-dozen or so layers define the metal wires between the transistors (the interconnect).


1.2    Physical Size of ICs.

  The physical size of a silicon die varies from a few millimeters on a side to over 1 inch on a side.

1.3    Measure of ICs

    we measure the size of an IC by the number of logic gates or the number of transistors that the  IC contains. As a unit of measure a gate equivalent corresponds to a two-input NAND gate (a circuit That performs the logic function, F = A • B ). Often we just use the term gates instead of gate equivalents when we are measuring chip size—not to be confused with the gate terminal of a transistor. For example, a 100 k-gate IC contains the equivalent of 100,000 two-input NAND gates.

1.4 Evolutions Of ICs.

The semiconductor industry has evolved from the first ICs of the early 1970s and matured Rapidly since then. Early small-scale integration ( SSI ) ICs contained a few (1 to 10) logic gates—NAND gates,  NOR gates, and so on—amounting to a few tens of transistors. The era of medium-scale integration ( MSI ) increased the range of integrated logic available to  counters and similar, larger scale, logic functions. The era of large-scale integration ( LSI ) packed even larger logic functions, such as the first microprocessors, into a single chip. The era of very large-scale integration ( VLSI ) now offers 64-bit microprocessors, complete with  cache memory and floating-point arithmetic units—well over a million transistors—on a single  piece of silicon. As CMOS process technology improves, transistors continue to get smaller and ICs   hold more and more transistors. Some people (especially in Japan) use the term ultralarge scale integration ( ULSI ), but most people stop at the term VLSI; otherwise we have to start inventing new words.

1.5 Technology Used In ICs.

The earliest ICs used bipolar technology and the majority of logic ICs used either Transistor  –transistor logic ( TTL ) or emitter-coupled logic (ECL). Although invented before the bipolar  transistor, the metal-oxide-silicon ( MOS ) transistor was initially difficult to manufacture  because of problems with the oxide interface. As these problems were gradually solved, metal- gate n -channel MOS ( nMOS or NMOS ) technology developed in the 1970s. At that time MOS technology required fewer masking steps, was denser, and consumed less power than equivalent  bipolar ICs. This meant that, for a given performance, an MOS IC was cheaper than a bipolar IC and led to investment and growth of the MOS IC market.

By the early 1980s the aluminum gates of the transistors were replaced by polysilicon gates, but  the name MOS remained. The introduction of polysilicon as a gate material was a major  improvement in CMOS technology, making it easier to make two types of transistors, n -channel MOS and p -channel MOS transistors, on the same IC—a complementary MOS ( CMOS , never cMOS) technology. The principal advantage of CMOS over NMOS is lower power consumption. Another advantage of a polysilicon gate was a simplification of the fabrication process, Allowing devices to be scaled down in size.

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