Integrated Circuits

Digital circuits are constructed with integrated circuits (ICs). An integrated circuit is a small silicon semiconductor crystal, known as a die, that contains the electronic components necessary for digital gates. These gates are interconnected within the chip to form the required circuit. The chip is mounted in a ceramic or plastic container, and connections are welded by thin gold wires to external pins to create the integrated circuit. The number of pins can range from 14 in small IC packages to 100 or more in larger packages. Each IC has a numeric designation printed on its surface for identification. Vendors publish data books or catalogs that contain detailed descriptions and all necessary information about the ICs they manufacture.

As IC technology has advanced, the number of gates that can be embedded in a single chip has increased significantly. The differentiation between chips with a few internal gates and those with hundreds or thousands of gates is made by referencing the package as either small-scale, medium-scale, or large-scale integration devices.

Scale of Integration

Small-Scale Integration (SSI)

SSI devices contain several independent gates in a single package. The inputs and outputs of these gates are connected directly to the pins in the package. The number of gates is usually fewer than 10, limited by the number of pins available in the IC.

Medium-Scale Integration (MSI)

MSI devices have a complexity of approximately 10 to 200 gates in a single package. These devices typically perform specific elementary digital functions such as decoders, adders, and registers.

Large-Scale Integration (LSI)

LSI devices contain between 200 and a few thousand gates in a single package. They include digital systems like processors, memory chips, and programmable modules.

Very-Large-Scale Integration (VLSI)

VLSI devices contain thousands of gates within a single package. Examples of VLSI devices include large memory arrays and complex microcomputer chips. The small size and low cost of VLSI devices have revolutionized computer system design, allowing designers to create structures that were previously not economical.

Digital Logic Families

Digital integrated circuits are classified not only by their logic operation but also by the specific circuit technology to which they belong. This classification is known as a digital logic family. Each logic family has its own basic electronic circuit, which forms the foundation upon which more complex digital circuits and functions are developed. The basic circuit in each technology is typically a NAND, NOR, or inverter gate. The electronic components used in constructing these basic circuits give the technology its name. The most popular logic families are:

• TTL: Transistor-Transistor Logic
• ECL: Emitter-Coupled Logic
• MOS: Metal-Oxide Semiconductor
• CMOS: Complementary Metal-Oxide Semiconductor

TTL (Transistor-Transistor Logic)

TTL is a widespread logic family that has been in operation for many years and is considered a standard. It evolved from a previous technology known as diode-transistor logic (DTL), which used diodes and transistors for the basic NAND gate. The diodes were later replaced by transistors to improve circuit operation, leading to the name "transistor-transistor logic." Variations of the TTL family include high-speed TTL, low-power TTL, Schottky TTL, low-power Schottky TTL, and advanced Schottky TTL. TTL circuits typically operate at a power supply voltage of 5 volts, with logic levels approximately 0 and 3.5 volts.

ECL (Emitter-Coupled Logic)

The ECL family provides the highest-speed digital circuits in integrated form. ECL is used in systems where high speed is essential, such as supercomputers and signal processors. The transistors in ECL gates operate in a nonsaturated state, enabling propagation delays of 1 to 2 nanoseconds.

MOS (Metal-Oxide Semiconductor)

MOS technology utilizes a unipolar transistor that depends on the flow of only one type of carrier—either electrons (n-channel) or holes (p-channel). This contrasts with the bipolar transistors used in TTL and ECL gates, which involve both types of carriers during normal operation. P-channel MOS is referred to as PMOS, and n-channel MOS as NMOS, with NMOS being the more commonly used type.

CMOS (Complementary Metal-Oxide Semiconductor)

CMOS technology uses both PMOS and NMOS transistors connected in a complementary fashion in all circuits. The significant advantages of CMOS over bipolar technologies include higher packing density of circuits, simpler fabrication processes, and lower power consumption.

Importance of Integrated Circuits

Integrated circuits are indispensable in providing the various digital components necessary for computer system design. Familiarity with these components is crucial for understanding the organization and design of digital computers. The most basic components are introduced here with explanations of their logical properties, providing a catalog of elementary digital functional units commonly used as basic building blocks in digital computer design.

By leveraging these advancements in integrated circuit technology, computer engineers can design and implement sophisticated digital systems with improved performance, reduced cost, and enhanced functionality.