Computer chip is a tiny piece of material, usually silicon, that contains a complex electronic circuit. These chips are essential in modern computers and a variety of other electronic devices. The circuit on a computer chip, sometimes called an integrated circuit, is made up of electronic components built into the chip. Most chips are no larger than a fingernail.
There are two main kinds of computer chips: (1) microprocessors, which carry out the instructions that make up computer programs, and (2) memory chips, which hold computer programs and other data. Both kinds are used in computers and many other products, including electronic games, digital watches, microwave ovens, cars, and telephones. A microprocessor serves as the “brain” of every personal computer. Some computer chips contain multiple microprocessors and memory chips packaged together.
Structure.
The body of most chips is made of silicon. This material is used because it is a semiconductor. In its pure form, silicon does not conduct electric current at room temperature. But if certain impurities are added to silicon, it can carry an electric current. Manufacturers “dope” silicon chips with such impurities as boron and phosphorus. The doped regions form the electronic components, which control the electric signals carried on the chip. The type and arrangement of the impurities determine how each component controls signals. Most components serve as switches called transistors. Others serve mainly as capacitors, which store an electric charge; diodes, which prevent current from flowing in one direction but not the other; and resistors, which control voltage.
Some chips contain billions of components. Certain parts of these components measure less than 50 nanometers (0.00000005 meters) across—about 1/2,000 the width of a human hair. Manufacturers create thin lines of metal—usually aluminum or copper—on the chip to connect these tiny components.
External connections.
Most chips connect with other devices by means of a container called a package. One common type of package is a pin grid array (PGA), typically square in shape. The underside of a PGA is covered with pins arranged in a regular grid, often 0.1 inches (0.25 centimeters) apart. Each pin is connected by a wire to an electric terminal on the chip inside the package.
In a computer, the packaged chip is mounted on a circuit board. Printed circuits on the board connect the package—and thus the chip—with other devices.
Characteristics of microprocessors.
Microprocessors perform essential computer operations. A microprocessor obtains instructions and data from an external memory device; performs arithmetic and logic operations with these data; and, after obtaining a result, moves the calculated data back to the memory device.
Word length.
Instructions and other data handled by computers are in the form of “words.” A word is a group of bits. A bit is a binary digit—a 0 or a 1. Computers operate on the basis of bits. For example, the presence of an electric charge in a capacitor can represent a “1” and the absence of a charge can represent a “0.”
The maximum word length that a microprocessor can handle helps determine how rapidly it can operate. The earliest microprocessors used 4-bit words. As microprocessors advanced, they were able to handle longer words—usually made up of 8, 16, 32, or 64 bits.
Clock speed
is another important characteristic of a microprocessor. Bits travel through a computer in pulses of electric current that occur at regular intervals called clock cycles. Today’s microprocessors run at speeds measured in billions of cycles per second. One billion cycles per second equals one gigahertz (GHz).
Instruction sets.
There are two basic types of microprocessors: (1) complex instruction set computer (CISC), and (2) reduced instruction set computer (RISC). Most personal computers use CISC chips. Many powerful workstations and some printers have RISC chips.
A CISC chip has a very large instruction set—that is, it has many ways to carry out each instruction. For example, it may be able to add two numbers by following any one of 10 procedures. These procedures take various numbers of clock cycles. The number of cycles depends on such factors as the size of the numbers to be added and the location of the numbers in the computer system. A RISC chip uses instructions that are always the same length and mostly can be executed in one clock cycle. By using its special circuits, a RISC chip can execute many times more instructions per second than can a comparable CISC chip.
Memory chips.
Most computers use three types of memory chips: (1) read-only memory (ROM), (2) random-access memory (RAM), and (3) nonvolatile memory (NVM). A ROM chip retains its stored values even when the computer is turned off, but the computer user normally cannot change the stored memory. In contrast, the user can change the values stored in a RAM chip with each keystroke made. But the chip holds these values only as long as power is on. An NVM chip retains its values even when the system is turned off, but it also allows users to modify its values. The most common type of NVM is called flash memory.
There are two main kinds of RAM: static RAM (SRAM) and dynamic RAM (DRAM). SRAM holds its memory until the microprocessor changes it, but DRAM can only hold its memory for a few thousandths of a second. Therefore, a DRAM chip must be refreshed regularly, or it will lose its data. To refresh a DRAM, the computer removes the information from each group of cells (memory storage units) in the chip, then puts the same information back. This may seem wasteful. But the time and expense involved are more than made up for by the amount of memory that can be fit into a small space on a DRAM and the cost of these devices. The majority of memory chips used in personal computers are DRAM’s because they are inexpensive to make and can store much memory.
Flash memory uses single transistors to store each bit. It writes values by trapping electric charge in the insulator of the transistor. This operation requires a high voltage. It can read values by using a lower voltage to sense if the transistor contains the charge or not. Extremely high density flash memories can store four different levels of charge in the insulator of each transistor, enabling a single transistor to store two bits of data.
Chip manufacture
begins with a wafer of doped silicon. The wafer measures from about 1 to 12 inches (25 to 300 millimeters) in diameter. A photographic process reduces a large master design for the integrated circuit to microscopic size. Technicians use these microscopic designs, called masks, as stencils to make hundreds of chips on one wafer. After the wafer has been processed, it is divided into individual chips.
History.
The first computer chips were patented in 1959 by two Americans—Jack Kilby, an engineer, and Robert Noyce, a physicist—who worked independently. During the 1960’s, scientists developed chips for guided missiles and satellites. Engineers soon began to build smaller and faster computers by using chips in place of conventional circuits. The first microprocessors were produced in 1971 for use in desktop calculators.
