Chapter 3: Microprocessors^47
photoresist is removed, which leaves ridges of silicon diode where the mask
shielded the wafer.- The wafer is covered with a layer of a conductive material calledpolysilicon,
which is used to connect the layers of the circuit being developed. - Another layer of photoresist covers the wafer and is bathed in UV light
through another mask. When the photoresist is removed, it exposes areas
of the silicon diode and polysilicon. - The wafer is blasted with ions (charged atoms), a process calleddoping, which
changes the exposed areas of the wafer to either conductors or insulators. - Steps 3 through 8 of this process are repeated as many as 20 times,
depending on the complexity of the circuit, with small windows left
open between the layers. - Metal is applied to the circuit, which flows down into the open windows and
connects the layers.
The circuit wafer actually contains hundreds of separate circuits. The last step of the
process is to cut the wafer into the microchips. The chips are then packaged with wiring
to connect the chip to the pins (legs) of the packaging, and the integrated circuit (IC) is
now ready for use.
For a more detailed explanation of how a microchip is manufactured, plus some other
very good information about the history of integrated circuits, visit thewww.techweb.
com/encyclopedia/Web site and search forchip.The Transistor
The size of microprocessors, relative to their computing power, is shrinking every day.
This is largely possible because of the technology advances that allow an increasing num-
ber of microscopic transistors to be placed closer together on a circuit. In the 1960s, the
transistor paved the way for radios, calculators, and other electronic devices to become
very small. Where the earliest computers filled up gymnasiums and had tens of thou-
sands of vacuum tubes, today a billion transistors would fit into a single vacuum tube.
Thetransistoris the primary circuit in a microprocessor and is used in several different
ways, but its basic function is to store the electrical voltage that represents one bit. A tran-
sistor works something like an electric light switch, in that it either has a high electrical
charge (on) or a low electrical charge (off). However, unlike the light switch, the transistor
has no moving parts; its charge is set electrically on or off to represent the 1s and 0s of
binary values (see the section “Binary Data” earlier in the chapter).
The number of transistors included in its circuitry determines the capacity of the pro-
cessor. Modern microprocessors include millions of transistors, which is the source of
their computing power. For example, the Intel Pentium III contains nearly ten million
transistors, and the processors used in larger mainframes and supercomputers have
billions of transistors.