Historical Background

The earliest electronic computers used bulky vacuum tubes resembling short neon lights to create the rapid on/off electric switching that is necessary to perform binary computations and logical functions. In the 19408,it took thousands of vacuum tubes to create the famous computers that occupied several rooms. Not surprisingly, this was a rather costly and tedious way to go about building a calculating machine.

In 1947, vacuum tube computing was rendered obsolete, ahnost overnight,by thetransistor. Three Bell Labs scientists-William Shockley, Walter Brattain, and John Bardeen-are credited with a series of inventions that introduced, refined, and then commercially launched the transistor.' Their invention was smaller, faster, and cheaper; handled more complex operations; and generated less heat than its predecessor. The transistor could amplify electric signals moving through circuits embedded in a solid piece of semiconductor material. Transistors were thus called

"solid-state" devices because electric current nows through a solid semiconductor rather than through a vacuum tube.

IIbe importance of the vision of M. Kelly, Bell Labs' research director at the time, is also usually stressed. He understood that vacuum tubes were holding back the electronics industry and fostered an innovative research atmosphere to find an alternative. A popular anecdote is that if one of today's cell phones were made from vacuum tubes, the device would be as big as the Washington Monument in the District of Columbia.

5.5 Transistors 177

Transistor technology started the microelectronics revolution by making high-performance inexpensive electronics possible. Transistors showed up in a burgeoning array of electronic products-from rockets to portable radios-throughout the 1950s. Also, with fewer power, heat, and size constraints, computer designers could build faster, more reliable computers that occupied much less space. But properly connecting hundreds of transistors with thousands of other electric circuit compo-nents was an enormous design, manufacturing, and performance problem.

The problems of interconnecting the discrete devices in computers were over-come with the invention of the integrated circuit in 1958 by Jack Kilby at Texas Instruments. This enabled the fabrication of circuit components and their intercon-nections on a single chip.

Integrated circuits are classified into analog and digital. Analog integrated cir-cuits include a large family of circir-cuits used in power electronics, instrumentation, telecommunications, and optics. Digital integrated circuits are usually classified into two types, memory and logic chips:

• Memory chips consist of memory cells and associated circuits for address selec-tion and amplificaselec-tion. Process technologies are extremely well developed for 16 and 64 megabyte dynamic random access memories (DRAMs). DRAMs are inexpensive commodity products differentiated by speed, power con-sumption, configuration, and package type. From an integrated design and fab-rication viewpoint, specialty DRAMs and video RAMs are the more emerging technologies of interest.

• Logic chips contain the circuits needed to petform arithmetic, logic, and con-trot functions central to the microprocessor. Application specific integrated circuits (ASles) are tailored to a customer's particular requirement, as opposed to one of the standard "Intel-inside" microprocessors.

Rapid advances in Ie design and process technologies meant that chips could be made at commercially viable scales by the early 19608.Improvements in minia-turization technology permitted ever-increasing numbers of components to fit on smaller and smaller chips (Table 5.1).

By1971, a single integrated circuit(Ie)was built that combined logic functions, arithmetic functions, memory registers, and the ability to send and receive data. This device was called the microprocessor. It was used in many applications and spurred

TABLE5.1 Trends in Ie Integration Levels

Integration scale Abbreviation Devices per chip

Small scele mtegration

178 Semiconductor Manufacturing Chap. 5

the factory-floor robotics revolution of the late 19708 (see Figure 1.2). For the robotics industry, the microcorurolter was a cheap and reasonably powerful special-ized control system built around the microprocessor. Of course, the microprocessor also made possible the development of the microcomputer-or the personal com-puter (PC).

5.5.2 Semiconductors:". Type and ~ Type

A semiconductor is a crystalline material (usually silicon) with electrical properties lying between conductors, such as aluminum and copper, and insulators, such as rubber and glass. Silicon crystallizes in a diamond-shaped lattice, with each atom surrounded by four other atoms in a tetrahedron. The atoms share valence electrons, which give each atom a complete valence shell. In its pure state, a semiconductor material exhibits relatively high electrical resistance. Adding controlled amounts of certain chemical impurities(dopanrs) to the crystal structure of the semiconductor lowers its resistivity and allows current to flow through the material. The atomic structure of the dopant determines whether the resulting material will be "a-type" or "p-type."

• n-type silicon is typically created by doping silicon with phosphorus. which has five electrons in its outer shell. In comparison with the four-electron silicon, this creates additional free electrons in the material, which readily move in response to a voltage. Since most of the conduction is carried by negatively charged electrons, the material is called n-type .

•p-type silicon is typically created by doping silicon with boron. Boron has only three electrons in its outer shell. Since all the silicon atoms were nicely bal-anced with four electrons in their outer shell, the presence of the boron intruder creates additional vacancies, or "holes," in the material. These holes can be thought of as positive charges. Surrounding electrons can move in and fill this hole and, in doing so, leave behind another hole. The holes thus seem to move in a direction opposite to the electron flow. Since most of the conduc-tion occurs by way of the positively charged holes, the material is called p-type.

Modifying the concentration of dopants controls the resulting change in semi-conductor conductivity. The process of doping semisemi-conductor materials to selec-tively increase their conductivity is fundamental to the manufacture of advanced semiconductor devices because it makes possible the fabrication of basic circuit substructures.

Silicon is the material of choice for microelectronics devices because of its numerous advantages. As one of the most abundant elements on the planet, silicon is cheap and readily available. It can be subjected to higher temperatures than ger-manium, the next most popular semiconductor resource. Silicon also has critical Pre-cessing advantages. It easily oxidizes to form silicon dioxide, an excellent insulator among circuit components. Silicon dioxide is also extremely useful during the fabri-cation process because it is an effective barrier layer during multiple doping

opera-2Strictly, each Si atom shares ita four electrons with irs neighbors, creating eight in the outer shells.

5.5 Transistors 17.

tiona Gallium arsenide rather than silicon is increasingly used in optoelectronic and high-frequency communication devices.

5.5.3The Transistor

The region where p-type and a-type semiconductors meet forms a crucial structure known as apnjunction (Figure 5.4). Apnjunction is basic to the operation of most electronic devices. For example, a diode is apnjunction that allows the flow of cur-rent in one direction and blocks it in the opposite direction. A bipolar junction tran-sistor(BIT) is made by sandwiching three different semiconductor slices into one solid block, such that the center slice is of one type and the two outer slices are of the opposite type. In effect, this creates twopnjunctions. Depending on how the junc-tions are combined, the transistor is either"npn" or "pnp" (see Figure 5.5). In an npn transistor, electrons can flow from the emitter(n),across the base(P),to the collector (rl).More significantly, applying a voltage to the base vigorously rips electrons from the emitter and sends them rocketing across the base into thecollector-s-in effect, amplifying the input current to the base. The stronger the voltage on the base, the stronger the resulting flow of current through the transistor. This amplification is more utilized in analog devices such as an electric guitar. For the ICs in computers, the primary function is the ultrafast switching ability for logic.

Figure 5.5 shows a simple sandwichlike npn arrangement. By contrast, Figure 5.6 shows the horizontal layout of the field effect transistor (FET). The termi-nology of the npn transistor-emitter, base, and collector-is now changed to source, gate,anddrain for the FET. To activate the transistor, voltage is applied to the polysilicon control gate (center of Figure 5.6). Electrons flow out of the source region (marked n+)through the channel (part of the p-type substrate) and into the drain (also markedn+).The amount of flow is precisely controlled by the voltage applied to the gate. For the n-type device (NMOS) a positive voltage is applied to the polysilicon gate'. The gate and the p-type substrate form the plates of a capac-itor with the gate oxide (Si02) as the dielectric of the capacitor. The reader is referred to a text such as Rabaey's (1996) for the relationship between the applied gate voltage and the current flow between the source and the drain.

Metallurgical junction

p-rype

••••• _{•• 1\••}

••••• _0.•••

•••b

•••••

•••••

• •• -Op.

a-type

Mobile holes Conduction electrons

JlIpre SA Schematic structure of apnsemiconductor junction in a silicon substrate (dopanticreate boles or free electrons, B=boroa,P. pbolphorul)

180 Semiconductor Manufacturing Chap. 5

This transistor is off

~-

Eminer Base Collector

This Iransistor is on

vouage apphed ro base

Elcrtron flow in

Ermtter Collector

flpre S.6 Basic structure of an a-type NMOS Ie (from Dtgital lmegmted Circuils by Rablley, <0 1996. Reprinted by permissionof Prentice-Hell, Inc., Upper Saddle Rlver.Nj).

5.5.4MOSFETs as the B ••ic Building Block of the Integrated