Transistor

Transistors replaced electronic components called vacuum tubes almost completely in the 1950's and 1960's because they have a number of advantages over vacuum tubes. For example, transistors are smaller, lighter, less expensive to produce, cheaper to operate, and more reliable than vacuum tubes. See Vacuum Tube.

Transistors are the main components built into computer chips, devices that carry out computer programs and store programs and other data. Some chips no larger than a fingernail contain millions of transistors. See Computer Chip.

Large, individual transistors are called discrete transistors. Because of their size, these units can handle many times the power of transistors in chips. Uses of discrete transistors include providing the power for the speakers of high-fidelity sound systems, the transmitters of citizens band radios, and the motors of small appliances; turning lights on and off; and controlling energy flow through electric power lines.

Transistors in computers perform rapid switching operations to manipulate electric charges that represent information as the 0's and 1's of the binary numeration system. As the transistors move the charges about, electronic circuits carry out calculations, solve problems in logic, form words and pictures on monitors, control printers, and perform all the other operations that we have come to associate with computers.

Their ability to amplify signals makes transistors essential parts of radios and television sets. The broadcast waves that travel through the air generate weak currents in a radio or TV antenna. Transistors in electronic circuits amplify these signals. Other components--including additional transistors--use the resulting strong currents to produce sounds and pictures.

A certain minimum voltage must be applied across a semiconductor before any current will flow. In a conductor, any voltage--no matter how small--will cause current to flow. In an insulator, the voltage required to start a current would be so strong that the material would be destroyed when current flow did start. See Volt.

Electric current is a flow of electric charges. In a semiconductor, current is a flow of free electrons or a flow of holes. A free electron is an electron that is not tightly bound to an atom. A hole is a positively charged, "empty" region near an atom that would normally be occupied by an electron.

In an atom, one or more negatively charged electrons orbit a positively charged nucleus. The electrons are arranged in shells. See Atom.

A silicon atom normally has four electrons in its outermost shell. In a pure crystal of silicon, however, there are always a very small number of free electrons and holes. This is so because a small percentage of electrons absorb enough heat energy to leave their shells, becoming free electrons--and leaving holes behind. The electrons quickly occupy holes--but in the meantime other electrons leave their shells.

Doping a semiconductor crystal--replacing some of the semiconductor's atoms with atoms of another substance--changes the crystal's ability to conduct current. In an n-type silicon crystal, a small number of silicon atoms are replaced by phosphorus atoms, which have five electrons in their outermost shells. One of these electrons is not tightly bound to the phosphorus nucleus, so the crystal has extra free electrons. In a p-type silicon crystal, a small number of silicon atoms are replaced by boron atoms, which have three electrons in their outermost shells. Thus, a p-type crystal has extra holes. See Semiconductor; Electronics.

Bipolar transistors. A simple bipolar transistor has a very thin region of one type of semiconductor material sandwiched between two thicker regions of the opposite type. If the middle region is p-type material, the outside regions are n-type. This design is known as NPN. A PNP transistor has an n-type inside region and p-type outside regions. In both designs, one outside region is called the emitter, and the other is known as the collector. The middle region is the base.

Connected to each region is an electric terminal. In a discrete transistor, the terminal is one end of a thin wire. In a transistor in a chip, it is a layer of metal.

The input signal is applied at the base terminal. The current that is switched off and on flows from the emitter to the collector.

Applied voltages. Before a transistor can operate, its terminals must receive certain voltages. To operate an NPN transistor in the normal way, a relatively high positive voltage is applied to the collector, and the emitter receives a voltage of zero. If the base's voltage is also zero, the transistor is off. Applying a small positive voltage to the base turns the transistor on.

When the voltages are applied, many free electrons and holes move to new positions throughout the transistor. These movements occur because positive voltages attract electrons and negative voltages attract holes.

Charge movement in the collector and base. Because the collector terminal is more positive than the base terminal, the voltage applied to the collector pulls free electrons in the collector toward the collector terminal. Because the base has a lower voltage than the collector, holes located in the base are pulled toward the base terminal. Thus, there are no free electrons or holes at the collector-base junction to flow as electric current.

Furthermore, when the holes in the base flow toward the base terminal, they leave behind a strong negative electric field on the base side of the collector-base junction. An electric field is a region in which electric force acts on a charged object.

Charge movement in the emitter. When the base is positive relative to the emitter, free electrons in the emitter are drawn toward the base-emitter junction and into the base. However, in a typical transistor in which the emitter voltage is zero, there is no significant flow of electrons across the junction until the base voltage reaches about 0.4 volt.

The bipolar transistor as a switch. When the base voltage is very low--from 0 to 0.3 volt in a typical silicon transistor--essentially no current flows from the emitter or the base to the collector. The base voltage is not high enough to pull electrons from the emitter across the base-emitter junction. Thus, the transistor is off.

Increasing the base voltage to about 0.6 volt causes large numbers of electrons to flow from the emitter into the base. Because the base is extremely thin, an electron that enters the base is already very close to the collector-base junction. As the concentration of electrons in the base increases, some of the electrons crowd others farther into the base. Many electrons penetrate all the way through the negative electric field at the collector-base junction, even though this field opposes a flow of electrons. Thus, these electrons are pushed through the collector-base junction.

Once the electrons are on the collector side of the junction, they pass easily to the collector terminal, leaving the transistor. Thus, current flows into the transistor via the emitter terminal, flows through the base region, and leaves the transistor through the collector terminal. The maximum flow of electrons occurs when the base voltage is about 0.7 volt.

The bipolar transistor as an amplifier. A transistor that functions as an amplifier remains in a conducting state, but the strength of the signal is varied. Increasing the strength of the signal causes free electrons in the emitter to flow into the base at a higher speed. More electrons therefore reach the collector. Thus, the current flowing from the emitter to the collector increases in proportion to the increased strength of the signal.

Decreasing the strength of the signal decreases electron speed. The current from the emitter to the collector therefore decreases in proportion to the decrease in signal strength.

The weak current flowing to the base also rises and falls in proportion to the changes in signal strength. The current flowing from the emitter to the collector is therefore a duplicate of this current, except for its strength. Thus, the transistor amplifies the weak current by producing a much stronger current that is a duplicate of the weak current.

A PNP transistor works on the same principles as an NPN transistor. However, the voltages in a PNP transistor are the reverse of those in an NPN transistor.

Metal oxide semiconductor field effect transistors (MOSFET's). The MOSFET's that are used in computer chips are the most common type of field effect transistors (FET's). A FET operates by creating an electric field that either attracts electrons to a gate region or repels electrons away from that region. If a sufficient number of electrons are attracted to the gate region, current will flow.

A MOSFET has three semiconductor layers--the source, the gate region, and the drain. The source and drain are made of the same type of semiconductor material--either n-type or p-type. The gate region, which lies between the source and the drain, is made of the opposite type of material.

In one kind of MOSFET, the gate region is p-type silicon, and the source and drain are n-type silicon. The gate region is a part of the basic chip material. The source and the drain are embedded in this material, with the gate region between them. The part of the chip material below the gate region is called the substrate.

Electric terminals are connected to the source and the drain. Above the gate region is a layer of silicon dioxide, an insulating material. Above this material is a third terminal, a metal layer called the gate.

A small voltage signal is usually applied at the gate. The current that is switched off and on usually flows from the source terminal to the drain terminal.

In normal operation, the drain is more positive than the source. Thus, current tends to flow from the source to the drain. Whether current actually flows depends upon whether the gate has a negative voltage or a positive voltage.

If the gate is given a negative voltage, the free electrons in the gate region will be repelled into the substrate. Thus, these electrons will not be available to flow as current. Current will be unable to flow from the source to the drain. The MOSFET will be off.

If the gate receives a positive voltage, free electrons will be attracted to the gate region. Thus, there will be a continuous band of material with extra free electrons underneath the gate's metal oxide layer, between the source terminal and the drain terminal. If the gate voltage is sufficiently high, current will flow. The MOSFET will be on.

Contributor: Charles Melear, M.S.E.E., Manager, Advanced Microcontroller Applications, Motorola Inc.

Olney, Ross R. and R. D. The Amazing Transistor. Atheneum, 1986

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