Transistor is a tiny device that controls the flow of electric current in radios, television sets, computers, and almost every other kind of electronic equipment. Transistors vary in size from about 1/20,000 of a millimeter—approximately 1/2,000 the width of a human hair— to a few centimeters across.
Transistors are the main components built into computer chips, devices that carry out computer programs and store data. Some chips no larger than a fingernail contain billions of microscopic transistors. Large, individual transistors are called discrete transistors. Because of their size, these units can handle many times the power of transistors in chips. Discrete transistors provide the power for stereo speakers and the motors of small appliances. Their other uses include turning lights on and off and controlling energy flow through electric relays and switches.
Three American physicists—John Bardeen, Walter H. Brattain, and William Shockley—invented the transistor in 1947. By the late 1960’s, transistors had replaced electronic components called vacuum tubes almost completely. Transistors offered numerous advantages over vacuum tubes. They were smaller, lighter, and cheaper to produce and operate. They also used less power and were more reliable than vacuum tubes. See Vacuum tube .
What transistors do
A transistor has two basic functions: (1) to switch electric current on and off and (2) to amplify (strengthen) electric current. A small voltage called the input signal controls both switching and amplification.
Transistors in computers perform rapid switching operations to manipulate electric charges. The charges represent information as the 0’s and 1’s of the binary number system. As the transistors move the charges around, electronic circuits carry out calculations, solve problems in logic, form images on screens, and perform other computer operations. See Computer (Computer hardware) .
The ability of transistors to amplify signals makes them 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 detect and amplify these signals. Other components—including additional transistors—use the resulting strong currents to produce sounds and pictures.
Transistor materials
Transistors are made from materials called semiconductors. A semiconductor conducts (carries) electric current better than an insulator, such as glass, but not as well as a conductor, such as silver or copper. Silicon is the most common semiconductor used for transistors.
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 is so strong that starting current flow would destroy the material.
Electric current is a flow of electric charges. In a regular conductor, current is a flow of free electrons. In a semiconductor, current can be a flow of free electrons or 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, a positively charged nucleus is surrounded by one or more negatively charged electrons. The electrons are arranged in groups called shells. (see Atom (Parts of an atom) . A silicon atom normally has four electrons in its outermost shell. In a pure crystal of silicon, however, there are always a 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. These free electrons quickly occupy holes—but in the meantime other electrons leave their shells.
Doping a semiconductor crystal—replacing some of its atoms with atoms of another chemical element—changes the way in which it conducts current. In an n-type silicon crystal, a small number of silicon atoms are replaced by phosphorus atoms. Phosphorus atoms have five electrons in their outermost shells. One of these electrons is not tightly bound to the phosphorus nucleus. Thus, the crystal has extra free electrons. In a p-type silicon crystal, a small number of silicon atoms are replaced by boron atoms. Boron atoms have only three electrons in their outermost shells. Thus, a p-type crystal has extra holes. An n-type crystal thus conducts electric current with _n_egative electrons, while p-type silicon conducts electric current with _p_ositive holes. See Electronics (Active components) ; Electronics (Transistors) ; Semiconductor .
How transistors work
There are two main kinds of transistors: (1) bipolar transistors and (2) metal oxide semiconductor field effect transistors (MOSFET’s). Bipolar transistors are able to work with large currents, but they consume energy when turned on or off. Most discrete transistors are bipolar. MOSFET’s are smaller than bipolar transistors, and turning them on and off consumes little energy. Almost all of the chips in computer circuits use MOSFET’s.
Bipolar transistors.
A simple bipolar transistor has a 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. The input signal is applied at the base terminal. A current then flows from the emitter to the collector. The transistor operates by amplifying this current or switching it on and off. The following sections describe the operation of an NPN transistor.
Applied voltages.
Before a transistor can operate, its terminals must receive certain voltages. To operate an NPN transistor in the normal way, a 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 current to the base turns the transistor on.
When the needed voltages are applied, many free electrons and holes move to new positions throughout the transistor. Positive voltages attract electrons. Negative voltages push away electrons and attract holes.
Charges in the collector and base.
Transistors work by varying the voltages in the emitter, collector, and base regions. When an NPN transistor is turned “off,” the collector terminal is more positive than the base terminal. The positive voltage at the collector attracts free electrons. Holes, meanwhile, are pulled toward the base region’s lower positive voltage. Thus, there are no free electrons or holes at the junction between the collector and base that can flow as electric current.
Charges in the emitter.
When the base is positive relative to the emitter, free electrons in the emitter are drawn toward the base-emitter junction. But 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 low—from 0 to 0.3 volt in a typical silicon transistor—essentially no current flows from the emitter through the base to the collector. The base voltage is too low 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 close to the collector-base junction. As the concentration of electrons in the base increases, some electrons penetrate all the way through the negative electric field at the collector-base junction, even though this field opposes a flow of electrons. Once the electrons are on the collector side of the junction, they pass easily to the collector terminal, leaving the transistor. Thus, current flows from the emitter terminal through the base 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 input signal at the base causes more free electrons in the emitter to flow into the base. 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 input signal decreases the flow of electrons across the base and into the collector. The current from the emitter to the collector decreases in proportion to the decrease in signal strength.
The current flowing across the transistor from the emitter to the collector is therefore a stronger copy of the weak input signal. The ratio of the strength of this output current to that of the input current is called gain. In a typical amplifying transistor, the gain may be 100.
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 changes how one of the transistor’s semiconductor regions, the gate region, conducts electric current. Current flows when enough charge carriers—either electrons or holes—are attracted to the gate region. A FET has three semiconductor regions—the source, the gate region, and the drain. In a MOSFET, 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. The gate region typically extends into the substrate, the underlying chip material. The source and drain are embedded in the substrate.
Electric terminals are connected to the source and the drain. Above the gate region is a thin layer of silicon dioxide, an insulating material. Above the insulating material is a layer called the gate, connected to a third terminal. The gate is often made of polycrystalline silicon. The smallest transistors have gates that are only a few atoms thick.
A small input voltage signal is applied at the gate terminal. The current that is switched off and on 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 the type of MOSFET, and whether the voltage to the gate is negative or positive. An n-channel MOSFET has a source and drain of n-type silicon on a p-type silicon substrate. A p-channel MOSFET has a p-type silicon source and drain on a substrate of n-type silicon. A negative voltage to the gate permits current to flow in an n-channel MOSFET, and a positive voltage has the same effect in a p-channel MOSFET. The voltage to the gate creates an electric field in the gate region. The field increases or reduces the number of charge carriers in that region, thus controlling current flow.
If the gate of an n-channel MOSFET 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 from the source to the drain. The MOSFET will be off. If the gate receives a positive voltage, free electrons will be attracted into the gate region. Thus, there will be a continuous band, or channel, of material with extra free electrons between the source and the drain. If the gate voltage is sufficiently high, current will flow. The MOSFET will be on. In p-channel MOSFET’s, opposite voltages are applied to produce the same effects.
