The transistor is a solid-state semiconductor device used for amplification and switching, and has three terminals: a small current or voltage applied to one terminal controls the current through the other two. It is the key component in all modern electronics. In digital circuits, transistors are used as very fast electrical switches, and arrangements of transistors can function as logic gates, RAM-type memory and other devices. In analog circuits, transistors are essentially used as amplifiers.
Transistor was also the common name in the sixties for a transistor radio, a pocket-sized portable radio that utilized transistors (rather than vacuum tubes) as its active electronics. This is still one of the dictionary definitions of transistor.

The transistor is considered by many to be one of the greatest discoveries or inventions in modern history, ranking with banking and the printing press. Key to the importance of the transistor in modern society is its ability to be produced in huge numbers using simple techniques, resulting in vanishingly small prices. Computer "chips" consist of millions of transistors and sell for dollars, with per-transistor costs in the thousanths-of-pennies. The average home might contain a few tens of light bulbs and perhaps 100 metres of paper, things many consider "cheap", but the computer you are using to read this contains millions of transistors.

The low cost has meant that the transistor has become an almost universal tool for non-mechanical tasks. Whereas a common device, say a refrigerator, would have used a mechanical device for control, today it is often less expensive to simply use a few million transistors and the appropriate computer program to carry out the same task through "brute force". Today transistors have replaced almost all electromechanical devices, most simple feedback systems, and appear in huge numbers in everything from computers to cars.

Hand-in-hand with low cost has been the increasing move to "digitizing" all information. With transistorized computers offering the ability to quickly find (and sort) digital information, more and more effort was put into making all information digital. Today almost all media in modern society is delivered in digital form, converted and presented by computers. Common "analog" forms of information such a television or newspapers spend the vast majority of their time as digitial information, being converted to analog only for a small portion of the time.

Invention:
The transistor was invented at Bell Laboratories in December 1947 (first demonstrated on December 23) by John Bardeen, Walter Houser Brattain, and William Bradford Shockley, who were awarded the Nobel Prize in physics in 1956. Ironically, they had set out to manufacture a field-effect transistor (FET) predicted by Julius Edgar Lilienfeld as early as 1925 but eventually discovered current amplification in the point-contact transistor that subsequently evolved to become the bipolar junction transistor (BJT).

How Does a Transistor Work?
A transistor is a three-terminal device. In a BJT, an electrical current is fed into the base (B) and modulates the current flow between the other two terminals known as the emitter (E) and collector (C). In FETs, the three terminals are called gate (G), source (S) and drain (D) respectively, and it is the voltage applied to the gate terminal that modulates the current between source and drain.

The schematic symbols for pnp- and npn-type BJTs.Conceptually, one can understand a bipolar junction transistor as two diodes placed back to back, connected so they share either their positive or their negative terminals. The forward-biased emitter-base junction allows charge carriers to easily flow out of the emitter. The base is made thin enough so that most of the injected carriers will reach the collector rather than recombining in the base. Since small changes in the base current affect the collector current significantly, the transistor can work as an electronic amplifier. The rate of amplification, usually called the current gain (ß), is roughly one hundred for most types of BJTs. That is, one milliampere of base current usually induces a collector current of about a hundred milliamperes. BJTs prevail in all sorts of amplifiers from audio to radio frequency applications and are also popular as electronic switching devices.

The schematic symbols for p- and n-channel MOSFETs. The symbols to the right include an extra terminal for the transistor body whereas in those to the left the body is implicitly connected to the ground.The most common variety of field-effect transistors, the enhancement-mode MOSFET (metal-oxide semiconductor field-effect transistor) can also be viewed as two back-to-back diodes that separate the source and drain terminals. The volume in between is covered by an extremely thin insulating layer that carries the gate electrode. When a voltage is applied between gate and source, an electric field is created in that volume, causing a thin conductive channel to form between the source and drain and allowing current to flow across. The amount of this current can be modulated, or completely turned off, by varying the gate voltage. Because the gate is insulated, no DC current flows to or from the gate electrode. This lack of a gate current (as compared to the BJT's base current), and the ability of the MOSFET to act like a switch, allows particularly efficient digital circuits to be created. Hence, MOSFETs have become the dominant technology used in computing hardware such as microprocessors and memory devices such as RAM.

The most common form of MOSFET transistor in use today is the CMOS (complementary metallic oxide semiconductor) which is the basis for virtually all integrated circuits produced.

Advantages of Transistors over Thermionic Valves:
Before the transistor, the thermionic valve or vacuum tube, was the main component of an amplifier. The key advantages that have allowed transistors to replace their valve predecessors in almost all applications are

smaller size,
simpler manufacture and, hence,
lower cost,
lower operating voltages,
absence of a heated filament and, as a consequence,
lower power dissipation,
higher reliability and greater endurance.

Valves are still used in very high-power applications such as broadcast radio signal amplification. Some audio amplifiers also use them, their enthusiasts claiming that their sound is superior. In particular, some argue that the larger numbers of electrons in a valve behave with greater statistical accuracy. Other detect a distinctive "warmth" to the tone. The "warmth" is actually distortion caused by the valves, but some audiophiles find a certain amount of "fuzziness" pleasing.

 

 

The second principal function of a transistor, after amplification, is its use as a switch. As you probably know by now, (see semiconductor triodes if you don't) transistors have three electrodes and so two p-n junctions. These p-n junctions can boost or oppose the flow of current. In modern transistors the functioning of the p-n junction can be controlled by properly affecting the voltages. Advanced graphs and equations are required to give the exact relations between the voltages and the state (biasing) of the junction.

However, these two junctions can together exhibit four states (each one can be either forward or reversed biased, as explained in the semiconductor diodes section of this sight):

One is forward-biased (boosts the flow of current) and other is reverse-biased (Opposes the flow of current) This is used in amplification.
One is reverse and other is forward biased. Also used for amplification
Both are forward biased. Full possible current flows
Both are reverse biased. No current flows
The working of the transistor as a switch is thus very simple. As you can see by the last two options, the transistor's two junctions can be manipulated to switch between full current and no current. This is the work of a switch, turning the current on and off. This function is useful in the working of modern computers.