TRANSISTOR

INTRODUCTION

It is a semiconductor device which is used for amplifying controlling and generating electrical signals. It converts audio waves into electronic waves transistor are the active component of integerated circuits or microchips and that is establishing in all over electronic circuits. They are used as amplifiers and switching apparatus. When it works as an amplifier it takes in a tiny electric current at one end(an input current) and produces a much bigger electric current(an output current) at the other transistor can also work as switches. An electric current flowing through one part of a transistor can make a much bigger current flow through another part of it transistor consists of three layers of a semiconductors material each capable of carrying current. A semiconductor is a material such as germanium and silicon that conducts electricity. The semiconductor material is given special properties by a chemical process called Doping. The doping results in a material that either adds extra electrons to the material (which is called N-type for the extra negative charge carriers) or creates holes in the materials crystal structure (which is then called P-type because it results in more positive charge carriers) The transistors three layer structure contains an N-type semiconductor layer b/w

P-type layer (a PNP configuration) or a P-type layer b/w N-type layers it is known as NPN transistor.

Basically, A transistor is a three terminal device. which is named aa,

=> Base - This is responsible for activating the transistor.

=> collector - This is the positive lead

=> emitter - This is the negative lead

The basic idea behind a transistor is that it lets you control the flow of current through one channel by varrying the intensity of a much smaller current that's flowing through a second channel.

HISTORY

The transistor was probably the most important invention of the 20th century . After the end of world war 2 kelly put together a team of scientists to develop a solid state semiconductor switch to replace the problematic vaccum tube. The team would use some of the the advances in semicondutor research during the war that had made radar possible. A young brilliant theoretician, Bill schokley was selected as the team leader and the other teammates are walter brattain and john bardeen .

 They starting the research in their tab ("Hells Bells Lab").

In the spring 1945, Shockley designed what he hoped his first semiconductor amplifier, relying on something called the field effect. His device was a small cylinder coated thinly with Silicon, mounted. Close to a small metal plate. Indeed, the device didn't work, yet. In the fall of dunking the entire apparatus into a tub of water and suprisingly it worked a littile bit. Brattain began to experiment with gold or germanium eliminating the liquid layer on the theory that it was slowing down the device. It didn't work but the team kept experimenting using that designs a discovered that starting point. But later bardeen and brattain researched and electrons forms a barrier the surface which is unknown until the this break through leads to the discovery of transistor. On december 16 1947,  they built the point contact transistor made from strips of gold foil on a plastic triangle, pushed down into contact with a slab of germanium. It was the first working transistor at Bell labs, This experiment first reported in june 1948.

Later silicon transistors are developed due to successful performance at high temperatures silicon transistor are more used from the year of 1954 extensive research is made form silicon transistor is lead to developement of integerated circuit and microprocessor devices. In 1956, he recieved a nobel prize with shockley and Brattian for his work on the transistor.

Transistor as a switch

BJT used as an electronic switch, in grounded-emitter configuration.

Transistors are commonly used in digital circuits as electronic switches which can be either in an "on" or "off" state, both for high-power applications such as switched-mode power supplies and for low-power applications such as logic gates. Important parameters for this application include the current switched, the voltage handled, and the switching speed, characterised by the rise and fall times.

In a grounded-emitter transistor circuit, such as the light-switch circuit shown, as the base voltage rises, the emitter and collector currents rise exponentially. The collector voltage drops because of reduced resistance from collector to emitter. If the voltage difference between the collector and emitter were zero (or near zero), the collector current would be limited only by the load resistance (light bulb) and the supply voltage. This is called saturation because current is flowing from collector to emitter freely. When saturated, the switch is said to be on.

Providing sufficient base drive current is a key problem in the use of bipolar transistors as switches. The transistor provides current gain, allowing a relatively large current in the collector to be switched by a much smaller current into the base terminal. The ratio of these currents varies depending on the type of transistor, and even for a particular type, varies depending on the collector current. In the example light-switch circuit shown, the resistor is chosen to provide enough base current to ensure the transistor will be saturated.

In a switching circuit, the idea is to simulate, as near as possible, the ideal switch having the properties of open circuit when off, short circuit when on, and an instantaneous transition between the two states. Parameters are chosen such that the "off" output is limited to leakage currents too small to affect connected circuitry, the resistance of the transistor in the "on" state is too small to affect circuitry, and the transition between the two states is fast enough not to have a detrimental effect.

Transistor as an amplifier

Amplifier circuit, common-emitter configuration with a voltage-divider bias circuit.

The common emitter amplifier is designed so that a small change in voltage (V_in) changes the small current through the base of the transistor whose current amplification combined with the properties of the circuit means that small swings in V_in produce large changes in V_out.

Various configurations of single transistor amplifier are possible, with some providing current gain, some voltage gain, and some both.

From mobile phones to televisions, vast numbers of products include amplifiers for sound reproduction, radio transmission, and signal proccesing. The first discrete-transistor audio amplifiers barely supplied a few hundred milliwatts, but power and audio fidelity gradually increased as better transistors became available and amplifier architecture evolved.

Modern transistor audio amplifiers of up to a few hundred watts are common and relatively inexpensive.

Comparison with vacuum tubes

Before transistors were developed, vaccum tubes (or in the UK "thermionic valves" or just "valves") were the main active components in electronic equipment.

Advantages

The key advantages that have allowed transistors to replace vacuum tubes in most applications are

• No cathode heater (which produces the characteristic orange glow of tubes), reducing power consumption, eliminating delay as tube heaters warm up, and immune from cathode poisoning and depletion.
• Very small size and weight, reducing equipment size.
• Large numbers of extremely small transistors can be manufactured as a single integerated circuit.
• Low operating voltages compatible with batteries of only a few cells.
• Circuits with greater energy efficiency are usually possible. For low-power applications (for example, voltage amplification) in particular, energy consumption can be very much less than for tubes.
• Complementary devices available, providing design flexibility including complementary-symmetry circuits, not possible with vacuum tubes.
• Very low sensitivity to mechanical shock and vibration, providing physical ruggedness and virtually eliminating shock-induced spurious signals (for example, microphonics in audio applications).
• Not susceptible to breakage of a glass envelope, leakage, outgassing, and other physical damage.

Limitations

Transistors have the following limitations:

• They lack the higher electron mobility afforded by the vacuum of vacuum tubes, which is desirable for high-power, high-frequency operation — such as that used in over-the-air television broadcasting.
• Transistors and other solid-state devices are susceptible to damage from very brief electrical and thermal events, including electrostatic discharge in handling. Vacuum tubes are electrically much more rugged.
• They are sensitive to radiation and cosmic rays (special radiation-hardened chips are used for spacecraft devices).
• In audio applications, transistors lack the lower-harmonic distortion — the so-called tube sound — which is characteristic of vacuum tubes, and is preferred by some.