Jun 152012
 

The name „transistor“ was introduced by John Pierce, who combined the words “transfer” and “resistor”. Regarding the invention of the first transistor things are a little controversial and are associated with the definition of the transistor – a semiconductor triode with three terminals, in which the input signal controls the current in the circuit. The transistor is an active electronic device which amplifies, switches and converts electrical signals. Over the years, different scientists in one way or another approached in their experiments a device with similar characteristics, so it is hard to say who exactly invented the modern transistor. What is certain is that in the laboratories of Bell (Bell labs) William Schottky, Walter Brattain and John Bardeen created the first bipolar transistor in 1947. In 1956 they received a Nobel Prize for physics for their research in the field of semiconductors and for the discovery of the transistor. A transistor consists of 3 zones in series connection with different PNP and NPN.

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Transistor

The three zones are called resectively an emitter, a collector and a base. Initially the name transistor was used for a resistor with controllable voltage. In fact, the transistor can be represented as a resistance, which can be adjusted by the voltage of one of the electrodes. The transistor is an active semiconductor device which is used in virtually all electronic devices. Most transistors are found embedded in integrated circuits and sometimes millions of transistors are integrated in a semiconductor chip. There is annual production of billions of individual transistors, as well as the same number of chips, containing not only transistors but also resistors, capacitors, etc.

The three main circuits for connecting a transistor are:
– a common emitter – it performs amplification in current and voltage (the most common circuit);

p-n-p-common-emitter-configuration
– a common collector – it performs amplification in current

n-p-n-common-collector-configuration
– a common base – it performs amplification in voltage

p-n-p-common-base-configuration
Depending on the type of the used semiconductor, transistors are divided into silicon, germanium, gallium, arsenide-gallium, semiconductor polymers, etc.
According to their power, transistors are divided into:
– low-power transistors- up to 100mW;
– medium-power transistors – 0.1 to 1W;
– high-power transistors – over 1W.
Types of transistors
Transistors can be extremely diverse, so we will focus only on the most common types:

  1. Bipolar transistors

These are the most popular discrete semiconductor devices. The name comes from the fact that their conductivity is provided by two (bi) types of current carriers – electrons and holes. A bipolar transistor consists of a semiconductor with two PN-junctions, and a three-layer semiconductor structure with different conductivity of the individual layers. There are two types of bipolar transistors: PNP and NPN. The two outer layers of the bipolar transistor are called an emitter and a collector, and the middle layer is called a base. The base has conductivity which is reverse to the conductivity of the emitter and the collector. As a rule, the bipolar transistor is a semiconductor device that is controlled by current.
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Regardless of the type of connection, the controlling circuit of the bipolar transistor is the transition base-emitter and the controlled circuit is respectively collector-emitter. Bipolar transistors are mainly used as amplifiers in electronic circuits. Two or more bipolar transistors can be connected in a way that forms an amplifier with three terminals. This is called a composite transistor. This circuit is applied when we want to achieve higher amplification rate by current. Another widely used circuit allowing amplification in current is the Darlington circuit. It comprises of connected transistors where each transistor is more powerful than the previous one. The circuit guarantees a high amplification rate by current, but it is difficult to achieve thermal stability.

    2.  Schottky Transistor

Named after the German physicist Walter Schottky, this transistor is created when a Schottky diode is connected between the collector and the base of an ordinary transistor. This aims at increasing the speed of the transistor. This circuit operates in pulse mode. In normal amp mode, the LED is blocked and does not affect the operation of the transistor. However, when short pulses with magnitude 3-5V are fed to the input, the Schottky diode is unblocked and part of the input current is diverted through it, thus preventing the base from high saturation with minor carriers, which would delay the switching of the transistor.
 

   3.  Field-effect transistor (FET)

The terminals of FETs are called sources, gates or drains. The electrical characteristics of FETs are similar to those of pentode electronic lamps. FETs have high input resistance and can be used as resistors, controlled by voltage. Field-effect transistors have electrical conductivity of the active area between two electrodes –a channel, deliberately created in a semiconductor material, controlled by an electric field created by the third electrode. When the channel in the FET is enriched with electrons, there is N conductivity. If is depleted of electrons – P conductivity.

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The conductivity surrounding the channel is converse and therefore PN transition occurs. Depending on the insolation between the gate and the source, there are transistors with insolated gate, the so called MOS transistors (metal oxid semiconductor) and transistors with PN transition, in which the gate and the channel form NP transition. Depending on the conductivity of the channel, there is a P channel (electronic conductivity), and N channel (foraminous conductivity). Depending on the number of gates, there are single-gate and double-gate transistors. A double gate transistor has characteristics that are similar to the heptod electronic lamp. There is also a TFT transistor – a very thin amorphous silicon transistor, a MOSFET field-effect transistor, etc. The most used, however, remains the MOS transistor.

   4. Other transistors
IGBT – Insulated-Gate Bipolar Transistor. IGBT is a three-electrode bipolar powerful electronic component used mainly as a powerful electronic switch in pulse power supplies, inverters and in systems for controlling electric drives.

JFET is a field-effect transistor with controlling PN – junction. JFET stands for Junction Field – Effect Transistor. In JFET transistors the output current is controlled through the input voltage.

MOSFET (metal-oxide-semiconductor field-effect transistor) is an electronic version of a switch. MOS switches are widely used in computers, microprocessors, storages, peripheral circuits, etc. Advantages: high input resistance, low power consumption, better temperature stability, low susceptibility to radiation. These transistors are used in power electronics, audio equipment, medical electronics, integrated circuits, computer, automotive, aerospace industries, household appliances, etc. They can be of two types: P-channel MOS-P-FET and N-channel MOS-N-FET.
Phototransistor – a highly sensitive semiconductor small-inertia converter of light signals into electrical ones. Phototransistors can enhance electrical current generated by light. Phototransistors are preferred over photodiodes when large output power is needed. The falling light beam is used as an emitter.

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There are other transistors which are used for making electronic circuits such as: VMOS, LDMOS, EOSFET, MODFET, MESFET, etc.

The MOSFET transistor is the one that is most used in modern digital equipment, its size steadily decreasing. The dimensions of current MOSFET transistors are from 90 to 8 nanometers. A crystal with dimensions of 1-2 cm² can collect several billion MOSFET. Reducing the size of transistors increases their speed, and hence the speed of processors, which helps to reduce power consumption and heat generation. Three-dimensional integrated circuits are also becoming widely used, which further improves the speed and increases the processor performance by more than 30%.

Transistors have successfully replaced the previously used vacuum tubes, because of their advantages such as small size, the possibility of a high degree of automation of manufacturing processes, leading to a reduction in the value of manufactured items, very long life, strength, ability to combine different additional devices, etc.

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