Importance And Characteristics Of A Transistor

Published Date: 02 Nov 2017

Elet 2420 | University of the West Indies mona

Semiconductor Devices

Evolution of transistors

Introduction – What is a Transistor?

"The Transistor was probably the most important invention of the 20th Century, and the story behind the invention is one of clashing egos and top secret research." This is a quote made famous by journalist Ira Flatow. A transistor is a semiconductor device that amplifies and switches electronic signals and electrical power. It consists of three layers of a semiconductor material, carrying a current. A voltage or current applied to one pair of the transistor's terminals changes the current through another pair of terminals. A transistor is able to amplify a signal due to the fact that the controlled power is able to be higher than that of the input power.

Importance and Characteristics of a Transistor

Transistors are central to the Integrated Circuit, meaning it is also an integral part of all electronic devices of the new age, such as computers and cellular telephones.

The transistor as switch shows a simple operation; it is either in one of two states, i.e. on or off. In its action as a switch a transistor allows the flow of electricity or not. This flow is a constant amount of current, it never fluctuates on the source side of the circuit.

A transistor as amplifier allows for what is called a metered flow of electricity. In this usage transistor allows a variable amount of electricity to flow from one point to another. In taking the form of an amplifier the transistor may be used to power audio circuitry such as producing music or sound through speakers.

A transistor must be able to control flow of electricity through it to be function as an amplifier or a switch.

The size characteristic is very important of transistors, depending on the application, transistors can be made very small. Masses of tiny transistors packed on silicon chips let us create pocket-sized cell phones and Mp3 players.

Transistors can be designed to use very little power which makes them very efficient. Millions of them in a watch or calculator can run for years on a small battery.

They are very rugged. Transistorized equipment is used in military, space and industrial applications. They can withstand extremes of shock and vibration.

Types of Transistors

Transistors may be classified or grouped by different features:- structure, material, Electrical polarity, Maximum power rating, Maximum operating frequency, Application, Physical packaging, Amplification factor.

The main way for distinguishing transistors is the use of structure. The two main types of transistors based on structure are the bipolar junction transistor and the field effect transistor.

Bipolar transistors possess the name bipolar because they conduct by using both minority and majority carriers. The bipolar junction transistor was the first type of transistor to be produced on a large scale. It is a combination of two junction diodes and is formed of either a thin layer of p-type semiconductor put between two n-type semiconductors (an n-p-n transistor), or two p-type semiconductors sandwiching a thin layer of n-type semiconductor (a p-n-p transistor). This design produces two p-n junctions: a base-collector junction and a base–emitter junction which separated by a thin region of semiconductor. This region is the base region.

The BJT has three terminals which is the three layers of semiconductors. These are an emitter, a base, and a collector. The main use of the bipolar transistor is in the application of an amplifier due to the fact that a small base current controls the current at the collector and emitter.

The field-effect transistor, which may be referred to as a unipolar transistor, uses electrons or holes for conduction. In the case of electrons it is termed the N-channel FET and P-channel FET when holes are used for conduction. The four terminals of the FET are the source, gate, drain, and body otherwise called the substrate.

In the Field Effect Transistor, the source to drain region is connected by a conducting channel in which current flows from the drain to source. An electric field is produced when a voltage is applied between the gate and source terminals and conductivity is varied by this field. This means that the current flowing is controlled by the voltage applied between gate and source.

FETs are divided into two groups they are: junction FET (JFET) and insulated gate FET (IGFET). The IGFET has a more common name of metal–oxide–semiconductor FET (MOSFET), which shows its original construction from layers of metal (the gate), oxide (the insulation), and semiconductor. The Junction FET gate forms a p-n diode with the channel which lies between the source and drain but the MOSFET doesn’t.

How a Transistor Works

The transistor can function as an insulator or a conductor. The transistor's ability to fluctuate between these two states that enables to switch or amplify. The transistor has many applications, but only two basic functions: switching and modulation (amplification).

History of Transistors

Before the transistor there were vacuum tubes. Vacuum tubes carried out the same function as the transistor, acting as switch or amplifier depending on the manner in which they were used. They were much larger and used a considerably more power to operate than the transistor.

This is because a heated surface that sat between the incoming current and outgoing leads regulated the amount of power flowing through them.

John Ambrose Fleming developed the Vacuum Tube in 1895. It is a device that modified a signal by controlling the movement of electrons in an evacuated space. The electrons flow only from filament to plate creating a diode (a device that can conduct current only in one direction).

In 1906 Lee De Forest developed the triode in vacuum tube which amplified signals allowing farther telephone conversations. The problems with this Triode is that it was unreliable and used a lot of power.

The first patent for the field-effect transistor principle was filed in Canada by Austrian-Hungarian physicist Julius Edgar Lilienfeld on October 22, 1925. Lilienfeld did not publish any research articles about his devices, and this caused his work to be ignored by industry. He proposed the basic principle behind the MOS field-effect transistor. In 1934 a German physicist by the name of Dr. Oskar Heil patented another field-effect transistor. Two years later, Bell Lab’s director of research Mervin Kelly felt that a better amplifier was needed to provide the best phone service so he formed a department dedicated to solid stated science. Shockley was designated team leader of the department and he hired Walter Brattain and John Bardeen assigning them to determine why the semiconductor amplifier he designed didn’t work. It was a small cylinder device coated thinly with silicon mounted close to a small, metal plate.

In 1947 Bardeen and Brattain built the point contact transistor. It was made from strips of gold foil on a plastic triangle, pushed down into contact with slab of germanium.

The main key to the development of the transistor was the further comprehension of the process of electron mobility in a semiconductor. It was noted that if the flow of electrons from the emitter to the collector could be controlled somehow, an amplifier could be constructed. Actually undertaking this procedure appeared to be very difficult. It was observed that the number of electrons or holes to be injected had to be very large if the crystal were of any reasonable size. The main problem appeared to be to place the input and output contacts very close together on the surface of the crystal on either side of this region.

Brattain started working on building such a device, and as the team worked on the problem hints of amplification appeared. The system worked at times but then ceased unexpectedly at other times. At one point a non-working system started working when placed in water. The electrons in any one piece of the crystal would migrate about due to nearby charges. Electrons would cluster at the surface of the crystal where they could find their opposite charge "floating around" in the air (or water). A small amount of charge from any location on the crystal would cause them to be pushed away from the surface. A small amount of electrons would accomplish what was needed instead of a large supply of these electrons.

Their understanding solved the problem of needing a very small control area to some degree. They figured a single large area could be used instead of needing two separate semiconductors connected by a common, but tiny, region. The emitter and collector leads would both be placed very close together on the top, with the control lead placed on the base of the crystal. The electrons or holes would be pushed out, when current was applied to the "base" lead, across the block of semiconductor, and collect on the far surface. Enough electrons or holes should be allowed between them to allow conduction to start as long as the emitter and conductor were very close together.

Brattain and H. R. Moore made a demonstration to several of their colleagues and managers at Bell Labs on the afternoon of 23 December 1947, which is recorded as the birth date of the transistor. Twelve people are mentioned as directly involved in the invention of the transistor in the Bell Laboratory.

The junction version known as the bipolar junction transistor, invented by Shockley in 1948, enjoyed three decades as the device of choice in the design of discrete and integrated circuits.

The transistor was announced to the public in June 1948. This new device had characteristics which could be used to overcome many of the fundamental limitations of vacuum tubes. Some of these characteristics were transistors had very long life, were small, lightweight and mechanically rugged, and required no filament current. The device was named transistor instead of Point-contact solid state amplifier. John Pierce invented the name, combining transresistance with the ending common to devices, like varistor and thermistor.

The commercial use of transistors increased dramatically in the 1950’s, beginning with telephone switching equipment and military computers in 1952, hearing aids in 1953, and portable radios in 1954. In 1953, over 1,000,000 transistors were manufactured; in 1955, this number increased drastically to 3,500,000 transistors and by 1957, annual production had increased to a whopping 29,000,000 units. The rapid rise of transistor technology in the 1950s can be attributed to the contributions of a few major companies which includes Bell Labs/Western Electric, Fairchild, General Electric, Motorola, Raytheon, Sylvania and Texas Instruments.

The first big change in transistors occurred in 1951 when William Shockley developed a junction transistor. The first junction transistors were sandwiches of  N- and P-type germanium (germanium with an excess and scarcity of electrons, respectively).  A weak voltage coming into the middle layer would affect another current traveling across the entire sandwich. Between 1956 and 1958 the methods for building transistors improved substantially as the decade went on.  Patterns were lay over the crystal so scientists could etch away specific parts of the crystal or add impurities to other parts as necessary. The first of these transistors which was developed at Bell, left a little protrusion sticking out of the middle, and so they were named "mesa" transistors after the Spanish word for table.  Later, Fairchild Semiconductor developed a version which was entirely flat -- these were called "planar" transistors. Fairchild Semiconductor was founded by a group of eight men known as the traitorous eight.

In 1958 Jack Kilby of Texas Instruments invented the Integrated circuit which is a single device that contains an interconnected array of elements like transistors, resistors, capacitors and electrical circuits contained in a silicon wafer. Also this was the same year Gordon Teal built the first silicon transistor, which worked just like a germanium junction transistor.

The next big jump in transistor evolution came in the 1960’s with the field-effect transistor.   Most transistors in today’s age are field-effect transistors -- specifically metal-oxide semiconductor field-effect transistors, or "MOSFETs."  Instead of being a sandwich, MOSFETs have a channel of either N- or P- type semiconductor running through a ridge on top of the other type as described earlier.  MOSFETs were not originally better than the junction transistor, but they are much easier to make on an integrated circuit or microprocessor, and so they soon became the preferred type of transistor.  

 

Future of Transistors

Since the invention of the first transistor in 1947 technology has come leaps and bounds and now the future of transistors has been unveiling in recent times. Some of developments of transistors in the future are listed below –

Molecular electronics

Carbon nanotubes transistors

Nanowire transistors

Quantum computing

CMOS devices will add functionality to CMOS non-volatile memory, opto-electronics, sensing….

CMOS technology will address new markets macroelectronics, bio-medical devices, …

Biology may provide inspiration for new technologies bottom-up assembly, human intelligence

Although much of these are not concrete some of them are in development or in the pipelines of development and some of the other technologies involving transistors have been surveyed and discussed below.

One future plan is to place graphene on silicon. Graphene-based transistors can run at higher frequencies and more efficiently than silicon transistors. Graphene has many uses, such as support membranes for transmission electron microscopy and in gas sensors because the effect of gas molecules that land on graphene are measureable. While graphene is seen as a material of the future, it has recently been revealed that it may not be a suitable replacement for silicon in CPUs. This is due to the fact that graphene has a very small energy state gap, meaning when it is used as a transistor it cannot be turned off. It is seen that molybdenite may be the perfect replacement.

Intel has developed 3D transistors , "Tri-Gate" transistor that will allow Intel to keep shrinking chips. Intel says the transistors will use 50 percent less power, conduct more current and provide 37 percent more speed than their 2D counterparts thanks to vertical fins of silicon substrate that stick up through the other layers. The fins could make for cheaper chips too -- currently, though, the tri-gate tech adds an estimated 2 to 3 percent cost to existing silicon wafers.

Researchers from Purdue and Harvard universities created the transistor. It is made from a material that could replace silicon within a decade. Each transistor contains three tiny nanowires made from indium-gallium-arsenide and not silicon as conventional transistors are. The three nanowires are progressively smaller, creating a tapered cross section resembling a Christmas tree. Indium-gallium-arsenide is among several promising semiconductors being studied to replace silicon.

DIODES – Esaki and IMPATT

A diode is a semiconductor device with two-terminals which has an asymmetric transfer characteristic. It allows the flow of current in one direction only. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals.

Esaki Diodes

Esaki diodes was named after Leo Esaki, who in 1973 received the Nobel Prize in Physics for discovering the electron tunneling effect used in these diodes. Esaki reported the first paper on tunnel diodes in Physical Review in 1958.

Esaki diode is also known as a tunnel diode. It is a type of semiconductor diode that is capable of rapid operation, well into the microwave frequency region, by using the quantum mechanical effect called tunneling.

It was invented in August 1957 by Leo Esaki when he was with Tokyo Tsushin Kogyo, now known as Sony.

These diodes have a heavily doped p–n junction only some 10 nm wide. A broken bandgap is a result of the heavy doping. The conduction band electron states on the n-side are more or less aligned with valence band hole states on the p-side.

Tunnel diodes were first manufactured by Sony in 1957. From 1960 General Electric and other companies started manufacturing these diodes. They are still made in low volume today. Tunnel diodes are usually made from germanium, but can also be made in gallium arsenide and silicon materials. They are used in frequency converters and detectors

Tunnel diodes can be operated in forward as well as in reverse bias. In forward bias operation as voltage increases, electrons tunnel through p-n junction barrier due to the fact that filled electron states in the conduction band in the n-side become aligned with empty valence band hole states in the p-side of the p-n junction. If the voltage increases further these states are more misaligned and the current drops which is known as negative resistance as current decreases with increasing voltage. However, if the voltage is increased further, its operation features in of a normal diode as electrons travel across the p-n junction by conduction instead of tunneling. In reverse direction, tunnel diodes act as fast rectifiers with zero offset voltage and extreme linearity for power signals. Filled states on the p-side become increasingly aligned with empty states on the n-side and electrons now tunnel through the p-n junction barrier in a reverse direction.

USES of ESAKI DIODES

They are used in low power amplifiers

Detector Log Video Amplifiers, microwave and RF power monitors, high-frequency triggers, ALC loops, zero bias detectors, ACP tunnel diode circuits, etc.

Since they are more resistant to nuclear radiation, tunnel diodes are used in space applications like amplifiers for satellite communications.

In technology development today, for circuit engineers, the question was, "Transistors or Esaki diodes?" Designers appear to be in the same position they were a decade ago, when they asked "Tubes or transistors?" Some stayed with "proved circuits," while others adopted the transistor and changed the industry. Today, designers familiar with standard solid-state technology are trying to adjust to new concepts forced on them by the Esaki diode: negative resistance, ultra-high speeds, and ultra-low power consumption.

IMPATT DIODE

IMPATT is an acronym of IMPact ionization Avalanche Transit Time, which reflects the mechanism of its operation. In its simplest form, an IMPATT is a p-n junction diode reversed biased to breakdown, in which an avalanche of electron-hole pair is produced in the high-field region of the device depletion layer by ‘impact ionization’. The transit of the carriers through the depletion layer leads to generation of microwave and MM-waves when the device is tuned in a suitable microwave and MM-wave cavity. These diodes exhibit negative resistance at microwave and MM-wave frequencies due to two electronic delays, ‘avalanche build-up delay’ due to ‘impact ionization’ leading to avalanche multiplication of charge carriers and ‘transit time delay’ due to the saturation of drift velocity of charge carriers moving under the influence of a high electric field. The working principles of the device were first described by Read in 1958. However, the idea of obtaining a negative resistance from a reversed biased p-n junction dates back to an earlier paper (1954) by Shockley, in which he showed that when an electron bunch from a forward biased cathode is injected into the depletion layer of a reversed biased p-n junction a ‘transit time negative resistance’ is produced as the electrons drift across the high field region. The negative resistance from such early devices was found to be small and microwave power output was low. Read showed that an improved negative resistance is obtained when impact ionization is used to inject the electrons. He showed that the properties of charge carriers in a semiconductor i.e. avalanche multiplication by impact ionization and transit time delay of charge carriers due to saturation of drift velocity at high electric fields, could be suitably combined in a reverse-biased p-n junction to produce a microwave negative resistance. By exploiting the time delay required to build up an avalanche discharge by impact ionization, coupled with Shockley’s transit time delay, he showed that efficient microwave oscillation could be realized in his proposed p+ n i n+ diode. However, due to the complicated nature of the Read structure, it was not until 1965 that the first experimental Read diode was fabricated. In the early 1965 Johnston et al., from Bell Laboratories, first made a successful experimental observation of microwave oscillations from a simple Si p-n junction diode. This study showed that the complicated Read structure was not essential required for generating microwave oscillations. On the basis of a small-signal analysis, T. Misawa showed that negative resistance would occur in a reverse biased p-n junction of any arbitrary doping profile. Since then, rapid advances have been made towards further development of various IMPATT structures, fabrication techniques as well as optimum circuit design for IMPATT oscillators and amplifiers. The frequency range of IMPATT devices can be pushed easily to MM and sub-MM wave ranges at which comparable amount of RF power generation is hardly possible by other two terminal solid-state devices.

The IMPATT diode family includes many different junctions and metal semiconductor devices. The first IMPATT oscillation was obtained from a simple silicon p-n junction diode biased into a reverse avalanche break down and mounted in a microwave cavity. Most of the electron–hole pairs are generated in the high field region due to the strong dependence of the ionization coefficient on the electric field. The generated holes drift across the P region while the generated electron immediately moves into the N region, while. The time required for the hole to reach the contact constitutes the transit time delay.

 

Operation

In an Impatt diode extremely high voltage gradient is applied (400kv/cm) which a normal pn junction can't withstand.

Such a high potential gradient, back-biasing the diode causes a flow of minority carrier across the junction.

The ac current is approximately 180 degrees out of phase with the applied voltage this gives rise to negative conduction and oscillation is resonant circuit.

 

Working

With a high bias threshold DC voltage, as the applied ac voltage goes positive, electron hole velocity becomes so high that these carriers form additional holes and electrons by knocking them out of the crystal structure by impatt ionization.

The original DC field is just at the threshold but this voltage is exceeded only during the positive half cycle of A.C voltage. It is a cumulative process and takes time. A 90 degree phase difference or a delay has taken place.

The holes produced in the avalanche rapidly reaches the p+ contact taking no part in the process but the electrons are released into n region where they do not combine with either donor or holes.

The electron drift is at their maximum velocity across the n region and current continues to flow in the external circuit which they are in transit.

When this current pulse actually arrives at the cathode terminal, the A.C voltage is at its negative peak and the second delay of 90 degree has taken place. This time depends upon the velocity and the thickness of the highly doped n+ layer.