Understanding The Terms Of Semiconductor Devices Information Technology Essay

Semiconductors are materials which have a conductivity between conductors generally metals and nonconductors or insulators (such as most ceramics). Semiconductors can be pure elements, such as silicon or germanium, or compounds such as gallium arsenide or cadmium selenide. In a process called doping, small amounts of impurities are added to pure semiconductors causing large changes in the conductivity of the material.

Semiconductor devices now influence our lives on a daily basis. Although insulators and conductors are useful in their own right, semiconductors such as silicon and gallium arsenide have dramatically changed the way in which billions of people live. Their intermediate ability to conduct electricity at room temperature makes them very useful for electronic applications. For example, the modern computing industry was made possible by the ability of silicon transistors to act as fast on/off switches.

1.2 History:

Fig 1: History of Semiconductor

Part 2: Project Overview

2.1 Types of Semiconductors

2.1.1 Intrinsic Semiconductors

An intrinsic semiconductor is one which is made up of a very pure semiconductor material. In more technical terminology it can stated that an intrinsic semiconductor is one where the number of holes is equal to the number of electrons in the conduction band.

The forbidden energy gap in case of such semiconductors is very minute and even the energy available at room temperature is sufficient for the valence electrons to jump across to the conduction band.

Another characteristic feature of an intrinsic semiconductor is that the Fermi level of such materials lies somewhere in between the valence band and the conduction band. This can be proved mathematically which is beyond the scope of discussion in this article. In case you are not familiar with the term Fermi level, it refers to that level of energy where the probability of finding an electron is 0.5 or half (remember probability is measured on a scale of 0 to 1).

2.1.2 Extrinsic Semiconductors

These are semiconductors in which the pure state of the semiconductor material is deliberately diluted by adding very minute quantities of impurities. To be more specific, the impurities are known as dopants or doping agents. It must be kept in mind that the addition of such impurities is really very miniscule and a typical dopant could have a concentration of the order of 1 part in a hundred million parts or it is equivalent to 0.01 ppm.

The materials chosen for doping are deliberately chosen in such a manner that either they have 5 electrons in their valence band, or they have just 3 electrons in their valence band. Accordingly such dopants are known as pentavalent or trivalent dopants respectively.

The type of dopant also gives rise to two types of extrinsic semiconductors namely P-type and N-type semiconductors.

A pentavalent dopant such as Antimony are known as donor impurities since they donate an extra electron in the crystal structure which is not required for covalent bonding purposes and is readily available to be shifted to the conduction band. This electron does not give rise to a corresponding hole in the valence band because it is already excess, therefore upon doping with such a material, the base material such as Germanium contains more electrons than holes, hence the nomenclature N-type intrinsic semiconductors.

On the other hand when a trivalent dopant such as Boron is added to Germanium additional or extra holes get formed due to the exactly reverse process of what was described in the upper section. Hence this dopant which is also known as acceptor creates a P-type semiconductor.

Hence electrons are the majority carriers (of current) in N-type while holes are minority carriers. The reverse is true of P-type semiconductors. Another difference is that whereas the Fermi level of intrinsic semiconductors is somewhere midway between the valence band and the conduction band, it shifts upwards in case of N-type while it drifts downward in case of P-type due to obvious reasons.

2.2 Semiconductor Device (Diode):

A diode is the simplest possible semiconductor device, and is therefore an excellent beginning point if you want to understand how semiconductors work. In this article, you’ll learn what a semiconductor is, how doping works and how a diode can be created using semiconductors. But first, let’s take a close look at silicon.

Silicon is a very common element — for example, it is the main element in sand and quartz. If you look “silicon” up in the periodic table, you will find that it sits next to aluminum, below carbon and above germanium.

A diode is the simplest possible semiconductor device. A diode allows current to flow in one direction but not the other. You may have seen turnstiles at a stadium or a subway station that let people go through in only one direction. A diode is a one-way turnstile for electrons.

When you put N-type and P-type silicon together as shown in this diagram, you get a very interesting phenomenon that gives a diode its unique properties.

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Fig 2: Diode

Even though N-type silicon by itself is a conductor, and P-type silicon by itself is also a conductor, the combination shown in the diagram does not conduct any electricity. The negative electrons in the N-type silicon get attracted to the positive terminal of the battery. The positive holes in the P-type silicon get attracted to the negative terminal of the battery. No current flows across the junction because the holes and the electrons are each moving in the wrong direction.

If you flip the battery around, the diode conducts electricity just fine. The free electrons in the N-type silicon are repelled by the negative terminal of the battery. The holes in the P-type silicon are repelled by the positive terminal. At the junction between the N-type and P-type silicon, holes and free electrons meet. The electrons fill the holes. Those holes and free electrons cease to exist, and new holes and electrons spring up to take their place. The effect is that current flows through the junction.

Part 3: Analysis

3.1 Application and Research

The building block of most semiconductor devices involves combining p-type and n-type regions into p-n junctions. Imagine bringing together two crystals where one is n-type and the other is p-type. A few of the electrons from the n-type flow toward the p-type material. At the point where the p-type and n-type meet (the interface) electrons from the n-side fill the holes on the p-side and a build-up of oppositely charged ions is generated, and thus a potential across the barrier forms. This build-up of charge is called the junction potential. The barrier prevents further migration of electrons and the net current is zero.

If a voltage is applied to the p-n junction with the negative terminal connected to the n-region and the p-region is connected to the positive terminal, the electrons will flow toward the positive terminal, while the holes will flow toward the negative terminal. This is called forward bias and current flows. However, if the positive terminal is connected to the n-type and the negative connected to the p-type, a reverse bias forms and no current flows due to the build up of the potential barrier. In other words, these devices must be placed in an electrical circuit with the correct polarity, or they will not function. This application of the p-n junction is used in many electronic devices. Figure 6 shows the formation of a potential at a p-n junction. Figure 7 shows the effect of forward and negative bias on the p-n junction.

Figure 3: A p-n junction before and after the two materials are brought in contact.

When the two materials are placed together, electrons from the n-side combine with the holes on the p-side. This results in a positive charge on the n-side of the junction and a negative charge accumulation on the p-side. This separation of charge creates a junction potential. Note: There are no electrons or holes at the junction, they have combined with each other.

Figure 4: A p-n junction under forward and reverse bias. Notice that in forward bias, the barrier is lowered, while in reverse bias, the barrier is raised.

Thought question: In each case in Figure 4, which side is connected to the positive terminal of the outside voltage source? Will electrons or holes carry current when the junction has this arrangement ?

3.1.1 Electronic Devices:

There are many electronic devices that function using combinations of p-n junctions such as diodes, solar cells and transistors. In this section a brief explanation of each of these basic devices will be given.

The diode is a p-n junction application that acts as a rectifier for converting alternating current to direct current. This is due to the ability of a diode to allow current flow in one direction but not in the other.

Solar cells are p-n junction devices which use sunlight to create electrical energy. It is the energy of the sun`s photons that causes the electrons to be promoted into the conduction bands and carry the current. However, the current derived from the solar cell is small. It takes many solar cells to produce enough current to do a large scale job. If the energy output from solar cells could be increased, solar energy could be used for more than individual, isolated applications.

Transistors are another application of the p-n junction. Transistors, unlike diodes, contain more than one p-n junction. Because of this, a transistor can be used in a circuit to amplify a small voltage or current into a larger one or function as an on-off switch. Transistors are of two main types: bipolar junction transistors (BJT’s) and field effect transistors (FET’s). Roughly 95% of all electronic systems utilize one or both of these types of devices.

BJTs are composed of three layers of doped materials, either n-p-n or p-n-p in configuration. The BJT acts like a bump or dam in an open stream to control the amount of current let by; thus as the bump is lowered, more current can flow. In the BJT, the height of the bump is controlled by the base current in the semiconductor. The BJT was invented in 1948 by John Bardeen, Walter Brittain and William Shockley using germanium. BJT’s remained the only important three terminal semiconductor devices for about a dozen years after their invention, and helped to launch the modern electronics era.

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Since the early 1960’s the FET has been considered one of the most important devices in solid state technology. At present, many of the applications of BJTs have been taken over by metal-oxide semiconductor FET’s (MOSFETs). MOSFETs were theorized for many years before they were able to be manufactured. The reason MOSFETs could not be made was that scientists had not yet developed techniques for growing high quality silicon dioxide (SiO2) on silicon. The FET functions more as a gate for controlling the flow of current (like a valve on a faucet). FET’s are relatively simple to fabricate compared to BJT’s, and they have proven to be extremely fast, reliable switches in miniaturized circuit components with much less power usage than BJT’s. Most modern microprocessors are based on FET devices–from pentium chips in PC’s to the CPU’s of super computers. Transistors, diodes, and other electronic devices are combined in many different patterns to form today’s integrated circuits.

The integrated circuit (IC) has been the workhorse of the “microelectronics era” which began in the late 1950’s. These chips, usually made of silicon, consist of combinations of four fundamental electrical regions. These regions contain resistors, capacitors, diodes and transistors. Since 1971, Very Large Scale Integration (VLSI) has allowed millions of such regions to be fabricated on a chip that is only one square centimeter. Not only are these circuit elements getting smaller, they are getting faster as well. For example today’s typical desktop pentium-based computer can perform tens of millions of operations per second, whereas contemporary super computers are rated in gigaflops (billions of operations per second). Teraflop (trillions of operations per second) machines will be ready for production by the year 2000.

3.2 Semiconductors Applications – Worldwide

We mentioned just a few of the many different applications of semiconductor devices. The use of these devices has become so widespread that it would be impossible to list all their different applications. Instead, a broad coverage of their specific application is presented.

Semiconductor devices are all around us. They can be found in just about every commercial product we touch, from the family car to the pocket calculator. Semiconductor devices are contained in television sets, portable radios, stereo equipment, and much more.

Science and industry also rely heavily on semiconductor devices. Research laboratories use these devices in all sorts of electronic instruments to perform tests, measurements, and numerous other experimental tasks. Industrial control systems (such as those used to manufacture automobiles) and automatic telephone exchanges also use semiconductors. Even today heavy-duty versions of the solid-state rectifier diode are being use to convert large amounts of power for electric railroads. Of the many different applications for solid-state devices, space systems, computers, and data processing equipment are some of the largest consumers.

The various types of modem military equipment are literally loaded with semiconductor devices. Many radars, communication, and airborne equipment are transistorized. Data display systems, data processing units, computers, and aircraft guidance-control assemblies are also good examples of electronic equipments that use semiconductor devices. All of the specific applications of semiconductor devices would make a long impressive list. The fact is, semiconductors are being used extensively in commercial products, industry, and the military.

3.3 Power semiconductor devices for DC/DC converters

As the performance of servers, notebook PCs and graphics cards increases, their power consumption grows as well. At the same time, the trend toward lower operating voltages for components such as CPUs, graphics processing units (GPUs), memory devices and ASICs results in increased current flow. This creates a need for DC/DC converters capable of handling low voltages and large currents.

 

Renesas 12th-generation power MOSFETs, the RJK0210DPA, RJK0211DPA and RJK0212DPA are now available for service in DC/DC converters, which operate by having two power MOSFETs, one for control and the other for synchronous rectification, switching on and off alternately to convert the voltage.  For example the new RJK0210DPA MOSFET is used for control and the Renesas Electronics 11th generation RJK0208DPA device can be used for synchronous rectification.

 

Refinements to the manufacturing process allow the new Renesas MOSFETs to achieve approximately 40 percent improvement in FOM (figure of merit; on-state resistance times gate charge) compared to the company’s existing products, which contributes to reduction of the power loss during voltage conversion and thereby enables highly efficient DC/DC converter performance.

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Using the Renesas Virtual Power Lab MOSFET Design Tool lets you check out these and other MOSFETs without the hassle of waiting for device samples, then having to solder the parts down on test boards. 

This tool lets engineers evaluate various solutions in a virtual real-time environment to facilitate the selection of optimum MOSFET combinations for synchronous buck-converter applications. Among its benefits: helping you find the right MOSFETs and interactively get help designing your sync buck converter application; analyzing performance, switching behavior and efficiency of your new buck converter design; evaluating MOSFET behavior under a variety of operating conditions using an interactive datasheet and downloading SPICE models.

3.4 Importance Of Semiconductor in Technologies:

Due to their role in the fabrication of electronic devices, semiconductors are an important part of our lives. Imagine life without electronic devices. There would be no radios, no TV’s, no computers, no video games, and poor medical diagnostic equipment. Although many electronic devices could be made using vacuum tube technology, the developments in semiconductor technology during the past 50 years have made electronic devices smaller, faster, and more reliable. If we think for a minute of all the encounters we have with electronic devices. How many of the following have we seen or used in the last twenty-four hours? Each has important components that have been manufactured with electronic materials.

microwave oven, electronic balance, video games, radio, television, VCR, watch, CD player, stereo, computer, lights, air conditioner, calculator, telephone, musical greeting cards, diagnostic equipment, clock, refrigerator, car, security devices , stove

Fig 5: Clockwise from top: A chip, an LED and a transistor are all made from semiconductor material 

Source: http://static.howstuffworks.com/gif/solid-state1.jpg

Semiconductors have had a monumental impact on our society. We find semiconductors at the heart of microprocessor chips as well as transistors. Anything that’s computerized or uses radio waves depends on semiconductors.

Today, most semiconductor chips and transistors are created with silicon. You may have heard expressions like “Silicon Valley” and the “silicon economy,” and that’s why — silicon is the heart of any electronic device.

3.5 Future Trends

Since the late 1950’s, the discovery and invention of new electronic semiconductor materials and the drastic reduction in the size of electronic devices has moved at a rapid pace. As a result, the speed of electronic devices (particularly integrated circuits) has grown exponentially over the same time period. Great strides have been made by companies such as Bell Laboratories, Intel, Western Electric, American Telephone and Telegraph, Motorola, Rockwell, and IBM.

In 1975, Gordon Moore gave a famous talk at the International Electronic Devices Meeting (IEDM) in which he predicted a growth in microchip complexity of roughly a factor of two every year. In most areas of electron device production, his predictions have been met or exceeded. The push for smaller dimensions, which allow for increased functionality and faster devices, also creates problems of long term reliability and heat dissipation. New device designs, new materials, and lower voltages are being employed to make the next generation of devices.

One extremely important area of semiconductor technology is the field of telecommunications. The new “Information Super Highway” requires technology which can transmit and receive information at high rates. One approach which is already being applied to this area is optoelectronics or the use of light to transmit information. Electrons are used to transfer information within computers, but most information sent over long distances uses light pulses traveling through fiber optic cables. The laser diodes which create these pulses and semiconductor receivers that detect the pulses are areas of intensive research.

It is clear that semiconductor technology has and will continue to play a major role in the development of the information age.

Part 4: Conclusion

After the completion of the term paper on “Importance Of Semiconductor discovery on Technology” I got many new things to learn about. The term paper includes brief description on semiconductors, it also contains the types of Semiconductors and brief description of it. The term paper also gives the importance of Semiconductor in our day today life.

It also explains the applications of semiconductor and its uses in some of the technologies. The term paper also gives future trends and research of semiconductor. Through this term paper I wanted to give the reader few ideas about what are semiconductors and about its importance.

I have worked very hard on this project and wanted to build it in a very simple and lucid manner so that it could be easy for the reader to go through and understand the term paper.

Hopefully, I think that you would have gained some knowledge on semiconductors and could have well understood it. I grant a sincere apologize if any mistake would have crept in my work.

Part 5: References

http://www.brighthub.com/engineering/electrical/articles/41694.aspx#ixzz145rf13AN

http://electronics.howstuffworks.com/diode2.htm

http://am.renesas.com/products/discrete/power_mos/vp/vp_landing.jsp

http://matse1.matse.illinois.edu/sc/prin.html

http://www.howstuffworks.com/diode.htm

http://www.tpub.com/neets/book7/24a.htm

http://www.gartner.com/it/products/research/asset_129174_2395.jsp

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