A chlorine molecule forms a covalent bond

Modern electronics can trace its roots to the first electronic devices called vacuum tubes. Although, today, solid state devices have totally replaced the vacuum tube, the fundamental principle as to its usage remains relatively unchanged. For more than 40 years, until the late 1960s, the most important part in a consumer electronics product was the vacuum tube. It is with this historical perspective in mind that this section is presented so that readers will not lose sight of where it all started.

The vacuum tube got its start in 1883, when Edison was developing the incandescent lamp. To correct the premature burnout of the red-hot filament in light bulbs, Edison tried a number of experiments, one of which was to place a metal plate sealed inside a bulb and connect it to a battery and ammeter, as shown in Fig. 1.4. Edison observed that, when the filament was hot and the plate was positively (+) charged by the battery, the ammeter indicated a current flow through the vacuum, across the gap between the filament and the plate. When the charge on the plate was reversed to negative (–), the current flow stopped. As interesting as this phenomena was, it did not improve the life of Edison’s lamps and, as a result, he lost interest in this experiment and went on to other bulb modifications that proved more successful. For about 20 years, Edison’s vacuum tube experiment remained a scientific curiosity. In 1903, as radios were coming into use, J. A. Fleming, in England, found just.

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Bohr model of silicon atom

Electrons are being forced into the next higher shell. An atom is chemically stable if its outer shell is either completely filled with electrons, based on the 2n2 rule, or has eight electrons in it. The electrons in the outer shell are called valence electrons and, if their number is less than eight, the atom will have a tendency to interact with other atoms either by losing, acquiring, or merging its electrons with other atoms. In the periodic table (Fig. 1.1), elements with the same number of valence electrons have similar properties and are placed in the same group. For example, elements in Group I have atoms with one electron in their outer shell. Group II shows elements that have atoms with two electrons in their outer shell, and so on. Elements on the left side of the periodic table have a tendency to lose their valence electrons to other atoms, thus becoming electropositive. The elements on the right side of the periodic table show a tendency to acquire electrons from other atoms and become electronegative.

The type of interaction occurring between atoms, as they are brought together, depends largely on the properties of the atoms themselves. The interaction may form bonds that can be classified as ionic, covalent, molecular, hydrogen bonded, or metallic. Since this chapter is concerned with semiconductors, which tend to form covalent bonds with other elements and with themselves, the emphasis will be on covalent bonding. Covalent bonds occur when two or more atoms jointly share each other’s valence electrons. If the outer shell is partially filled with electrons, the atom will be attracted to other atoms also having a deficiency of electrons, so sharing each other’s valence electrons will result in a more stable condition. As an example, two chlorine atoms will attract and share each other’s single electron to orm a stable covalent bond with eight electrons in each shell (Fig. 1.3).

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IDEAL DIODE

The first electronic device to be introduced is called the diode. It is the simplest of semiconductor devices but plays a very vital role in electronic systems, having characteristics that closely match those of a simple switch. It will appear in a range of applications, extending from the simple to the very complex. In addition to the details of its construction and characteristics, the very important data and graphs to be found on specification sheets will also be covered to ensure an understanding of the terminologyemployed and to demonstrate the wealth of information typically available from manufacturers.
The term ideal will be used frequently in this text as new devices are introduced. It refers to any device or system that has ideal characteristics—perfect in every way. It provides a basis for comparison, and it reveals where improvements can still be made. The ideal diode is a two-terminal device having the symbol and characteristics shown in Figs. 1.1a and b, respectively. Ideally, a diode will conduct current in the direction defined by the arrow in the symbol and act like an open circuit to any attempt to establish current in the opposite direction. In essence:

The characteristics of an ideal diode are those of a switch that can conduct current in only one direction.


In the description of the elements to follow, it is critical that the various letter symbols, voltage polarities, and current directions be defined. If the polarity of the applied voltage is consistent with that shown in Fig. 1.1a, the portion of the characteristics to be considered in Fig. 1.1b is to the right of the vertical axis. If a reverse voltage is applied, the characteristics to the left are pertinent. If the current throughthe diode has the direction indicated in Fig. 1.1a, the portion of the characteristics to be considered is above the horizontal axis, while a reversal in direction would require the use of the characteristics below the axis. For the majority of the device characteristics hat appear in this book, the ordinate (or “y” axis) will be the current axis, while the abscissa (or “x” axis) will be the voltage axis.
One of the important parameters for the diode is the resistance at the point or region of operation. If we consider the conduction region defined by the direction of ID and polar  of VD in Fig. 1.1a (upper-right quadrant of Fig. 1.1b), we will find that the value of the forward resistance, RF, as defined by Ohm’s law is

where VF is the forward voltage across the diode and IF is the forward current through The ideal diode, The ideal diode, therefore, is a short circuit for the region of conduction. Consider the region of negatively applied potential (third quadrant) of Fig. 1.1b,

where VR is reverse voltage across the diode and IR is reverse current in the diode.The ideal diode, therefore, is an open circuit in the region of nonconduction. In review, the conditions depicted in Fig. 1.2 are applicable.

In general, it is relatively simple to determine whether a diode is in the region of conduction or nonconduction simply by noting the direction of the current ID established by an applied voltage. For conventional flow (opposite to that of electron flow), if the resultant diode current has the same direction as the arrowhead of the diode symbol, the diode is operating in the conducting region as depicted in Fig. 1.3a. If  the resulting current has the opposite direction, as shown in Fig. 1.3b, the opencircuit equivalent is appropriate.

As indicated earlier, the primary purpose of this section is to introduce the characteristics of an ideal device for comparison with the characteristics of the commercial variety. As we progress through the next few sections, keep the following questions in mind:

How close will the forward or “on” resistance of a practical diode compare with the desired 0- level? Is the reverse-bias resistance sufficiently large to permit an open-circuit approximation?

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Semiconductor Diodes INTRODUCTION

It is now some 50 years since the first transistor was introduced on December 23, 1947. For those of us who experienced the change from glass envelope tubes to the solid-state era, it still seems like a few short years ago. The first edition of this text contained heavy coverage of tubes, with succeeding editions involving the important decision of how much coverage should be dedicated to tubes and how much to semiconductor devices. It no longer seems valid to mention tubes at all or to compare the advantages of one over the other—we are firmly in the solid-state era. The miniaturization that has resulted leaves us to wonder about its limits. Complete systems now appear on wafers thousands of times smaller than the single element of earlier networks. New designs and systems surface weekly. The engineer becomes more and more limited in his or her knowledge of the broad range of advances— it is difficult enough simply to stay abreast of the changes in one area of research or development. We have also reached a point at which the primary purpose of the container is simply to provide some means of handling the device or system and to provide a mechanism for attachment to the remainder of the network. Miniaturization appears to be limited by three factors (each of which will be addressed in this text): the quality of the semiconductor material itself, the network design technique, and the limits of the manufacturing and processing equipment.

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Sharp to join e-reader business war

A man looks at e-book products on display at a booth of Taiwan's Green Book Inc, during the five-day Computex Taipei in June. Sharp said it would launch an e-reader this year able to handle text as well as video and audio content, in a bid to challenge Apple and other rivals in the lucrative market.

Sharp said Tuesday it would launch an e-reader this year able to handle text as well as video and audio content, in a bid to challenge Apple and other rivals in the lucrative market.


The Japanese electronics giant said it had updated its e-book format with the "next-generation XMDF" platform, an advanced multimedia version of the XMDF format for text and still images that it launched in 2001.
"The next-generation XMDF enables easy viewing of digital content including video and audio and allows automatic adjustment of the layout to match and meet publishers' needs," Sharp said in a statement.
Sharp plans to begin the service and sell two types of e-readers, which resemble Apple's iPhone and iPad, by the end of the year in Japan and will then also export the gadgets.
"Now there is a lot of attention on the e-publishing business," Masami Obatake, a senior Sharp official, told a news conference. "Launching it by the end of this year will be good timing."
Asked if Sharp can cope with the competition, Obatake said: "Since we have a new system, I think we will be able to compete sufficiently."
Sharp said it had already reached basic accords with major Japanese publishers and newspaper companies on content, adding it would be open to further collaboration to establish an e-book market.
In late May, Sony announced a similar plan jointly with telecoms operator KDDI, the Asahi Shimbun newspaper company and the Toppan printing company, with each company taking a 25 percent stake.
That came just a day before the launch of the iPad in Japan and other countries outside the United States, where print media face a steady decline in advertising and have turned to e-readers as a way to win new revenue.
The Japanese electronic book market is now estimated to be worth 46 billion yen (about 500 million dollars), with most titles distributed via mobile telephones and conventional computers.
Japanese news media had until this year taken a wait-and-see approach to the devices, contrary to US peers.
Newspaper circulation has held up better than in the United States, having fallen only six percent between 1999 and 2009 to 50.3 million sales daily, the Japan Newspaper Publishers & Editors Association said. However, magazine circulation in Japan has slumped by a third over the decade.

Sourch

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