Basic Electronics Essay Example for Free

Basic Electronics Essay The p-n junction is a homojunction between a p-type and an n-type semiconductor. It acts as a diode, which can serve in electronics as a rectifier, logic gate, voltage regulator (Zener diode), switching or tuner (varactor diode); and in optoelectronics as a light-emitting diode (LED), laser diode, photodetector, or solar cell. In a relatively simplified view of semiconductor materials, we can envision a semiconductor as having two types of charge carriers-holes and free electrons which travel in opposite directions when the semiconductor is subject to an external electric field, giving rise to a net flow of current in the direction of the electric field. Figure 1 illustrates the concept. A p-n junction consists of a p-type and n-type section of the same semiconductor materials in metallurgical contact. The p-type region has an abundance of holes (majority carriers) and a few mobile electrons (minority carriers); the n-type region has an abundance of mobile electrons and a few holes (Fig. 2). Both charge carriers are in continuous random thermal motion in all directions. Fig. 2. Energy levels and carrier concentrations for a p-type and n-type semiconductor before contact. 1 When a section of p-type material and a section of n-type material are brought in contact to form a pn junction, a number of interesting properties arise. The pn junction forms the basis of the semiconductor diode. Electrons and holes diffuse from areas of high concentration toward areas of low concentration. Thus, electrons diffuse from the n-region to the p-region. , leaving behind positively charged ionized donor atoms. In the p-region the electrons recombine with the abundant holes. Similarly, holes diffuse from the p-region into the n-region, leaving behind negatively charged ionized acceptor atoms. In the n-region the holes recombine with the abundant mobile electrons. This diffusion process does not continue indefinitely, however, because it causes a disruption of the charge balance in the two regions. As a result, a narrow region on both sides of the junction becomes nearly depleted of the mobile charge carriers. This region is called the depletion layer. It contains only the fixed charges (positive ions on the n-side and negative ions on the p-side). The thickness of the depletion layer in each region is inversely proportional to the concentration of dopants in the region. The net effect is that, the depletion region sees a separation of charge, giving rise to an electric field pointing from the n side to the p side. The fixed charges create an electric field in the depletion layer that points from the n-side towards the p-side of the junction. The charge separation therefore causes a contact potential (also known as built-in potential) to exist at the junction. This built-in field obstructs the diffusion of further mobile carriers through the junction region. An equilibrium condition is established that results in a net contact potential difference Vo between the two sides of the depletion layer, with the n-side exhibiting a higher potential than the p-side. This contact potential is typically on the order of a few tenths of a volt and depends on the material (about 0. 5 to 0. 7 V for silicon). The built-in potential provides a lower potential energy for an electron on the n-side relative to the p-side. As a result, the energy bands bend as shown in Fig. 3. In thermal equilibrium there is only a single Fermi function for the entire structure so that the Fermi levels in the p- and the n-regions must align. No net current flows across the junction. The currents associated with the diffusion and built-in field (drift current) cancel for both the electrons and holes. Fig. 3. A p-n junction in the Thermal equilibrium at T gt; 0? K. The depletion-layer, energy-band diagram, and concentrations (on a logarithmic scale) of the mobile electrons n(x) and holes p(x) are shown as a functions of the position x. The built-in potential difference V corresponds to the energy eV where e is the electron charge. 0 0 2 The Biased p-n Junction An externally applied potential will alter the potential difference between the p- and n-regions. This in turn will modify the flow of majority carriers, so that the junction can be used as a â€Å"gate†. If the junction is forward biased by applying a positive voltage V to the p-region (Fig. 4), its potential is increased with respect to the n-region, so that an electric field is produced in a direction opposite to that of the built-in field. The presence of the external bias voltage causes a departure from equilibrium and a misalignment of the Fermi levels in the p- and n-regions, as well as in the depletion layer. The presence of the two Fermi levels in the depletion layer, Efc and Efv represents a state of quasi-equilibrium. Fig. 4. Energy band diagram and carrier concentrations for a forward-biased p-n junction. In effect, then, if one were to connect the two terminals of the p-n junction to form a closed circuit, two currents would be present. First, a small current, called reverse saturation current, is, exists because of the presence of the contact potential and the associated electric field. In addition, it also happens that holes and free electrons with sufficient thermal energy can cross the junction. This current across the junction flows opposite to the reverse saturation current and is called diffusion current. Of course, if a hole from the p side enters, it is quite likely that it will quickly recombine with one of the n-type carriers on the n side. (Fig. 4) The net effect of the forward bias is to reduce the height of the potential-energy hill by an amount eV. The majority carrier current turns out to increase by an exponential factor exp(eV/kT). So that the net current becomes i = isexp(eV/kT) – is, where is is nearly a constant. The excess majority carrier holes and electrons that enter the n and p regions, respectively, become minority carriers and recombine with the local majority carriers. To explain the mechanism of reverse conduction, one needs to visualize the phenomenon of avalanche breakdown. When a very large negative bias is applied to the p-n junction, sufficient energy is imparted to charge carriers that reverse current can flow, well beyond the normal reverse, saturation current. In addition, because of the large electric field, electrons are energized to such levels that if they collide with other charge carriers at a lower energy level, some of their energy is transferred to the carriers with low energy, 4 and these can now contribute to the reverse conduction process, as well. This process is called impact ionization. Now, these new carriers may also have enough energy to energize other lowenergy electrons by impact ionization, so that once a sufficiently high reverse bias is provided, this process of conduction takes place very much like an avalanche: a single electron can ionize several others. Fig. 6. The reverse breakdown region The phenomenon of Zener breakdown is related to avalanche breakdown. It is usually achieved by means of heavily doped regions in the neighbourhood of the metal-semiconductor junction (the ohmic contact) . The high density of charge carriers provides the means for a substantial reverse breakdown current to be sustained at a much lower specific voltage than normal diode, at a nearly constant reverse bias known as the Zener voltage, Vz. This phenomenon is very useful in applications where one would like to hold some load voltage constant for example, in voltage regulators. The response time of a p-n junction to a dynamic (ac) applied voltage is determined by solving the set of differential equations governing the processes of electrons and hole diffusion, drift (under the influence of the built-in and external electric fields), and recombination. These effects are important for determining the speed at which the diode can be operated. They may be conveniently modeled by two capacitances, a junction capacitance and diffusion capacitance, in parallel with an ideal diode. The junction capacitance for the time necessary to change the fixed positive and negative charges stored in the depletion layer when the applied voltage changes. The thickness l of the depletion layer turns out to be proportional to v(Vo-V); it therefore increases under the reverse-bias conditions (negative V) and decreases under the forward-bias conditions (positive V). The junction capacitance C=?A/l (where A is the area of the junction) is therefore inversely proportional to v(VoV). The junction capacitance of a reverse-biased diode is smaller (and the RC response time is therefore shorter) than that of a forward-biased diode. The dependence of C on V is used to make voltage-variable capacitors (varactors). 5 Experiment l(a) : i-v characteristics of a semiconductor diode Procedure Connect the diode according to the circuit diagram as shown in Fig. 8. Fig 8 Vary the voltage V on the power supply between 0-30V. Alternately, the second concept is that the blocking action of an inductor stops the a. c. portion while the d. c. portion passes without much attenuation. Note: For filtering, large capacitance (hundreds to tens of hundreds microfarad) is needed. These are generally electrolytic capacitors, which consist of a repeating sandwich of aluminum sheets and a conducting paste, rolled into a cylinder for miminmun size. The aluminum sheets are polarized to form thin layers of aluminum oxide, a dielectric insulating material. The thinner the the dielectric the higher the capacitance will be.

The Plug-In Drug by Marie Winn Essay -- essays research papers

In an article ' The Plug-In Drug ' the author Marie Winn discusses the bad influence of television on today's society. Television is a ' drug ' that interfere with family ritual, destroys human relationships and undermines the family.   Ã‚  Ã‚  Ã‚  Ã‚  Marie Winn claims that television over the years have effected many American family life. Since television is everyday ritual, many American tend to spent more time with television than they do with their family and this result in unhealthy relation in family. She also acknowledge that television destroy family unique quality that they carry, such reading, cooking, games, songs and other special rituals.   Ã‚  Ã‚  Ã‚  Ã‚     Ã‚  Ã‚  Ã‚  Ã‚  The author claims television also destroys a human relationship. During free time instead working our difference with conversing each other, we are absorbed by imaginary world of television. Therefor we fail to interact through the real world leading to the ' distortion of real life relationship.' Television gives an escape root for people who need to be spending time on relationships with their family as well as others. Because children have ' one way relationships ' with the television, they fail to fully develop their communication and social skills. This further intensifies the decline of the family relationships.   Ã‚  Ã‚  Ã‚  Ã‚  Author also point out television undermines the family. Most parents are now relying on outside sources such... The Plug-In Drug by Marie Winn Essay -- essays research papers In an article ' The Plug-In Drug ' the author Marie Winn discusses the bad influence of television on today's society. Television is a ' drug ' that interfere with family ritual, destroys human relationships and undermines the family.   Ã‚  Ã‚  Ã‚  Ã‚  Marie Winn claims that television over the years have effected many American family life. Since television is everyday ritual, many American tend to spent more time with television than they do with their family and this result in unhealthy relation in family. She also acknowledge that television destroy family unique quality that they carry, such reading, cooking, games, songs and other special rituals.   Ã‚  Ã‚  Ã‚  Ã‚     Ã‚  Ã‚  Ã‚  Ã‚  The author claims television also destroys a human relationship. During free time instead working our difference with conversing each other, we are absorbed by imaginary world of television. Therefor we fail to interact through the real world leading to the ' distortion of real life relationship.' Television gives an escape root for people who need to be spending time on relationships with their family as well as others. Because children have ' one way relationships ' with the television, they fail to fully develop their communication and social skills. This further intensifies the decline of the family relationships.   Ã‚  Ã‚  Ã‚  Ã‚  Author also point out television undermines the family. Most parents are now relying on outside sources such...

Dbq- Scientific Revolution

DBQ – The Scientific Revolution The Scientific Revolution of the sixteen and seventeenth century were affected greatly from the contributions of the opposing voice and ideas of the Church and their disagreement with the uprising of scientific studies. Despite the rejection from the Church, the Scientific Revolution was heavily influenced by those in society who felt differently, and believed the benefits the Scientific Revolution would bring. This view however, was unequally agreed in when it came to the view of it politically.Still during the sixteenth and seventeenth century, religious and the Church played a great role in the ways of people. Till the time of the Scientific Revolution, many things were not questioned, but once scientists began to question the traditional beliefs, many people of the church were outraged and spoke openly against it. Even people like Copernicus, who was the great contributor to the heliocentric idea, denied himself and submitted to the church e ven dedicating a part of his book to Pope III which showed his fear and actions in pleasing the pope to avoid condemnation. doc. 1) His situation greatly exemplified how the Scientific Revolution although was growing but many times stunted because of fear from the disapproval from the Church. Italian monk Giovanni Ciampoli also expressed his disapproval in a letter to Galileo stating with much urgency that the nature of the world should just be left alone for the Scripture to explain it and that man should not go about their ways to reason why. (doc. 3) Similar to Copernicus, Walter Charleton a English doctor and natural philosopher who studied the balance of science and religious.He makes it clear that science is only possible with religion. Although he does not completely push the idea of scientific studies, he does believe it is only possible with the power of God. (doc. 8) The conclusion as you interpret out of Charleton is that no matter what man upholds through science or idea s are not sufficient enough to differ or oppose what God determines it to be. The views of secular people and society also bought about significant influence upon the Scientific Revolution, but just on the other side of the scale.Francis Bacon, a English philosopher of science made it evident in his advocacy of science. In fact, in the document, he expresses his eagerness of the goal people need to bring achievement in human society. (doc. 4) The document shows that despite the lack of support from the church was given, many people chose to stray away tradition and venture out into new ideas. Another document that poses the same suggestion as Bacon as to improving the community of scientific studies was that of Henry Oldenbury, Secretary of the English Royal Society in his letter to Johannes Hevelius.Oldenbury emphasizes the need in cooperation, and that scientists shouldn’t just focus on one’s study, put into consideration with the studies of others to develop the edu cation of science. (doc. 6) Oldenbury in saying â€Å"friendship among learned men is a great aid to the investigation and elucidation of the truth† only shows him hinting that a society of scientist should be raised. Both Bacon and Oldenbury were men during the seventeenth century who viewed science as a way to improve and enhance society, but others had a different view.Margaret Cavendish’s Observations on Experimental, shows her demand in questioning why women were not allowed to be a part of the revolution and contribute as men could, and her willingness in building upon the study of natural philosophy of women if she were allowed. (doc. 9) Her partake show’s that the influences of the Scientific Revolution did not solely come from different men of different social communities, but it had even spread to the other gender, which shows involvement. Political figures serve as a voice and power of the people in their society. In this case, politics took a great p art in the uprise of the Scientific Revolution.Document 5 is a letter from French monk Marin Mersenne to his noble patron in which he asks for the approval of his patron on his statements based on his experiments. His letter expresses a sense of meekness and humbleness Mersenne has for his patrons. He feels pressure in doing things correctly for his patrons, and is hindered from sharing what he has experimented on without winning the approval from his patrons. The power of political figures still remained throughout the sixteenth and seventeenth century, which from this document, shows how some studies were hindered because of the political authority.Political influences on scientific studies included personal beliefs on how scientific research should be controlled and suppressed. Thomas Hobbes, an English philosopher believed that any scientific findings that would interfere with the authority of rulers should be rid of. (doc. 7) Hobbes, a strong believer in an absolute monarch emp hasized the power of rulers to overpower the research of scientists. Unlike other documents, the depiction of the drawing that commemorated Louis XIV’s visit to the French Royal Academy, although not a very reliable source still portrayed how Louis XIV’s support in the scientific studies.Since a ruler takes up such a huge part in a country, the people of the country, in this case France, would naturally follow in the steps of their ruler. Louis XIV from this painting would’ve initiated a message that showed The work of scientists were affected by religious, social, and political ideas and influences in the sixteenth and seventeenth century. All these affected in ways that encouraged the study of science, while others felt that all scientific research should be stopped and suppressed.

Using French Verbs with Prepositions

In English, many verbs require a certain preposition in order for the meaning of the verb to be complete, such as to look at, to take care of, etc. The same is true in French, but unfortunately, the prepositions required for French verbs are often not the same as the ones required by their English counterparts. In addition, some verbs that require a preposition in English dont take one in French, and vice versa. De  and  Ãƒ  Ã‚  are by far the most common French prepositions for verbs. Because there are so many, these are divided into those that are followed by an infinitive and those that are followed by an indirect object. à  Ã‚   infinitiveà  Ã‚   indirect objectde   infinitivede   indirect object Some verbs have a different meaning depending on whether they are followed by  Ãƒ  Ã‚  or  de, while other verbs require both prepositions:  Ãƒ  Ã‚  and/or  de The expressions  cest  and  il est  have their own rules about which preposition follows:  cest  /  il est   prepositions. Note:  There are also constructions with no verb   Ãƒ  Ã‚  or  de   infinitive, a structure known as  passive infinitive. While  Ãƒ  Ã‚  and  de  are the most common prepositions required after verbs, there are others as well: contredansenparpoursurvers And finally, a number of French verbs dont require a preposition whereas their English equivalents do: no preposition   infinitiveno preposition   direct object Some French learners find it helpful to memorize lists of verbs by the prepositions they require, as provided above, while others prefer a master list of  alphabetized verbs.