vb.net barcode reader code Figure 83 Current ow in a semiconductor in Software

Generate QR in Software Figure 83 Current ow in a semiconductor

Figure 83 Current ow in a semiconductor
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Figure 84 Doped semiconductor
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Semiconductors and Diodes
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Thus, free electrons are the majority carriers in an n-type material, while holes are the minority carriers In a p-type material, the majority and minority carriers are reversed Doping is a standard practice for a number of reasons Among these are the ability to control the concentration of charge carriers, and the increase in the conductivity of the material that results from doping
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THE pn JUNCTION AND THE SEMICONDUCTOR DIODE
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A simple section of semiconductor material does not in and of itself possess properties that make it useful for the construction of electronic circuits However, 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, a widely used circuit element Figure 85 depicts an idealized pn junction, where on the p side, we see a dominance of positive charge carriers, or holes, and on the n side, the free electrons dominate Now, in the neighborhood of the junction, in a small section called the depletion region, the mobile charge carriers (holes and free electrons) come into contact with each other and recombine, thus leaving virtually no charge carriers at the junction What is left in the depletion region, in the absence of the charge carriers, is the lattice structure of the n-type material on the right, and of the ptype material on the left But the n-type material, deprived of the free electrons, which have recombined with holes in the neighborhood of the junction, is now positively ionized Similarly, the p-type material at the junction is negatively ionized, because holes have been lost to recombination The net effect is that, while most of the material (p- or n-type) is charge-neutral because the lattice structure and the charge carriers neutralize each other (on average), the depletion region sees a separation of charge, giving rise to an electric eld pointing from the n side to the p side The charge separation therefore causes a contact potential to exist at the junction This potential is typically on the order of a few tenths of a volt and depends on the material (about 06 to 07 V for silicon) The contact potential is also called the offset voltage, V
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+ + p + + + Depletion region _ + _ + _ + _ + _ + _ + _ + _ _ n _
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The p-side depletion region is negatively ionized because its holes have recombined with free electrons from the n-side
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The n-side depletion region is positively ionized because its free electrons have recombined with holes from the p-side
Figure 85 A pn junction
In effect, then, if one were to connect the two terminals of the pn junction to each other, to form a closed circuit, two currents would be present First, a small current, called reverse saturation current, I0 , exists because of the presence of the contact potential and the associated electric eld In addition, it also happens
Part II
Electronics
that holes and free electrons with suf cient thermal energy can cross the junction This current across the junction ows opposite to the reverse saturation current and is called diffusion current, Id Of course, if a hole from the p side enters the n side, it is quite likely that it will quickly recombine with one of the n-type carriers on the n side One way to explain diffusion current is to visualize the diffusion of a gas in a room: gas molecules naturally tend to diffuse from a region of higher concentration to one of lower concentration Similarly, the p-type material for example has a much greater concentration of holes than the n-type material Thus, some holes will tend to diffuse into the n-type material across the junction, although only those that have suf cient (thermal) energy to do so will succeed Figure 86 illustrates this process The phenomena of drift and diffusion help explain how a pn junction behaves when it is connected to an external energy source Consider the diagrams of Figure 87, where a battery has been connected to a pn junction in the reverse-biased direction (Figure 87(a)), and in the forward-biased direction (Figure 87(b)) We assume that some suitable form of contact between the battery wires and the semiconductor material can be established (this is called an ohmic contact) The effect of a reverse bias is to increase the contact potential at the junction Now, the majority carriers trying to diffuse across the junction need to overcome a greater barrier (a larger potential) and a wider depletion region Thus, the diffusion current becomes negligible The only current that ows under reverse bias is the very small reverse saturation current, so that the diode current, iD (de ned in the gure), is iD = I0 (84)
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