Diodes and Circuit Applications

Diodes and Circuit - Diodes and Circuit Applications 1 pn junction When p-type and n-type semiconductors are brought in contact a pn-junction is

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Diodes and Circuit Applications
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1- pn junction When p-type and n-type semiconductors are brought in contact a pn-junction is formed which is the basis of the semiconductor diode, a widely used circuit element.
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The charge separation causes a contact potential to exist at the junction. This potential is typically on the order of few tenths a volt and depends on the material (for Silicon about 0.6 to 0.7 V). The contact potential is called the offset voltage V γ .
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The existing of electric field across the junction helps the minority carriers (holes in the n-type and electron in the p-type) to drift across the depletion region creating a small reverse saturation current I s flows in the reverse direction when the diode is reverse biased. I s is independent of the junction voltage and is determined by thermal carrier generation. At room temperature, I s is in the order of nanoamperes (10 -9 A) for Silicon. Another current caused by diffusion of majority carriers (holes in the p-type and electrons in the n- type) across the junction called diffusion current I d . The diffusion current flows in the forward direction and is largely dependent on junction voltage, thus, it increases as the forward bias voltage increases.
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S D I I i = = 0 Reverse-biased diode The effect of the reverse bias is to increase the contact potential. The only current is the drift current Forward-biased diode The forward voltage acts in opposition to the contact potential therefore, the diffusion current is aided by the applied voltage KT qv d D e I I / 0 =
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The net forward current is, diode equation where, v D is the voltage across the pn-junction, K=1.38 x 10 -23 J/K is Boltizman’s constant, q is the charge of one electron, and T is the temperature in kelvins. Since I 0 is very small, i D is approximated to, The diode is essentially conducts current in only one direction that is when the junction is forward-biased ) 1 ( / 0 0 = = KT qv d D D e I I I i KT qv D D e I i / 0 =
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Diode i-v characteristics
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We can summarize the diode behaviour as follows: v D V γ the diode is forward-biased (on), conducts current, and acts as a short circuit. -V Z v D V the diode is reverse-biased (off), acts as an open circuit, and allows a very small reverse current to flow I 0 . v D -V Z the diode operates in the reverse breakdown region, conducts a current in the reverse direction. Note that the reverse breakdown region represents the behaviour of the diode when a sufficiently high reverse bias voltage is applied. Aided by the electric field, sufficient energy is imparted to the charge carriers that reverse current can flow. The process of conduction is very like an avalanche breakdown in which a single electron can ionize others.
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2-Diode circuit models 2.1-Ideal diode model Ideal diode is a theoretical model for the practical diode. The diode is modelled as a simple on/off device.
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To determine the conduction state of an ideal diode: 1. assume a diode conduction state (on or off).
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This note was uploaded on 02/20/2011 for the course EES 612 taught by Professor Karim during the Fall '10 term at Ryerson.

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Diodes and Circuit - Diodes and Circuit Applications 1 pn junction When p-type and n-type semiconductors are brought in contact a pn-junction is

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