Handout_4_-_Diode_Circuits

Handout_4_-_Diode_Circuits - EE 101A Winter 10 EE 101A...

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1 EE 101A / Winter 10 Handout #4 EE 101A Analysis of Diode Circuits Reading – Sedra & Smith, Sections 3.1 – 3.4 1. Semiconductor Diode (reference : Sedra & Smith, Section 3.7) 1.1 Semiconductor An atom is composed of one or more shells of (negatively charged) electrons surrounding a (positively charged) nucleus. The electrons in the inner shells are tightly bonded to the nucleus. Hydrogen Atom Silicon Atom When atoms are structured to form a crystal, the bonding of the electrons in the outermost shell (valence electrons) determines the electrical conductivity of the crystal. In diamond (carbon crystal), the valence electrons are tightly bonded and cannot move around. Diamond is an insulator. In graphite, a different form of carbon crystal, the valence electrons are loosely bonded and can easily move around. Graphite is a conductor. In aluminum, the valence electrons are loosely bonded and can easily move around under an electric field. Aluminum is a conductor.
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2 Crystal Structure of Silicon and Diamond In silicon, the valence electrons are intermediately bonded. At room temperature, about 1 in 10 12 can move around. Silicon is a semi-conductor. Note that aluminum, diamond and silicon all have face-centered cubic (FCC) structure; diamond and silicon have the same exact crystal structure. Silicon is in column IV of the periodic table. Each silicon atom has 4 valence electrons. If a small amount of column V impurity (with 5 valence electrons, e.g., phosphorus P, arsenic As, antinomy Sb) is introduced to replace silicon atoms in a crystal, the 5 th electron is loosely bonded and can move around. The silicon now has excess FREE electrons, and is called n-type (n stands for negative free charge). The resistively of a n-type silicon is determined by the amount of impurity and can be varied over many orders of magnitudes. If a small amount of column III impurity (with 3 valence electrons, e.g.,
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Handout_4_-_Diode_Circuits - EE 101A Winter 10 EE 101A...

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