note2 - Handout 2 Overview of Semiconductors and P-N...

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Handout 2 ECE 315, Cornell University 1 Overview of Semiconductors and P-N Junctions What you will learn: • Intrinsic (pure) and extrinsic (doped) semiconductors. • Carrier statistics in equilibrium and under bias. • Basic differential equations to model the semiconductor devices. • Electrostatics of P-N junction diodes • Diode IV: reverse bias, reverse-bias with injection, weak forward-bias, resistive. Handout 2
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Handout 2 ECE 315, Cornell University 2 2.1. Statistics of Electrons and holes in homogeneous semiconductors • Electrons and holes in intrinsic semiconductors • N-type and P-type doping semiconductors • Electrons and holes in equilibrium and quasi-neutral relations. • Carrier statistics in N-type and P-type semiconductors. Sedra and Smith: Sec. 3.7
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Handout 2 ECE 315, Cornell University 3 • Semiconductor is the class of material where the conductivity of the material can be controlled to vary a large orders of magnitude, i.e., > 10 10 , (metal: conductivity always high; insulator: conductivity always low). – Elemental semiconductor: Si, Ge – Compound semiconductor (fixed composition): SiC, GaAs, GaN – Alloy: Si 1-x Ge x , Al 1-x Ga x As, Hg 1-x Cd x Te, etc. • Purity: semiconductor devices are made of semiconductors with ultra purity: unintentional doping < 10 -9 in part, i.e., 0.001 ppm in solids – To increase range of control in conductivity (lower leakage when devices are switched off, faster switching when devices are on) – To increase mobility (faster and less power consumption from lower resistivity) Introduction to Semiconductors
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Handout 2 ECE 315, Cornell University 4 • We ignore the transition metals here (things with d orbital in the valence band). They are surely important, just more complex. The Simple Periodic Table
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Handout 2 ECE 315, Cornell University 5 Actual 3D Structure of a Silicon Crystal Lattice covalent bonds • Each Silicon atom is surrounded by 4 other Silicon atoms in a tetrahedral configuration. • Silicon atomic density 5 × 10 22 cm -3
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Handout 2 ECE 315, Cornell University 6 Intrinsic Si with Thermally Generated Electrons and Holes This picture is known as the bond model… There is another model, which is quantitatively more elegant, called the band model. Please take 306 then 457 to know more. Intrinsic (undoped): n = p n : electron concentration (cm -3 ) p : hole concentration (cm -3 ). Fig. 3.41
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Handout 2 ECE 315, Cornell University 7 n = p by this thermal process! • The energy required to break up the electrons and holes is a measure of the “bandgap”: E gap . • Insulator (in room temperature) : E gap > 4eV (SiO 2 : 9.1eV, Si 3 N 4 : ~5eV). The larger the E gap , the better the isolation. • Conductor: E gap < 3 kT ~ 0.1eV, or even negative bandgap Semiconductor : anything in between, depending on what temperature we are talking about.
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This note was uploaded on 02/02/2008 for the course ECE 3150 taught by Professor Spencer during the Fall '07 term at Cornell University (Engineering School).

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note2 - Handout 2 Overview of Semiconductors and P-N...

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