CHAPTER3ChapterIII-EE477-SNAZ-Spring09

CHAPTER3ChapterIII-EE477-SNAZ-Spring09 - University of...

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University of Southern California OS VLSI Circuit Design: MOS VLSI Circuit Design: MOS Transistor Nazarian EE477L – Spring 2009
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MOSFET M etal O xide S emiconductor F ield E ffect T ransistor (MOSFET) is a unipolar: single carrier-type device The dominant dc current is carried by only one type: electrons (in nMOS) and holes (in pMOS) y contrast a BJT ( olar nction nsistor) depend By contrast, a BJT ( B ipolar J unction T ransistor) depend on both electrons and holes (but it is a minority carrier device) Because its output current is carried by injected minority carriers which pass through a thin base yer & are collected by a reverse iased p/n layer & are collected by a reverse-biased p/n junction in the output MOS occupies smaller silicon area than BJT and with USC/EE477L/Spring 2009 2 p fewer fabrication steps
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Two-Terminal MOS Structure Under thermal equilibrium (Mass Action Law) ) 10 45 . 1 ( 3 10 2 × = cm n n np If substrate is uniformly doped @ NA (Typically 10 15 to 10 16 ) p i i n 2 USC/EE477L/Spring 2009 3 A i p A p N n N p 0 0 =
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NMOS – Accumulation USC/EE477L/Spring 2009 4
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NMOS – Depletion USC/EE477L/Spring 2009 5
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NMOS – Inversion USC/EE477L/Spring 2009 6
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Band Theory – Background I Valence band : The band made up of the occupied molecular rbitals and is lower in energy orbitals and is lower in energy than conduction band. It is generally completely full in semi- conductors. When heated, , electrons from this band jump out across the band gap and into the conduction band, making the material conductive Conduction band : The band of rbitals that are high in energy orbitals that are high in energy and generally empty. In semiconductors, it is the band at accepts electrons from the USC/EE477L/Spring 2009 7 that accepts electrons from the valence band
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Band Theory – Background II Band gap : For insulators and semiconductors, the band gap generally refers to the energy difference btn the top of the valence band and the bottom of the conduction band; it is the amount of energy required to free an outer hell electron from its orbit about shell electron from its orbit about the nucleus to the conduction band A material with a small, positive band gap (< 3 eV) is referred to as a semiconductor. A material with a rge band gap is called an USC/EE477L/Spring 2009 8 large band gap is called an insulator
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Band Theory – Fermi Level I Fermi Level Each of the many distinct energies with which an electron can be held within a solid is called an energy level . According to the quantum mechanic ws each energy level can accommodate only a laws, each energy level can accommodate only a limited number of electrons Fermi level : A measure of the energy of the least ghtly held electrons within a solid named for tightly held electrons within a solid, named for Enrico Fermi Fermi level is important in determining the lectrical and thermal properties of solids electrical and thermal properties of solids The value of the Fermi level at absolute 0K (i.e., 273.15 °C) is called Fermi energy and is a constant
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This note was uploaded on 07/21/2010 for the course EE 477L taught by Professor Parker during the Spring '08 term at USC.

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CHAPTER3ChapterIII-EE477-SNAZ-Spring09 - University of...

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