ECE3080-L-11b-SchottkyDiodesAndHeterojunctions

ECE3080-L-11b-SchottkyDiodesAndHeterojunctions - Lecture...

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ECE 3080 - Dr. Alan Doolittle Georgia Tech Lecture 11b Schottky Diodes Heterojunctions and NDR Diodes Reading: (Cont’d) Notes and Anderson 2 Chapter 6 Some images from Anderson and Anderson text and Dr. Fred Schubert
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ECE 3080 - Dr. Alan Doolittle Georgia Tech •Why a Schottky? •Minority Carrier Charge storage in p-n junctions tends to limit the switching times of p-n junction diodes •Turn off times limited by minority carrier lifetimes •Schottky Diodes have little (no) minority carrier stored charge and thus have application in fast switching applications (motors, etc…) •Cheaper •Disadvantages: •Generally higher leakage currents •Generally lower breakdown voltages
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ECE 3080 - Dr. Alan Doolittle Georgia Tech Metal-semiconductor (MS) junctions •P-N junctions formed depletion regions by bringing together two materials with dissimilar fermi energies, allowing charge transfer and subsequent alignment of the energy bands. •Several other combinations of such materials can also form “useful junctions”. – Schottky Diodes (metal-semiconductor junction) – Ohmic contacts (metal-semiconductor junction) – Thermocouples (metal-metal junction)
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ECE 3080 - Dr. Alan Doolittle Georgia Tech Ideal Metal-Semiconductor Contacts Assumptions - Ideal MS contacts •Metal (M) and Semiconductor (S) are in intimate contact, on atomic scale •No oxides or charges at the interface (very bad assumption in some cases – some interfaces are dominated by interfacial oxides or interface charge). •No intermixing at the interface (in some cases, it is impossible to put a metal on a semiconductor without some exchange of atoms – intermixing- occurring) •These assumptions require ultra-clean interfaces otherwise non-ideal behavior results (fermi-level pinning of III-V compounds is common for example)
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ECE 3080 - Dr. Alan Doolittle Georgia Tech Definitions Vacuum level, E 0 - corresponds to energy of free electrons in vacuum. • The difference between vacuum level and Fermi-level is called workfunction, of materials. Workfunction, M is an invariant property of a given metal . It is the minimum energy required to remove electrons from the metal. (Lowest value is 1.95eV for Cs, 3.66eV for Mg, 5.15eV for Ni, and highest value is 5.7eV for Pt, etc.). Electron density varies with crystallographic orientation so the work function varies with orientation as well. • However, since the electron concentration depends on doping in a semiconductor, the semiconductor workfunction, s , depends on the doping. where = ( E 0 E C )| SURFACE is a a fundamental property of the semiconductor. (Example: = 4.0 eV, 4.03 eV and 4.07 eV for Ge, Si and GaAs respectively) FB F C s ) ( E E
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ECE 3080 - Dr. Alan Doolittle Georgia Tech Energy band diagrams for ideal MS contacts M > S M < S An instant after contact formation Under equilibrium conditions Schottky Ohmic
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ECE 3080 - Dr. Alan Doolittle Georgia Tech MS (n-type) contact with M > S • Soon after the contact formation, electrons will begin to flow from the semiconductor to the metal.
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This note was uploaded on 08/23/2011 for the course ECE 3080 taught by Professor Staff during the Spring '08 term at Georgia Institute of Technology.

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ECE3080-L-11b-SchottkyDiodesAndHeterojunctions - Lecture...

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