Lecture 18 - MS Contacts and Schottky Diodes

Lecture 18 - MS Contacts and Schottky Diodes - 1 Metal...

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Unformatted text preview: 1 Metal Semiconductor Contacts and Diodes Objective: Understand the physics and models of semiconductor devices. Questions to be answered: How do we describe an ideal metal-semiconductor contact electrostatically? How do we differentiate between a rectifying and an ohmic contact? EE360 Lecture 18 ohmic contact? What phenomena determine the IV characteristics of an MS contact? Why do we need metal-semiconductor (MS) contacts? Metal-Semiconductor Contact: Metal p- or n- type semi. Ohmic contacts: MS contacts that behave like resistors connect semiconductor materials to the outside world Schottky (or MS) diodes: EE360 Lecture 18 MS contacts that behave like pn junctions (rectifying) low turn-on voltage, high speed, sometimes used for high-power devices 2 New Elements in the Band Diagram Vacuum level, E : an energy reference level that is the same for all materials. It represents the minimum energy of an electron From Pierret, pg. 478. removed from the material. Electron affinity, : the amount of energy required for an electron to move from the bottom of the conduction band to the vacuum level. (Given for a material) S = semiconductor work function EE360 Lecture 18 Work function, : the amount of energy required for an electron to move from the Fermi level to the vacuum level. (Depends on doping) = electron affinity, i.e. Si: = 4.03eV work function ) ( S FS C E E + = Band Diagram for a Metal Fermi level, E FM , is positioned inside the conduction band for a metal. Almost all states are filled up to E Almost all states are filled up to E FM. Vacuum Level Fermi Level M = metal work function, i.e. Al: M = 4.3eV EE360 Lecture 18 Au: M = 5.1eV From Pierret, pg. 478. 3 Band Diagrams for MS Contact 1. Consider metal and semiconductor separately (Is M > S or M < S ? How to form the band diagram for a MS contact: (Is M > S or M < S ?) 2. Use the vacuum level as a reference. 3. Align Fermi-levels in equilibrium. (Conduction and valence bands must bend) Preview: n-type EE360 Lecture 18 semiconductor, M > S From Pierret, pg. 479....
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Lecture 18 - MS Contacts and Schottky Diodes - 1 Metal...

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