lecture19

lecture19 - 6.720J/3.43J Integrated Microelectronic Devices...

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Unformatted text preview: 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 Lecture 19-1 Lecture 19 - Metal-Semiconductor Junction (cont.) March 19, 2007 Contents: 1. Schottky diode 2. Ohmic contact Reading assignment: del Alamo, Ch. 7, §§ 7.3-7.5 Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 Lecture 19-2 Key questions • What is the basic structure of a Schottky diode? What are its most important parasitics? • What are key technological constraints in the design and fabri- cation of Schottky diodes? • How are Schottky diodes modeled for circuit design? • How do Schottky diodes switch? What sets their time response? • What does one have to do for a metal-semiconductor junction to become an ohmic contact? • Why do ohmic contacts look as S = ∞ for minority carriers? Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 Lecture 19-3 1. Schottky diode Key uniqueness: fast switching from ON to OFF and back. Widely used: • in analog circuits: in track and hold circuits in A/D converters, pin drivers of IC test equipment • in communications and radar applications: as detectors and mix- ers, also as varactors Technological constraint: Schottky diodes engineered using process modules developed for other circuit elements → demands resource- fulness and imagination from device designer. Typical implementations: n + n + n Schottky metal n + n Schottky junction ohmic contact ohmic contact semi-insulating GaAs p Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 Lecture 19-4 Parasitics Series resistance due to QNR ohmic drop Voltage across junction is reduced and I-V characteristics modified: q ( V − IR s ) I = I S [exp − 1] kT ideal with series resistance I log |I| IR s 1/R s 0 V V linear scale semilogarithmic scale R s bad because: • for given forward current, V increased and harder to control • it degrades dynamic response of diode Cite as: Jesús del Alamo, course materials for 6.720J Integrated Microelectronic Devices, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. 6.720J/3.43J - Integrated Microelectronic Devices - Spring 2007 Lecture 19-5 Substrate capacitance I s C n + n R s n + p parasitic substrate p-n diode Also degrades dynamics of diode....
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lecture19 - 6.720J/3.43J Integrated Microelectronic Devices...

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