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Unformatted text preview: 6.720J/3.43J Integrated Microelectronic Devices Spring 2007 Lecture 81 Lecture 8 Carrier Drift and Diffusion (cont.) , Carrier Flow February 21, 2007 Contents: 1. QuasiFermi levels 2. Continuity equations 3. Surface continuity equations Reading assignment: del Alamo, Ch. 4, § 4.6; Ch. 5, §§ 5.1, 5.2 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 82 Key questions • Is there something equivalent to the Fermi level that can be used outside equilibrium? • How do carrier distributions in energy look like outside equilib rium? • In the presence of carrier ﬂow in the bulk of a semiconductor, how does one formulate bookkeeping relationships for carriers? • How about at surfaces? 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 83 1. QuasiFermi levels Interested in energy band diagram representations of complex sit uations in semiconductors outside thermal equilibrium . In TE, Fermi level makes statement about energy distribution of carriers in bands ⇒ E F relates n o with N c and p o with N v : E F − E c E v − E F n o = N c F 1 / 2 ( ) p o = N v F 1 / 2 ( ) kT kT Outside TE, E F cannot be used. Define two ”quasiFermi levels” such that: E fe − E c E v − E fh n = N c F 1 / 2 ( ) p = N v F 1 / 2 ( ) kT kT Under MaxwellBoltzmann statistics ( n N c , p N v ): E fe − E c n = N c exp kT E v − E fh p = N v exp kT What are quasiFermi levels good for? 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 84 Take derivative of n = f ( E fe ) with respect to x : E fe − E c n = N c exp kT dn n dE fe dE c n dE fe q = ( − ) = − n E dx kT dx dx kT dx kT Plug into current equation: dn J e = qμ e n E + qD e dx To get: dE fe J e = μ e n dx Similarly for holes: dE fh J h = μ h p dx Gradient of quasiFermi level: unifying driving force for carrier ﬂow ....
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This note was uploaded on 09/24/2010 for the course EECS 6.720J taught by Professor Jesúsdelalamo during the Spring '07 term at MIT.
 Spring '07
 JesúsdelAlamo

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