lecture3anotat - 6.012 - Microelectronic Devices and...

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6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 3-1 Lecture 3 - Semiconductor Physics (II) Carrier Transport September 15, 2005 Contents : 1. Thermal motion 2. Carrier drift 3. Carrier diffusion Reading assignment: Howe and Sodini, Ch. 2, §§ 2.4-2.6
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6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 3-2 Key questions What are the physical mechanisms responsible for cur- rent flow in semiconductors? How do electrons and holes in a semiconductor behave in an electric field? How do electrons and holes in a semiconductor behave if their concentration is non-uniform in space?
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6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 3-3 1. Thermal Motion In thermal equilibrium, carriers are not sitting still: undergo collisions with vibrating Si atoms ( Brownian motion ) electrostatically interact with charged dopants and with each other Characteristic time constant of thermal motion - mean free time between collisions: τ c collision time [ s ] In between collisions, carriers acquire high velocity: v th thermal velocity [ cm/s ] ...but get nowhere!
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6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 3-4 Characteristic length of thermal motion: λ mean free path [ cm ] λ = v th τ c Put numbers for Si at 300 K: τ c ± 10 14 10 13 s v th ± 10 7 cm/s λ ± 1 10 nm For reference, state-of-the-art MOSFETs today: L g ± 0 . 1 µm carriers undergo many collisions in modern devices
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6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 3-5 2. Carrier Drift Apply electric field to semiconductor: E electric field [ V /cm ] net force on carrier F = ± qE E - Between collisions, carriers accelerate in direction of field: v ( t )= at = t for electrons m n v ( t t for holes m p
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6.012 - Microelectronic Devices and Circuits
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This note was uploaded on 07/20/2009 for the course ELECTRICAL 6.012 taught by Professor Prof.jesúsdelalamo during the Fall '05 term at MIT.

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lecture3anotat - 6.012 - Microelectronic Devices and...

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