lec3 - 6.012 - Microelectronic Devices and Circuits - Fall...

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Unformatted text preview: 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 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? 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: c collision time [ s ] In between collisions, carriers acquire high velocity: v th thermal velocity [ cm/s ] ...but get nowhere! 6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 3-4 Characteristic length of thermal motion: mean free path [ cm ] = v th c Put numbers for Si at 300 K: c 10- 14 10- 13 s v th 10 7 cm/s 1 10 nm For reference, state-of-the-art MOSFETs today: L g 50 nm carriers undergo many collisions in modern devices 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 =- qE m n t for electrons v ( t ) = qE m p t for holes 6.012 - Microelectronic Devices and Circuits - Fall 20056....
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lec3 - 6.012 - Microelectronic Devices and Circuits - Fall...

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