Lecture 3 - MIT

Spring 2007 Lecture 31 Lecture 3 Semiconductor Physics (II)Carrier TransportOutline Thermal Motion Carrier Drift Carrier DiffusionReading Assignment:Howe and Sodini; Chapter 2, Sect. Spring 2007 Lecture 321. Thermal Motion Undergo collisions with vibrating Si atoms (Brownian motion) Electrostatically interact with each other and with ionized (charged) dopantsIn thermal equilibrium, carriers are not sitting still:Characteristic time constant of thermal motion: mean free time between collisions c collison time[s]In between collisions, carriers acquire high velocity:vth thermal velocity[cms 1].. but get nowhere! Spring 2007 Lecture 33 Characteristic length of thermal motion: mean free path[cm] =vth cPut numbers for Si at room temperature: c 10 13svth 107cms 1 mFor reference, state-of-the-art production MOSFET:Lg m Carriers undergo many collisions as they travel through Spring 2007 Lecture 342.

of field time τc. 6.012 Spring 2007 Lecture 3 6 ... (Si Crystal) • A gradient of particles (electrons and holes) inside the medium • Collisions between particles and medium send particles off in random directions – Overall result is to erase gradient n x. 6.012 Spring 2007 Lecture 3 12 Fick’s first law-Key diffusion relationship

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