Lecture 3 - MIT

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Spring 2007Lecture 31Lecture 3Semiconductor Physics (II)Carrier TransportOutline Thermal Motion Carrier Drift Carrier DiffusionReading Assignment:Howe and Sodini; Chapter 2, Sect. Spring 2007Lecture 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].

6.012 Spring 2007 Lecture 3 10 Numerical Example: Si with Nd = 3 x 1016 cm-3 at room temperature µn ≈1000 cm2 / V •s ρncm n ≈3X10 16 cm−3 Apply E = 1 kV/cm 3 2 6 4.8 10 / 10 / J A cm E J qnv qn E E v cm s v drift n dn n drift n dn th ≈ × ≈ = µ = = ≈ << ρ σ Time to drift through L = 0.1 µm td = L vdn =10 ps fast!

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