{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

MIT6_012S09_lec03

# MIT6_012S09_lec03 - Lecture 3 Semiconductor Physics(II...

This preview shows pages 1–6. Sign up to view the full content.

Lecture 3 Semiconductor Physics (II) Carrier Transport Outline • Thermal Motion • Carrier Drift • Carrier Diffusion Reading Assignment: Howe and Sodini; Chapter 2, Sect. 2.4-2.6 6.012 Spring 2009 Lecture 3 1

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
1. Thermal Motion Undergo collisions with vibrating Si atoms ( Brownian motion ) Electrostatically interact with each other and with ionized (charged) dopants In thermal equilibrium, carriers are not sitting still: Characteristic time constant of thermal motion: mean free time between collisions τ c collison time [ s ] In between collisions, carriers acquire high velocity: v th thermal velocity [ cms 1 ] …. but get nowhere! 6.012 Spring 2009 Lecture 3 2
Characteristic length of thermal motion: λ ≡ mean free path [cm] λ = v th τ c Put numbers for Si at room temperature: τ c 10 13 s v th 10 7 cms 1 λ 0.01 μ m For reference, state-of-the-art production MOSFET: L g ≈ 0.1 µm Carriers undergo many collisions as they travel through devices 6.012 Spring 2009 Lecture 3 3

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
2. Carrier Drift Apply electric field to semiconductor: E electric field [V cm -1 ] net force on carrier F = ±qE Between collisions, carriers accelerate in the direction of the electrostatic field: v ( t ) = a t = ± qE m n , p t E 6.012 Spring 2009 Lecture 3 4
But there is (on the average) a collision every τ c and the velocity is randomized: The average net velocity in direction of the field: v = v d = ± qE 2 m n , p τ c = ± q τ c 2 m n , p E This is called drift velocity [cm s -1 ]

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}