Ee3161 spring10 hw2

Info iconThis preview shows page 1. Sign up to view the full content.

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

Unformatted text preview: Pierret pp. 105-138 Lecture Note 5 Carrier Dynamics Light absorption Generation/recombination Capacitor analogy Photo-generation Minority carrier diffusion Examples Quasi Fermi level EE3161 Semiconductor Devices Sang-Hyun Oh 1 UNIVERSITY OF MINNESOTA Cover Image: Carrier Dynamics “Crossing the border” Positive holes from the p-side (red) of a semiconductor (GaAs) p-n junction increasingly diffuse into the n-side (blue) as voltage across the junction is increased. Imaged with a scanning tunneling microscope (STM) at a resolution of ~10 nanometers. Shoji Yoshida et al. Phys. Rev. Lett. 98, 026802 (2007) EE3161 Semiconductor Devices Sang-Hyun Oh 2 UNIVERSITY OF MINNESOTA Themes The main topic of our course is to apply a voltage to a device to inject holes and electrons; then watch how the conductivity is changed. It’s simpler to first put the excess carriers in the semiconductor by photo-excitation. From Pierret: “Minority carrier diffusion into a sea of majority carriers might be likened to a small group of animals attempting to cross a piranha-infested stretch of the Amazon River.” Their distribution falls off exponentially. EE3161 Semiconductor Devices Sang-Hyun Oh 3 UNIVERSITY OF MINNESOTA Light Absorption I (x) − I (x + dx) = α dx I (x) dI = −αdx I I = I0 e−αx EE3161 Semiconductor Devices Sang-Hyun Oh 4 UNIVERSITY OF MINNESOTA Illumination Assume that light is uniformly incident on the surface of a semiconductor. If αL ≪ 1, then light is absorbed uniformly throughout the semiconductor GL= #/vol/sec of electron-hole pairs generated. EE3161 Semiconductor Devices Sang-Hyun Oh 5 UNIVERSITY OF MINNESOTA Absorption vs. Bandgap Ephoton Eg ? GaAs Eg=1.42 eV Ge Eg=0.66 eV Silicon Eg=1.12 eV Quiz: When each semiconductor material is illuminated with photons with energy=1.2 eV, which materials are transparent? Why? EE3161 Semiconductor Devices Sang-Hyun Oh 6 UNIVERSITY OF MINNESOTA Notation n = carrier concentration under arbitrary conditions. n0 = equilibrium carrier concentration ∆n = (n - n0) = excess carrier concentration GL=external light that generates electron-hole pairs EE3161 Semiconductor Devices Sang-Hyun Oh 7 UNIVERSITY OF MINNESOTA Energy Picture ∂ n(x, t) ￿ ∂ p(x, t) ￿ ￿ ￿ GL = GL (x, λ) = = ￿ ￿ ∂t ∂t light light (# additional electrons) = (# additional holes) EE3161 Semiconductor Devices Sang-Hyun Oh 8 UNIVERSITY OF MINNESOTA Law of Mass Action: Revisited The equilibrium carrier concentration is not a...
View Full Document

This note was uploaded on 02/24/2010 for the course EE 3161 taught by Professor Prof.sang-­hyunoh during the Spring '10 term at University of Minnesota Crookston.

Ask a homework question - tutors are online