This preview shows page 1. Sign up to view the full content.
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.
- Spring '10