Heterostructures

1
Heterostructures Optoelectronic Devices

• Light generation in semiconductor
• Light emission diode (LED)
• Semiconductor laser
• Heterojunction transistors
• Photo-detector

2
Compound Semiconductor DeviceMarket
3
Light Emission in Semiconductors
E
electron
conduction band
-
h?
Band gap
Electron-hole recombination

valence band
hole
Si Eg 1.1 eV GaAs Eg 1.4 eV, ? 880
nm AlAs Eg 2.2 eV, ? 565 nm
Si indirect bandgap, ineffective GaAs direct
bandgap, effective
4
Electron-Hole Recombinationand photon Energy
If the electron-hole recombination occurs not at
the minimum gap point, the emitted photon can
have a higher energy. When the recombination
involves an impurity state in the gap,
lower-energy photon is generated.
h? gt Eg
5
Light Emission Diode
More efficient heterojunction LED
Simple PN Junction LED
6
Lattice Matching in Heteroepitaxy
7
Semiconductor Laser
Lateral confinement by proper electrode design
8
Heterojunction and HEMT
Band Offset
High electron-mobility transistor (HEMT) with
electron gas from the heavily-doped AlGaAs layer
moving in the undoped GaAs channel
9
Quantum Well and Superlattice
Bound states in quantum well to mini-band in
superlattice
Strained superlattice
Adjustable bandgap
10
III-V Nitride Optoelectronic Devices
(see S.C. Jain et al., J. Appl. Phys. 87 (2000)
965)
11
III-V Nitride Optoelectronic Devices
GaN on sapphire with an AlN buffer layer
GaN-InGaN DH LED
12
Large-scale application of LED
13
Photo-detector
E
electron
conduction band
-
h?
Band gap
Reverse biased!
Electron-hole generation

valence band
hole
Si Eg 1.1 eV, ?c 1130 nm
Simple detector conductivity increase of
semiconductor when illuminated.
P-I-N photo-detector low dark current,
quick response.