SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

194 CHAPTER 4. JUNCTIONS IN SEMICONDUCTORS:P-NDIODES

InSb PbS Ge Si GaAs CdSe GaP CdS SiC GaN ZnS

HgCdTe GaAs1-yPy

Infrared Red Green Violet Ultraviolet

Orange Yellow Blue

λ (μm)

Human eye

repsonse

Eg(eV)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6

6.0 3.0 2.0 1.5 1.0 0.9 0.8 0.7 0.6 0.5 0.45 0.4 0.35

CdTe

Figure 4.29: The bandgap and cutoff wavelengths for several semiconductors. The semiconduc-
tor bandgaps range from 0 (for Hg 0. 84 Cd 0. 15 Te) to well above 3eV, providing versatile detection
systems.

materials using InGaN as the emission region for blue and green and GaAs-based AlGaInP for
the red region.

Substrate Availability:

Almost all optoelectronic light sources depend upon epitaxial crystal growth techniques where
a thin active layer (a few microns) is grown on a substrate (which is∼ 200 μm). The availabil-
ity of a high-quality substrate is extremely important in epitaxial technology. If a substrate that
lattice-matches to the active device layer is not available, the device layer may have dislocations
and other defects in it. These can seriously hurt device performance. One of the most important
opto-electronic materials for LEDs that has emerged lately is GaN. In spite of the lack of a native
substrate, GaN-based LEDs grown on either sapphire or SiC have become multi-billion dollar
industry. The reason is that the InGaN quantum well which is used as the emission region has
fluctuations which cause local energy minima for electron and holes. Thus radiative recombi-
nation is encouraged within this region and diffusion to and non-radiative recombination at a
dislocation minimized. This is shown schematically in figure 4.31. Furthermore, the dislocation
propagation and generation of dislocation sin GaN is very high because of the high bond energies
in the material. This eliminates one of the failure mechanisms in conventional LEDs and lasers,
that of generation and propagation of dislocations caused by absorption of emitted the photon
energy. The important substrates that are available for conventional light-emitting technology