3.6. CARRIER TRANSPORT BY DIFFUSION 119
a) b)Figure 3.15: Temperature dependence of scattering rates in 2DEGs for (a) InGaAs and (b) GaN
Table 1: Scattering Comparison
Point A B
GaN GaAs InGaAs GaN GaAs InGaAs
Energy (eV) 0.2 0.2 0.2 0.8 0.8 0.8
Optical phonon emission 8.85x 10^13 6.60x 10^12 6.33x 10^12 8.95x 10^13 6.35x 10^12 7.00x 10^12
Optical phonon absorption 3.48x 10^12 1.79x 10^12 2.08x 10^12 2.88x 10^12 1.61x 10^12 2.11x 10^12
acoustic Phonon 1.50x 10^12 3.44x 10^11 1.64x 10^11 3.23x 10^12 9.06x 10^11 5.69x 10^11
Alloy scattering 0.0 0.0 9.56x 10^10 0.0 0.0 4.23x 10^11
ionized impurity 1.47x 10^13 7.41x 10^12 6.41x 10^12 8.47x 10^12 5.11x 10^12 5.08x 10^12
dislocation 2.33x 10^12 7.35x 10^8 4.90x 10^8 2.33x 10^12 7.35x 10^8 4.90x 10^8
Nonequivalent intervalley
emission(Γ–L) 0.0 0.0 0.0 0.0 3.42x 10(^13) 1.23x 10 13
Nonequivalent intervalley
absorption(Γ–L) 0.0 0.0 0.0 0.0 1.15x 10
(^13) 5.46x 10 12
Total (s-1) 1.11x 10^14 1.61x 10^13 1.51x 10^13 1.06x 10^14 5.97x 10^13 3.30x 10^13
Table 3.3: Scattering rate mechanisms in InGaAs, GaAs, and GaN 2-DEG channels
there is a gradient in the concentration of a species of mobile particles, the particles diffuse from
the regions of high concentration to the regions of low concentration. As the mobile charges
move they suffer random collisions, as discussed in the previous section. The collision process
can be described by the mean free path and the mean collision timeτsc. The mean free path
is the average distance the electron (hole) travels between successive collisions. In between the
collisions the electrons move randomly, with equal probability of moving in any direction (there
is no electric field). We are interested in finding out how the electrons move (diffuse) when there
is a concentration gradient in space.