234
8 Transport
(a)
(b)
Fig. 8.8 a Carrier concentration and b conductivity of n-type Ge as a function of temperature. The doping level varies
from N D ≈ 10 13 to 10 18 (samples A–F as in Fig. 8.5a where the mobility of the same samples is shown). The dashed
lines are for intrinsic Ge. The solid lines are guides to the eye. Adapted from [594]
Fig. 8.9 a Electron mobility in Si:P at room temperature over a wide range of carrier concentrations. b Electron mobility
in Si:P and hole mobility in Si:B for various high carrier concentrations. Adapted from [739]
to ionized impurity scattering. At high doping level, it becomes more important at room temperature
than (acoustical or optical) phonon scattering [738]. The mobility of carriers in n- and p-type silicon
with very high carrier concentrations is depicted in Fig. 8.9b.
Thus, for bulk material high carrier density and high mobility are contrary targets and cannot be
achieved simultaneously. A solution will be provided with the concept of modulation doping where
the dopants and the (two-dimensional) carrier gas will be spatially separated in a heterostructure (cf.
Sect. 12.3.4).
At high doping, the substitutional character of the impurities may be lost and secondary phases
can arise, e.g. as observed for highly doped ZnO:Ga, exhibiting octahedral coordination of gallium
8 Transport
(a)
(b)
Fig. 8.8 a Carrier concentration and b conductivity of n-type Ge as a function of temperature. The doping level varies
from N D ≈ 10 13 to 10 18 (samples A–F as in Fig. 8.5a where the mobility of the same samples is shown). The dashed
lines are for intrinsic Ge. The solid lines are guides to the eye. Adapted from [594]
Fig. 8.9 a Electron mobility in Si:P at room temperature over a wide range of carrier concentrations. b Electron mobility
in Si:P and hole mobility in Si:B for various high carrier concentrations. Adapted from [739]
to ionized impurity scattering. At high doping level, it becomes more important at room temperature
than (acoustical or optical) phonon scattering [738]. The mobility of carriers in n- and p-type silicon
with very high carrier concentrations is depicted in Fig. 8.9b.
Thus, for bulk material high carrier density and high mobility are contrary targets and cannot be
achieved simultaneously. A solution will be provided with the concept of modulation doping where
the dopants and the (two-dimensional) carrier gas will be spatially separated in a heterostructure (cf.
Sect. 12.3.4).
At high doping, the substitutional character of the impurities may be lost and secondary phases
can arise, e.g. as observed for highly doped ZnO:Ga, exhibiting octahedral coordination of gallium