Due to the temperature dependence of the generation rate, it is beneficial to maintain large areas of high temperature, as pointed out in the last section. However, carrier generation can be further improved by the introduction of traps in the forbidden energy gap.
According to the Shockley-Read-Hall formalism [289], thermal generation is affected by local temperature as well as the amount and energy level of present traps. For trap energy levels at the mid band gap, the thermal generation reaches its maximum. For silicon, gold can be used as additional dopant in the generation region of the device to introduce deep levels close to mid band gap [300]. Since the impurity state can absorb differences in momentum between the carriers, this generation process is the dominant one in silicon and other indirect semiconductors. To some extent, the device performance of a pn-junction thermoelectric generator at a certain temperature can be shifted to lower temperatures by adaption of the additional trap density and distribution.
![]() |
As an example, a thin film thermoelectric generator based on silicon is
investigated. The device consists of a p-doped substrate with a dopant
concentration of
and an n-doped layer with a dopant
concentration of
, resulting in a pn-junction, which is
located at a depth of
. In order to increase the power output,
gold has been implanted as additional generation centers at the hotter end of
the device. The device considered is
long and has a width of
.
Fig. 6.35 illustrates the power output of the device
at different thermal conditions. The dashed line represents the variation of
the maximum power output with temperature. For higher temperatures, carrier
generation is more pronounced and thus the power output increases
significantly, while at the same time, the inner resistance reduces. The
temperature scale along the dashed line has been calibrated by the external
thermal resistances as well as the exact trap density within the device. A
maximum power output of
has been measured for a hot end
temperature of
, while a temperature of
increases
the output to a maximum of
. The dotted line depicts the
simulated power output of the similar structure without additional traps. For
this device configuration, a maximum power output of
is
predicted.
M. Wagner: Simulation of Thermoelectric Devices