6.5.2 Generation Enhancement by Additional Traps

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.

Figure 6.35: Power output of a thin film thermoelectric generator at different hot end temperatures.

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 $10^{19}\,\ensuremath{\mathrm{cm}}^{-3}$ and an n-doped layer with a dopant concentration of $10^{20}\,\ensuremath{\mathrm{cm}}^{-3}$ , resulting in a pn-junction, which is located at a depth of $2.5\,\mu\ensuremath{\mathrm{m}}$ . 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 $30\,\ensuremath{\mathrm{mm}}$ long and has a width of $20\,\ensuremath{\mathrm{mm}}$ .

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 $19\,\mu\ensuremath{\mathrm{W}}$ has been measured for a hot end temperature of $535\,\ensuremath{\mathrm{K}}$ , while a temperature of $596\,\ensuremath{\mathrm{K}}$ increases the output to a maximum of $115\,\mu\ensuremath{\mathrm{W}}$ . The dotted line depicts the simulated power output of the similar structure without additional traps. For this device configuration, a maximum power output of $91\,\mu\ensuremath{\mathrm{W}}$ is predicted.

M. Wagner: Simulation of Thermoelectric Devices