A thermoelectric energy generator (TEG) with high area density, high Seebeck coefficient, and high-temperature gradient of thermocouples is developed by standard bipolar complementary metal–oxide–semiconductor (BiCMOS) process. The TEG has the stacked thermocouple design for high area density, the polysilicon germanium (poly-SiGe) thermocouple for better thermoelectric conversion efficiency, and the double cavity design for sufficient thermal gradient. It is shown that a $5\times $ 5 mm2 TEG chip with stacked thermocouples in optimal size $65\times 2\,\,\mu \text{m}$ at $2 \mu \text{m}$ width spacing, or about 3512 thermocouple/mm2, can achieve 0.131- $\mu \text{W}$ /cm $^{{2}}\text{K}^{{2}}$ power factor and 37.01-V/cm $^{{2}}\text{K}$ voltage factor in analysis, and $0.105 \mu \text{W}$ /cm $^{{2}}\text{K}^{{2}}$ and 33.91 V/cm $^{{2}}\text{K}$ in measurement. The thermocouple area density can be increased further by reducing the width spacing to $1.067 \mu \text{m}$ , or about 4802 thermocouple/mm2, to achieve $0.103 \mu \text{W}$ /cm $^{{2}}\text{K}^{{2}}$ and 36.19 V/cm $^{{2}}\text{K}$ in measurement. The optimal thermocouple size is $78\times 2\,\,\mu \text{m}$ to achieve $0.131 \mu \text{W}$ /cm $^{{2}}\text{K}^{{2}}$ and 44.95 V/cm $^{{2}}\text{K}$ in analysis. The performance increase is shown to be superior to all the other semiconductor TEGs.