A SiGe/Si Heterojunction Phototransistor for High Sensitivity Light Detection

Silicon-based heterojunction phototransistors (HPTs) with their advantages of high internal gain, high responsivity, and compatibility with CMOS processes have attracted much attention in high-sensitivity light detection. In this article, the SiGe/Si HPT with an illuminated area of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$50\times 50~\mu $ </tex-math></inline-formula>m2 for high responsivity and sensitivity was designed and fabricated. The optical responsivity of the fabricated SiGe/Si HPT reached 1.717 and 12.379 A/W for 405 and 650 nm, their specific detectivity values were <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.54\times 10^{{10}}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$11.16\times 10^{{10}}~\text {cm}\cdot \text {Hz}^{{0.5}}\cdot \text {W}^{-{1}}$ </tex-math></inline-formula>, respectively. An analytic model was developed to discuss current amplification for different wavelengths when considering absorption efficiency and carrier transportation. The emitter thickness was optimized as 60 nm to significantly improve the current amplification under short wavelengths. The achieved optical responsivity of the optimized SiGe/Si HPT for 405 and 650 nm respectively were 13.756 and 13.904 A/W, and the specific detectivity were <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$12.41\times 10^{{10}}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$12.54\times 10^{{10}}~\text {cm}\cdot \text {Hz}^{{0.5}} \cdot \text {W}^{-{1}}$ </tex-math></inline-formula>.