Scattering-Engineered Ge-on-Si Avalanche Photodiodes with Low Operating Voltage and Ionization Coefficient Ratio of 0.003

The avalanche photodiode (APD) is an optoelectronic detector with superior sensitivity that typically requires a high operating voltage. Reducing the avalanche region size could lower the breakdown voltage, but at the cost of increased excess noise. In this work, a scattering-engineered APD (SE APD) is proposed to reduce the operating voltage while maintaining a low excess noise merit. In SE APD, a special staggered-finger doping profile is used to spatially separate the electron-ionization path and the hole-transport path via scattering mechanisms, thereby enabling electron unipolar ionization. In SE APD, avalanche occurs at a very low reverse bias of ~2 V in the current–gain–voltage test; an extremely low ionization coefficient ratio (k-value) of 0.003 @ gain = 40 is verified by the noise power spectral density test. A low temperature-dependent breakdown drift coefficient of ~10 mV/K is obtained, indicating good thermal stability. Through comparative experiments, it is demonstrated that the SE APD can increase the gain–bandwidth product by fivefold when compared with the traditional APD structure. Furthermore, the width of the electron ionization multiplication path should be moderate, allowing holes to scatter into the transport path while retaining more electrons within the multiplication path.