With exponential data growth driven by cloud computing, artificial intelligence, and big data, optical interconnects are gradually replacing traditional electrical interconnects in data centers and chip-to-chip communication systems. Among the core components of optical interconnects, photodetectors (PDs) are responsible for the crucial optical-to-electrical signal conversion. The performance of photodetectors directly determines the data transmission rate and system efficiency. Germanium-silicon photodetectors (Ge/Si PDs) have attracted considerable attention for their high responsivity, broad bandwidth, and complementary metal oxide semiconductor (CMOS) compatibility, making them highly suitable for large-scale integration and low-cost fabrication. Despite extensive efforts, the frequency response of Ge/Si PDs has become a critical bottleneck in achieving ultrafast signal processing and Tbit-level data transmission. As integrated photonics advances toward higher speeds and greater density, there is an urgent need to overcome the physical and structural limitations of Ge/Si PDs. Key challenges include minimizing carrier transit time, reducing RC time constants, and enhancing bandwidth without compromising responsivity or increasing dark current. Innovations such as lateral PIN (LPIN) and vertical PIN (VPIN) architectures, gain peaking techniques, and hybrid material integration have been explored to push device performance boundaries.

We systematically review the structural designs and physical mechanisms affecting the high-speed performance of Ge/Si PDs, including discussions on both waveguide-coupled and normally incident structures (Figs. 1?5). To improve frequency response, strategies focus on three major physical bottlenecks: 1) reducing carrier transit time by optimizing the intrinsic region thickness and electric field distribution; 2) lowering RC time constants through device miniaturization and low-capacitance design; 3) enhancing gain peaking using LC resonance circuits (Fig. 5). Waveguide-coupled structures have become the dominant design due to their high efficiency and strong integration potential. LPIN structures, in particular, have demonstrated bandwidths exceeding 265 GHz (Table 1), with optimized doping profiles and lateral carrier collection paths minimizing transit delays. In contrast, VPIN designs offer better stability and manufacturability, but their vertical carrier transport path limits the achievable speed. Gain peaking, achieved using on-chip spiral inductors or off-chip wire bonding, has proven effective in extending the 3-dB bandwidth without significantly influencing dark current or responsivity. Devices using this method have achieved bandwidth improvements of up to 103 GHz (Fig. 7), validating the feasibility of external circuit compensation. Recent studies have also addressed trade-offs among speed, responsivity, and noise. For instance, LPIN devices achieve high speed at the cost of elevated dark current density, while VPIN devices generally maintain lower dark current at moderate speeds. Design efforts such as minimizing optical loss through metal contact optimization and improving absorption using Bragg reflectors or micro-resonators have further enhanced responsivity. In application, Ge/Si PDs have been successfully integrated into 3D optoelectronic transceivers for 800 Gbit/s transmission [Fig. 10(a)], photonic neural networks [Fig. 10 (b)?(c)], and millimeter-wave wireless communication systems [Fig. 10(d)]. Their role in emerging optical computing and microwave photonic systems highlights their strategic importance in future data infrastructure.

In this paper, we present a comprehensive review of the current status and key challenges in the development of Ge/Si PDs for high-speed, high-capacity integrated photonics. It covers structural types, physical limitations, and performance evolution. By analyzing carrier transit time compression, RC constant optimization, and gain peaking strategies, we reveal the influence of different architectures, such as waveguide-coupled LPIN and VPIN on bandwidth, dark current, and responsivity. With their high responsivity, ultra-wide bandwidth (up to 265 GHz), and CMOS compatibility, Ge/Si PDs have become core components in optical interconnects, photonic computing, and microwave photonic systems. However, enhancing frequency response often results in reduced responsivity and increased fabrication complexity, representing a critical bottleneck yet to be addressed. Future development of Ge/Si PDs will focus on four key directions: 1) resolving the trade-off between responsivity and bandwidth; 2) suppressing dark current through high-quality material growth and effective passivation; 3) extending spectral response into the mid-infrared range to meet broader application demands; 4) promoting heterogeneous integration with high-performance electro-optic materials to enhance overall system performance. Ultimately, advances in material quality, structural design, and multi-scale integration will drive the development of Ge/Si PDs, supporting the evolution of next-generation high-speed optical communication and optoelectronic systems.