## 1 Introduction  
The swift growth in the capacity of communication networks has led to new services, such as mobile broadband for smart devices, social networking, cloud computing, and online streaming. To assist in this growth, the Ethernet standards have repeatedly been extended from 100 Gb/s to 400 Gb/s. In optical fiber communication systems, there is always a tradeoff between the transmission capacity and the transmission distance. The optical receiver is a major factor in extending the distance limited by the power transmitted from laser sources. In comparison to the conventional p-i-n photodiodes, high-sensitivity avalanche photodiodes (APDs) are a prerequisite for >25 Gbit/s operation across the telecommunication wavelength (1.31 to 1.55 \(\upmu\)m), for the optical signal receivers over 40-80 km fibers in 100 and 400 Gbit/s ER-4 systems [1]. Significant research has been done on possible alternative materials like silicon-germanium (Si/Ge) and the III-V compounds, such as InP, InGaAs, and InAlAs, for the fabrication of high-speed APDs for high data rate applications [2]. However, the III-V based APDs have numerous advantages in terms of band engineering flexibility in which the materials can be lattice matched with the InP substrate during growth. Thus, high performance can be achieved by modifying the different stacked layers in the APD, varying the distinguishing features such as the bandgaps, electron, hole velocities, and diffusion coefficients. For example, by varying the thickness of the hybrid absorber layer, we can boost the speed of an APD for a given responsivity and vice versa. APDs with In\({}_{0.52}\)Al\({}_{0.48}\)As-based M-layers have attracted significant attention for 10 Gb/s and >25 Gb/s applications because the benefits of a large gain-bandwidth product, high temperature stability, and reasonably low dark current characteristics at the 1.31-1.55 \(\upmu\)m wavelengths. In order to meet the requirements for high-speed transmission, a lot of effort has been made to improve the bandwidth and gain bandwidth product (GBP) of the device. For APDs >25 Gb/s operation, thin absorption (<1 \(\upmu\)m) and M (<100 nm) layers with a small diameter of active mesa (<20 \(\upmu\)m) are requisite [3; 4; 5; 6], but this small diameter mesa leads to a degradation in the unit gain responsivity and offers less tolerance for optical alignment. Further improvement in the GBP, up to 140 GHz, has been reported using a 200 nm InAlAs avalanche layer [7]. [PERSON] et al., proposed an APD with a 100 nm avalanche layer, which could reach a GBP of 235 GHz with the diameter of 20 \(\upmu\)m. Later, they further downscaled the multiplication layer thickness to 90 nm and achieved a GBP up to 270 GHz with the same 20 \(\upmu\)m diameter [5; 8; 9]. There are various factors limiting the speed, gain, and noise in the receiver performance in high-speed systems, for instance, the avalanche build-up time, which depends on the material and the thickness of the avalanche layer. Moreover, to further improve APD speed performance, very thin avalanche layers are generally required, to minimize the avalanche buildup time [10]. However very thin avalanche layers are vulnerable to band-to-band tunneling currents, which increase exponentially with the electric field and add to the receiver noise. Thus, to avoid excessive tunneling currents, the thickness of the avalanche layer should be within the lower limits. In order to achieve a large GBP and low noise, it is necessary to adopt the appropriate thickness for the avalanche layer. In this work, we demonstrate a novel design for In\({}_{0.52}\)Al\({}_{0.48}\)As based vertical illuminated APDs with dual M-layers. Separate absorption, grading, and multiplication (SAGM) structures with thin absorber and dual M-layer (Device A) versus thick absorber and dual M-layer (Device B) APDs are fabricated.