The surge in popularity of over-the-top (OTT) media services and 5G mobile front-haul networks has driven up the demand for optical communication channel bandwidth. The 400Gb/s Ethernet system which uses a pulse-amplitude modulation (PAM-4) format with a 53 Gbaud per channel has been developed to meet the requirements for faster data rates. However, when the linking distance exceeds 40 km, the limited output optical power of the electro-absorption-modulated laser (EML) transmitter and the sensitivity of the p-i-n PD based receiver place limitations upon the optical power budget needed to maintain such a high data rate. Avalanche photodiodes (APDs) with wide optical-to-electrical (O-E) bandwidths and higher sensitivity than that of conventional PDs, have proven an effective way to alleviate the aforementioned problems on the receiver side . Recently, the Si/Ge based APDs have demonstrated excellent dynamic and static performance for > 106 Gbit/sec transmissions per lane. Compared with their III-V counterparts, the Si/Ge APDs show improved dynamic performance, mainly due to the superior carrier multiplication process inside the Silicon M- layer over the III-V M-layer, which occurs in In0.52Al0.48As. However, this kind of APD, in which the active Ge photo-absorption layer is usually grown on lattice-mismatched silicon substrates, interface defects become a challenge affecting the reliability under harsh operation conditions, e.g., in uncooled environments or for high optical power illuminations (~mW). In addition to the PAM-4 modulation formats, coherent communication schemes have become an alternative solution for >106 Gbit/sec transmissions. However, the PDs or APDs in a coherent receiver need to sustain high-speed and high-linearity performance under strong (~ mW) optical local oscillator (LO) pumping powers to ensure high sensitivity performance. It has been demonstrated that with the In0.52Al0.48As based APDs one can attain a larger signal-to-noise (S/N) ratio with a lower optical LO power compared with the traditional p-i-n PDs used for coherent applications, such as FMCW lidar. Such requirements have driven the development of high-speed III-V APDs with high linearity and reliable high-power performance. In order to ensure an increase in the bandwidth and saturation power of APDs, a gradual decrease in the thickness of both their absorber and M-layers is necessary, but this comes at the cost of lower responsivity. A relaxation in the trade-off between the bandwidth and responsivity and further improvement in the GBP has been reported for waveguide type APDs (WGAPD) using thinner absorbers. High-responsivity performance in such devices can be maintained by properly increasing the absorption length. However, the edge-coupled waveguide APD structure typically has a substantially narrower alignment tolerance than its vertically illuminated counterparts (5 vs. 25 um), which is due to the smaller aperture size of the optical waveguides. The backside-illuminated ADP structure is another possibility for further enhancing the responsivity of a top-illuminated structure because of the double pass of the incident optical signal through the topmost contact metal, which serves as a reflector. However, the flip-chip bonding package for backside illumination usually induces parasitic capacitance, which degrades the net O-E bandwidth of the PD. In the recent years, we demonstrate a In0.52Al0.48As based backside-illuminated APD with a novel M-layer designs and flip-chip bonding package which are targeted for relaxing the fundamental trade-offs among responsivity and bandwidth. The packaged device exhibits a more moderate damping O-E frequency response and superior bandwidth (36 vs. 31 GHz) and responsivity (3.4 vs. 2.3 A/W) to those of the top-illuminated reference device under 0.9 Vbr operation. In terms of high-power performance, with this device package, we are also able to attain a record-high millimeter wave output power (0 dBm) at 40 GHz with a high saturation current (12.5 mA at +8.8 dBm optical power) among all those reported for high-speed APDs. These excellent performances in terms of speed, responsivity, dark current (175 nA), gain-bandwidth product (>1 THz) and saturation current of these APDs ensure thier applicability for advanced high-speed receivers in PON or coherent system.