This paper proposes a novel type of avalanche photodiode-the separate-absorption-transport-charge-multiplication (SATCM) avalanche photodiode (APD). The novel design of photoabsorption and multiplication layers of APDs can avoid the photoabsorption layer breakdown and hole-transport problems, exhibit low operation voltage, and achieve ultra-high-gain bandwidth product performances. To achieve low excess noise and ultra-high-speed performance in the fiber communication regime (1.3 1.55 m), the simulated APD is Si-based with an SiGe-Si superlattice (SL) as the photoabsorption layer and traveling-wave geometric structures. The frequency response is simulated by means of a photo-distributed current model, which includes all the bandwidth-limiting factors, such as the dispersion of microwave propagation loss, velocity mismatch, boundary reflection, and multiplication/transport of photogenerated carriers. By properly choosing the thicknesses of the transport and multiplication layers, microwave propagation effects in the traveling-wave structure can be minimized without increasing the operation voltage significantly. A near 30-Gb/s electrical bandwidth and 10 avalanche gain can be achieved simultaneously, even with a long device absorption length (150 m) and low operation voltage ( 12 V). In addition, the ultrahigh output saturation power bandwidth product of this simulated TWAPD structure can also be expected due to the large photoabsorption volume and superior microwave-guiding structure. Index Terms-Avalanche photodetector, avalanche photodiode (APD), high gain-bandwidth-product avalanche photodetector, high-saturation-power photodetector, Si-SiGe-based photodetector, Si-SiGe-based superlattice (SL), traveling-wave photodetector, ultra-high-speed photodetector. I. INTRODUCTIONT HE rapid growth in Internet traffic has created an urgent need to increase the channel bandwidth capacity of fiber-optic communication systems. As a key component in these transmission systems, ultra-high-speed photodetectors are able to not only increase the transmission capability in a single channel, but also reduce the required number of channels and complexity of management in a wavelength-division-multiplexing (WDM) fiber communication system [1]. There

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