A low-noise and wide-bandwidth InGaAs/InAlAs superlattice APD

An InGaAs-InAlAs superlattice avalanche photodiode (APD) was fabricated by MBE (molecular beam epitaxy), and impact ionization rates, excess multiplication noise, and high-frequency response were measured. The electron ionization rate ( alpha ) is enhanced by the large conduction band offset. The ratio of alpha / beta is as large as 29 at an electric field of 2.2*10/sup 5/ V/cm, where beta is the hole ionization rate. As a result of the enhanced ionization rate ratio, the excess multiplication noise is quite small and fits the theoretical curve with an effective ionization rate ratio of 0.05. The 3-dB bandwidth is 4 GHz at a multiplication factor of 16. This shows that the carrier pileup is negligible in a high electric field. The 3-dB bandwidth is limited not by the gain-bandwidth limit but by the electron diffusion time in the photoabsorption layer.<
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