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Photo detector (PD) converts light into current, thus when it is exposed to light, it passes current. To increase sensitivity of PD, avalanche photo detector (APD) which multiplies current by impact ionization is developed. Because of high gain property, APD has various application: medical, remote sensing, and commercial.
With the emergence of ~1.5 µm eye-safe laser sources, low-noise and sensitive compound semiconductor photo detectors at around 1.5 µm wavelengths are needed.
Figure 1 is our design which is InP-InGaAs separate absorption, grading, charge, and multiplication (SAGCM) APD. Light is absorbed in the absorption region and is swept into the high field, high bandgap multiplication layer where avalanche effect occurs. There are grading layer which reduces noise and charge layer which controls electric field between absorption and multiplication layers. With the epitaxial layer, it is etched to form mesa structure (figure 2.). To provide straight sidewalls, proper etching condition was developed.