The first superlattice avalanche photodiode (APD) is reported. The high field region of thisp‐i‐nstructure consists of 50 alternating Al0.45Ga0.55As (550 A˚) and GaAs (450 A˚) layers. A large ionization rate ratio has been measured in the field range (2.1–2.7)×105V/cm, with &agr;/&bgr;≃10 at a gain of 10 giving a McIntyre noise factorFn= 3. The ionization rate ratio enhancement with respect to bulk GaAs and AlGaAs is attributed to the large difference in the band edge discontinuities for electrons and holes at the heterojunction interfaces. The superlattice APD is a new device concept which can be used to develop low noise APD’s in a variety of III‐V materials including long wavelength 1.3–1.6‐&mgr;m semiconductors.