Fig. 2: The APD’s operating principle.
From: Infrared avalanche photodiodes from bulk to 2D materials

a Electron and hole multiplication mechanisms, schematic of multiplication mechanism for b k = 0 (αh = 0) and c k = 1 (αe = αh), where k = αh/αe – αe, αh represent electron and hole multiplication coefficients. d αe, αh ionization coefficients versus electric field for selected semiconductors used for APDs’ fabrication26