Fig. 4

a Schematic and b SEM cross-sections of the normal incident Si-Ge vertical SACM APD [72]; c Measured I-V characteristics of the negative differential resistance NI Si-Ge SACM APD. Inset is the device schematic cross-section [88]; d Schematic cross-section of the normal incident Si-Ge vertical SACM APD integrated with a MIM capacitor [89]