04-01-2013, 11:13 AM
Semiconductor Photodiodes With INTERNAL GAIN
Semiconductor Photodiodes.pptx (Size: 540.43 KB / Downloads: 28)
Semiconductor Photodiodes
without internal gain is
single electron hole pair generated
per absorbed photon
with internal gain is
multiple electron hole pairs
per absorbed photon
Working:
Light enters through p+ region
Absorbed in material (high resistivity p type material)
Avalanche carrier Multiplication in high electric field
pn+ junction region
Germanium APD
Used for longer wavelength
n+ p structure same as Silicon APD
Low avalanche breakdown voltage due to high absorption coefficient of Ge (1.6 um)
Reduced excess noise factor as multiplication process is initiated by holes
Comparison of APDs
Wavelength range : 300nm to 1700nm.
Silicon APDs used between 300nm to 1100nm,
Germanium between 800nm and 1600nm
InGaAs from 900nm to 1700nm.
InGaAs APDs more expensive than germanium
APDs
InGaAs APDs are available with much lower noise currents, and provide higher frequency bandwidth for a given active area.
A germanium APD is recommended for environments applications where amplifier noise is significantly higher than the noise from an InGaAs APD, or for applications where cost is of basic consideration
PHOTOTRANSISTORS
Ordinary transistor with two modification-
(1) a transparent window so that light can shine on the junctions
(2) the structure has been modified to maximize the light capture area.
BASE current is multiplied to give the COLLECTOR current.
The BASE current is controlled by the amount of light, which is why the device only needs 2 pins.
Change in base current cause a significant change (increase) in collector current.
Using Ohm's law, V=I*R, therefore the voltage at Vout will move up & down based on the amount of light.