600
22 Light-to-Electricity Conversion
(a)
(b)
Fig. 22.14 a Quantum efficiency of various photodetectors. The dashed lines depict lines of equal responsivity (R λ in
A/W) as labeled on top of the panel. b Detectivity D ∗ of various photoconductors and photodiodes (PD). The lighter
(darker) shaded area indicates the range unachievable at 300 K (77 K) due to background radiation. Adapted from [574]
(a)
(b)
photoelectric
effect
interaction
circuit
signal
background
currents
dark
background
signal
thermal
noise
Fig. 22.15 a Schematic dark and illuminated I –V -characteristics of a photodiode (for the case j p = −2 j s ). b Schematic
representation of currents in a photodiode and equivalent circuit. Part b adapted from [1838]
22.3.2 Pn Photodiodes
The most important figures of merit are the quantum efficiency, responsivity, noise equivalent power
(NEP) and the response speed.
If the depletion layer is hit by a photon flux with a generation rate G 0 (i.e. electron-hole pairs per unit
volume per unit time) the photogenerated current is added to the diffusion current. The photocurrent
density j p (per unit area) is
j p = −e G 0 L p
(22.22)
for a p
+ n-diode. In order to obtain this result the diffusion and continuity equations have to be solved
for the depletion region.
6 Equation (22.22) means that the dark I –V characteristic is shifted by j p as
shown in Fig. 22.15a. The number of electron-hole pairs that are generated per photon of energy hν
by the absorption of the (monochromatic) light power P opt is
6 This derivation is done in Sect. 22.3.3. Equation (22.22) is obtained from (22.33) for vanishing thickness w and αL p 1.
22 Light-to-Electricity Conversion
(a)
(b)
Fig. 22.14 a Quantum efficiency of various photodetectors. The dashed lines depict lines of equal responsivity (R λ in
A/W) as labeled on top of the panel. b Detectivity D ∗ of various photoconductors and photodiodes (PD). The lighter
(darker) shaded area indicates the range unachievable at 300 K (77 K) due to background radiation. Adapted from [574]
(a)
(b)
photoelectric
effect
interaction
circuit
signal
background
currents
dark
background
signal
thermal
noise
Fig. 22.15 a Schematic dark and illuminated I –V -characteristics of a photodiode (for the case j p = −2 j s ). b Schematic
representation of currents in a photodiode and equivalent circuit. Part b adapted from [1838]
22.3.2 Pn Photodiodes
The most important figures of merit are the quantum efficiency, responsivity, noise equivalent power
(NEP) and the response speed.
If the depletion layer is hit by a photon flux with a generation rate G 0 (i.e. electron-hole pairs per unit
volume per unit time) the photogenerated current is added to the diffusion current. The photocurrent
density j p (per unit area) is
j p = −e G 0 L p
(22.22)
for a p
+ n-diode. In order to obtain this result the diffusion and continuity equations have to be solved
for the depletion region.
6 Equation (22.22) means that the dark I –V characteristic is shifted by j p as
shown in Fig. 22.15a. The number of electron-hole pairs that are generated per photon of energy hν
by the absorption of the (monochromatic) light power P opt is
6 This derivation is done in Sect. 22.3.3. Equation (22.22) is obtained from (22.33) for vanishing thickness w and αL p 1.