22.3 Photodiodes
603
Fig. 22.17 a Schematic
cross section of pin-diode,
b profile of carrier
generation due to light
absorption and c schematic
band structure under
reverse bias. The
generation of three
electron-hole pairs is
shown; subsequent drift
(diffusive) transport is
indicated with solid
(dashed) arrows
(a)
(b)
(c)
j tot = e 0
1 −
exp(−α w)
1 + α L p
+ e p n 0
D p
L p
.
(22.33)
The first term is due to the photocurrent, the second term is due to the diffusion current known from
the p
+ n-diode. In normal operation, the second can be neglected compared to the first. The quantum
efficiency is
η =
j tot /e
P opt / hν
= (1 − R)
1 −
exp(−α w)
1 + α L p
.
(22.34)
For a high quantum efficiency, of course low reflectance and high absorption coefficient, i.e. αw 1,
are necessary.
However, for w 1/α the transit time through the depletion layer t r ≈ w/v s (at sufficiently
high field, v s being the drift-saturation velocity) increases too much. The 3 dB cutoff frequency f 3 dB
(Fig. 22.18) is
f 3 dB ∼ =
2.4
2π t r
∼ =
0.4 v s
w
.
(22.35)
Therefore a tradeoff exists between the quantum efficiency and the response speed of the pin-photodiode
(Fig. 22.18). Choosing w ∼ = 1/α is a good compromise.
22.3.4 Position-Sensing Detector
In a position-sensing detector (PSD) two electrodes are placed at opposite edges of a photodetector.
The current output depends linearly on the beam position in between the electrodes, similar to a voltage
603
Fig. 22.17 a Schematic
cross section of pin-diode,
b profile of carrier
generation due to light
absorption and c schematic
band structure under
reverse bias. The
generation of three
electron-hole pairs is
shown; subsequent drift
(diffusive) transport is
indicated with solid
(dashed) arrows
(a)
(b)
(c)
j tot = e 0
1 −
exp(−α w)
1 + α L p
+ e p n 0
D p
L p
.
(22.33)
The first term is due to the photocurrent, the second term is due to the diffusion current known from
the p
+ n-diode. In normal operation, the second can be neglected compared to the first. The quantum
efficiency is
η =
j tot /e
P opt / hν
= (1 − R)
1 −
exp(−α w)
1 + α L p
.
(22.34)
For a high quantum efficiency, of course low reflectance and high absorption coefficient, i.e. αw 1,
are necessary.
However, for w 1/α the transit time through the depletion layer t r ≈ w/v s (at sufficiently
high field, v s being the drift-saturation velocity) increases too much. The 3 dB cutoff frequency f 3 dB
(Fig. 22.18) is
f 3 dB ∼ =
2.4
2π t r
∼ =
0.4 v s
w
.
(22.35)
Therefore a tradeoff exists between the quantum efficiency and the response speed of the pin-photodiode
(Fig. 22.18). Choosing w ∼ = 1/α is a good compromise.
22.3.4 Position-Sensing Detector
In a position-sensing detector (PSD) two electrodes are placed at opposite edges of a photodetector.
The current output depends linearly on the beam position in between the electrodes, similar to a voltage