Эквивалентная электрическая схема
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Rd |
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Iph=Photocurrent generated by detector |
Iph |
Id |
Cd |
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Cd=Detector Capacitance |
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Id=Dark Current |
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PIN |
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In=Multiplied noise current in APD |
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R =Bulk and contact resistance |
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Rd d |
Iph |
Id |
In |
Cd |
APD
Ёмкость фотоприемника
P |
N |
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C |
A |
p-n |
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junction |
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w xp xn |
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W |
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For a uniformly doped junction
Capacitance must be minimized for high sensitivity (low noise) and for high speed operation
Minimize by using the smallest light collecting area consistent with efficient collection of the incident light
Minimize by putting low doped “I” region between the P and N doped regions to increase W, the depletion width
W can be increased until field required to fully deplete causes excessive dark current, or carrier transit time begins to limit speed.
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A |
2q |
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1/ 2 |
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C |
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Nd |
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Where: =permitivity |
q=electron charge |
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2 |
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V |
bi |
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V |
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Nd=Active dopant density |
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1/ 2 |
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2 (Vo |
Vbi) |
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W |
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qNd |
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Vo=Applied voltage |
Vbi=Built in potential |
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A=Junction area
Полоса пропускания
C= 0 A/w; |
B = 1/2 RC |
Transit time
=W/vsat
vsat=saturation velocity=2x107 cm/s
R-C Limitation |
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R A |
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RC |
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in W |
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Responsivity |
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R |
q |
1 Rp 1 e W |
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h |
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Diffusion
=4 ns/µm (slow)
Искажения сигнала
Темновой ток
Surface Leakage
Bulk Leakage
Surface Leakage |
Bulk Leakage |
Ohmic Conduction |
Diffusion |
Generation-recombination |
Generation-Recombination |
via surface states |
Tunneling |
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Usually not a significant noise source at high bandwidths for PIN Structures High dark current can indicate poor potential reliability
In APDs its multiplication can be significant
Типовые характеритики фотодиодов
Функциональная схема фотоприемника
Типы фотоприемников
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+Bias |
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+Bias |
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+Bias |
Rf |
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Is |
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Is |
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Is |
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Output |
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Output |
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Output |
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RL |
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RL |
Ct |
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Ct |
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Amplifier |
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50 |
Amplifier |
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Amplifier |
Equalizer |
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Low Impedance |
High Impedance |
Transimpedance |
Low Sensitivity |
Requires Equalizer for high BW |
High Dynamic Range |
Easily Made |
High Sensitivity |
High Sensitivity |
Wide Band |
Low Dynamic Range |
Stability Problems |
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Careful Equalizer Placement Required Difficult to equalize |
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Регенерация и усиление
Энергетический бюджет |
Линейная система |