Transcript Application of photodiodes - Oklahoma State University

Application of photodiodes
A brief overview
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Quantum devices
• Absorption of a photon of sufficient energy elevates
an electron into the conduction band and leaves a
hole in the valence band.
• Conductivity of semi-conductor is increased.
• Current flow in the semi-conductor is induced.
Conduction band
- Electron
Energy gap
Energy
level
+
Hole
Valence band
Photon
(hv)
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Photodiode structure
Incident light
Front
Contact
Insulation
p+ Active Area
Depletion region
n- region
n+ Back Diffusion
Back Metalization
Rear
Contact
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Photodiode fundamentals
• Based on PN or PIN junction diode
– photon absorption in the depletion
region induces current flow
P
+
hole
h
I
IL
electron
N
• Spectral sensitivity
Material
Band gap
(eV)
Spectral sensitivity
silicon (Si)
1.12
250 to 1100 nm
indium arsenide (InGaAs)
~0.35
1000 to 2200 nm
Germanium (Ge)
.67
900 to 1600 nm
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RL
Photodiode characteristics
• Circuit model
– I0 Dark current (thermal)
– Ip Photon flux related current
Rs
Ip
I0
Rj
Cj
• Noise characterization
– Shot noise (signal current related)
– q = 1.602 x 10–19 coulombs
– I = bias (or signal) current (A)
– is = noise current (A rms)
is  2qi
– Johnson noise (Temperature related)
–
–
–
–
–
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k = Boltzman’s constant = 1.38 x 10–23 J/K
T = temperature (°K)
B = noise bandwidth (Hz)
R = feedback resistor (W)
out
eOUT = noise voltage (Vrms)
e
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Dark current
Photovo
Voltage
Photodiode current/voltage characteristics
Isc (light level dependent)
ltaic mo
de load
line
Current
Increasing Light level
Photoco
nductiv
e
load lin mode
e
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Trans-impedance amplifier function
• Current to voltage converter (amplifier)
• Does not bias the photodiode with a voltage as
current flows from the photodiode (V1 = 0)
• Circuit analysis
Io  0
Vf
I f  I s
V1  0
V1
V f  R f I f  R f I s
Is
Io
-
-
+
+
+
If
Vout
Vout  V f  R f I s
–Note: current to voltage conversion
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Diode operating modes
• Photovoltaic mode
–
–
–
–
Photodiode has no bias voltage
Lower noise
Lower bandwidth
Logarithmic output with light intensity
+
+
Vout
• Photoconductive mode
– Higher bandwidth
– Higher noise
– Linear output with light intensity
+
Vs
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Vout
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For the photovoltaic mode
• I = thermal component + photon flux related current
 eV
 heP
kT
I  I 0  e  1 

 h
• where
I = photodiode current
V = photodiode voltage
I0 = reverse saturation current of diode
e = electron charge
k = Boltzman's constant
T = temperature (K)
 = frequency of light
h = Plank’s constant
P = optical power
h = probability that hv will elevate an electron across the band gap
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Circuit Optimization
• Burr-Brown recommendations (TI)
• Photodiode capacitance should be as low as possible.
• Photodiode active area should be as small as possible so that
CJ is small and RJ is high.
• Photodiode shunt resistance (RJ ) should be as high as
possible.
• For highest sensitivity use the photodiode in a “photovoltaic
mode”.
• Use as large a feedback resistor as possible (consistent with
bandwidth requirements) to minimize noise.
• Shield the photodetector circuit in a metal housing.
• A small capacitor across RF is frequently required to suppress
oscillation or gain peaking.
• A low bias current op amp is needed to achieve highest
sensitivity
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