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Application of photodiodesA brief overview
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Diode devicesCheck valve behaviorDiffusion at the PN junction of
P into N and N into P causes a depleted non-conductive
regionDepletion is enhanced by reverse biasDepletion is broken down
by forward biasWhen forward biasedHigh current flow junction
voltageWhen reverse biasedVery low current flow unless above peak
inverse voltage (PIV) (damaging to rectifying diodes, OK for
zeners)
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D1
Text
cathode-
anode+
+
-
Depletion region
1N412
Diode
Schematic Symbol
Semiconductor Elements
Typical Component Appearance
P -doped
N -doped
Breakdown voltage (PIV)
V
I
JunctionVoltage0.7 - silicon0.3 - germanium
Forward biascurrent
Reverse biascurrent
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Quantum devicesAbsorption 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.
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Conduction band
Energy gap
Valence band
Energy level
+
-
Photon(hv)
Hole
Electron
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Photodiode structureAbsorbtion in the depletion layer causses
current to flow across the photodiode and if the diode is reverse
biased considerable current flow will be induced
BAE 5413
Insulation
n- region
Back Metalization
n+ Back Diffusion
Front Contact
RearContact
Depletion region
p+ Active Area
Incident light
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Photodiode fundamentalsBased on PN or PIN junction diodephoton
absorption in the depletion region induces current flowDepletion
layer must be exposed optically to source light and thick enough to
interact with the lightSpectral sensitivity
MaterialBand gap (eV)Spectral sensitivitysilicon (Si)1.12250 to
1100 nmindium arsenide (InGaAs)~0.351000 to 2200 nmGermanium
(Ge).67900 to 1600 nm
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I
P
N
+
-
hn
RL
IL
electron
hole
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Photodiode characteristicsCircuit modelI0 Dark current
(thermal)Ip Photon flux related currentNoise characterizationShot
noise (signal current related)q = 1.602 x 1019 coulombsI = bias (or
signal) current (A)is = noise current (A rms)Johnson noise
(Temperature related)k = Boltzmans constant = 1.38 x 1023 J/KT =
temperature (K)B = noise bandwidth (Hz)R = feedback resistor
(W)eOUT = noise voltage (Vrms)
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Ip
Rj
Cj
Rs
I0
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Photodiode current/voltage characteristics
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Current
Voltage
Dark current
Photoconductive mode load line
Photovoltaic mode load line
Increasing Light level
Isc (light level dependent)
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Trans-impedance amplifier functionCurrent to voltage converter
(amplifier)Does not bias the photodiode with a voltage as current
flows from the photodiode (V1 = 0)Circuit analysisNote: current to
voltage conversion
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+
-
+
-
Is
Vout
Vf
Io
+
If
V1
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Diode operating modesPhotovoltaic modePhotodiode has no bias
voltageLower noiseLower bandwidthLogarithmic output with light
intensity
Photoconductive modeHigher bandwidthHigher noiseLinear output
with light intensity
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+
-
+
-
Vout
+
-
+
-
Vout
Vs
-
-
For the photovoltaic modeI = thermal component + photon flux
related current
where I = photodiode currentV = photodiode voltageI0 = reverse
saturation current of diodee = electron chargek = Boltzman's
constantT = temperature (K)n = frequency of lighth = Planks
constantP = optical powerh = probability that hv will elevate an
electron across the band gap
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Circuit OptimizationBurr-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
BAE 5413
A semiconductor diode's current-voltage, or I-V, characteristic
curve is ascribed to the behavior of the so-called depletion layer
or depletion zone which exists at the p-n junction between the
differing semiconductors. When a p-n junction is first created,
conduction band (mobile) electrons from the N-doped region diffuse
into the P-doped region where there is a large population of holes
(places for electrons in which no electron is present) with which
the electrons "recombine". When a mobile electron recombines with a
hole, the hole vanishes and the electron is no longer mobile. Thus,
two charge carriers have vanished. The region around the p-n
junction becomes depleted of charge carriers and thus behaves as an
insulator. However, the depletion width cannot grow without limit.
For each electron-hole pair that recombines, a positively-charged
dopant ion is left behind in the N-doped region, and a negatively
charged dopant ion is left behind in the P-doped region. As
recombination proceeds and more ions are created, an increasing
electric field develops through the depletion zone which acts to
slow and then finally stop recombination. At this point, there is a
'built-in' potential across the depletion zone. If an external
voltage is placed across the diode with the same polarity as the
built-in potential, the depletion zone continues to act as an
insulator preventing a significant electric current. However, if
the polarity of the external voltage opposes the built-in
potential, recombination can once again proceed resulting in
substantial electric current through the p-n junction. For silicon
diodes, the built-in potential is approximately 0.6 V. Thus, if an
external current is passed through the diode, about 0.6 V will be
developed across the diode such that the P-doped region is positive
with respect to the N-doped region and the diode is said to be
'turned on'. Wikipedia
Absorbtion of photodiodes in the depletion layer causes current
to flow across the diode and if the diode is reverse biased
considerable current will flow.
