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P-N Junction Effects and Other Detectors
S W McKnight and
C A DiMarzio
![Page 2: P-N Junction Effects and Other Detectors S W McKnight and C A DiMarzio](https://reader036.vdocuments.pub/reader036/viewer/2022081802/5697c02e1a28abf838cda0b1/html5/thumbnails/2.jpg)
P-N Junction
Ef
electrons“holes”
E
x
+-
Na=density of acceptors (m-3)
Nd=density of donors (m-3)
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P-N Junction
Ef
electrons“holes”
E
x
+-
Depletion Region
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Abrupt Junction
x
ρ
-qNa
qNd
P-side N-side
-xop
xon
Charge conservation → q Na xop = q Nd xon
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Electric Field Calculation
Poisson’s Equation:
1D Junction:
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Abrupt Junction
x
P-side N-side
-xop
xon
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P-N Junction
Ef
E
x
Ev
Ec
Junction “built-in” voltageVo
-xpo xno0
w
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Built-in Voltage and Electric Field
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Drift and Diffusion Currents
Hole Drift Current:
Hole Diffusion Current:
Diffusion Constant:
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Built-in Voltage from Current Balance
Jp (drift) + Jp (diffusion) = 0
Equilibrium condition:
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Built-in Voltage from Current Balance
Electric field and Voltage:
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Built-in Voltage from Current Balance
Since:
pp ≈ Na; nn ≈ Nd
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Depletion Region Width
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Junction Capacitance
1. Reverse bias: Q = depletion charge (ionized donors and acceptors)
2. Forward bias: Q = diffusion charge storage
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Reverse Bias Junction Capacitance
A=junction cross-section area
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Junction Capacitance
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Thermal Detectors
Absorbing film
Temperature Sensitive Device
Electric leads/ Heat sinks
Incident Radiation
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Thermal Detectors
• Thermocouple/Thermopile Detectors
• Semiconductor Bolometers
• Golay Cells
• Pyroelectric detectors
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Thermopile Detectors
Absorbing film
Electric leads/ Heat sinks
Incident Radiation
Bi Sb
(Thermocouples in series)
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Golay Cell
Absorber
Mirror
Deformable membrane
Gas
Light Source
DetectorIncident radiation
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Pyroelectric Detector
+
+
+
+
+
-
-
-
-
-
P(T) = Polarization
A
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Ferroelectric Materials
P
Ea
Pr
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Ferroelectric Domains
Unpoled P=0
Poled P=Pr
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Temperature Dependence of Ferroelectric Effect
Tc = Ferroelectric Curie Temperature
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Pyroelectric Detectors
Pyroelectric coefficent:
Pyroelectric detector current:
Ad = detector area
= temperature rate of change
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Pyroelectric Materials
Material Pyroelectric Coefficient
BaTiO3 2 x 10-8 C cm-2 K-1
Triglycine Sulfate (TGS) 2-3.5 x 10-8
LiNbO3 4 x 10-9
LiTaO3 6 x 10-9
SrBaNbO3 1 x 10-7
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Pyroelectric Detectors
• Room temperature operation
• Good frequency response
• Low sensitivity