p-n junction effects and other detectors s w mcknight and c a dimarzio
TRANSCRIPT
P-N Junction Effects and Other Detectors
S W McKnight and
C A DiMarzio
P-N Junction
Ef
electrons“holes”
E
x
+-
Na=density of acceptors (m-3)
Nd=density of donors (m-3)
P-N Junction
Ef
electrons“holes”
E
x
+-
Depletion Region
Abrupt Junction
x
ρ
-qNa
qNd
P-side N-side
-xop
xon
Charge conservation → q Na xop = q Nd xon
Electric Field Calculation
Poisson’s Equation:
1D Junction:
Abrupt Junction
x
P-side N-side
-xop
xon
P-N Junction
Ef
E
x
Ev
Ec
Junction “built-in” voltageVo
-xpo xno0
w
Built-in Voltage and Electric Field
Drift and Diffusion Currents
Hole Drift Current:
Hole Diffusion Current:
Diffusion Constant:
Built-in Voltage from Current Balance
Jp (drift) + Jp (diffusion) = 0
Equilibrium condition:
Built-in Voltage from Current Balance
Electric field and Voltage:
Built-in Voltage from Current Balance
Since:
pp ≈ Na; nn ≈ Nd
Depletion Region Width
Junction Capacitance
1. Reverse bias: Q = depletion charge (ionized donors and acceptors)
2. Forward bias: Q = diffusion charge storage
Reverse Bias Junction Capacitance
A=junction cross-section area
Junction Capacitance
Thermal Detectors
Absorbing film
Temperature Sensitive Device
Electric leads/ Heat sinks
Incident Radiation
Thermal Detectors
• Thermocouple/Thermopile Detectors
• Semiconductor Bolometers
• Golay Cells
• Pyroelectric detectors
Thermopile Detectors
Absorbing film
Electric leads/ Heat sinks
Incident Radiation
Bi Sb
(Thermocouples in series)
Golay Cell
Absorber
Mirror
Deformable membrane
Gas
Light Source
DetectorIncident radiation
Pyroelectric Detector
+
+
+
+
+
-
-
-
-
-
P(T) = Polarization
A
Ferroelectric Materials
P
Ea
Pr
Ferroelectric Domains
Unpoled P=0
Poled P=Pr
Temperature Dependence of Ferroelectric Effect
Tc = Ferroelectric Curie Temperature
Pyroelectric Detectors
Pyroelectric coefficent:
Pyroelectric detector current:
Ad = detector area
= temperature rate of change
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
Pyroelectric Detectors
• Room temperature operation
• Good frequency response
• Low sensitivity