Transcript Lecture Notes 8 - P-N Junction Effects and Other Detectors

P-N Junction Effects and
Other Detectors
S W McKnight and
C A DiMarzio
P-N Junction
- +
E
electrons
“holes”
Ef
Na=density of
Nd=density of
acceptors (m-3) donors (m-3)
x
P-N Junction
E
- +
electrons
“holes”
Ef
Depletion
Region
x
Abrupt Junction
ρ
P-side
N-side
-xop
qNd
xon
-qNa
Charge conservation → q Na xop = q Nd xon
x
Electric Field Calculation
Poisson’s Equation:
1D Junction:
Abrupt Junction
P-side
N-side
-xop
xon
x
P-N Junction
E
Vo
Junction “built-in”
voltage
Ec
Ef
w
-xpo 0 xno
Ev
x
Built-in Voltage and Electric Field
Drift and Diffusion Currents
Hole Drift Current:
Hole Diffusion Current:
Diffusion Constant:
Built-in Voltage from Current
Balance
Equilibrium condition:
Jp (drift) + Jp (diffusion) = 0
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
Incident Radiation
Absorbing film
Temperature
Sensitive
Device
Electric leads/
Heat sinks
Thermal Detectors
•
•
•
•
Thermocouple/Thermopile Detectors
Semiconductor Bolometers
Golay Cells
Pyroelectric detectors
Thermopile Detectors
Incident Radiation
Absorbing film
Bi
Sb
(Thermocouples in series)
Electric leads/
Heat sinks
Deformable
membrane
Golay Cell
Detector
Incident
radiation
Gas
Mirror
Absorber
Light
Source
Pyroelectric Detector
P(T) = Polarization
+
-
+
-
+
-
+
-
+
-
A
Ferroelectric Materials
P
Pr
Ea
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
BaTiO3
Pyroelectric Coefficient
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