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