Semiconductor Photoconductive Detectors

S W McKnight and

C A DiMarzio

Types of Photoconductivity

• “Intrinsic photoconductors”– Absorption across primary band-gap, Eg,

creates electron and hole photocarriers

• “Extrinsic photoconductors”– Absorption from (or to) impurity site in gap

creates photocarriers in conduction or valence band

Intrinsic and Extrinsic PhotoconductorsE

Intrinsic Photoconductor

Extrinsic Photoconductor

Ef1

Ef2

1

2

1. Donor level to conduction band

2. Valence band to acceptor level

Eg

Impurities Levels in Si

PhotoconductorsMaterial Eg (max) Material Eg (max)

Si 1.1eV(i) (1.2μ) PbS 0.37eV (3.3μ)

GaAs 1.43eV (0.87μ) InSb 0.18eV (6.9μ)

Ge 0.67eV(i) (1.8μ) PbTe 0.29eV (4.3μ)

CdS 2.42eV (0.51μ) Hg0.3Cd0.7Te

0.24eV (5.2μ) (77K)

CdTe 1.58eV (0.78μ) Hg0.2Cd0.8 Te

0.083eV (15μ) (77K)

Indirect Gap Semiconductors

Eghνphoton

hνphonon

Direct Gap Semiconductors

Eghνphotonk

E

Optical Constants of Silicon

0

1

2

3

4

5

6

7

8

0 200 400 600 800 1000 1200

Wavelength (nm)

Op

tic

al

Co

ns

tan

ts (

n,

k)

n

k

k*1000

GaAs Optical Constants

0

1

2

3

4

5

6

0 200 400 600 800 1000 1200

Wavelength (nm)

n, k

n

k

100*k

Optical Electric Field and Power

q=ω (ε)1/2 = (ω/c) (n+ik)

Optical Electric Field and Power

A x (B x C) = B(A·C) – C(A·B)

α = absorption coefficient = 2 ω k/c

Absorption Coefficient for Si and GaAs

Reflection at Front Surface

For Silicon, near 600 nm: n=3.95 k=0.026

→ R = 0.35

(Can be reduced by anti-reflection coating)

Absorption in Semiconductorα = 2 ω k / c

For Silicon near 600 nm: α = 4 π 0.026 / 600 x 10-9 = 5.44 x 105 m-1

For GaAs near 600 nm: α = 4.76 x 106 m-1

0 1 2 3 4 5 6 7 8 9 100

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Z (microns)

Op

tica

l Po

we

r In(z)=Io e- z

Si

GaAs

Carrier Generation/Recombination

1. Thermal Equilibrium:

2. Direct recombination of excess carriers:

Units: g = e-h excitations/sec/m3

r = m3/sec

Direct Recombination of Excess Carriers

Direct recombination (low level)→ δn = δp << no

Photogenerated Carriers3. Steady-state optical excitation:

Neglect for δn<<no

Differential Optical Excitation Rate

Photoconductivity

Φp = photon flux (photon/sec)

Area=A

length=l

η = quantum efficiency

Hole Trapping

Hole trapping at recombination centers:

a. hole is trapped

b. electron trapped, completing recombination

c. hole detraps to valence band

(c)

Photoconductivity with Hole Trapping

# of current-carrying photoelectrons = # of trapped holes

(Steady-state)

Photoconductive Gain

G = photocurrent (electron/sec) / rate of e-h generation

Area=A

length=l

Photoconductive Gain

→

Effect of Carrier Lifetime on Detector Frequency Response

Photoconductor Bias Circuit

Photoconductive Voltage

Photoconductor Responsivity

Responsivity Factors• Photocarrier lifetime

– Tradeoff with response frequency

• Quantum efficiency (anti-reflection coating)

• Carrier mobility• Detector current• Dark resistance

– R= ℓ / σ A– Detector area: Ad = ℓ w– Sample thickness

length=ℓ

Cross-section area=A

Detector area=Ad w

tDetector current, i

Photoconductive Noise Factors• 1/f Noise

– Contact related

• Thermal noise (Johnson noise)– Statistical effect of thermal fluctuations– <In

2> ~ kT/R

• Generation-Recombination noise– Statistical fluctuations in detector current– Dark current (thermal electron-hole pairs)– Background photogenerated carriers– <In

2> ~ Id / e

Noise Sources

Johnson noise:

G-R noise:

Ep = photon irradiance=Φp / Ad

G = photoconductive gain

Background-Limited Photoconductive Detection

Johnson-Noise-Limited Photoconductive Detection

Noise Sources for IR Detectors