Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells
EE529 Semiconductor Optoelectronics
Photodetectors and Solar Cells 1. Photodetector noise 2. Performance parameters 3. Photoconductors 4. Junction photodiodes 5. Solar cells
Reading: Liu, Chapter 14: Photodetectors; Bhattacharya, Chapter 10: Solar Cells Ref: Bhattacharya, Sec. 8.2-8.3
Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells
Photodetector Noise
2
Shot noise: 2, 2 ( )n sh s b di eB i i i= + +
2, 2 ( )n sh s b di eBGF i i i= + +: signal currentsi: background radiation currentbi: dark currentdi
Thermal noise: 2 2, , ,4 /n th B n th n thP k TB i R v R= = =
Exercise: A photodetector without internal gain has a load resistance of 50 R = Ω and a bandwidth of B = 100 MHz. Input optical power is adjusted to generate photocurrent ranging from 1 µA to 10 mA. Discuss the behavior of its SNR vs photocurrent. At what photocurrent is the shot noise equal to thermal noise?
for photodetectors with internal gain G
2 2/ :F G G= Excess noise factor
Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells
Noise Characteristics of Photodetectors
3
Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells Discussion: Noise Characterization
for a QD Photoconductor
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This figure is from the paper “Ultrasensitive solution-cast quantum dot photodetectors” published in Nature in 2006. The device structure is shown in Slide 7. The noise characterization was done using a lock-in amplifier, which reported a noise current in A/Hz1/2. From the experimental results presented in Figure (b), determine the NEP and root-mean-square noise current at various modulation frequencies.
1/21/2
rms( )
(2 2 4 / ) = (W)
n
b d B
iNEP
ei ei k T R B
=
+ +R
R1/2
1/2 1( )* (cm Hz )( )
BD WNEP
−= ⋅ ⋅A
Normalized detectivity
Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells
Linearity and Dynamic Range
5
Dynamic Range (DR) 10logsat
sPNEP
=
Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells
Speed and Frequency Response
6
3
0.35dB
r
ft
=
Considering the rectangular time interval used to define the electrical bandwidth B when discussing noise,
3
0.443 0.886dBf BT
= =
Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells
Photoconductor Structure and Principle
“Ultrasensitive solution-cast quantum dot photodetectors,” Nature (2006)
Photogenerated carriers drift across the photoconductor multiple times during their lifetime. Gain
7
Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells
Exercise: Photoconductor Gain
8
An n-type GaAs intrinsic photoconductor for 850λ = nm has the following parameters: 100 ml w= = µ , 1 md = µ , 4 11 10 cm−α = × at 850 nm, 1collη = , and 1tη = with
antireflection coating on the incident surface. It’s lightly doped with 12 30 1 10 cmn −= × .
GaAs has the following characteristic parameters at 300 K: 013.2ε = ε at DC or low frequencies, 2 1 18500 cm V se
− −µ = , 2 1 1400 cm V sh− −µ = , 6 32.33 10 cmin −= × . The
bimolecular recombination coefficient 11 3 18 10 cm sB − −= × . (a) Find the external quantum efficiency for this device. (b) Under an incident optical power of 1sP = µW on the detection area, what is the carrier lifetime assuming bimolecular recombination dominates? (c) Find the dark conductivity. The device is biased at V = 2 V. Is the device limted by a
space-charge effect at any level of input optical signal? (d) What are the gain and the responsivity of this device? (e) What is the space charge-limited gain?
Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells
Exercise: Photoconductor Noise
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The photoconductor considered in the previous exercise is loaded with a sufficiently large resistance such that the resistive thermal noise is negligible compared to the shot noise from its dark current at the operating temperature of 300 K. The background radiation noise is also negligible. The incident wavelength 850λ = nm. (a) Find the dark resistance of the device. Then, find its dark current at 2V bias. (b) Find the NEP of the device for a bandwidth of 1 Hz, NEP/ 1/2B (W Hz-1/2). (c) Find the specific detectivity D* for the device. (d) Discuss how gain affects the NEP for a photoconductor.
Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells
p-n Junction Photodiode
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Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells
p-i-n Photodiode
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p+
i-S i n+
SiO2E lec trode
ρnet
–eNa
eNd
x
x
E(x)
R
Eo
E
e–h+
Iph
hυ > Eg
W
(a)
(b)
(c)
(d)
Vr
Vo ut
E lec trode
Drawbacks of p-n junction photodiode: (1) High junction capacitance → long RC time. (2) Thin depletion layer → low quantum efficiency. (3) Depletion width changes as bias changes. (4) Non-uniform e-field in the depletion region. Transit time across the depletion layer /tr dW vτ =→ Desirable to operate at saturation velocity (vd = vsat)
[ ](1 )1 exp( )s
pheP R
i Wh
−= − −α
ν
Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells
Photodetection Modes
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Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells
Exercise: Si p-i-n Photodiode
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Discuss the responsivity of a Si p-i-n photodiode at λ = 900 nm, given Ps = 100 nW and the reflection coefficient of the top surface = 32%. What would be the photocurrent and responsivity if the depletion layer thickness is 20 µm? What would be the maximum responsivity given an ideal device structure? Discuss the possible drawbacks of such a structure. For 20 µm-thick depletion layer, what’s the 3-dB cutoff frequency assuming saturation velocities are achieved for both electrons and holes, and the photodiode bandwidth is limited by its transit time?
Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells
Solar Radiation Spectrum
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AM0: Solar spectrum in outer space
AM1: Solar spectrum at sea level under normal light incidence
AM2: Solar spectrum at an incident angle resulting in twice the path length through the atmosphere
Solar cell aircraft Helios (Source: NASA Dryden Research Center)
Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells
Example: Solar Cell Driving a Load
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Solar cell area: 1cm x 1cm Illumination light intensity: 900 W m-2
Load resistance: 16 Ohm
What are the current and voltage in the circuit? What is the power delivered to the load? What is the efficiency of the solar cell? Assume it is operating close to the maximum efficiency point, what is the fill factor?
Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells
Absorption isn’t the Whole Story
16 (Atwater and Polman, “Plasmonics for improved PV devices,” Nature Materials 2010)
Light-trapping structures are desirable Anti-reflection surface is necessary
Si nanoshells
Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells
Utilizing the Full Solar Spectrum
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Multi-junction or tandem structure
Tandem colloidal QD solar cell
Sargent group, Nature Photonics (2011)
(b)