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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 d

i eB i i i = + +

2 ,

2 ( )

n sh s b d

i eBGF i i i = + + : signal current

s

i : background radiation current

b

i : dark current

d

i

Thermal noise:

2 2 , , ,

4 /

n th B n th n th

P k TB i R v R = = =

Exercise: A photodetector without internal gain has a load resistance

• f

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

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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.

1/2 1/2

rms( ) (2 2 4 / ) = (W)

n b d B

i NEP ei ei k T R B = + + R R

1/2 1/2 1

( ) * (cm Hz ) ( ) B D W NEP

−

= ⋅ ⋅ A

Normalized detectivity

Lih Y. Lin EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells

Linearity and Dynamic Range

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Dynamic Range (DR)

10log

sat s

P NEP =

Lih Y. Lin EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells

Speed and Frequency Response

6 3

0.35

dB r

f t =

Considering the rectangular time interval used to define the electrical bandwidth B when discussing noise,

3

0.443 0.886

dB

f B T = =

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

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Lih Y. Lin EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells

Exercise: Photoconductor Gain

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An n-type GaAs intrinsic photoconductor for 850 λ = nm has the following parameters: 100 m l w = = µ , 1 m d = µ ,

4 1

1 10 cm

−

α = × at 850 nm, 1

coll

η = , and 1

t

η = with antireflection coating on the incident surface. It’s lightly doped with

12 3

1 10 cm n

−

= × . GaAs has the following characteristic parameters at 300 K: 13.2 ε = ε at DC or low frequencies,

2 1 1

8500 cm V s

e − −

µ = ,

2 1 1

400 cm V s

h − −

µ = ,

6 3

2.33 10 cm

i

n

−

= × . The bimolecular recombination coefficient

11 3 1

8 10 cm s B

− −

= × . (a) Find the external quantum efficiency for this device. (b) Under an incident optical power of 1

s

P = 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 850 λ = 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/

1/2

B (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

11 p+ i-Si n+ SiO

2

Electrode ρnet –eNa eNd x x E(x) R Eo E e– h+ Iph hυ > Eg W (a) (b) (c) (d) Vr Vout Electrode

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 d

W v τ =

→ Desirable to operate at saturation velocity (vd = vsat)

[ ]

(1 ) 1 exp( )

s ph

eP R i W h − = − −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

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(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)