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: i 2 eB i ( i i ) n sh , s b d = + + 2 i 2 eBGF i ( i i ) for photodetectors with internal gain G n sh , s b d i : signal current s i : background radiation current b i : dark current d = 2 2 F G / G : Excess noise factor = = = 2 2 P 4 k TB i R v / R Thermal noise: n th , B n th , n th , Exercise: A photodetector without internal gain has a load resistance R = Ω and a bandwidth of B = 100 MHz. Input optical power is of 50 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? 2 Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells Noise Characteristics of Photodetectors 3 Lih Y. Lin
EE 529 Semiconductor Optoelectronics Discussion: Noise Characterization – Photodetectors and Solar Cells for a QD Photoconductor 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/Hz 1/2 . From the experimental results presented in Figure (b), determine the NEP and root- mean-square noise current at various modulation frequencies. rms( ) i = n NEP R + + 1/2 (2 ei 2 ei 4 k T / R ) 1/2 b d B = B (W) R A 1/2 ( B ) = ⋅ ⋅ − 1/2 1 D * (cm Hz W ) Normalized detectivity ( NEP ) 4 Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells Linearity and Dynamic Range sat P = s 10log Dynamic Range (DR) NEP 5 Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells Speed and Frequency Response 0.35 = f 3 dB t r Considering the rectangular time interval used to define the electrical bandwidth B when discussing noise, 0.443 = = f 0.886 B 3 dB T 6 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 λ = An n-type GaAs intrinsic photoconductor for 850 nm has the following parameters: = = µ d = µ α = × − η = η = 4 1 l w 100 m , 1 m , 1 10 cm at 850 nm, 1 , and 1 with coll t = × − 12 3 antireflection coating on the incident surface. It’s lightly doped with n 1 10 cm . 0 ε = ε at DC or low GaAs has the following characteristic parameters at 300 K: 13.2 0 µ = µ = = × − − − − − 2 1 1 2 1 1 6 3 frequencies, , , . The 8500 cm V s 400 cm V s n 2.33 10 cm e h i = × − − 11 3 1 bimolecular recombination coefficient B 8 10 cm s . (a) Find the external quantum efficiency for this device. P = µW on the detection area, what is the carrier (b) Under an incident optical power of 1 s 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? 8 Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells Exercise: Photoconductor Noise 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 λ = noise is also negligible. The incident wavelength 850 nm. (a) Find the dark resistance of the device. Then, find its dark current at 2V bias. (W Hz -1/2 ). 1/2 (b) Find the NEP of the device for a bandwidth of 1 Hz, NEP/ B (c) Find the specific detectivity D * for the device. (d) Discuss how gain affects the NEP for a photoconductor. 9 Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells p-n Junction Photodiode 10 Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells p-i-n Photodiode Drawbacks of p-n junction photodiode: SiO 2 Electrode Electrode p + (1) High junction capacitance → long RC time. (2) Thin depletion layer → low quantum efficiency. (3) Depletion width changes as bias changes. n + i -Si (a) (4) Non-uniform e-field in the depletion region. τ = Transit time across the depletion layer W / v ρ net tr d → Desirable to operate at saturation velocity (v d = v sat ) eN d (b) x – eN a E ( x ) x (c) E o W h υ > E g E e – h + (d) − eP (1 R ) 1 [ ] I ph R V out = − −α s i exp( W ) ν ph h V r 11 Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells Photodetection Modes 12 Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells Exercise: Si p-i-n Photodiode Discuss the responsivity of a Si p-i-n photodiode at λ = 900 nm, given P s = 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 µ m? What would be the maximum responsivity given an ideal device structure? Discuss the possible drawbacks of such a structure. For 20 µ m-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? 13 Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells Solar Radiation Spectrum Solar cell aircraft Helios (Source: NASA Dryden Research Center) 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 14 Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells Example: Solar Cell Driving a Load 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? 15 Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells Absorption isn’t the Whole Story Anti-reflection surface is necessary Light-trapping structures are desirable Si nanoshells (Atwater and Polman, “Plasmonics for improved PV devices,” Nature Materials 2010) 16 Lih Y. Lin
EE 529 Semiconductor Optoelectronics – Photodetectors and Solar Cells Utilizing the Full Solar Spectrum Multi-junction or tandem structure Tandem colloidal QD solar cell (b) Sargent group, Nature Photonics (2011) 17 Lih Y. Lin
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