opti 202l lab #12 – p-n junctions: photodiodes, solar cells led’s, and laser diodes dr. mike...
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OPTI 202LLab #12 – p-n Junctions: Photodiodes, Solar CellsLED’s, and Laser Diodes
Dr. Mike Nofziger
Professor
College of Optical Sciences
University of Arizona
Dr. Mike Nofziger 2014
Lecture #12 Outline:
● Photodiodes—Physical Construction—Basic Properties—Basic Physics—Current Voltage Characteristics—Use in an Electrical Circuit
● Solar Cells—Basic Properties
● LED’s—Basic Properties
Dr. Mike Nofziger 2014Lecture 12
Photodiodes—Physical Construction:
A 40 Gb/s “Optical Receiver” !!
“Official Symbol”
Dr. Mike Nofziger 2014Lecture 12
Photodiodes—Basic Properties:
Dr. Mike Nofziger 2014Lecture 12
● p-n junction (p-side ≡ “anode”, n-side ≡ “cathode”)
● Built-in electric field (depletion region) separates the electrons and holes (electrons → p-side, holes → n-side)
● Photons absorbed (ideally in or near the depletion region) create electron-hole pairs
● Built-in electric field separates the electrons and holes before they recombine, producing a photocurrent (electrons → n-side, holes → p-side)
● I-V curve is very non-linear
● The photocurrent is linear with photon flux over 7-decades!
● Most common semiconductor material used to make photodiodes (for detection of visible light) is Silicon (Si).
Photodiodes—Basic Physics:
Dr. Mike Nofziger 2014Lecture 12
g
hch E
max
1.24
g g
hc
E E
max
For Si, 1.12
1.1 m
gE eV
u
METAL CONTACT
N-TYPE BULK SILICON
A-R COAT
ACTIVE AREA
SiO 2
P+ DIFFUSION
DEPLETION REGION
Photodiodes—Current-Voltage Characteristics:
Dr. Mike Nofziger 2014Lecture 12
1qV kTdark oI I e
p e d
e
I qE Ahc
qhc
1qV kTTOTAL o pI I e I
The “Shockley diode equation”
Io is the reverse saturation currentV is the voltage across the junction
Photocurrent generated by irradiance Ee (W/m2)
Photocurrent generated by optical power ϕe (W)
Photodiodes—Current-Voltage Characteristics:
Dr. Mike Nofziger 2014Lecture 12
1qV kTTOTAL o pI I e I
Photodiodes—Use in an Electrical Circuit:
Dr. Mike Nofziger 2014Lecture 12
Operated at V = 0 “zero-bias”: Output is very linear over 7-decades of fluxOperated at –V “reverse-bias”: Capacitance decreases, speed increases
Operated at I≈0 “open-circuit”: The open-circuit voltage is logarithmic with flux:
o
phooc I
II
q
kTV ln
NOT the preferred way to operate a photodiode!
Basics of Solar Cells:
● A solar cell is a Photovoltaic (“PV”) detector: - is made of Silicon (not silicone!!) - absorbs light from ≈ 350nm – 1100nm
- the absorption of light “frees up” electrons
- This creates a voltage at the terminals of the cell(the “Open-Circuit” voltage)
- If the cell’s terminals are shorted, the maximum current will flow
(the “Short-Circuit” current)
- If a load resistor is connected to the cell, a current will flow(the “Photocurrent”)
Dr. Mike Nofziger 2014Lecture 12
Basics of Solar Cells:
● The Power (Watts) that the cell can produce is given by:
● Because of internal resistance in the cell, the maximum power you can generate is across a load resistance equal to the internal resistance.
www.keithley.com
P V I
maxFill Factorsc oc
P
I V
Dr. Mike Nofziger 2014Lecture 12
LED – Light Emitting Diode– solid-state, semiconductor p-n junction– GaAs, AlGaAs, GaP, AlGaInP, GaN, InGaN,etc.– 3mm, 5mm, or 8mm dia. plastic packages
“Sources of Light”
Low-Power LED’s 1-20mA of electrical current “on/off indicators”
High-power LED’s 500mA to >1A Lumileds, Osram, Cree Lighting!! Replace incandescent bulbs
Dr. Mike Nofziger 2014Lecture 12
“Sources of Light”
LED – Light Emitting Diode
Dr. Mike Nofziger 2014Lecture 12
“Sources of Light”
White-Light LED’s: General Approaches
Reference
Dr. Mike Nofziger 2014Lecture 12
“Sources of Light”White-Light LED’s (3 phosphors, “tri-phosphor”)
– AlGaInN LED is the source of light – emits light in the near-UV spectral region
– 380-430 nm– 3 phosphor coatings on top
– Europium-based red and blue phosphors – Copper and aluminum doped zinc sulfide green phosphor
– Color-mixing more easily controlled to produce a pure white light. – Example: CREE
Reference
Dr. Mike Nofziger 2014Lecture 12
“Sources of Light”
White-Light LED’s (1 phosphor)
– GaN semiconductor LED is the source of light – emits blue light 450 – 470 nm
– Yellow phosphor coating on top – Cerium-doped YAG: Y3Al5O12:Ce3+
– light is emitted by scintillation (no afterglow)
Example: CREE
Dr. Mike Nofziger 2014Lecture 12
“Sources of Light”
White-Light LED’s: TRENDS
Reference
Reference
Dr. Mike Nofziger 2014Lecture 12
“Sources of Light”
White-Light LED’s: Applications
vs.
Dr. Mike Nofziger 2014Lecture 12
“Sources of Light”
White-Light LED’s: Applications
Uses a CREE XM-L 1000 lumen LED
Dr. Mike Nofziger 2014Lecture 12
Laser Diodes—LI Curve
Dr. Mike Nofziger 2014Lecture 12