OPTI 202L
Lab #12 – p-n Junctions:
Photodiodes, Solar Cells
LED’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 2014
Lecture 12
Photodiodes—Physical Construction:
“Official Symbol”
A 40 Gb/s “Optical Receiver” !!
Dr. Mike Nofziger 2014
Lecture 12
Photodiodes—Basic Properties:
● 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).
Dr. Mike Nofziger 2014
Lecture 12
Photodiodes—Basic Physics:
h 
hc
 m ax 

 Eg
hc
Eg

1.24
Eg
For S i, E g  1.12 eV
  m ax  1.1 u m
ACTIVE A REA
A-R C OAT
P+ D IFFUSION
DEPLETION REGION
SiO
2
N-TYPE BU LK SILIC ON
METAL CON TACT
Dr. Mike Nofziger 2014
Lecture 12
Photodiodes—Current-Voltage Characteristics:
qV
I dark  I o  e
kT
 1 
The “Shockley diode equation”
Io is the reverse saturation current
V is the voltage across the junction
I p   qE e Ad
  q e

Photocurrent generated by irradiance Ee (W/m2)
hc

Photocurrent generated by optical power ϕe (W)
hc
qV
I T O T A L  I o  e
kT
 1   I p
Dr. Mike Nofziger 2014
Lecture 12
Photodiodes—Current-Voltage Characteristics:
qV
I T O T A L  I o  e
kT
 1   I p
Dr. Mike Nofziger 2014
Lecture 12
Photodiodes—Use in an Electrical Circuit:
Operated at V = 0 “zero-bias”: Output is very linear over 7-decades of flux
Operated at –V “reverse-bias”: Capacitance decreases, speed increases
Operated at I≈0 “open-circuit”: The open-circuit voltage is logarithmic with flux:
V oc 
 kT
q
 I o  I ph
ln 
Io





NOT the preferred way to operate a photodiode!
Dr. Mike Nofziger 2014
Lecture 12
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 2014
Lecture 12
Basics of Solar Cells:
● The Power (Watts) that the cell can produce is
given by:
P V I
● Because of internal resistance in the cell, the
maximum power you can generate is across a load
resistance equal to the internal resistance.
Fill Factor 
P m ax
I sc  V oc
www.keithley.com
Dr. Mike Nofziger 2014
Lecture 12
“Sources of Light”
LED – Light Emitting Diode
– solid-state, semiconductor p-n junction
– GaAs, AlGaAs, GaP, AlGaInP, GaN, InGaN,etc.
– 3mm, 5mm, or 8mm dia. plastic packages
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 2014
Lecture 12
“Sources of Light”
LED – Light Emitting Diode
Dr. Mike Nofziger 2014
Lecture 12
“Sources of Light”
White-Light LED’s: General Approaches
Reference
Dr. Mike Nofziger 2014
Lecture 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
Reference
– Color-mixing more easily controlled to produce
a pure white light.
– Example: CREE
Dr. Mike Nofziger 2014
Lecture 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 2014
Lecture 12
“Sources of Light”
White-Light LED’s: TRENDS
Reference
Reference
Dr. Mike Nofziger 2014
Lecture 12
“Sources of Light”
White-Light LED’s: Applications
vs.
Dr. Mike Nofziger 2014
Lecture 12
“Sources of Light”
White-Light LED’s: Applications
Uses a CREE XM-L 1000 lumen LED
Dr. Mike Nofziger 2014
Lecture 12
Laser Diodes—LI Curve
Dr. Mike Nofziger 2014
Lecture 12