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

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max

1.24

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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

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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

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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