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PHYSICAL ELECTRONICS(ECE3540) APPLICATIONS OF APPLICATIONS OF PHYSICAL ELECTRONICS PART I PHYSICAL ELECTRONICS PART I 1 Tennessee Technological University Monday, October 28, 2013 Introduction In the following slides, we will

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SLIDE 1

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PHYSICAL ELECTRONICS(ECE3540)

APPLICATIONS OF PHYSICAL ELECTRONICS – PART I APPLICATIONS OF PHYSICAL ELECTRONICS – PART I

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SLIDE 2

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Introduction

In the following slides, we will discuss the summary

  • f the Reading Assignment:

the concepts of a large reverse-bias voltage that cause a Junction Breakdown, the Zener Effect and the Avalanche Effect.

  • 1. Junction

Breakdown, Avalanche Breakdown, Tunneling Breakdown

  • 2. Zener Diodes
  • 3. Tunnel Diodes
  • 4. Applications of Physical Electronics I:

PN Junction Diodes. More discussion on these concepts in Chapter 12.

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SLIDE 3

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PHYSICAL ELECTRONICS(ECE3540) Explanation of the Reading Assignment Zener Diodes and Tunnel Diodes

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SLIDE 4

Junction Breakdown

Dominant if both sides of a junction are very heavily doped. Can be classified into two: 1. Zener Breakdown 2. Avalanche Breakdown

V/cm 10

6

 

crit p

E E

V I

Breakdown

Empty States Filled States - Ev Ec

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SLIDE 5

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  • 1. Zener Breakdown

A Zener diode is designed to operate in the breakdown mode.

V I VB, breakdown P N A R Forward Current Small leakage Current voltage

3.7 V

R IC A B C D Zener diode

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SLIDE 6

Peak Electric Field

2 / 1

|) | ( 2 ) (         

r bi s p

V qN   E E

bi crit s B

qN V     2

2

E

N+ P Na

Neutral Region

xp

(a) increasing reverse bias Deletion layer

x E xp

(b)

increasing reverse bias Ep

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SLIDE 7
  • 2. Avalanche Breakdown
  • Impact ionization: an energetic

electron generating electron and hole, which can also cause impact ionization.

qN V

crit s B

2

2

E  

  • Impact ionization + positive

feedbackavalanche breakdown

d a B

N 1 N 1 N 1 V   

Ec EFn Ec Ev EFp

  • riginal

electron electron-hole pair generation

Ev

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SLIDE 8

Quantum Mechanical Tunneling

) (

) ( 8 2 exp

2 2

E V h m T P

H 

  

Tunneling probability:

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A tunnel diode or Esaki diode is a type of semiconductor that is capable

  • f

very fast

  • peration,

well into the microwave frequency region, made possible by the use of the quantum mechanical effect called tunneling.

  • Fig. 1 Quantum Mechanical Tunneling
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SLIDE 9

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Tunnel Diode

  • Under normal forward bias operation, as voltage

begins to increase, electrons at first tunnel through the very narrow p–n junction barrier because filled electron states in the conduction band on the n-side become aligned with empty valence band hole states

  • n the p-side of the p-n junction.
  • As voltage increases further these states become more

misaligned and the current drops – this is called negative resistance because current decreases with increasing voltage. As voltage increases yet further, the diode begins to operate as a normal diode.

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SLIDE 10

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Tunnel Diode

  • In

the reverse direction, tunnel diodes are called back diodes (or backward diodes) and can act as fast rectifiers with zero offset voltage and extreme linearity for power signals (they have an accurate square law characteristic in the reverse direction).

  • Under reverse bias, filled states on the p-side

become increasingly aligned with empty states on the n-side and electrons now tunnel through the PN junction barrier in reverse direction.

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SLIDE 11

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Tunnel Diode

  • Fig. 2:

a) Simplified Energy band diagram of a tunnel diode with a reverse bias voltage b) I-V Characteristic of a Tunnel Diode with a reverse-bias voltage

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SLIDE 12

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PHYSICAL ELECTRONICS(ECE3540)

APPLICATIONS OF PN JUNCTION DIODES APPLICATIONS OF PN JUNCTION DIODES

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SLIDE 13

The PN Junction as a Temperature Sensor

What causes the IV curves to shift to lower V at higher T ?

) 1 (  

kT V q

e I I          

a n n d p p i

N L D N L D Aqn I

2

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  • Fig. 3: PN Junction diode as a Temperature Sensor
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SLIDE 14
  • Solar Cells are also known as

photovoltaic cells (PV).

  • Convert sunlight to electricity

with 10-30% conversion efficiency.

  • 1 m2 solar cell generate about

150 W peak

  • r

25 W continuous power.

  • Low cost and high efficiency

are needed for wide deployment.

Solar Cells

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  • Fig. 4: World Energy Consumption
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SLIDE 15

Solar Cell Basics

sc kT V q

I e I I    ) 1 (

V 0.7 V –Isc Maximum power-output Solar Cell IV I Dark IV Eq.(4.9.4) N P

  • Short Circuit

light Isc

+

(a)

Ec Ev

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SLIDE 16

Light Absorption

) ( 24 . 1 (eV) Energy Photon m hc     

x

  • e

(x) intensity Light

α(1/cm): absorption coefficient A thinner layer of direct-gap semiconductor can absorb most of solar radiation than indirect-gap semiconductor. Compare Si and Ge.

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  • Fig. 5: Photon Energy vs. Absorption

Coefficient

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SLIDE 17

Output Power

FF V I

  • c

sc

   Power Output

  • Theoretically, the highest efficiency (~24%) can be obtained with

1.9eV >Eg>1.2eV. Larger Eg lead to too low Isc (low light absorption); smaller Eg leads to too low Voc.

  • Tandem solar cells gets 35% efficiency using large and small Eg

materials tailored to the short and long wavelength solar light. A particular operating point on the solar cell I-V curve maximizes the

  • utput power (I x V).
  • Si solar cell with 15-20% efficiency

dominates the market now

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SLIDE 18

Light emitting diodes (LEDs)

  • LEDs are made of compound semiconductors such as InP and

GaN.

  • Light is emitted when electron and hole undergo radiative

recombination.

Ec Ev

Radiative recombination Non-radiative recombination through traps

Light Emitting Diodes and Solid-State Lighting

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SLIDE 19

LED Materials and Structure

) ( 24 . 1 energy photon 24 . 1 m) ( h wavelengt LED eV Eg   

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  • Fig. 7: LED Materials and Structure
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SLIDE 20

LED Materials and Structure

) (eV E

g

red yellow blue

Wavelength (μm)

Color

Lattice constant (Å)

InAs 0.36 3.44 6.05 InN 0.65 1.91

infrared

3.45 InP 1.36 0.92 5.87 GaAs 1.42 0.87 5.66 GaP 2.26 0.55

5.46

AlP 3.39 0.51 5.45 GaN 2.45 0.37 3.19 AlN 6.20 0.20

UV

3.11

Table: Light-emitting diode materials

compound semiconductors binary semiconductors:

  • Ex: GaAs, efficient emitter

ternary semiconductor :

  • Ex: GaAs1-xPx , tunable Eg (to vary

the color)

quaternary semiconductors:

  • Ex: AlInGaP , tunable Eg and lattice

constant (for growing high quality epitaxial films on inexpensive substrates)

violet

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SLIDE 21

Common LEDs

Spectral range Material System Substrate Example Applications Infrared InGaAsP InP Optical communication Infrared- Red GaAsP GaAs Indicator lamps. Remote control Red- Yellow AlInGaP GaA or GaP Optical communication. High-brightness traffic signal lights Green- Blue InGaN GaN or sapphire High brightness signal lights. Video billboards Blue-UV AlInGaN GaN or sapphire Solid-state lighting Red- Blue Organic semicon- ductors glass Displays

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SLIDE 22

Solid-State Lighting

Incandescent lamp Compact fluorescent lamp Tube fluorescent lamp White LED Theoretical limit at peak of eye sensitivity ( λ=555nm) Theoretical limit (white light)

17 60 50-100 90 683 ~340

luminosity (lumen, lm): a measure of visible light energy normalized to the sensitivity of the human eye at different wavelengths

Luminous efficacy of lamps in lumen/watt

Terms: luminosity measured in lumens, luminous efficacy

Organic Light Emitting Diodes (OLED) :

has lower efficacy than nitride or aluminide based compound semiconductor LEDs.

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SLIDE 23

Diode Lasers

(d) Net Light Absorption (e) Net Light Amplification Stimulated emission: emitted photon has identical frequency and directionality as the stimulating photon; light wave is amplified. (b) Spontaneous Emission (c) Stimulated Emission (a) Absorption

Light Amplification

Light amplification requires population inversion: electron

  • ccupation probability is larger

for higher E states than lower E states.

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SLIDE 24

Laser Applications

Red diode lasers: CD, DVD reader/writer Blue diode lasers: Blu-ray DVD (higher storage density) 1.55 m infrared diode lasers: Fiber-optic communication Photodiodes: Reverse biased PN diode. Detects photo-generated current (similar to Isc of solar cell) for optical communication, DVD reader, etc. Avalanche photodiodes: Photodiodes operating near avalanche breakdown amplifies photocurrent by impact ionization.

Photodiodes

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SLIDE 25

Picture Credits

  • Semiconductor Physics and Devices, Donald Neaman, 4th

Edition, McGraw Hill Publications.

  • Modern Semiconductor Devices for Integrated Circuits, Prof.

Chenming Calvin Hu, UC Berkeley.

http://www.eecs.berkeley.edu/~hu/Book-Chapters-and-Lecture-Slides- download.html

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