B

EvBE

+

–

iB

IBS

bFib

B

E

C

vBE

+

–

iFIES

aFiF

vBC

+

–

iR

ICS

aRiR

vCE

+

–

6.012 - Electronic Devices and Circuits Lecture 10 - Junction Device Wrap-up - Outline

• Announcements Handout - Lecture Outline and Summary First Hour Exam - Tomorrow!! Rm. 34-101, 7:30-9:30 pm Recitations - no recitations tomorrow but instructors will have office hours

Forward active region: vBE > 0.6 V vCE > 0.2 V

(i.e. vCB < 0.4V)iR is negligible

Other regionsCutoff:

vBE < 0.6 V Saturation: vCE < 0.2 V

B

E

C

vBE +

–

iBB

E

C

vBE

+

–

iF IES

aFiF

vBC

+

–

iR

ICS

aRiR

C• Review/BJT model wrap-upaFIESeqVBE/kTRev. biased junctions as sinks

or60 mV rule for diodes, BJTs bFiB

Limitations of BJT model IES

• Photodiodes and solar cells Optical and electrical excitation: superposition, iD(vAB, L)Detecting lightGenerating electrical power: optical-to-electrical conversionVideo: "Solar cell electricity is better electricity - putting 6.012 to work

improving our world (a true story)"

• Light emitting diodes; laser diodesDiode design for efficient light emission: materials, structure The LED renaissance: red, amber, yellow, green, blue, white Video: "Prof. Fonstad goes to Newton to watch the traffic lights change"Laser diodes

Clif Fonstad, 10/03 Lecture 10 - Slide 1

Bipolar transistors in history: An early bipolar integrated

circuit

Part of a series of USpostage stampscommemoratingthe decade of the1960's

C. G. Fonstad, 4/03 Lecture 10 - Slide 2

Integrated circuit bipolar transistors: An early bipolar integrated circuit

E C E BA Fairchild

Semiconductor digital (DTL) IC

from 1964

B

(before most of us were born!)

C. G. Fonstad, 4/03 Lecture 10 - Slide 3

Images courtesy of Fairchild Semiconductor. Used with permission.

BJT's, review: Looking at BJT characteristics

Regions of operation, Gummel plots (and the 60 mV rule for junctions)

•Regions of operation: forward and reverse active, saturation, cutoff

(above)

•Gummel plots and the 60 mV rule for junction diodes and BJTs (at right)

Clif Fonstad, 9/03 Lecture 10 - Slide 4

BJT's, review: Limitations of the large signal model Limitations of our junction model - their impact on BJT characteristics

We looked at this figure when we discussed our diode junction models:

•Large forward bias: High level injection (c) Series voltage drop (d)

• Large reverse bias: See Sze, S.M. Physics of Semiconductor Devices.Reverse breakdown pg. 105. ISBN 471-842-90-7.

• Very low bias levels: SCL generation and recombination (a, e)

Ref: Figure 18 in S. M. Sze,“Physics of SemiconductorDevices” 1st. Ed (Wiley,1969)

Clif Fonstad, 9/03 Lecture 10 - Slide 5

BJT's, review: Limitations of the large signal model-

•

•

Limitations of our junction model their impact on BJT characteristics

Beta roll-off at high and low collector currents

B-C junction breakdown; Clif Fonstad, 9/03 base punch through Lecture 10 - Slide 6

BJT's, review: Limitations of the large signal model Limitations of our junction model - their impact on BJT characteristics

iC

vCEvA •Base width modulation, the

Early effect and Early voltage

•Punch through: When the delpletion region at the B-C junction extends

through the base all the way to the collector (has a similar effect on the characteristics as does B-C junction reverse breakdown.

Clif Fonstad, 9/03 Lecture 10 - Slide 7

700Light emitting diodes

- human eye response

400 500 550 650 700 750

510 nm 610 nm

Viol

et

Blu

e

Gre

en

Yello

w

Ora

nge

Red

400

500

600

300

200

100

Φv:

Lum

inou

s flu

x (lm

)

450 600

C. G. Fonstad, 4/03 Lecture 10 - Slide 8

Light emitting diodes ­typical spectra

Relative Photon Intensity vs Wavelength • LED emission - typ. 20 nm wide

GaAsP red LED

InP • Important spectra for

comparison with LEDspectrum

Relative Spectral Response or Output vs Wavelength

C. G. Fonstad, 4/03 Lecture 10 - Slide 9

Light emitting diodes: fighting total internal reflection

Total internal reflection can be alleviated if the device is packaged in a domed shaped, high index plastic package:

If the device is fabricated with a substrate that is transparent to the emitted radiation, then light canbe extracted from the 4 sides and bottom of the device as well as from the top. This increases the extraction efficiency by a factor of 6!

C. G. Fonstad, 4/03 Lecture 10 - Slide 10

Light emitting diodes:fighting total internal

reflection, and keeping it cool (getting the light and the heat both out).

C. G. Fonstad, 4/03 Lecture 10 - Slide 11

6.012 - Electronic Devices and Circuits Lecture 10 - Junction Device Wrap-up - Summary

• BJT model wrap-upLimitations of BJT model:

@ low current levels: SCL g-r can be significant and reduce bF @ high current levels: series resistance and high level injection reduce bF w. base width modulation: iC will not truly saturate in FAR @ large vCE: B-C junction can break down and/or base punch throughDevices and model optimized for FAR

• Photodiodes and solar cellsCharacteristic: iD(vAB, L) = IS(eqvAB/kT -1) - IL

Reverse or zero bias: iD(vAB < 0) ≈ – IL (detects the presence of light) In fourth quadrant: iD x vAB < 0 (power is being produced!!)

• Light emitting diodes; laser diodesMaterials: red: GaAlAs, GaAsP, GaP amber: GaAsP

yellow: GaInN green: GaP, GaN blue: GaN white: GaN w. a phosphor

The LED renaissance: new materials (phosphides, nitrides)new applications (fibers, lighting, displays, etc)

Laser diodes: CD players, fiber optics, pointersCheck out: http://www.britneyspears.ac/lasers.htm

Clif Fonstad, 10/03 Lecture 10 - Slide 12