Spring 2007EE130 Lecture 21, Slide 1 Lecture #21 ANNOUNCEMENTS No coffee hour today Quiz #3 on Friday (March 9) –Material of HW #5 & #6 (Lectures 13-17)

EE130 Lecture 21, Slide 1

Spring 2007

Lecture #21

ANNOUNCEMENTS

• No coffee hour today • Quiz #3 on Friday (March 9)

– Material of HW #5 & #6 (Lectures 13-17)– Closed book, no calculators; 3 pages of notes allowed

OUTLINE

• pn Junctions:– transient response: turn-on

• Summary of important pn-diode concepts

• pn diode applications

Reading: Chapters 8 & 9

EE130 Lecture 21, Slide 2

Spring 2007

x

Turn-On TransientAgain, consider a p+n diode (Qp >> Qn):

t

i(t)

t

vA(t)

0 pxx

n

qAD

i

dx

dp

n

For t > 0:

pn(x)

xn

EE130 Lecture 21, Slide 3

Spring 2007

• By separation of variables and integration, we have

• If we assume that the build-up of stored charge occurs quasi-statically so that

then

0for ττ

tQ

IQ

idt

dQ

p

pF

p

pp

ptpFp eItQ τ/1τ)(

pkTqv

pdiffusionpAeIItQ τ1τ)( /

0

ptF

A eI

I

q

kTtv τ/

0

11ln)(

EE130 Lecture 21, Slide 4

Spring 2007

• If p is large, then the time required to turn on the diode is approximately Q/IF

jp QQQ where

EE130 Lecture 21, Slide 5

Spring 2007

Summary of Important Concepts

• Under forward bias, minority carriers are injected into the quasi-neutral regions of the diode

• Current flowing across junction is comprised of hole and electron components

• In order for one of these components to be dominant, the junction must be asymmetrically doped

EE130 Lecture 21, Slide 6

Spring 2007

Summary of Important Concepts (cont.)

• The ideal diode equation stipulates the relationship between JN(-xp) and JP(xn)

If holes are forced to flow across a forward-biased junction, then electrons must also be injected across the junction

EE130 Lecture 21, Slide 7

Spring 2007

Summary of Important Concepts (cont.)• Under reverse bias, minority carriers are collected

into the quasi-neutral regions of the diode• Minority carriers within a diffusion length of the

depletion region will diffuse into the depletion region and then be swept across the junction by the electric field.

Current flowing in a reverse-biased diode depends on the rate at which minority carriers are supplied in the quasi-neutral regions

EE130 Lecture 21, Slide 8

Spring 2007

Varactor Diodes

• Voltage-controlled capacitor– Used in oscillators and detectors

(e.g. FM demodulation circuits in your radios)

– Response changes by tailoring doping profile:

21mn

biVV

for

VC

r

nrj

EE130 Lecture 21, Slide 9

Spring 2007

Tunnel Diodes

• Degenerately doped such that EFp < Ev and EFn > Ec

• Can achieve negative differential resistance– useful in high-speed

circuits and perhaps static memories

EE130 Lecture 21, Slide 10

Spring 2007

Tunnel Diodes (cont.)

EE130 Lecture 21, Slide 11

Spring 2007

)(~)(

)1( A0

NPopNPopop

opkTVq

LLqAgLWLqAgI

IeII

Optoelectronic Diodes

EE130 Lecture 21, Slide 12

Spring 2007

1ln

0A opnLp

L

LL

qkT

Ioc gVVp

n

nn

p

p

np

EE130 Lecture 21, Slide 13

Spring 2007

p-i-n Photodiodes

• W Wi-region, so most carriers are generated in

the depletion region faster response time (~10 GHz operation)

• Operate near avalanche to amplify signal

EE130 Lecture 21, Slide 14

Spring 2007

Light Emitting Diodes (LEDs)

•LEDs are typically made of compound semiconductors

(direct bandgap)

EE130 Lecture 21, Slide 15

Spring 2007

Organic LEDs

• Some organic materials exhibit semiconducting properties– OLEDs are attractive for

low-cost, high-quality flat-panel displays