SEMICONDUCTOR DEVICES

Ho Kyung Kim, Ph.D.

hokyung@pusan.ac.kr

School of Mechanical Engineering

Pusan National University

Basic Experiment and Design of Electronics

Outline

• Fundamentals of semiconductors

• Diode I-V characteristics

• Photodiodes

2

• Intrinsic semiconductor

– elements from group IV of the periodic table

– 4 valence electrons in the outer (valence shell)

– crystal structure by covalent bonds

– intrinsic (carrier) concentration: number of free electrons

• ni = 1.51016 electrons/cm3

Fundamentals of semiconductors

SiSi Si

Si

Si

e-e-

e-

e-

e-

e-

e- e-

e-

e-e-

e-

e-

e-

e-e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-e-

e-

+14 e-

e-

e-

e-

e-e-

e- e-e- e-

e- e-e-e-

e- e-e-

Conduction band

Energy

Valence band

Second band (shell 2)

First band (shell 1)

Energy gap

Energy gap

Energy gap

• Energy band diagram

– conduction band, valence band, forbidden gap (or energy gap)

– electron conduction; recombination

e-

Energy

e- Free electron

Hole

Electron-hole pair

– Hole conduction

e-Si

e-

e-

Si Si

e-

e-

SiSi

e-

e-

Si

e-

e-

Si

e-

e-

Si

e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

• Conductors, semiconductors, insulators

Valence band

Energy

Energy gap

Conduction band

Valence band

Energy gap

Conduction band

Energy

Valence band

Overlap

Conduction band

Energy

Insulators Semiconductors Conductors

• Doping

– a process to control the number of charge carriers

– adding impurities (or dopants) in the crystalline structure

– two kinds of dopants

• donors: group V (e.g., P, As, Sb) additional (free) electrons

n-type (n >> ni and p << ni) ; majority carrier n and minority carrier p

• acceptors: group III (e.g, B, Al, Ga, In) additional (free) holes

p-type (p >> ni and n << ni) ; ; majority carrier p and minority carrier n

• PN junction

p n

p n

electron hole

positive ion negative ion

Diffusion

Electron diffusion current

Hole diffusion current

p n

DriftElectric field

Electron drift current

Hole drift current

Electron diffusion current

Hole diffusion current

Depletion (or space charge) region

p n

+ -VgRp Rn

Potential barrier, Vg

or contact potential

or offset voltage = 0.6 ~ 0.7 V

• Minority carriers

– thermally generated holes in n-type semiconductors (or electrons in p-type semiconductors)

– recombining with free electrons (or holes)

– some of minority hole carriers (or electrons) drifting into the depletion region and pushed across the junction by E field (small) reverse saturation current, IS

• independent of the junction voltage

• determined by thermal carrier generation (i.e., dependent on the temp.)

• e.g., 1 nA in Si at room temp.

p n

x

Charge density

x

Electric field

r+

r-

E = dE/dx = r/e

x

Electric potential

(for holes)

vD

E = -V or V = -Edx

x

Electric potential

(for electrons)

• What happens when applying a bias at PN junction?

p n p np n

vD vD - VBvD + VB

VB -VB

Electric field Electric field Electric field

Electron drift current

Hole drift current

Electron diffusion current

Hole diffusion current

Forward-biased Reverse-biased

• Reverse-biased diodes

– negligible diffusion current because of a large potential barrier

(contact potential diffusion of majority carrier )

–

Diode I-V characteristics

p n

-VB

iD = -I0 = IS

SD IIi 0

• Forward-biased diodes

– lowering the potential barrier

(contact potential diffusion of majority carrier )

– increasing diffusion current as a function of external voltage

–

p n

VB

iD = id – I0 id

kTqv

dDeII

/

0

)1(/

00 kTqv

dDDeIIIi diode equation

where k = 1.38110-23 J/K = Boltzmann's constant

kTqv

DDeIi

/

0 ( I0 = 10-9 ~ 10-15 A)

Breakdown voltage ~ 20 V but Zener breakdown voltage ~ 5 V

Zener diode

• Photoionization

– light photons depletion region of PN junction generation of electron-hole pairs

Photodiodes

Conduction band

Energy gap

Valence band

Energy

level

+

-

Photon

(hv)

Hole

Electron

VR

RL

Slope = -1/RL

Current

Voltage

l

VD

ID

load line for normal operation

load line for solar cell

• Electroluminescence

– reversed operation of the photodiode

– large recombination in the depletion region of PN junction

– energy release in the form of heat and light from recombining electrons

Light-emitting diodes