SEMICONDUCTOR DEVICES Ho Kyung Kim, Ph.D. [email protected] School of Mechanical Engineering Pusan National University Basic Experiment and Design of Electronics
SEMICONDUCTOR DEVICES
Ho Kyung Kim, Ph.D.
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