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SEMICONDUCTOR DEVICES
Ho Kyung Kim, Ph.D.
School of Mechanical Engineering
Pusan National University
Basic Experiment and Design of Electronics
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Outline
• Fundamentals of semiconductors
• Diode I-V characteristics
• Photodiodes
2
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• 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-
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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
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– 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-
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• 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
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• 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
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• PN junction
p n
p n
electron hole
positive ion negative ion
Diffusion
Electron diffusion current
Hole diffusion current
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p n
DriftElectric field
Electron drift current
Hole drift current
Electron diffusion current
Hole diffusion current
Depletion (or space charge) region
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p n
+ -VgRp Rn
Potential barrier, Vg
or contact potential
or offset voltage = 0.6 ~ 0.7 V
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• 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.
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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)
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• 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
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• 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)
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Breakdown voltage ~ 20 V but Zener breakdown voltage ~ 5 V
Zener diode
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• 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
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VR
RL
Slope = -1/RL
Current
Voltage
l
VD
ID
load line for normal operation
load line for solar cell
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• 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