ECE685 Nanoelectronics –
Semiconductor Devices
Lecture given by Qiliang Li
Silicon Structure
• Unit cell of silicon crystal is
cubic.
• Each Si atom has 4 nearest
neighbors.
Dopants, Electrons and holes
Si
Si
Si
Si
Si
Si
Si
As
Si
Si
B
Si
Si
Si
Si
Si
Si
Si
DOPANT DENSITY cm-3
Relationship between Resistivity and Dopant Density
P-type
N-type
RESISTIVITY (cm)
 = 1/
GaAs, III-V Compound Semiconductors, and Their Dopants
Ga As
Ga
As Ga
As
Ga As
Ga
GaAs has the same crystal structure as Si.
GaAs, GaP, GaN are III-V compound semiconductors, important for
optoelectronics.
Wich group of elements are candidates for donors? acceptors?
Energy Band Model
}
Empty upper bands
(conduction band)
2p
2s
(valence band)
}
(a)
Filled lower bands
(b)
Energy states of Si atom (a) expand into energy bands of Si crystal (b).
 The lower bands are filled and higher bands are empty in a semiconductor.
The highest filled band is the valence band.
The lowest empty band is the conduction band .
Energy Band Diagram
Conduction band
Ec
Band gap
Eg
Ev
Valence band
Energy band diagram shows the bottom edge of conduction band,
Ec , and top edge of valence band, Ev .
 Ec and Ev are separated by the band gap energy, Eg .
Donor and Acceptor in the Band Model
Conduction Band
Ed
Donor Level
Ec
Donor ionization energy
Acceptor ionization energy
Acceptor Level
Ea
Valence Band
Ev
Ionization energy of selected donors and acceptors in silicon
Donors
Dopant
Sb
Ionization energy, E c –E d or E a –E v (meV) 39
P
44
Acceptors
As
54
B
45
Al
57
In
160
Device Fabrication
Oxidation
Lithography &
Etching
Ion Implantation
Annealing &
Diffusion
Beginning from a silicon wafer
Side View
Top View
Thermal Oxidation
Side View
Top View
Spin-on Photo Resist (PR)
Side View
Top View
Alignment, UV Expose and Develop Photo Resist (PR)
Side View
Top View
Oxide Etched
Side View
Top View
Remove Photo Resist (PR)
Side View
Top View
Doping (implantation or diffusion)
Side View
Top View
Grow Field Oxide (wet/dry) and dopant diffusion
Side View
Top View
Spin-on Photo Resist (PR)
Side View
Top View
Alignment, UV Expose and Develop Photo Resist (PR)
Side View
Top View
Oxide Etched
Side View
Top View
Remove Photo Resist (PR)
Side View
Top View
Grow Gate Oxide (dry)
Side View
Top View
Spin-on Photo Resist (PR)
Side View
Top View
Alignment, UV Expose and Develop Photo Resist (PR)
Side View
Top View
Field Oxide Etched
Side View
Top View
Field Oxide Etched
Side View
Top View
Metal (e.g., Aluminum) deposition
Side View
Top View
Spin-on Photo Resist (PR)
Side View
Top View
Alignment, UV Expose and Develop Photo Resist (PR)
Side View
Top View
Aluminum Etched
Side View
Top View
Remove Photo Resist (PR), annealing - complete
Side View
Top View
PN Junction
– V +
I
Donor ions
N
P
N-type
I
P-type
V
Reverse bias
Forward bias
PN junction is present in perhaps every semiconductor device.
Energy Band Diagram of a PN Junction
N-region
P-region
Ef
(a)
Ef is constant at
equilibrium
Ec
(b)
Ec
Ef
Ev
Ec and Ev are known
relative to Ef
Ev
Ec
Ef
Ev
(c)
Neutral
N-region
Depletion
layer
Neutral
P-region
Ec
(d)
Ef
Ev
Ec and Ev are smooth,
the exact shape to be
determined.
A depletion layer
exists at the PN
junction where n  0
and p  0.
Light emitting diodes (LEDs)
•LEDs are made of compound semiconductors such as InP and
GaN.
• Light is emitted when electron and hole undergo radiative
recombination.
Ec
Radiative
recombination
Ev
Non-radiative
recombination
through traps
LED Materials and Structure
1.24
1.24
LED wavelengt h (  m) 

photon energy Eg (eV )
Common LEDs
AlInGaP
Quantun Well
Schottky Diodes
Forward
biased
V=0
I
Reverse
biased
Reverse bias
V
Forward bias
MOS: Metal-Oxide-Semiconductor
Vg
Vg
gate
gate
metal
SiO2
SiO2
N+
Si body
MOS capacitor
N+
P-body
MOS transistor
Surface Accumulation
Vox  Vg  V fb
Vg <Vt
Gauss’s Law
Vox  Qacc / Cox
Qacc  Cox (Vg  V fb )
Vox  Qs / Cox
Surface Depletion (
V
g
> V
fb
)
qVox
q s
gate
++++++
V
-- -- -- -- -- -- --------
depletion layer
charge, Q dep
P-Si body
2
qVg
Wdep
depletion
region
Ec, E f
Ev
M
Ef
Ev
--
SiO
O
qN a 2 s s
Qdep qN aWdep
Qs
Vox  



Cox
Cox
Cox
Cox
Ec
S
Threshold Condition and Threshold Voltage
Ec
Threshold (of inversion):
ns = Na , or
st
(Ec–Ef)surface= (Ef – Ev)bulk , or
 A=B, and C = D
kT  N a 
st  2B  2 ln  
q  ni 
qVg = qVt
A
D
Ei
C =qB
B
Ec, Ef
Ev
M
O
S
kT  N v  kT  N v  kT  N a 
 

q B 
 ( E f  Ev ) |bulk 
ln   
ln 
ln 
2
q  ni  q  N a  q  ni 
Eg
qN a 2 s 2B
Vt  Vg at threshold  V fb  2B 
Cox
Ef
Ev
V t(V), N+ gate/P-body
Tox = 20nm
Vt (V), P+ gate/N-body
Threshold Voltage
Body Doping Density (cm -3 )
Cox
ody
Tox = 20nm
+ for P-body,
– for N-body
ody
Vt  V fb  2 B 
qN sub
(a)2 s 2 B
Strong Inversion–Beyond Threshold
Vg > V t
Wdep
2 s 2B
 Wdmax 
qN a
Vg > Vt
-
gate
---
++++++++++
V
- - - - - - Q dep
Q inv
P - Si substrate
Ef
Ev
qVg
SiO2
- - - - - - - -
Ec
E c, Ef
Ev
M
O
S
Basic MOSFET structure and IV characteristics
+
+
dI ds
W
 0  Coxe  ns (Vgs  Vt  mVds )
dVds
L
Vdsat 
Vgs  Vt
m