2. Impact of doping on silicon resistivity
silicon
4.995×1022 atoms in cm3
Resistivity 3.2 × 105 Ωcm
dope with
dope with
phosphorous
boron →
or arsenic →
p-type
n-type
1 atom in billion ⇒ 88.6 Ωcm 1 atom in billion ⇒ 266.14 Ωcm
1 atom in million ⇒ 0.114 Ωcm 1 atom in million ⇒ 0.344 Ωcm
1 atom in thousand ⇒ 0.00174 Ωcm 1 atom in thousand ⇒ 0.00233 Ωcm
⇒ Electrons are more mobile/faster than holes
3. What happens if we sandwich p & n types?
A Al
p
n
B
One-dimensional
representation
In equilibrium, the drift and diffusion components of current
are balanced; therefore the net current flowing across the
junction is zero.
4. PN-junction regions of operation
A forward bias
In reverse bias, the width decreases the potential
of the depletion region drop across the
increases. The diode acts junction. As a result,
as voltage-controlled the magnitude of the
electric field decreases
capacitor. and the width of the
depletion region
narrows.
5. nMOS and pMOS transistors
Each transistor consists of a stack of a conducting gate, an insulating
layer of silicon dioxide and a semiconductor substrate (body or bulk)
nMOS transistor pMOS transistor
Source Gate Drain
Source Gate Drain Polysilicon
Polysilicon
SiO2
SiO2
polysilicon
gate
W
n+ n+ p+ p+
tox
p bulk Si L SiO2 gate oxide n bulk Si
n+ n+ (good insulator, εox = 3.9)
p-type body
Body is typically grounded Body is typically at supply voltage
6. nMOS transistor
Source Gate Drain
Polysilicon
SiO2
n+ n+
p bulk Si
g=0: When the gate is at a low voltage (VGS < VTN):
p-type body is at low voltage
source and drain-junctions diodes are OFF
transistor is OFF, no current flows
g=1: When the gate is at a high voltage (VGS ≥ VTN):
negative charge attracted to body
inverts a channel under gate to n-type
transistor ON, current flows, transistor
can be viewed as a resistor
7. nMOS pass ‘0’ more strongly than ‘1’
Source Gate Drain
Polysilicon
SiO2
n+ n+
p bulk Si
• Why does ‘1’ pass degraded?
8. pMOS transistor
Source Gate Drain
Polysilicon
SiO2
p+ p+
n bulk Si
g=0: When the gate is at a low voltage (VGS < VTP):
positive charge attracted to body
inverts a channel under gate to p-type
transistor ON, current flows
g=1: When the gate is at a high voltage (VGS ≥ VTP):
negative charge attracted to body
source and drain junctions are OFF
transistor OFF, no current flows
9. pMOS pass ‘1’ more strongly than ‘0’
Source Gate Drain
Polysilicon
SiO2
p+ p+
n bulk Si
• Why does ‘0’ pass degraded?
10. An nMOS and pMOS make up an inverter
pMOS + nMOS = CMOS
11. More CMOS gates
B
A
B
F = AB
0
What is this gate function? What’s wrong about this design?
12. 3-input NANDs
What are the advantages of CMOS circuit style?
