EE 434 Lecture 12

1
EE 434Lecture 12

• Devices in Semiconductor Processes
• Diodes
• Capacitors
• MOS Transistors

2
Quiz 10
A 10K resistor has a temperature coefficient of
80ppm/oC If the resistor was measured to be
9.83K at 20oC, what would be the resistor value
at 80oC?
3
And the number is .
4
Quiz 10
A 10K resistor has a temperature coefficient of
80ppm/oC If the resistor was measured to be
9.83K at 20oC, what would be the resistor value
at 80oC?
Solution
5
Review from Last Time

• Process Flow is a recipe for the process
• Shows what can and can not be made
• Gives insight into performance capabilities and
  limitations
• Back-End Processes
• Die attach options (eutectic, preform,conductive
  epoxy)
• Stresses the die
• Bonding
• Wire bonding
• Bump bonding
• Packaging
• Many packaging options
• Package Costs can be large so defective die
  should be eliminated before packaging

6
Basic Devices and Device Models

• Resistor
• Diode
• Capacitor
• MOSFET
• BJT

7
Diodes (pn junctions)
N
P
Depletion region created that is ionized but void
of carriers
8
pn Junctions
N
P
Physical Boundary Separating n-type and p-type
regions
If doping levels identical, depletion region
extends equally into n-type and p-type regions
9
pn Junctions
N
P-
Physical Boundary Separating n-type and p-type
regions
Extends farther into p-type region if p-doping
lower than n-doping
10
pn Junctions
N-
P
Physical Boundary Separating n-type and p-type
regions
Extends farther into n-type region if n-doping
lower than p-doping
11
pn Junctions
N
P
I
V
12
pn Junctions
I
N
P
V
I
V
Diode Equation
JS Sat Current Density A Junction Cross Section
Area VTkT/q n is approximately 1
13
Basic Devices and Device Models

• Resistor
• Diode
• Capacitor
• MOSFET
• BJT

14
Capacitors

• Types
• Parallel Plate
• Fringe
• Junction

15
Parallel Plate Capacitors
A2
C
A1
cond1
cond2
d
insulator
A area of intersection of A1 A2
One (top) plate intentionally sized smaller to
determine C
? Dielectric constant
16
Parallel Plate Capacitors
where
17
Fringe Capacitors
C
d
A is the area where the two plates are parallel
Only a single layer is needed to make fringe
capacitors
18
Fringe Capacitors
C
19
Capacitance
Junction Capacitor
C
VD
p
d
d
n
depletion region

• Note d is voltage dependent
• capacitance is voltage dependent
• usually parasitic caps
• varicaps or varactor diodes exploit
• voltage dep. of C

Cj0 junction capacitance at VD 0V ?B barrier
or built-in potential
20
Basic Devices and Device Models

• Resistor
• Diode
• Capacitor
• MOSFET
• BJT

21
n-Channel MOSFET
Poly
Gate oxide
n-active
p-sub
22
n-Channel MOSFET
Drain
Gate
Source
L
W
LEFF
Bulk
23
n-Channel MOSFET
Poly
Gate oxide
n-active
p-sub
depletion region (electrically induced)
24
n-Channel MOSFET Operation and Model
VDS
ID
VGS
IG
VBS
IB
Apply small VGS (VDS and VBS assumed to be small)
ID0 IG0 IB0
Depletion region electrically induced in channel
Termed cutoff region of operation
25
n-Channel MOSFET Operation and Model
VDS
ID
VGS
IG
VBS
IB
Increase VGS (VDS and VBS assumed to be small)
ID0 IG0 IB0
Depletion region in channel becomes larger
26
n-Channel MOSFET Operation and Model
VDS
ID
Critical value of VGS that creates inversion
layer termed threshold voltage, VT)
VGS
IG
VBS
IB
(VDS and VBS small)
Increase VGS more
IDRCHVDS IG0 IB0
Inversion layer forms in channel
Inversion layer will support current flow from D
to S
Channel behaves as thin-film resistor
27
n-Channel MOSFET Operation and Model
VDS
ID
VGS
IG
VBS
IB
(VDS and VBS small)
Increase VGS more
IDRCHVDS IG0 IB0
Inversion layer in channel thickens
RCH will decrease
Termed ohmic or triode region of operation
28
Triode Region of Operation
For VDS small
29
n-Channel MOSFET Operation and Model
VDS
ID
VGS
IG
VBS
IB
(VBS small)
Increase VDS
ID? IG0 IB0
Inversion layer thins near drain
ID no longer linearly dependent upon VDS
Still termed ohmic or triode region of
operation
30
Triode Region of Operation
For VDS larger
31
n-Channel MOSFET Operation and Model
VDS
ID
VGS
IG
VBS
IB
(VBS small)
Increase VDS even more
ID? IG0 IB0
Inversion layer disappears near drain
Termed saturationregion of operation
Saturation first occurs when VDSVGS-VT
32
Saturation Region of Operation
For VDS at saturation
33
n-Channel MOSFET Operation and Model
VDS
ID
VGS
IG
VBS
IB
(VBS small)
Increase VDS even more (beyond VGS-VT)
ID? IG0 IB0
Nothing much changes !!
Termed saturationregion of operation
34
Saturation Region of Operation
For VDS in Saturation
35
Model Summary
Note This is the third model we have introduced
for the MOSFET