Metal Oxide TFTs
for Circuit Applications
Arokia Nathan
an299@cam.ac.uk
Engineering Department
Cambridge University
Orama
Summer School: 21 Oct 2012
Contents
•
Applications of Oxide Transistors
•
Part 1. Device Physics
•
Part 2. I

V Relations
•
Part 3. Parameter Extraction
•
Part 4. Compact Modeling
•
Part 5. Simulation for Display and Sensor Applications
•
Summary and Conclusion
Applications of Oxide Transistors
Tfts
must perform analog and digital functions
Physics

based compact models needed for circuit design
f
system
(Hz)
frame

rate
µ
TFT
(cm
2
/V

s)
Application
< 100
< 1
AMLCD, AMOLED, AMFPI
< 1 kHz
< 10
3

D displays, RF ID tags, smart cards
< 100 kHz
10
–
100
adaptable surfaces, embedded imaging,
interactive screens, HDTV, 3D
< 1 MHz
100
–
1000
distributed sensors/networks, low frequency RF,
NW TFTs and flex computers, holographic displays
Part 1: Device Physics
Density of States
Reduced density of tail states
l敳s trapping.
Pot敮eial barri敲s abov攠E
m
s捡l敤eband mobility by p敲捯cation 捯cdu捴ion
P.V. Erslev et al., Appl. Phys. Lett. 95 (2009) 192115.
Localized Tail States (kT
t
<kT)
• Exponential distribution of tail states.
• Steep tail states: characteristic temperature of tail states (kT
t
) = 20meV < kT of 26meV.
Conduction Mechanisms
M
odified band mobility while considering potential barriers
E
m
E
T
E
E
F
filled tail/deep
states
W
B
D
B
q
B0
q
B
E
f(E)
1
0.5
at T=300K
E
F0
unfilled tail states
Lee
et al.
, Applied Physics Letters 98, 203508 2011.
TLC (Trap

limited conduction)
Percolation
Mobility with TLC & Percolation
Modes
Equations for kT
t
< kT
Free
Carrier
Trapped
Carrier
Mobility
where
TLC (Trap

limited conduction)
Percolation
V
GS

Dependent Mobility
where
If N
C
>> 0.5N
tc
kT
t
Gauss’s law:
Solution of Poisson’s Eq. and Gauss’s law
yields
Δ
E
F
as a function of V
GS
, allowing
Part 2: I

V Relations
Above

threshold Regime
where
Approximations for Linear Regime
(ignoring higher order terms)
Linear regime
Source
Drain
Gate
Insulator
Oxide semiconductor
Accumulation
Analytical Expression for V’
DS
C
GDO
C
GSO
R
D
R
S
R
dyn
C
t
R
t
C
t
R
t
I
Dt
I
St
I
DS
+

+

Drain
V
C
/2
V’
DS
Source
Gate
V
C
/2
Pinch

off and Saturation Regime
where
Source
Drain
Gate
Insulator
Oxide semiconductor
Pinch

off
Source
Drain
Gate
Insulator
Oxide semiconductor
Sub

T Current due to Interface States
For Sub

Threshold Regime at V
GS
close to V
FB
;
Diffusion current predominant by Interface states (D
it
) (I
it
).
where
Interface state density
E
[eV]
DOS [cm

3
eV

1
]
E
m
N
dc
(log

scale)
tail
deep
E
F0
E
F
Interface states
Sub

T Current due to Deep States
For Sub

Threshold Regime at V
GS
close to V
T
;
Trap

limited conduction due to deep states at bulk (I
deep
).
where
E
[eV]
DOS [cm

3
eV

1
]
E
m
N
dc
(log

scale)
tail
deep
kT
d
E
F
Total Drain Current
1) Linear regime:
2) Saturation regime:
h
armonic average of three
components for each regime
Summary of I

V Relations
V
FB
V
T
I
DS
(log

scale)
V
GS
Sub

T Regime
Above

T regime
Sub

T current due to interface states
Above

T current due to percolation and tail states
Sub

T current due to deep states (dangling bonds at bulk)
Part 3: Parameter Extraction
Extraction Procedure
C

V Measurements
(at low freq. for a different L)
I

V Measurements
(Transfer/Output for a different L)
DOS Extraction
Contact Resistance
Extraction
Extraction of Model Parameters
(power parameters and constants)
C
ox
and
C
ov
Extraction
Parameters for Pinch

off Voltage, V
DS
’, etc.
Simulation and Verification (VerilogA)
Q
ref
,
p
Measurements
Device
Characterization
Extraction of Device
Parameters (V
T
, SS, V
FB
)
Model Parameter
Extraction
Circuit Design
for linear regime
for saturation regime
N
tc
, kT
t
R
SD
,
L
sat
m
A
C
Extraction of Tail State DOS
I

V Measurement
and Free /Total Charges
Mapping between Surface
Potential and Gate Voltage
Extraction of
Trapped Carrier Density
First Derivative of
Trapped Carrier Density
Sub

gap DOS
When kT
t
> kT :
Lee et al.
, Electron Device Letters,
33 (7), 2012
.
Parameter
Value
N
tc
2x10
19
cm

3
eV

1
kT
t
20meV
S. Lee
et al.
,
Applied Physics Letters
,
101,
036238
, 2012
.
Extracted Tail State DOS
Extraction of R
SD
&
L
V
GS
20
17.5
15
12.5
10
L [
浝
R
tot
[M
W
z
L [
浝
R
tot
[M
W
z
R
SD
=9637
W
L
G
=

3.5
m
at V
DS
= 0.1V
R
SD
= 9637
W
R
SD
W= 96.37
W

捭
L㴠

3.5
m(數eansin)
L [
浝
R
SD
L’ [
m]
15
9637
W
18.5
25
9637
W
28.5
50
9637
W
53.5
100
9637
W
103.5
Negative
L(㴠
3.5
m)
捨cnnel length e硰xnsi潮.
W= 100
m
Extraction of C
ov
& C
ox
V
GS
[V]
C
gs
[pF]
L=100
m
L=50
m
L=25
m
L=15
m
f=1kHz
V
S
= V
D
= 0V
C
gs
[pF]
L’
[
浝
C
ov
= 0.45pF
L’ [
m]
103.5
53.5
28.5
18.5
C
meas
[F/cm
2
]
1.60pF
1.03pF
0.741pF
0.637pF
C
ov
[F/cm
2
]
0.45pF
0.45pF
0.45pF
0.45pF
C
ox
[F/cm
2
]
11.5nF
11.6nF
11.6nF
12.5nF
Linear extrapolation for V
T
extraction
log(I
DS
) vs. V
GS
plot for S extraction
L=15
m
L㴲5
m
L㴵0
m
L㴱00
m
at
DS
= 0.1V
GS
[V]
I
DS
[
䅝
V
GS
[V]
Log(I
DS
) [A]
L=15
m
L㴲5
m
L㴵0
m
L㴱00
m
at
DS
= 0.1V
Parameters
L=15 um
L=25 um
50 um
100 um
V
T
[V]
4.34
3.86
3.98
4.33
S
[V/dec]
0.15
0.16
0.17
0.19
D
it
[cm

2
eV

1
]
1.07x10
11
1.19x10
11
1.31x10
11
1.55x10
11
Extraction of
V
T
, S,
&
D
it
Non

iterative solution of V’
DS
log(V
GS

V
T
)
log(
z
⡖
DS
,I
DS
))
V
GS
[V]
V’
DS
[V]
V
GS
[V]
error
[%]
Parameter
Value
A
C
1.63x10

7
R
SD
9637
0.9954
Error Less than 0.1%
Extraction of
p
log(V
GS

V
T
)
log(I
DS
/(V
DS

R
SD
I
DS
))
slope = 1.0206
=
p
+1
L= 100
m
at
DS
= 0.1V
slope
Parameters
Value
Slope
1.0206
p
0.0206
K (Constant)
Constant
Extraction of Q
ref
V
GS
[V]
Q
ref
[C/cm
2
]
Q
ref
= 7.28
x10

8
L= 100
m
at V
DS
= 0.1V
Parameters
Value
Q
ref
7.28x10

8
Model Parameters (Above

T)
Parameters
Description
Value
Unit
0
*
Effective band mobility scaled by percolation term
15
[cm
2
/V

s]
R
SD
S/D total contact resistance for W=100
m
9637
[ohm]
R
SD
W
S/D total contact resistance (standardized)
96.37
[ohm

cm]
L
Channel length reduction or expansion

3.5
[mm]
C
ov
Total overlap parasitic capacitance
0.45p
[F/cm
2
]
C
ox
Gate capacitance
11.5n
[F/cm
2
]
Q
ref
Reference charge density for above

threshold regime
72.8n
[C/cm
2
]
p
Power parameter for above

threshold regime
0.0206
unitless
sat
Power parameter for saturation regime
1.08
unitless
n
Harmonic averaging
parameter
5
unitless
Ee捴ive灯per灡rameter
0.9954
畮tless
A
C
Constant for effective drain voltage
0.163m
[ohm

1
V

h
]
for linear regime
for saturation regime
Model Parameters (Sub

T)
Parameters
Description
Value
Unit
Q
fi
Intrinsic charge density
7.05X10

14
C/cm
2
V
FB
Flat band voltage
0.55
V
D
it
Interface state density
1.64X10
11
cm

2
eV

1
C
ox
Gate

insulator capacitance
1.15
F/cm
2
d
Exponent for linear regime
2.26
Unitless
Q
d
Reference charge for linear regime
7.80X10

8
C/cm
2
Material Parameters
Parameters
Description
Value
Unit
T
Ambient Temperature
300
[K]
kT
Thermal Energy
26m
[eV]
kT
t
Tail State Energy
20m
[eV]
N
C
Effective DOS
5x10
18
[cm

3
]
N
tc
Density of Tail States at E
m
2x10
19
[cm

3
eV

1
]
m
n
Effective Mass for Electrons
0.34m
0
[kg]
S
Channel Permittivity
11.8e
0
[F/cm]
E
F0

E
m
Fermi Energy at Flat Band

1.175
[
eV
]
Part 4: Compact Modeling
Equivalent Circuit
Static
I
DS
= voltage

dependent current source.
R
D
, R
S
= contact resistances (=0.5R
SD
).
Dynamic
C
GDO
, C
GSO
= overlap capacitances (=0.5C
OV
).
I
Dt
= current associated with traps near drain.
I
St
= current associated with traps near source.
R
dyn
= for the transit time for carriers.
C
t
, R
t
= for time constants associated with defects.
Implementation in
VerilogA
or VHDL
C
GDO
C
GSO
R
D
R
S
R
dyn
C
t
R
t
C
t
R
t
I
Dt
I
St
Drain
Source
Gate
V’
DS
I
DS
Transfer Characteristics
at V
DS
= 0.1V
V
GS
[V]
I
DS
[
䅝
V
GS
[V]
Error [%]
Measured
Modeled
L=100
m
I
DS
[A] (log

scale)
at V
DS
= 20V
I
DS
[
䅝
Measured
Modeled
L=100
m
I
DS
[A] (log

scale)
V
GS
[V]
Linear regime
Saturation regime
V
GS
[V]
Error [%]
Sub

T
Sub

T
Above

T
Above

T
Transfer Characteristics for Different L’s
100
m
㔰
m
L㴠25
m
1
m
㔰
m
L= 25
m
V
GS
[V]
I
DS
[
䅝
V
GS
[V]
I
DS
[
䅝
Measured
deled
easred
deled
V
GS
[V]
Error [%]
at V
DS
= 0.1V
L=25
m
L㴵0
m
L㴱00
m
V
GS
[V]
Error [%]
at V
DS
= 20V
L=25
m
L㴵0
m
L㴱00
m
at V
DS
= 0.1V
at
DS
= 20V
Linearregime
atratinr敧ime
Measured
deledFr䰽100
m
V
GS
= 20V
17.5
15V
12.5
1
V
DS
[V]
I
DS
[
䅝
Error [%]
V
DS
[V]
V
GS
=10V
GS
=17.5V
GS
=12.5V
GS
=20V
V
GS
=15V
Output Characteristics
Error less
than
3%
Part 5: Circuit Simulation
Examples
OLED Display Pixel
Δ
V
T
[V]
Δ
I
OLED
[
䅝
2

T Pixel Structure
3

T Pixel
OLED
Sel.
VDD
VDATA
I
OLED
2

T Pixel
time [sec]
I
OLED
[
䅝
V
DATA
[V]
Sel. (i+1) [V]
Sel. (i) [V]
3

T Pixel Structure
OLED
Sel. (i)
VDD
VDATA
I
OLED
Sel. (i+1)
hv
Image Sensor Pixel
time [sec]
Vout [V]
Vout [V]
Vout [V]
Vout [V]
Reset
at I
ph
= 5nA
at I
ph
= 10nA
at I
ph
= 15nA
at I
ph
= 20nA
I
ph
[nA]

Δ
V
out
 [V]
Dark Signal
Δ
V
out
Δ
V
out
Δ
V
out
Δ
V
out
3

T Pixel Structure
PD
VDD
I
ph
Reset
hv
I
Bias
Sel.
V
out
I
ph
C
PD
•
Material and physical properties of oxide TFTs considered for
modeling
.
•
Percolation mobility model combined with trap

limited
conductiondeduced
from DOS of oxide TFTs.
•
Current

voltage relation derived based on these mobility models
and
device physics, allowing derivation of physically

based
compact
model for oxide transistors.
•
Model parameters extracted using measured I

V and C

V.
•
Static and dynamic results of the model with a good agreement
with
measured results.
•
Simulation for
display
and
sensor applications
performed
based
on
the developed compact model.
Conclusions
Thank
you!
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