Modeling of TFTs Part-­‐I: DC Behavior and implica=ons for biosensing Henrique Leonel Gomes Universidade do Algarve, FCT, Campus de
Gambelas, 8000 Faro, Portugal
([email protected])
Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Objec3ve Small-­‐signal model for the BioFET is given, with emphasis on its design and opera=ng condi=ons for op=mum signal-­‐to-­‐noise ra=o. Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Outline
•  DC models
-Review of the available models.
-Parameter extraction
•  Dynamic behaviour and models
-Review of available models
-Equivalent circuits
- Modelling challenges
- Open problems
Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Mo=va=on The threshold voltage shiO can depend in a significant manner on various parameters or experimental condi=ons other than the substance or cell to detect. Examples include: •  Electrolyte concentra=on. •  pH of the solu=on and surface •  adsorp=on affinity, which can all lead to different shiOs in the threshold voltage. •  OOen the transistors themsell suffur also from To predict the amount of voltage shi= in response to the moelecuel or cell to measure, it is necessary to include all these effects when modelling the BioFET Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Outline •  Commercial TCAD Models •  Non ideal behaviour, contact effects •  The Unified Compact Model •  Parameter extrac=on Approxima=ons to the BioFET’s numerical model that allow us to develop an analy=c model for its flatband voltage is presented and discussed Equivalent circuit model of the BioFET, Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Challenges in TFT Circuits Design
•  Material instabilities
Electrical degradation (ex. bias-stress)
Chemical degradation (ex. oxygen, water vapor)
Process variations
More than 50% variations in key device parameters (ex. Vth)
Device mismatches are common
Unable to have high quality VIAs for multiple interconnects
•  Device limitations
Mono-type only p-type
Lower mobility
Higher supply voltage
EDA supports
Lack of trustworthy and compact
device models
Lack of design verifications
Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Commercial TCAD Models
•  Silvaco ATLAS 2D Simulator – TFT module
β and γ are fi_ng parameters (Pool-­‐Frenkel Mobility model) •  Infiniscale GREENLys
•  Simucad UOTFT – Universal Organic TFT SPICE Model
Based on a exponen=ally distributed density of states in the organic semiconductor and opera=ng in the channel accumula=on mode in the presence of interface traps It includes and extrinsic gate RC network for the modeling of frequency dispersion effects. Extrinsic overlap capacitances ,Thermal RC network for the modeling of self-­‐
hea=ng effects. Noise model. Geometrical scalability. Compact Analytical Models
•  M. Fadlallah model DC/AC unified OTFT compact model
Based on variable range hopping (VRH) theory ( 2007)
•  Marinov and Deen generic analy3cal model for Organic TFTs Incorporates channel length modula=on, subthreshold opera=on, and contact effects in the TFT •  E. Calvetti et al. DC/dynamic analytical model (2005)
•  Ling Li et al. compact model for polycrystalline pentacene TFT
(2010)
(VRH model, dynamic operation are reported as well as the expressions of the
capacitances. Suited for implementation in Spice. )
Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Compact Analytical Models •  Estrada and Cerdeira model (UMEM OTFT model) ((2007)
•  Fabrizio Torricelli et al. model (*)
MTR, and a double exponen=al density of trap states is considered.
•  Algarve Model (Peter Stallinga and Henrique Gomes) (+)
(*) Fabrizio Torricelli et al. IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 58, NO. 9, SEPTEMBER 2011 (+) Stallinga P and Gomes et al. Synthe=c Metals 156 (2006) 1316–1326 Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Compact Analytical Models
(Fabrizio Torricelli et al. Model (*)) Large trap distribu=on in the regions of semiconductor close to the source and drain electrodes can explain the contact resistance. The drain current of contact transistors M2 and M3 is modeled assuming a different trap disorder with respect to the channel region, i.e., The tail states are an intrinsic property of the organic semiconductor, whereas the deep states are induced by the gate insulator. (*) Fabrizio Torricelli et al. IEEE TRANSACTIONS ON ELECTRON DEVICES (2012) Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Fabrizio Torricelli model
Higher density of traps Controled by the dielectric Controled by the semiconductor Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) How to model contact effects Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Looking at TFT I-­‐V curves Contact problems 13 Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Looking at TFT I-­‐V curves Contact problems www.noe-­‐flexnet.eu 14 Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Looking at TFT I-­‐V curves Meter on red Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Looking at TFT I-­‐V curves /Users/hgomes/Desktop/POLAND/jiri/I7_D14/D2/DAY2 Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Unified compact model
The model assumes that:
γ
µ FET
⎛ VGS − VT ⎞
⎟⎟
= µ0 ⎜⎜
⎝ VAA ⎠
I DSlin
K
1+γ
= γ (VGS − VT ) VDS
V AA
Where:
V AA
⎡ KVDS ⎤
= ⎢ 1+γ ⎥
⎣ S1 ⎦
1γ
Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Unified compact model
The model assumes that:
γ
µ FET
⎛ VGS − VT ⎞
⎟⎟
= µ0 ⎜⎜
⎝ VAA ⎠
I DSlin
K
1+γ
= γ (VGS − VT ) VDS
V AA
Where:
V AA
⎡ KVDS ⎤
= ⎢ 1+γ ⎥
⎣ S1 ⎦
1γ
Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Unified compact model
The model assumes that:
γ
µ FET
⎛ VGS − VT ⎞
⎟⎟
= µ0 ⎜⎜
⎝ VAA ⎠
I DSlin
Where:
V AA
⎡ KVDS ⎤
= ⎢ 1+γ ⎥
⎣ S1 ⎦
1γ
K
1+γ
= γ (VGS − VT ) VDS
V AA
Unified compact model
The model assumes that:
γ
µ FET
⎛ VGS − VT ⎞
⎟⎟
= µ0 ⎜⎜
⎝ VAA ⎠
I DSlin
K
1+γ
= γ (VGS − VT ) VDS
V AA
Where:
V AA
⎡ KVDS ⎤
= ⎢ 1+γ ⎥
⎣ S1 ⎦
1γ
Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Unified compact model
The model assumes that:
γ
µ FET
⎛ VGS − VT ⎞
⎟⎟
= µ0 ⎜⎜
⎝ VAA ⎠
I DSlin
K
1+γ
= γ (VGS − VT ) VDS
V AA
Where:
V AA
⎡ KVDS ⎤
= ⎢ 1+γ ⎥
⎣ S1 ⎦
1γ
Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Extrac3on procedure VGS
H (VGS ) =
∫I
DS
( x)dx
(1) 0
I DS (VGS )
1
(VGS − VT )
H (VGS ) =
2+γ
IDS 1/(1+ γ) VT is determined from the intercept, and γ from the slope (2) S1 VGS Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Extrac3on procedure VGS
H (VGS ) =
∫I
DS
( x)dx
(1) 0
I DS (VGS )
1
(VGS − VT )
H (VGS ) =
2+γ
IDS 1/(1+ γ) S1 VGS (2) VT is determined from the intercept, and γ from the slope Density of states
Using an exponen=al DOS and considering that Qfree<< Qloc the following expression for mobility was derived: gdo can also be extracted as: [1] M. Estrada et all, “Mobility model for compact device modeling of OTFTs made with different materials”, Solid State Electronics 52 (2008) 787-­‐794. Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Example
2
x 10
-6
device 35
7
x 10
-4
device 35
1.5
Drain-Source Current (A)
- Drain-Source Current (A)
6
1
0.5
γ=0.81 5
4
3
2
1
0
0
0
2
4
6
- Gate Voltage Vg (V)
8
I DSlin = Kµ (VGS − VT )VDS
0
2
10
I DSlin
4
6
Gate Voltage Vg (V)
8
10
K
= γ (VGS − VT )1+γ VDS
V AA
Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Evolution of the linear transfer curves with
increasing stress time
8
x 10
-4
Time 7
-­‐IDS 1/(1+ 0.81)(A 0.55) 6
5
4
3
2
5x104 s 1
0
0
5
10
15
-­‐VGS Curves ploUed as IDS1/(1+γ) vs. VGS. With γ=0.81. Vg=-­‐10V . Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) 10
12
8
10
6
8
V (V)
4
2
0
0
th
Drain-source current ( µA)
Bias-­‐induced threshold voltage shi= t= 106 s
t= 0 s
2
4
6
8
Gate voltage Vg (V)
10
Stretched exponen=al decay 6
4
12
2
2
10
4
10
Log (time)(s)
6
10
Stressing/Figure of merit τ τIs a figure of merit that measures stability β is related with the material Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Incorporating effects of bias-stress in the
TFT model •  The gate bias-stress strongly degrades the switching ability of an individual
transistor.
•  Different transistors can be exposed to different degrees of degradation.
•  Therefore, the input/output characteristics of a degraded transistor influence its
neighboring transistors.
•  This problem imposes great design burdens on circuit designers to ensure
reliable operation throughout the expected lifetime.
Can we analyze the degradation profile for each TFT, perform reliability
simulation, and predict the circuit lifetime accordingly by incorporating transient
simulators such as SPICE?
Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Analy3cal Model for the change in Vth under digital pulses Tsung-­‐Ching Huang et al. where α is a process-­‐dependent parameter; t is total opera=on =me; Dc is the duty-­‐cycle of the bias pulses; the minus sign shown between posi=ve and nega=ve ΔVTH (t) reflects the fact that VTH shiOs in opposite direc=ons for posi=vely and nega=vely pulsed biases; f(PW) is a func=on characterizing the pulse-­‐width dependence under nega=vely pulsed biases. Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Equivalent circuits to simulate Vth changes
The model must be easy to incorporate
into a circuit simulator, and yet be
physically meaningful by drawing on the
relevance to the physics of the device.
Sanjiv Sambandan and Arokia Nathan IEEE TRANSACTIONS ON
ELECTRON DEVICES, VOL. 53, NO. 9, SEPTEMBER 2006
Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Equivalent circuit Small-­‐signal equivalent circuit diagram for the BioFET Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Further reading Shinwari, M.W., Deen, M.J., and Landheer, D.: ‘Study of the electrolyte–insulator–
semiconductor field-­‐effect transistor with applica=ons in biosensor design’, Microelectron. Reliab., 2007, 47, (12), pp. 2025–2057. Shinwari, M.W.: ‘Modeling and simula=on of electrochemical DNA biosensors in CMOS technology’. M.A.Sc. thesis, Department of Electrical and Computer Engineering, McMaster University, Hamilton, April 2007. Landheer, D., Aers, G., McKinnon, W.R., Deen, M.J., and Ranua´rez, J.C.: ‘Model for the field effect from layers of biological macromolecules on the gates of metal oxide semiconductor transistors’, J. Appl. Phys., 2005, 98, (4), p. 044701 (15 pp)b Bousse, L., de Rooij, N.F., and Bergveld, P.: ‘Opera=on of chemically sensi=ve field-­‐effect sensors as a func=on of the insulator–electrolyte interface’, IEEE Trans. Electron Devices, 1983, 30, (10), pp. 1263–1270. Landheer, D., McKinnon, W.R., Aers, G., Jiang, W., Deen, M.J., and Shinwari, M.W.: ‘Calcula=on of the response of field-­‐effect transistors to charged biological molecules’, IEEE Sens. J., 2007, 7, (9), pp. 1233–1242 Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00) Further reading Deen, M.J., Shinwari, M.W., Ranua´rez, J.C., and Landheer, D.: ‘Noise considera=ons in field-­‐effect biosensors’, J. Appl. Phys., 2006, 100, p. 074703 (8pp) Delahay, P.: ‘Double layer and electrode kine=cs’ (Interscience, New York, 1965). Wei, F., Sun, B., Guo, Y., and Zhao, X.S.: ‘Monitoring DNA hybridiza=on on alkyl modified silicon surface through capacitance measurement’, Biosens. Bioelectron., 2003, 18, pp. 1157–1163 Biossensores, Mestrado Integrado em Engª. Electrónica e Telecomunicações (MIEET-­‐2009/00)