a 28.4 pj per conversion isfet-based ph sensing design...
TRANSCRIPT
Jian Sen Teh1, Anh-Tuan Do2, Tony Tae-Hyoung Kim1 and Liter Siek1
1Centre of Excellence in IC Design (VIRTUS) Nanyang Technological University
2Institute of Microelectronics Agency for Science, Technology and Research (A*STAR)
A 28.4 pJ per Conversion ISFET-based pH Sensing Design for Low-Energy Applications
Outline
Introduction
Proposed ISFET-based pH sensor Design
Simulation Results
Conclusion
2
ISFET Ion Sensitive Field Effect Transistor Sense the presence and concentration of ions in
a solution Replaces the metal gate of a MOSFET with a
complex gate consisting of an electrode, the solution and a passivation layer
3
n+ n+
P
Metal Gate DrainSource
(Bergveld, 1970)
n+ n+
P
Passivation
SolutionRef
Electrode
DrainSource
MOSFET ISFET
n+ n+
P-substrate
Passivation
SolutionRef Electrode
Drain Source
Metal Layers
ISFET
Cpass
CHelm
CGouy
Velectrode
Vchem
H+ H+ H+
VGate
H+ H+ H+
ISFET Operation
𝑉𝑉𝑇𝑇𝑇𝑇(𝐼𝐼𝐼𝐼) = 𝐸𝐸𝑟𝑟𝑟𝑟𝑟𝑟 − 𝛹𝛹𝑜𝑜 + 𝜒𝜒𝑠𝑠𝑜𝑜𝑠𝑠 − 𝜙𝜙𝐼𝐼𝑆𝑆 −𝑄𝑄𝑜𝑜𝑜𝑜 + 𝑄𝑄𝐼𝐼𝐼𝐼 + 𝑄𝑄𝐵𝐵
𝐶𝐶𝑜𝑜𝑜𝑜+ 2𝜙𝜙𝑟𝑟
(Dutta, 2012) 𝐼𝐼𝐷𝐷 =
12𝐶𝐶𝑜𝑜𝑜𝑜µ
𝑊𝑊𝐿𝐿
𝑉𝑉𝐺𝐺𝐼𝐼 − 𝑉𝑉𝑇𝑇𝑇𝑇 2
4
Site binding theory
(Fung, Cheung, & Ko, 1986; Yates, Levine, & Healy, 1974)
ISFET Applications
General pH sensing, applied in water quality monitoring
Detection of the ratio of methylated DNA which can be used in the detection of cancers
Measurement of Urea, Creatinine concentration and their ratio in the prediction of renal failure
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(Chen, Chan, & Tse, 2005)
(Kalofonou, Georgiou, Ou, & Toumazou, 2012)
(Pookaiyaudom, Seelanan, Lidgey, Hayatleh, & Toumazou, 2011)
ISFET Read-out Circuits Bridge-type floating drain source follower (BFDSF) design Output proportional to Vth of MISFET, which is proportional
to pH of solution Higher system complexity Required more power consumption
6
(Wen-Yaw et al., 2010)
ISFET Read-out Circuits
Differential current mode design
pH of solution proportional to the difference in current flowing through M1 & M2
Temperature compensated Critical matching of
transistors Hard to generate Vfloat = Vds
of M2
7
(Nobpakoon, Pijitrojana, & Poyai, 2013)
1
2
Motivations
Most ISFET read-out circuits utilizes analog intensive circuits Proposed digital intensive design
Reduce analog circuitry to minimize common
issues faced such as mismatches
Do not require ADC to convert analog output into digital bits
8
ISFET-Based pH Sensing System
ISFET
Analog Front-End
pH input
Coarse & Fine Digital Block
Control Logic
Binary Output
9
pH range of 4-12 Converts pH directly to binary output (10-bits) Minimal analogue front end circuitry
Based on storing and discharging charges of a capacitor
pH of solution proportional to time taken to discharge the capacitor
pH-to-time Conversion
𝐐𝐐 = 𝐂𝐂∆𝐕𝐕 = 𝐈𝐈∆𝐭𝐭
10
C0
S2
VDD
S1
ISFET
Node A
Charge to VDD
C0
S2
VDD
S1
ISFET
Node A
Discharge by ISFET
Precharge Evaluation
pH-to-time Conversion
Time (µs)0.5 1.00.0 1.5 2.0
V cap
(V)
0.0
0.25
0.5
0.75
1.0
1.25
Lower pH
Higher pH
High resolution
Increasing pH
Low
er p
H
Hig
her p
H
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Characteristics of time taken for capacitor to discharge across pH inputs
High non-linearity across the full pH range
High resolution with good linearity can still be achieved by using only a portion of the graph
Proposed Design Circuit Operation
12
Divide full pH range into sub-ranges Each fine sensing ISFET is responsible for a pH sub-
range
C0
S2 S3 S4 S5 S6
VDD
Coarse sensing Fine sensing
Coarse sensing D9-D8
Fine sensing D7-D0ISFET
S1 VcapNode A
C0
S2 S3 S4 S5 S6
VDD
Coarse sensing Fine sensing
Coarse sensing D9-D8
Fine sensing D7-D0ISFET
S1 VcapNode A
Proposed Design Circuit Operation
Precharge to VDD
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C0
S2 S3 S4 S5 S6
VDD
Coarse sensing Fine sensing
Coarse sensing D9-D8
Fine sensing D7-D0ISFET
S1 VcapNode A
Proposed Design Circuit Operation
Coarse ISFET discharges the
capacitor
Process the time taken &
determine the sub pH
range
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Coarse Sensing Structure
CMP
CMP
CMP
Thermometer to Binary Decoder
C1
C2
C3
Clk
Clk
Clk
Vph10
Vph8
Vph6
Vcap
D9 – D8
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Time (ns)
Vca
p (V
)
5 100 15 20 25 300.0
0.25
0.5
0.75
1.0
1.25
Precharge
pH 10: 1.09VpH 8: 884mV
pH 6: 545mV
Flash-type structure Senses pH boundary
of pH 6, 8 and 10
Proposed Design Circuit Operation
C0
S2 S3 S4 S5 S6
VDD
Coarse sensing Fine sensing
Coarse sensing D9-D8
Fine sensing D7-D0ISFET
S1 VcapNode A
Precharge to VDD
16
Proposed Design Circuit Operation
C0
S2 S3 S4 S5 S6
VDD
Coarse sensing Fine sensing
Coarse sensing D9-D8
Fine sensing D7-D0ISFET
S1 VcapNode A
Fine ISFET discharges the
capacitor
Process the time taken to resolve the
pH value
17
Fine Sensing Structure
CMP
8-bits Counter
ENClk
Vcap
Clk
D7 – D0
Vref
18
Basic binary counter Counts the time taken
for the capacitor to discharge from VDD to Vref
CLK
Vcap
EN
Time
Vref
D7 – D0
Digital Circuits VDD
Pre
Pre
Vin+ Vin-
Vout
EN EN ENQ0 Q1 Q7
CLKJ
Q
Q
K
SET
CLR
J
Q
Q
K
SET
CLR
J
Q
Q
K
SET
CLR
Latch-based comparator JK Flip Flop-based binary counter
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Output precharged to VDD Cross coupled inverter pair
latches output depending on the input differential pair
Standard asynchronous binary counter
When counting is enabled, it counts the number of rising clock edges
Design Considerations
20
The capacitor is a main component in the design Affects the sizing of the ISFETs, and hence the pH
resolution
Determines the area consumed by the circuit
Trade off between area, resolution, speed and power
Design Considerations
pH range Type of ISFET Region of operation W/L ratio Resolution
(pH/LSB)
Coarse (4-12) Low VT Subthreshold - Saturation 10 -
Fine (4-6) High VT Subthreshold 7.7 0.0348
Fine (6-8) Standard VT Subthreshold 2 0.0335 Fine (8-10) Standard VT Subthreshold 23.4 0.0437
Fine (10-12) Low VT Subthreshold 4.3 0.0382
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ISFET were sized such that the highest pH of each sub range will allow the capacitor to discharge for the full sensing operation
Biased in the subthreshold region
Simulation Environment
Standard 65nm/1.2V CMOS technology Assumed sensitivity of 50mV/pH Sensing cycle of 1.29µs or Sampling rate of
775kS/s Clock frequency = 200MHz Vbias = 500mV Vref = 400mV Capacitor = 615fF
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(Tomaszewski et al., 2007)
Linearity of Proposed Design
Time (µs)0.0 0.25 0.5 0.75 1.0
Vcap
(V)
1.25
Higher pH
Lower pH
400mV
Increasing pH
7.57ns
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40 pH levels with resolution of 0.05pH
Lines distributed evenly, showing good linearity
Counter with clock period of 5ns able to distinguish the different pH levels
Simulation Example
Time (µs)0.0 0.25 0.5 0.75 1.0 1.25
D7 – D0
Fine Sensing (S3/4/5/6)
D9 – D8
Coarse Sensing (S2)
Precharge (S1)
Vcap
D7 – D0 = 1100 1101
D9 – D8 = 00
S3 closed S4/5/6 open
1072.53ns 400mV
• pH = 5.83• Sensing period = 1290ns• Result = 00 1100 1101• Energy = 24.78pJ
18.75ns each
11.25ns
1241.25ns
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As clock frequency increases, digital energy consumption per conversion remains approximately constant
However, smaller energy consumption from capacitor
Trend saturates, 200MHz clock was chosen
Maximum energy consumption of 28.43pJ/Conversion
Energy Consumption
35
30
25
10
5
0
Ener
gy (p
J)
Freq (MHz)20 70 120 170 220
Ecap
Edigital
Etotal
Almost constant
… ..
.
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𝑬𝑬𝑻𝑻𝒐𝒐𝒐𝒐𝒐𝒐𝒐𝒐 = 𝑬𝑬𝑪𝑪𝒐𝒐𝑪𝑪 + 𝑬𝑬𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝒐𝒐𝒐𝒐𝒐𝒐
𝑬𝑬𝑪𝑪𝒐𝒐𝑪𝑪 = 𝟏𝟏𝟐𝟐
𝑪𝑪𝑽𝑽𝑫𝑫𝑫𝑫𝟐𝟐
𝑬𝑬𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝒐𝒐𝒐𝒐𝒐𝒐 = 𝑪𝑪𝒆𝒆𝒆𝒆𝒆𝒆𝑽𝑽𝒅𝒅𝒅𝒅𝟐𝟐 𝒆𝒆𝒄𝒄𝒐𝒐𝒄𝒄𝑻𝑻𝒐𝒐𝒐𝒐𝒐𝒐𝒐𝒐𝒐𝒐
Conclusion
Digital Based ISFET pH sensor which converts pH into a 10-bits output
High resolution (0.0437pH/LSB)
Low energy consumption (28.43pJ/Conv)
Sampling rate of 775kS/s with a digital clock
frequency of 200MHz
Highly flexible & reconfigurable
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The End
References 1. Bergveld, P. (1970). Development of an Ion-Sensitive Solid-State Device for Neurophysiological Measurements.
Biomedical Engineering, IEEE Transactions on, BME-17(1), 70-71. doi: 10.1109/tbme.1970.4502688 2. Chen, D. Y., Chan, P. K., & Tse, M. S. (2005, Oct. 30 2005-Nov. 3 2005). A CMOS ISFET interface circuit for water
quality monitoring. Paper presented at the Sensors, 2005 IEEE. 3. Dutta, J. C. (2012). Ion sensitive field effect transistor for applications in bioelectronic sensors: A research review.
Paper presented at the 2012 2nd National Conference on Computational Intelligence and Signal Processing, CISP 2012, March 2, 2012 - March 3, 2012, Guwahati, India.
4. Fung, C. D., Cheung, P. W., & Ko, W. H. (1986). A generalized theory of an electrolyte-insulator-semiconductor field-effect transistor. IEEE Transactions on Electron Devices, ED-33(1), 8-18. doi: 10.1109/T-ED.1986.22429
5. Kalofonou, M., Georgiou, P., Ou, C.-P., & Toumazou, C. (2012). An ISFET based translinear sensor for DNA methylation detection. Sensors and Actuators, B: Chemical, 161(1), 156-162. doi: 10.1016/j.snb.2011.09.089
6. Nobpakoon, T., Pijitrojana, W., & Poyai, A. (2013). A New Method for Current Differential ISFET/REFET Readout Circuit. International Journal of Information and Electronics Engineering, 3(2), 12-14. doi: 10.7763/IJIEE.2013.V3.284
7. Pookaiyaudom, P., Seelanan, P., Lidgey, F. J., Hayatleh, K., & Toumazou, C. (2011). Measurement of urea, creatinine and urea to creatinine ratio using enzyme based chemical current conveyor (CCCII+). Sensors and Actuators, B: Chemical, 153(2), 453-459. doi: 10.1016/j.snb.2010.11.015
8. Tomaszewski, D., Yang, C.-M., Jaroszewicz, B., Zaborowski, M., Grabiec, P., & Pijanowska, D. (2007). Electrical characterization of ISFETs. Journal of telecommunications and information technology, 55-60.
9. Wen-Yaw, C., Cruz, F. R. G., Chung-Huang, Y., Fu-Shun, H., Tai-Tsun, L., Pijanowska, D. G., . . . Jarosewicz, B. (2010). CMOS readout circuit developments for ion sensitive field effect transistor based sensor applications Solid State Circuits Technologies (pp. 421-444). Vukovar, Croatia: InTech.
10. Yates, D. E., Levine, S., & Healy, T. W. (1974). Site-binding model of the electrical double layer at the oxide/water interface. Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases, 70, 1807.
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Q & A
ISFET Applications
30
Conventional pH sensor ISFET-based pH sensor
http://www.titralo.hu/WEBSET_DOWNLOADS/613/GLP%2022_Bench-pH-Ion_e.pdf
http://s.campbellsci.com/documents/au/product-brochures/b_cs526.pdf
Comparisons with other ISFET designs
Specification BFDSF (Wen-
Yaw et al., 2010)
Differential Current Mode (Nobpakoon, Pijitrojana, & Poyai, 2013)
Inverter-based (Al-Ahdal & Toumazou,
2012)
This Work
Output mode Analog Analog Analog Digital
Process CMOS 0.35µm CMOS 0.35µm CMOS 0.35µm CMOS 65nm
Number of bits N.A. N.A. N.A. 10
pH resolution -50.68mV/pH 25.6µA/pH 3.79V/pH 0.0437pH/LSB*
Clock frequency (MHz)
N.A. N.A. N.A. 200
Sampling rate (kS/s) - - - 775
Power (µW) -** 937.6 -** 22.03
Energy/Conv (pJ) - - - 28.43
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*Assumed pH sensitivity of 50mV/pH **Power consumption not reported
ISFET Read-out Circuits
Inverter-based ISFET design pH of solution proportional to trip point of the inverter Additional Vin2 & C2 allows a high gain to be achieved pH measurement is not straightforward, as Vin2 has to be
swept to find the trip point
32
(Al-Ahdal & Toumazou, 2012)
Occurs when input pH level is highest Capacitor requires the full sensing operation to
discharge, causing the counter to operate for the full sensing operation
Maximum Energy Consumption Breakdown
Component Energy per
conversion (pJ) Charging of capacitor 0.88
Coarse sensing digital block 0.20
Fine sensing digital block 27.17 Digital Control block 0.18
Total 28.43
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Future Work
Improve linearity
Improve coarse sensing architecture
Study other non-ideal effects (e.g PVT variation, mismatch effects)
Use of TDCs to improve resolution or reduce area
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Transmission Gate Switch
𝑰𝑰𝑫𝑫 =𝟏𝟏𝟐𝟐𝑪𝑪𝒐𝒐𝒐𝒐µ
𝑾𝑾𝑳𝑳 𝑽𝑽𝑮𝑮𝑮𝑮 − 𝑽𝑽𝑻𝑻𝑻𝑻 𝟐𝟐
𝑰𝑰𝑫𝑫 = 𝑪𝑪𝒐𝒐𝒐𝒐µ𝑾𝑾𝑳𝑳 [ 𝑽𝑽𝑮𝑮𝑮𝑮 − 𝑽𝑽𝑻𝑻𝑻𝑻 𝑽𝑽𝑫𝑫𝑮𝑮 −
𝟏𝟏𝟐𝟐𝑽𝑽𝑫𝑫𝑮𝑮
𝟐𝟐]
35
High Low
Node A
VDS
Transmission Gate Switch
High Low
Node A
VDS
𝑹𝑹𝑮𝑮𝑺𝑺𝑻𝑻 =𝟐𝟐𝑽𝑽𝑫𝑫𝑮𝑮
𝑪𝑪𝒐𝒐𝒐𝒐µ𝑾𝑾𝑳𝑳 𝑽𝑽𝑮𝑮𝑮𝑮 − 𝑽𝑽𝑻𝑻𝑻𝑻 𝟐𝟐
𝑹𝑹𝑻𝑻𝑹𝑹𝑰𝑰 = 𝟐𝟐
𝑪𝑪𝒐𝒐𝒐𝒐µ𝑾𝑾𝑳𝑳 [𝟐𝟐 𝑽𝑽𝑮𝑮𝑮𝑮 − 𝑽𝑽𝑻𝑻𝑻𝑻 − 𝑽𝑽𝑫𝑫𝑮𝑮]
𝑹𝑹𝑪𝑪𝑪𝑪𝒐𝒐−𝒐𝒐𝒆𝒆𝒆𝒆 = ∞
36
Transmission Gate Switch
Res
(Ω)
nmospmos
Overall
37