A 28.4 pJ per Conversion ISFET-based pH Sensing Design …news.ntu.edu.sg/rc-VIRTUS/Documents/2015-4.05.pdf · ISFET Ion Sensitive Field Effect Transistor Sense the presence and concentration

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)

[email protected]

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

5

(Chen, Chan, & Tse, 2005)

(Kalofonou, Georgiou, Ou, & Toumazou, 2012)

(Pookaiyaudom, Seelanan, Lidgey, Hayatleh, & Toumazou, 2011)

Presenter

Presentation Notes

General pH sensing can be used for water quality monitoring Dna methylation detects the ratio of methylation, can be used for cancer detection Urea concentration, creatinine concentration, urea to creatinine ratio for the prediction of renal failure

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)

Presenter

Presentation Notes

- CVCC structure - Current mirror with M313 compensates body effect of MISFET since they are matched with the same drain current having same body effects -Additional performance enchancement circuit using a differential circuit with VoutS and VoutT not shown here which compensates temperature, long-term drift and common noises

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

Presenter

Presentation Notes

Current flowing through ISFET and REFET controls Vgs of M1 & M2. At same pH, same current flows through M1 & M2. At higher / lower pH input of ISFET, there will be output current. Differential current achieved between M1 & M2. Since it is differential, it achieves temperature compensation.

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

Presenter

Presentation Notes

device mismatch, offset voltage, biasing, power consumption and stability

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

11

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




S1 VcapNode A


Precharge to VDD

13

C0

S2 S3 S4 S5 S6

VDD




S1 VcapNode A


Coarse ISFET discharges the

capacitor

Process the time taken &

determine the sub pH

range

14

Coarse Sensing Structure

CMP

CMP

CMP

Thermometer to Binary Decoder

C1

C2

C3

Clk

Clk

Clk

Vph10

Vph8

Vph6

Vcap

D9 – D8

15

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


C0

S2 S3 S4 S5 S6

VDD




S1 VcapNode A

Precharge to VDD

16


C0

S2 S3 S4 S5 S6

VDD




S1 VcapNode A

Fine ISFET discharges the

capacitor

Process the time taken to resolve the

pH value

17

Presenter

Presentation Notes

Switches implemented by transmission gates

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

19

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

Presenter

Presentation Notes

No fix rule in deciding the value of the capacitor. It depends on what performance you want to achieve trade offs between area, resolution, speed and power. For e.g, if the capacitor value is increased, with all else remains constant, the time taken for the capacitor to discharge will increase, achieving a higher resolution. However, the speed of the system will be reduced and it will consume more 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

21

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

Presenter

Presentation Notes

Biased in the subthreshold region to minimise the discharging current, allowing us to use a smaller capacitor as compared to biasing the ISFETs in the saturation 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

22

(Tomaszewski et al., 2007)

Presenter

Presentation Notes

59.2mV Nerstein voltage

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

23

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

24

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

… ..

.

25

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

26

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

31

*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)

Presenter

Presentation Notes

C2 designed to be small compared to Cpass. Amplification factor of Cpass/C2. To measure voltage, Vin2 is swept with Vref constant. Trip point of inverter corresponds to pH of solution.

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

33

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

34

Transmission Gate Switch

𝑰𝑰𝑫𝑫 =𝟏𝟏𝟐𝟐𝑪𝑪𝒐𝒐𝒐𝒐µ

𝑾𝑾𝑳𝑳 𝑽𝑽𝑮𝑮𝑮𝑮 − 𝑽𝑽𝑻𝑻𝑻𝑻 𝟐𝟐

𝑰𝑰𝑫𝑫 = 𝑪𝑪𝒐𝒐𝒐𝒐µ𝑾𝑾𝑳𝑳 [ 𝑽𝑽𝑮𝑮𝑮𝑮 − 𝑽𝑽𝑻𝑻𝑻𝑻 𝑽𝑽𝑫𝑫𝑮𝑮 −

𝟏𝟏𝟐𝟐𝑽𝑽𝑫𝑫𝑮𝑮

𝟐𝟐]

35

High Low

Node A

VDS


High Low

Node A

VDS

𝑹𝑹𝑮𝑮𝑺𝑺𝑻𝑻 =𝟐𝟐𝑽𝑽𝑫𝑫𝑮𝑮

𝑪𝑪𝒐𝒐𝒐𝒐µ𝑾𝑾𝑳𝑳 𝑽𝑽𝑮𝑮𝑮𝑮 − 𝑽𝑽𝑻𝑻𝑻𝑻 𝟐𝟐

𝑹𝑹𝑻𝑻𝑹𝑹𝑰𝑰 = 𝟐𝟐

𝑪𝑪𝒐𝒐𝒐𝒐µ𝑾𝑾𝑳𝑳 [𝟐𝟐 𝑽𝑽𝑮𝑮𝑮𝑮 − 𝑽𝑽𝑻𝑻𝑻𝑻 − 𝑽𝑽𝑫𝑫𝑮𝑮]

𝑹𝑹𝑪𝑪𝑪𝑪𝒐𝒐−𝒐𝒐𝒆𝒆𝒆𝒆 = ∞

36


Res

(Ω)

nmospmos

Overall

37