final_chemical isfet

Ion-sensitive field-effect transistors

Presented By

Naveen Kaushik Arnab Bose

Subrat Mishra PrasannaThengodkar

Electrical Engineering IIT Bombay

Contents

• Motivation

• Existing Techniques

• Introduction to ISFET

• ISFET with membranes

• Challenges

• Novel approach

• Conclusion

Introduction

• Chemical sensors are micro-devices that connect the

chemical and electrical domains

• The response of the sensors should be fast and

selective for the analyte.

• Measurement of pH is a very common task of chemical

senors required for many environmental and biomedical

applications

Existing techniques Glass membrane electrode

Limitations :

1. the inability to operate at high temperatures,

2. being a bulky device

3. manufacturing difficulties

4. low durability

Introduction to ISFET

ISFET- Ion Sensitive Field Effect Transistor

Why FET ? Small size

Fast response time

Reliability Of IC

• In the ISFET, the gate metal electrode of the MOSFET is

replaced by an electrolyte solution which is contacted by

reference electrode

• The metal part of reference electrode can be considered as

the gate of the MOSFET.

Dissolved free ions surrounded by water molecules

Change of Current(Id) due to change of Vth

• But putting Voltage of opposite polarity we can neutralize surface band bending keeping current constant

• Vref ∞ Reaction near gate-oxide

ISFETs

Basic Idea: removal of the metal plate of an

MOSFET and expose the oxide to an

electrolyte

Important:

Vgs - Potential applied between reference

electrode

• Possible Respond mechanisms:

1. Interfacial potential at electrolyte-oxide interface(MOSFET)

2. Diffusion of species through the

oxide Diffusion:

ISFET

• Potential drop across:-Solution

• (Bulk diffuse layer OHP IHP

• -Oxide /Electrolyte surface dipoles

• -Capacitance of the oxide

• -Oxide/Semiconductor interface

dipoles

• -Semiconductor

Effect of PH on current

Challenges:

• Good interface between Gate oxide and Membrane.

• Size and immobilization of Antibody.

• Temperature stability.

• Proper Controlling of Feedback circuit & maintaining pH stability.

• Lithographic challenges

Proposed Solution

• Surface to Volume Ratio should be high

• Solution: 2- D channel sensors

• Example : Graphene sheet, MoS2

Conclusion

• High-performance

• Reliable

• Fast Response time

• Small Size

• Respond to any compound By using particular membrane

• low power consumption

• robustness,

• Sensing and analysing DNA, Protein, Enzymes ,Cells

References

• Rothberg, Johnathan M (2011). "An integrated semiconductor device enabling non-optical genome sequencing". Nature 475 (7356): 348–52. doi:10.1038/nature10242. ISSN 1476-4687.

• Chang-Soo Lee 1, Sang Kyu Kim 1,2 and Moonil Kim 1 “Ion-Sensitive Field-Effect Transistor for Biological Sensing” Sensors 2009, 9, 7111-7131; doi:10.3390/s90907111

• IEEE Sensor Conference Torneto October 2003 on ISFET, Theory and Practice by Prof.Dr.Ir.P.Bergveld Em, University of Twente, Fac.EE, MESA+ Research Institute

Thank you

Use of Membrane

• Sensitivity depends upon SiO2 & Electrolyte interface

• Gate materials are: SiO2, Si3N4, Al2O3 and Ta2O5 • Surface Reaction: —Si–OH + H2O ↔ —Si–O– + H3O+ —Si–OH + H3O+ ↔ —Si–OH2

+ + H2O • Stabilization of Membrane is important and challenging

ISFETS

• Electrolyte/Oxide/Semiconductor Interface

• Inner Helmholtz Plane (IHP)

• Specifically adsorbed ions

• amphoteric hydroxyl groups

• Outer Helmholtz Plane (OHP)

• closest approach of solvated ions

• Diffuse (Gouy-Chapman) Layer

• diffuse charge region into the bulk

electrolyte

Introduction of potentiometric sensors –Measuring the electrical potential difference at a solid/liquid interface –Nernst Equation

Δφ= RT/F ln ai1/ai2 ai1,2 = fi*ci = activity of ions i

–Constant potential drop at the inner surface of the bulb –Contact between inner KCl solution and the outer solution –Electrochemical couple

Existing Technique

Application of membranes for ISFETs sensitive to different ions

Introduction

• Problem of miniaturizing

–Less stable

Problematic for in vivo measurements

• Bergveld 1970: Development of an Ion-Sensitive

Solid State Device for Neurophysiologic Measurements

• Advantage of chip technology

• –cheaper

• –Improved characteristics

• –Reproducibility

• Ion-Sensitive Field-Effect Transistor (ISFET)

• –small and rigid

• –fast response