Ion Sensitive Field-Effect Transistor – ISFET Working Principle

The ion sensitive field effect transistors are the novel integrated devices in the micro electrochemical lab on chip systems. These are the common type of chemically sensitive field effect transistors, and the structure is same as the general metal oxide semiconductor field effect transistor. The sensitive area represents a transistor gate and incorporates the means of transduction from an ion concentration to a voltage. In the case of ISFET the metal oxide and the metal gates are of general MOSFET are replaced by the simple solution with the reference electrodes deep in the solutions and the insulating layers are for detecting the specific analyte. The nature of the insulating layers is defined as the functionality and sensitivity if the ISFET sensor.

What is an ISFET?

The abbreviation of the ISFET is Ion Sensitive Field Effect Transistor. It is a field effect transistor, used for the measuring the concentration of ion solutions. The ion concentration like H+ is changed as pH then there is a change in current through the transistor consequently. Here the gate electrode is the solution and the voltage between the oxide surface and the substrate are due to the ion sheath.


Working Principle of ISFET

The working principle of an ISFET pH electrode is a change of normal field effect transistor and they are used in many amplifier circuits. In the ISFET normally the input is used as metal gates, which are replaced by the ion-sensitive membrane. Hence the ISFET gather in one device the sensing surface and a single amplifier gives the high current, low impedance output and it allows the use of connecting cables without unnecessary shielding. The following diagram shows the illustration of ISFET pH electrode.

Working Principle of ISFET
Working Principle of ISFET

There are different machines for the measurement of the pH from the traditional glass electrode. The measurement principle is based on the control of current flowing between the two semiconductors, they are drain and source. These two semiconductors are placed together to a third electrode and it behaves is like a gate terminal. The gate terminal is directly contacted to the solution to be measured.

Construction of ISFET
Construction of ISFET

Fabrication Steps for ISFET

  • The following step by step process shows the fabrication of the ISFET
  • ISFET is fabricated with the help of the CMOS technology and without any post processing steps
  • All the fabrication is done in the in house in the micro fabrication Lab
  • The material should be 4inch p-type silicon wafer
  • In the ISFET the gate terminal is prepared with the material of SiO2, Si3N4, both COMS computable materials.
  • There are six masking steps which are a creation of n-well, n and p source drains, gate, contact and material.
  • The design of Si3N4 and SiO2 is through the buffer oxide etch solutions

The following fabrication steps show the standard MOSFET process and up to the time of the deposition of silicon nitride as an ion sensing film. The performance of the deposition of the silicon nitride is with the help of the plasma enhanced chemical vapor deposition method. The thickness of the film is measured with the ellipsometer. After the nitride deposition, the process is continued to contact form by using the contact mask.

Fabrication Steps for ISFET

fabrication steps show the standard MOSFET process

design of Si3N4 and SiO2 is through the buffer oxide etch solutions

 etching step for silicon nitride

The wet chemical etch BHF is used for the etching and underlying nitride and oxide films to from the source and drain region. The custom of BHF helps to eradicate additional etching step for silicon nitride. The last and final step is the metallization in the ISFET fabrications. Near the gate region the Ion Sensitive Field-Effect Transistor does not have the metal layer, the metallization is provided at the source and drain contacts. The simple and main steps of Ion Sensitive Field-Effect Transistor fabrications are shown in the following diagram.

ISFET pH Sensor

These types of sensors are the choice for pH measurement and it is required for higher level performance. The size of the sensor is very small and sensors are used for the study of medical applications. The ISFET pH sensor is used in the FDA and CE which approves medical devices and they are also best for the food applications because the glass free and fitted in probes with the help of small profile that minimize damage to produce. The ISFET pH sensor is applicable in many environments, and industrial situations which vary for wet and dry conditions and also in some physical conditions like pressure make conventional glass pH electrodes will be suited.

ISFET pH Sensor
ISFET pH Sensor

Characteristics of ISFET pH

The general characteristics of the pH ISFET are the following

  • The chemical sensitivity of the ISFET is totally controlled by the properties of electrolyte
  • There are different types of organic materials for pH sensor like Al2O3, Si3N4, Ta2O5 have better properties than the SiO2 and with more sensitivity, low drift.

Advantages of ISFET

  • The response is very fast
  • It is a simple integration with measurement electronics
  • Reduce the dimension of the probe biology.

Applications of ISFET

The main advantage of the ISFET is, it can integrate with the MOSFET and the standard transistors of integrated circuits.

Disadvantages of ISFET

  • The large drift requires inflexible encapsulation of the chip edges and with binding leads
  • Even though the amplification properties of transistor of this device are very good looking. For sensing chemicals, the liability of the insulating membrane to the ecological poisoning and subsequent transistor breakdown has prohibited the ISFE from gaining popularity in commercial markets.

This article describes about the working principle of the ISFET and its fabrication step by step process. The given information in the article has given the basics of the Ion Sensitive Field-Effect Transistor and if you have any regarding this article or about the CMOS and NMOS fabrications please comment in the below section. Here is the question for you, what is the function of the ISFET?

Photo Credits:

  • Working principle of ISFET hon
  • ISFET pH Sensor microfluidics

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