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Lecture 8 Notes.Pages !1 Special Lecture Series Biosensors and Instrumentation Lecture 8 - FET Based Sensors This lecture looks at the Ion Sensitive Field Effect Transistor (ISFET) which is the basic chemical sensor that can be made using the same basic structure as a MOSFET, but with some important changes. The initial implementation of the ISFET was a pH sensor operating in a potentiometric mode and that’s what this lecture presents first. We will then go on to look at different instrumentation methods for the ISFET, some of which are suitable for integration with CMOS processes for single chip microsystems or the creation of sensor arrays. Issues with ISFET measurement will be introduced along with techniques to deal with these with differential instrumentation. In the subsequent lecture we will look at some alternative implementations, for example using weak inversion mode of operation, additional sensing modes, or nano-wire structures. An example of a sensor chip developed at Edinburgh which combines pH, pO2 and amperometric biosensing capabilities will also be covered. Ion Sensitive Electrodes We begin by reviewing the concept of ion selective or ion sensitive electrodes, potentiometric devices where the electrode potential is measured when there’s no current flowing. They are characterised by the the Nernst equation: RT a E = E0 + ln i1 nF a ! ✓ i2 ◆ This depends on the ratio of activities (approximated as concentrations in dilute solutions) of the oxidised (ai1) and reduced species (ai2) being measured. Glass membrane electrodes, which you should be somewhat familiar with are a common form of potentiometric ISE but they’re difficult to miniaturise and so there’s been a lot of activity looking at trying to make a solid state equivalent. The ISFET The first solid state ISE was developed by Prof. Piet Bergveld at the University of Twente in the 1970s. The Ion Sensitive Field Effect Transistor (ISFET) is a standard MOSFET with the gate metal/polysilicon removed and replaced by the solution being measured and an electrochemical reference electrode. The gate dielectric, commonly SiO2, acts as the ion sensitive membrane where the surface potential is controlled by the pH of the solution. We will begin by revising some of the formulae for the operation of a standard MOSFET, that also apply for the ISFET. The drain-source current of the device when it’s operating in the linear region is defined in terms of the gate source voltage VGS, the threshold VT, the drain source voltage VDS and a device dependent gain β which is made up of terms representing carrier mobility μ, the gate oxide capacitance Cox and the dimensions of the channel region. V 2 I = β (V V )V DS W D GS − T DS − 2 β = µCox ! ! L With the potential difference between the source and the substrate/bulk set to 0V, the threshold voltage is made up of the flatband voltage, a term representing the gate voltage due to the charge in the depletion region QB and the potential due to the doping of the bulk ϕF. !2 QB VT = VFB + +2φF ! Cox Finally, the flatband voltage is made up of terms representing the work function difference between the “metal” gate and the silicon substrate and the potential due to fixed charges at the silicon/oxide interface (Qf or sometimes Qss) and charge incorporated into the oxide Qox. Qf + Qox VFB = ΦMS ! − Cox In the ISFET the effective gate source voltage is controlled by the reference electrode connection to the solution but the characteristics of the solution will also affect the threshold voltage. The expression for the VFB of the ISFET is: Qf + Qox VFB = Eref + ⇥0 + χsol ΦSi ! − − Cox The first three terms here are the contribution from the “gate” side of the device, equivalent to "M in a MOSFET. This includes figures for the potential of the reference electrode (Eref) and the solution dipole potential (χsol), but the most important for operation as an ion sensor is the surface potential #0. The final two terms are due to the work function of the silicon and the oxide charges as before. The interface between the thin gate oxide over the channel region of the transistor and the solution will consist of hydroxyl (OH) groups, which can either accept or donate protons (H+ ions) from the solution. They are “amphoteric” sites meaning they can exist in acidic, basic and neutral forms. The balance of these charges at the surface will depend on the pH in the solution and will effectively change the surface potential of the oxide. The equilibrium reactions between the surface and the solution are as follows: + SiOH SiO− +H , B SiOH+ SiOH + H+ ! 2 , B The surface acts as a buffer for changes in the pH of the bulk solution. When this increases the surface will donate protons, becoming more negatively charged whereas when the pH reduces the surface will accept protons and become more positively charged. The theory is developed in more depth than is appropriate for this course in [1] and you are directed there if you’re interested in finding out more. Basically though the surface potential at the oxide solution interface (#0) is dependent on the pH of the bulk solution (pHB) with the following function: ⇥Ψ kT 0 = 2.3 α !⇥pHB − q This is Nernstian in nature but includes a sensitivity parameter α which can vary between 0 and 1. The closer to 1 it is then the more Nernstian the pH response, i.e. it’s closer to acting like a glass membrane ISE,where $#0 = -59.2 mV/pH at 298K. Theoretically the sensitivity % will have the following formula: 1 ↵ = 2.3kT Cdl 2 +1 ! q βint This is a function of physical constants (k, T & q) as well as the variables Cdl and βint which represent firstly the double layer capacitance at the solution/oxide interface and secondly the buffer capacity of the oxide surface. !3 The capacitance Cdl is a result of the electrical double-layer that forms at the surface of any object when it is submerged in a liquid. In the most common model of this, known as the Gouy-Chapman-Stern model, this is made up from two parallel layers of charges. For our ISFET, the inner part is a tightly bound layer of ions at the oxide surface referred to as the compact Stern or Helmholtz layer. This is surrounded by a diffuse layer of charges that balance the surface potential or charge of an electrode or other charged surface when in contact with a solution. These are effectively two capacitances in series, one with a fixed value representing the Stern/Helmholtz layer (CH) and one representing the diffuse layer (Cd) which will vary with the solution concentration. The characteristic thickness of the double- layer is defined by the Debye length which is inversely proportional to the square root of the concentration. Therefore, the overall DL reduces in thickness as the concentration increases and, as it’s considered to have a constant permeability, the capacitance of the layer will also increase. So Cdl will vary with changes in the ionic concentration of a solution that are unrelated to pH, which will change α and affect the ISFET performance. The intrinsic buffer capacity of the oxide surface (βint) is a measure of the ability of the surface to accept or donate protons to and from the solution. It should be maximised to increase the sensitivity by making α closer to unity. Silicon dioxide isn’t the best material to use for an ISFET as the buffer capacity is relatively low. This means the sensitivity is sub- Nernstian and the ISFET made with SiO2 is more sensitive to changes in Cdl. Other materials like silicon nitride (Si3N4), aluminium oxide (Al2O3) and tantalum pentoxide (Ta2O5) are better for a variety of reasons and have higher βint and α. One reason for the better performance of the aluminium and tantalum oxides might be the greater number of oxygen atoms involved in the oxides, meaning a greater density of amphoteric sites at the surface. The graph on slide 12 is taken from [2] and shows the response curves of ISFETs made with the different materials mentioned previously. It shows how increasing the buffer capacity through choice of the gate dielectric can make the ISFET response more linear and more Nernstian. The best result is obtained for tantalum pentoxide which has a sensitivity that is almost Nernstian and appears to be very linear. The graph on slide 13, taken from the same paper, shows the affect of changing the ionic concentration of sodium chloride in the solution while keeping the pH constant. Silicon dioxide shows the worst cross correlation while Ta2O5 is much more insensitive, as the very large buffer capacity prevents Cdl affecting the ISFET response. As we’ve seen, the oxide/solution surface potential (Ψ0) is dependent on pH and therefore so are VFB and VT. If all else is constant the threshold voltage will have the same dependence on pH as the surface potential: ⇥V kT T = 2.3 α !⇥pHB − q This means that the ISFET can be completely controlled by the solution pH, though setting it up for a controlled measurement can be quite complicated as we’ll see in the next section. ISFET Instrumentation A standard method for measuring the ISFET is to bias it in the linear region of operation, so VDS is set to be some small, constant value so that the transistor channel isn’t pinched. Then the current through the device, IDS, is also controlled so that it is kept constant. If we remember the equation for the current in a MOSFET in the linear mode given earlier in this document, it can be rearranged to give an equation for the gate-source voltage, VGS.
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