Instrumentation
J. Ν. Zemel The Moore School of Electrical Engineering Department of Electrical Engineering and Science University of Pennsylvania Philadelphia, Pa. 19174
Ion-Sensitive Field Effect Transistors and Related Devices
From the beginning of its develop ment, the metal-oxide semiconductor field effect transistor (MOSFET) has been plagued by pronounced sensitivi ty to various surface impurities. The elimination or neutralization of this sensitivity has been a primary goal of the semiconductor industries process development. After 15 years and mil lions of dollars, present MOSFET in tegrated circuits can be manufactured with excellent stability. However, the details of why a given processing pro cedure actually works are still poorly understood, and from time to time, processes go out of control, leading to a variety of instabilities. Although these instabilities are a highly undesirable state of affairs to the electronic circuit manufacturer, they do raise the possibility that single crystal semiconductor devices can be developed with specificity and sensi tivity. For example, as a result of in tensive research, it has been possible to associate particular ionic species with different types of instabilities. By far the best known is the N a + instabil ity. When the oxide of a MOSFET is contaminated with N a + , the operating point of the MOSFET becomes a function of prior heat and bias treat ment (1 ). Many authors have described ways to produce instabilities in semicon ductor devices, but none was directly concerned with using the instability to measure the presence of an impurity (1-11). The main thrust of their work was passivation. Of particular interest is the study by Raider et al. (12) on
Studies of the basic physical processes involved in the operation of chemically sensitive semiconductor devices such as ion-selective field effect transistors, MOS capacitors, and gatecontrolled diodes suggest the possible development of these devices as specific, sensitive ionic or molecular sensors
ion exchange at the Si0 2 -fluid inter face and a recent study on the gettering of N a + in S1O2 by Ligenza (13 ). So far as we know, Bergveld (14, 15 ) is the first author to describe an ionsensitive field effect transistor (ISFET) manufactured with present day semiconductor technology in any detail. His work makes it clear that substantially more research will be needed before such elements will be reduced to practical operational de vices. In this article, we present selected information on the design of chemicalsensitive semiconductor devices (CSSD) based on charge transfer from the semiconductor to a suitable sur face layer. The physical principles we will use are well known, being based on the operation of metal-oxide semi conductor field effect transistors (16). We will stress important differences between the proposed CSSD and the ion-sensitive electrode (ISE), both
with regard to ultimate sensitivity and ease of operation. Principles of Operation There are two classes of CSSD's that will be considered. The first em ploys an insulating layer in contact with the semiconductor. Examples of this type of device include those pre pared by Bergveld (14,15 ). The de vice shows extremely weak electronic or hole conductivity through the insu lator, though it must have a respecta ble mobility for either the molecular or ionic species of interest to respond in a reasonable time. The second class of device uses a layer having relatively good conduc tivity to form a junction (Schottky or hetero) with the semiconductor. The junction characteristics can be in fluenced by the electrochemical po tential of the layer which, in turn, de pends on the level of doping by vari ous mobile impurities. The bare sili con surface is not considered at all be cause it is reactive, readily poisoned, and far more difficult to "clean u p " than the layers we visualize, e.g., S1O2, glasses, PbS. One structure that has been long sought after employs a suit able glass layer deposited by direct chemical reaction or sputtering to pro duce a pH-sensitive element. The properties of ion-sensitive glasses, being reasonably well understood, could, in principle, be coupled with our knowledge of the surface space charge region (SSCR) to produce the device of interest. To date, this has not been done. Similarly, one could
ANALYTICAL CHEMISTRY, VOL. 47, NO. 2, FEBRUARY 1975 · 255 A
use the general theory of the heterojunction or the Schottky barrier junc tion as the basis of other devices. An example of this latter class of device will be presented later. In the case of the ISE, ideal opera tion produces a Nernst potential of (KT/e ) In (10) V/decade change in the activity of the ion of interest. This po tential source, which must be mea sured with respect to a suitable refer ence electrode, acts like an internal voltage generator and requires a highimpedance measuring system to avoid loading of the source. The CSSD, on the other hand, will operate in re sponse to a charge transfer from the semiconductor to the surface layer, as indicated in Figure 1. Because the semiconductor can be selected to have a low density of free carriers, even small changes in the surface charge can be readily measured internally. In Figure 1 we show the CSSD as a composite system consisting of the semiconductor itself, a transition re gion where charge sites (e.g., surface states) can reside, and the surface layer. The semiconductor can have a surface-conducting region made up of carriers of opposite sign to those in the bulk (an inversion layer). This inver sion layer can be contacted by either a source or drain (see Figures 2 and 4). It is separated from the bulk by a de pletion region of high resistivity. As a result, even small amounts of inver sion can be easily measured. In actual operation, MOSFET's can detect 109 charges/cm 2 with ease. Therefore, the possibility exists for lower impedance measurements than are possible with the ISE, with consequent reduction in noise and difficulties in transmitting
Figure 1. Schematic diagram of Si0 2 -Si interface. Transition region may arise from nonstoichiometry at interface or in-diffused oxygen
Gate Ον·ιΙ( Substrate
Figure 2. Schematic of η-channel enhancement mode MOSFET
the data. We note that the fluid need not be a liquid and could well be a gas or vapor. As a result, these devices could be employed to detect neutral gas or liquid molecules as well as ions. For one class of devices the silicon (to be specific) has either a grown or deposited insulating layer on its surface. As a rule, the transition region will not be truly uniform, and its properties will undoubtedly vary on an atomic scale. The sensing process involves the sorption of a particular species on the layer surface. Then, depending on the relative ionic or electronic mobilities, charge will migrate to form an appropriate double layer either in the transition region between the silicon and the surface layer or between the fluid and the surface layer. In either event, the silicon surface is charged, and the change in surface charge can be measured by one of several methods. Unlike the case of the uniform ion : exchange glass electrode, the ISFET structure is composed of a multilayer system where charge exchange sites may be distributed within the surface layer and transition region. Operation of MOSFET
Sèmiûoncluéloi
The standard MOSFET is shown schematically in Figure 2. The application of a positive gate voltage Va between the gate and substrate generates surface space charge layer in the semiconductor. Since the substrate is ρ type in the case treated, a suffi ciently large positive voltage induces negative charge at the silicon surface. This negative charge, or inversion layer, is directly related to any exter nal charges whether generated by a
battery, the capture of positive charge from the silicon, or the diffusion of charges in the oxide. If a very small positive voltage, V-o, is applied to the drain, a current, ID, will be drawn, given by (15 ) ID = {Wi±nCjL)(VG
-
VT)VD(l) 2
where C ox is the capacity/cm of the gate oxide, μη is the electron mobility in the surface region, and VT is de fined as V
T
=
ms +
2
ΦF
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