Sensors for selectively determining liquid-phase or gas-phase co

Details Sensors for selectively determining liquid-phase or gas-phase co Sensors for selectively determining liquid-phase or gas-phase co

1987-03-23

1989-10-31

Eisenzopf, Reinhard J.

Electricity: measuring and testing

Determining nonelectric properties by measuring electric...

Semiconductors for nonelectrical property

204418, G01N 2700

Patent

active

048780154

DESCRIPTION:

BRIEF SUMMARY
The invention relates to sensors for selectively determining liquid-phase or gas-phase components; in particular, the invention relates to field effect transistors and capacitors for qualitatively or quantitatively determining species to be analyzed (molecules, ions etc.) in liquids or gases.
In a MOS field effect transistor (Metal Oxide Semiconductor Field Effect Transistor; MOSFET), an embodiment of which is shown diagrammatically in FIG. 1, the gate electrode (e.g. aluminum) is separated from the semiconductor (e.g. silicon) by a single-layer or multi-layer gate insulator (e.g. silicon dioxide.) The voltage U.sub.G on the gate controls the current flow between drain and source electrode.
A distinction is made between n-channel and p-channel transistors: drain region. The drain electrode is at a higher potential than the source electrode. region. The source electrode is at a higher potential than the drain electrode.
The transistors are furthermore divided into depletion and enhancement types: conduct even with a gate voltage of 0 V. transistors) have no current flow at a gate voltage of 0 V.
Two operating conditions (a) blocking and (b) conducting are discussed below using an n-channel FET as the example and making reference to FIG. 2:
(a) The voltage U.sub.G on the gate electrode is less than the threshold voltage U.sub.th : U.sub.G <U.sub.th. The source and drain regions (n-type silicon) have electrons as mobile majority carriers and the semiconductor in between (p-type silicon) has holes as mobile majority carriers. Since one of the two pn-junctions is always reverse biased, no current flows in this operating condition.
(b) The gate voltage is larger than the threshold voltage: U.sub.G>U.sub.th The gate is positively charged to such an extent that an inversion layer consisting of electrons is produced at the semiconductor/insulator boundary layer. A channel has consequently formed underneath the gate so that electrons can now travel from the source region through this channel beneath the gate electrode to the drain. The more positive the gate voltage is, the more electrodes there are in the channel and the greater is the current flow.
The simplest formulae for an n-channel FET (current flow from U.sub.G >U.sub.th upwards) are as follows, it being necessary to distinguish between the triode region and the saturation region (see FIG. 3): triode region: ##EQU1##
The drain current I.sub.D therefore depends on the following parameters: the higher the current. Since electrons have a greater mobility than holes, an n-channel MOSFET conducts more current than a p-channel MOSFET for otherwise equal parameters. the current flow. between source region and drain region is, the greater the electrical field is between drain and source for a given voltage U.sub.D and the greater the current. For a high current, therefore, the ratio W/L must be large. /d.sub.is ( 3)
.epsilon..sub.is : Dielectric constant of the gate insulator thickness produces a higher insulator capacitance so that, for the same gate voltage, there is a greater charge in the channel, i.e. a greater current flows. On the other hand, the insulator capacitance is proportional to the dielectric constant of the gate insulator so that if silicon nitride is used as insulator instead of silicon dioxide, a higher current is established. n-channel FET. but reaches a saturation value for D.sub.D >U.sub.D sat.
The threshold value U.sub.th is made up of the following components: ##EQU2## O.sub.m -O.sub.si : Difference in the work functions of the metal (gate electrode) and silicon manufacturing process, the silicon dioxide contains positive charges. The more positive charges there are in the oxide, the fewer the positive charges which have to be supplied to the gate for a certain current to flow; U.sub.th is consequently lower. area The magnitude of Q.sub.b depends on the doping of the silicon and the voltage U.sub.B on the substrate connection. ##EQU3## .epsilon..sub.si : Dielectric constant of silicon N.sub.A : Density of dopant atoms (acceptors)

REFERENCES:
patent: 3337497 (1967-08-01), Bostick
patent: 3400145 (1965-12-01), Wu
patent: 4241019 (1980-12-01), Nakatani et al.
patent: 4269682 (1981-05-01), Yano et al.
Journal of Applied Physics, vol. 51, No. 9, Sep. 1980 (US) M. Aktik et al.: A New Polymer Insulated Gate Field-effect Transistor", pp. 5055-5057, see the whole article

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