FET type sensor and a method for driving the same

A field effect transistor-type sensor comprising a field effect transistor device incorporated with a sensitive means exhibiting electric variation due to a physical or chemical interaction with the physical quantity to be detected, wherein an auxiliary electrode for the application of a drift-cancellation voltage to said sensitive means is located on said sensitive means, thereby suppressing the influence of impurities and/or ions contained in the sensitive means and/or the interface between the sensitive means and the field effect transistor device, or impurities and/or ions contaminating the device from the external atmosphere during use thereof, on the operation and/or the output of the field effect transistor device, and maintaining the stable output characteristic over a long period of time.

BACKGROUND OF THE INVENTION 
1. Field of the invention: 
The present invention relates to a field effect transistor-type sensor for 
detecting a variation of the gate operation of a field effect transistor 
due to a variation of external factors, by a sensitive means formed on the 
gate insulating film of an MOS- or MIS-field effect transistor. 
2. Description of the prior art: 
A field effect transistor (hereinafter, referred to as FET)-type sensor, 
which comprises an FET device incorporated with a sensitive means 
exhibiting an electric variation of electrostatic capacity, electric 
conductivity, electrostatic potential, etc., due to a physical or chemical 
interaction with the physical quantity to be detected, detects the said 
physical quantity as a variation of the gate operation of the said FET 
device. Taking advantage of the high input impedance and the amplifying 
function of the FET device, such an FET type sensor can exhibit a high 
output, even though its size is extremely small, and thus is advantageous 
in actual use. Particularly, an FET type sensor which is constructed in 
such a manner to have a sensitive means on the gate region of the FET 
device is advantageous practically and economically since the FET device 
can be small, and a number of devices can be formed on the same substrate. 
However, such an FET type sensor containing the FET device therein is 
inferior to an ordinary FET device alone in the operation stability of the 
FET device. It is also inferior to an FET device with regard to the output 
stability and the reproducibility of the output characteristic. Depending 
upon the kind of the FET type sensor required, materials and production 
processes of the sensitive means are so different that the operation 
characteristic of the FET device can be remarkably varied. As compared 
with an ordinary FET device, a large amount of impurities and/or ions are 
apt to appear in the sensitive means or may contaminate the interface 
between the sensitive means and the gate insulating film during the 
formation of the sensitive means on the FET device, causing instability 
not only in the operation characteristic of the FET device but also in the 
output characteristic of the FET type sensor. Moreover, since the FET type 
sensor, which is designed to be used as an atmosphere sensor such as a gas 
sensor, a moisture sensor, etc., is exposed to an atmosphere, it will be 
contaminated by impurities in the atmosphere, causing variation and/or 
deterioration of the FET characteristic and/or deterioration of the sensor 
itself. Accordingly, an FET type sensor must be provide with an 
arrangement which will suppress the influence of impurities and/or ions 
contained in the materials of the sensitive means, or impurities and/or 
ions contaminating the interface between the sensitive means and the gate 
insulating film during the formation of the sensitive means on the FET 
device and/or during operation of the FET device, thereby providing for a 
stable output characteristic over a long period of time. If such an FET 
type sensor is designed, a variety of sensors such as gas sensors, 
moisture sensors, ion sensors, biological sensors, infrared-ray sensors, 
etc. will be able to be produced in an FET type format. FET type gas 
sensors, moisture sensors, ion sensors and biological sensors cannot avoid 
direct interaction of the sensitive means with the atmosphere so that the 
device therein cannot be covered with a package, etc. Therefore, the 
above-mentioned problems deriving from the contamination etc. of 
impurities and/or ions from the outside must be solved for FET type 
sensors. To solve these problems, a silicon nitride film having a small 
diffusion coefficient for ions, moisture, etc. has been used as the gate 
insulating film, or used to cover the surface of the FET device. The 
resulting FET sensors are, however, still inferior in their output 
stability over a long period of time. 
SUMMARY OF THE INVENTION 
The FET type sensor of this invention which overcomes the above-discussed 
and numerous other disadvantages and deficiencies of the prior art, 
comprises a field effect transistor device incorporated with a sensitive 
means exhibiting electric variation due to a physical or chemical 
interaction with the physical quantity to be detected, wherein an 
auxiliary electrode for the application of a drift-cancellation voltage to 
said sensitive means is located on said sensitive means. 
The auxiliary electrode is located between the gate insulating film of said 
field effect transistor device and the sensitive means, and the gate 
electrode is located on the other surface of said sensitive means. 
The sensitive means is a moisture sensitive means, the electrostatic 
capacity or the electric conductivity of which varies with the absorption 
and the desorption of water vapor or moisture. The moisture sensitive 
means is at least one selected from the group consisting of a cellulose 
derivative film, a vinyl derivative film, an organic or an inorganic solid 
electrolyte film and a metal oxide film. 
The method for driving the FET type sensor of this invention which also 
overcomes the above-discussed and numerous other disadvantages and 
deficiencies of the prior art, comprises a field effect transistor device, 
a sensitive means located between the gate insulating film of said field 
effect transistor device and the gate electrode, an auxiliary electrode 
located between said gate insulating film and said sensitive means, and a 
fixed resistor located between said gate electrode and said auxiliary 
electrode, wherein said method includes applying a DC voltage and an AC 
voltage superposed thereon to said gate insulating film and said sensitive 
means through said gate electrode and said auxiliary electrode while 
maintaining the resistance of said fixed resistor at a sufficiently high 
level over the AC impedance of said sensitive means in the frequency of 
said AC voltage, and obtaining as a detecting signal an AC component 
resulting from the drain current of said field effect transistor device. 
Thus, the invention described herein makes possible the objects of (1) 
providing an FET type sensor which suppresses the influence of impurities 
and/or ions contained in the sensitive means and/or the inferface between 
the sensitive means and the FET device or impurities and/or ions 
contaminating same from the external atmosphere during the use of the 
sensor, on the operation and/or the output of the FET device, thereby 
maintaining the stable output characteristic over a long period of time; 
(2) providing an FET type sensor which includes a moisture sensor for 
detecting moisture, a gas sensor for detecting gases, an ion sensor for 
detecting ions, a biological sensor for detecting organic substances, 
etc., which maintains stable operation and output characteristics over a 
long period of time and attains the reproducibility of the 
characteristics; and (3) providing a method for driving the 
above-mentioned FET type sensors.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows an FET type moisture sensor as an embodiment of the FET type 
sensor according to this invention, which comprises an FET device 11 
incorporated with a moisture sensitive means 9. 
The FET device 11 is a MOS-type n-channel FET in which an n-type source 2 
and an n-type drain 3 are formed in a row by the diffusion of phosphorus 
into the surface of a p-type silicon substrate 1. The surface of the 
silicon substrate 1 is covered by a silicon dioxide film 5 having 
through-holes for the source 2 and the drain 3. Double layers of the 
silicon dioxide film (Si0.sub.2)5 and a silicon nitride film (Si.sub.3 
N.sub.4)7 disposed the silicon substrate 1 form, between the source 2 and 
the drain 3, a gate insulating film 100. The silicon nitride film 7 
serving to protect the FET device 11 covers a portion of the upper face of 
each of the conductive electrode films 6, which are formed on the silicon 
substrate 1 and the silicon dioxide film 5, and which come into contact 
with their respective source 2 and the drain 3 at the ends which extend 
through the holes in the film 5. On the gate insulating film 100, the 
moisture sensitive means 9 and a moisture permeable gate electrode film 10 
are successively formed. A blocking film 8 made of a conductive film is 
located between the moisture sensitive means 9 and the silicon nitride 
film 7. The blocking film 8 serves as an auxiliary electrode which applies 
a drift-cancellation voltage to the moisture sensitive means 9. 
The moisture sensitive means 9 is made of polyvinylalcohol or cellulose 
acetate crystallized by a baking treatment, but is not limited thereto. An 
organic or inorganic solid electrolyte film, or a metal oxide film such as 
an aluminium oxide film, etc. can be used therefor. The moisture permeable 
gate electrode film 10 is made of au gold evaporation film having a 
thickness of about 100 .ANG., but is not limited thereto. The blocking 
film 8 is made of a gold or aluminum evaporation film having a thickness 
of about 2,000 .ANG., but is not limited thereto. The sensitive means 9 is 
not limited to a moisture sensitive means, but may be a gas sensitive 
means, an ion sensitive means, another chemical substance sensitive means, 
a heat sensitive means, a light sensitive means, etc. As the FET device, a 
MIS-type FET can be used. 
FIG. 2 shows an equivalent network of the above-mentioned FET type moisture 
sensor, wherein references Cs and Ci the are electrostatic capacities of 
the moisture sensitive means 9 and the double layered gate insulating film 
100, respectively; reference R.sub.L is a load resistor connected in 
series with the drain electrode 6; and reference R.sub.B is a resistor 
connected in series with the blocking film 8. 
The basic operation of the FET type moisture sensor according to this 
invention is explained as follows: In order to simplify the explanation, 
the case where the moisture sensitive means 9 is directly formed on the 
gate insulating film 100 without the blocking film 8, that is, the 
resistor RB is omitted in the equivalent network in FIG. 2, is described, 
first. 
Given that the voltage to be applied to the moisture permeable gate 
electrode film 10 is V.sub.A and the threshold voltage of the FET device 
11 is V.sub.th, the drain current I.sub.D can be represented by the 
following equation (1): 
EQU I.sub.D =(.beta./2)(V.sub.A -V.sub.th).sup.2, .beta.=(.mu.nCW/L) (1) 
wherein .mu.n is a carrier mobility; L and W are the channel length and the 
channel width of the FET device, respectively; and C is an electrostatic 
capacity, in the case where an electrostatic capacity Ci of the gate 
insulating film is connected in series with an electrostatic capacity 
C.sub.s of the moisture sensitive means 9, and is represented by the 
equation (2): 
EQU C=(CsCi/Cs+Ci) (2) 
Thus, given that V.sub.A is a constant value, moisture can be detected a 
variation of the drain current I.sub.D with the variation of the 
electrostatic capacity C.sub.s of the moisture sensitive means 9 depending 
upon the moisture of the external atmosphere. 
Since a DC potential difference exists between both surfaces of the 
moisture sensitive means 9, impurities and/or ions contained in the 
moisture sensitive means 9 migrate by the action of an electric field, 
thereby attaining a rearrangement and/or a localization thereof which have 
a remarkable effect on the device characteristic in the channel region of 
the FET device, causing a variation of the threshold voltage V.sub.th and 
the drift of the operation characteristic i.e., operating point, of the 
FET device, and further causing a drift of the output signal as a moisture 
sensor. In the case where impurities and/or ions are contained in the 
interface between the moisture sensitive means 9 and the moisture 
permeable gate electrode film 10 and/or the interface between the moisture 
sensitive means 9 and the gate insulating film 100, the same phenomenon as 
the above-mentioned occurs as well. As described before, the contamination 
by impurities and/or ions from the external atmosphere into the device is 
unavoidable, and accordingly the solution of such problems is of great 
importance in providing the desired FET type moisture sensor. 
In order to solve such problems and thereby provide an FET type moisture 
sensor which can operate stably over a long period of time, an FET type 
moisture sensor according to this invention comprises a conductive 
blocking film 8 located between the moisture sensitive means 9 and the 
gate insulating film 100, as shown in FIG. 1. The blocking film 8 is 
connected with the moisture permeable gate electrode film 10 on the 
moisture sensitive means 9 by the resistor R.sub.B as shown in FIG. 2. A 
voltage V.sub.A, which is composed of a DC voltage V.sub.A (DC) and an AC 
voltage of frequency f superposed thereon, is applied to the gate 
insulating film 100 and the moisture sensitive means 9 through the 
moisture permeable gate electrode film 10 and the blocking film 8 to 
thereby drive this FET type moisture sensor. In the case where the DC 
voltage V.sub.A (DC) is smaller than the withstand voltage of the gate 
insulating film 100 and a leakage current does not occur through the gate 
insulating film 100, the DC voltage component V.sub.G (DC) of the 
effective gate voltage V.sub.G applied to the blocking film 8 becomes 
equal to the DC voltage V.sub.A (DC), resulting in no DC potential 
difference between both surfaces of the moisture sensitive means 9, so 
that the above-mentioned phenomenon that impurities and/or ions migrate 
within the moisture sensitive means 9 causing their rearrangement and/or 
localization can be suppressed and, additionally, the diffusion of these 
impurities and/or ions into the gate insulating film 100 can be suppressed 
because of the incorporation of the blocking film 8. Since the DC voltage 
V.sub.G (DC) is equal to the DC voltage V.sub.A (DC), this FET type 
moisture sensor cannot, of course, operate as a moisture sensor by the 
application of the DC voltage V.sub.A (DC) alone. The DC voltage V.sub.A 
(DC) functions to give an optimum bias voltage in the I.sub.D -V.sub.G 
characteristic of the FET device. 
In order that the FET type moisture sensor operates as a moisture sensor, 
that is, it detects the variation of an electrostatic capacity C.sub.s of 
the moisture sensitive means due to the moisture in an atmosphere, an AC 
voltage V.sub.A (AC) is essential. 
In the case where the resistor R.sub.B, having a resistance value which is 
sufficiently great as compared with the impedance (2.pi.fC.sub.S).sup.-1 
of the moisture sensitive means at a frequency f, is connected to the 
blocking film 8 and the moisture permeable gate electrode film 10, the 
resistance of resistor R.sub.B is negligible and the AC voltage component 
V.sub.G (AC) of the gate voltage V.sub.G can be represented by the 
equation (3): 
EQU V.sub.G (AC)=Cs/Cs+Ci)V.sub.A (AC) (3) 
This indicates that since V.sub.G (AC) varies with the values of the 
electrostatic capacity C.sub.s of the moisture sensitive means at the 
application of V.sub.A (AC) with a given amplitude, the output signal 
required for a moisture sensor can be detected as the AC amplitude of the 
drain current I.sub.D. Thus, the modulation of an electric current flowing 
from the source 2 to the drain 3 while applying V.sub.A (AC) to the gate 
electrode film 10, can be detected by the conductive electrode film 6 as a 
detecting signal. 
FIG. 3 shows the output i.e., the relative humidity characteristic 
experimentally measured, while the above-mentioned FET type moisture 
sensor operated under the conditions that the moisture sensitive means 9 
was made of a baked cellulose acetate film; the values of the fixed 
resistors R.sub.B and R.sub.L, respectively, are 10 M.OMEGA. and 1 
K.OMEGA.; that V.sub.A (DC) is 5 V; and that V.sub.A (AC) is 100 mV rms 
(10 KHz). 
In order to reveal the output stability of the above-mentioned FET type 
moisture sensor, the relationship between the time for which the FET 
device was allowed to stand in an atmosphere having a relative humidity of 
60% and the output of the FET type moisture sensor was examined and is 
shown in FIG. 4, wherein the characteristic curve A shows the drift of the 
output of the test sensor containing the blocking film 8, while the 
characteristic curve B shows the drift of the output of the control sensor 
containing no blocking film 8. Both sensors were subjected to examination 
under the same operating and measuring conditions, and their outputs, 
respectively, were expressed by a relative value on the basis of the 
initial output value. FIG. 4 indicates that the use of the blocking film 8 
is significantly effective to maintain the output of the FET type moisture 
sensor stably for a long period of time and that the drain current 
(I.sub.D)--drain voltage (V.sub.DS) characteristic, and the drain current 
(I.sub.D)--the gate voltage (V.sub.G) characteristic, etc. of the FET 
device are stable, do not drift and exhibit excellent reproducibility. On 
the contrary, in the case where the blocking film 8 is not employed as 
shown by the characteristic curve B in FIG. 4, both the I.sub.D -V.sub.DS 
characteristic and the I.sub.D -V.sub.G characteristic of the FET device 
exhibit great drift and are extremely inferior in reproducibility. 
Moreover, it can be observed that the I.sub.D -I.sub.DS characteristic 
and/or the I.sub.D -V.sub.G characteristic are greatly different from the 
initial characteristic even when the ON-OFF operation or the polarity at 
the applicaiton of V.sub.G is reversed. This phenomenon indicates that the 
migration and the distribution (rearrangement) of impurities and/or ions 
in the moisture sensitive means and/or the interface between the moisture 
sensitive means and the gate insulating film by the action of an electric 
field have remarkable effect on the characteristics of the FET device. 
It is understood that various other modifications will be apparent to and 
can be readily made by those skilled in the art without departing from the 
scope and spirit of this invention. Accordingly, it is not intended that 
the scope of the claims appended hereto be limited to the description as 
set forth herein, but rather that the claims be construed as encompassing 
all the features of patentable novelty which reside in the present 
invention, including all features which would be treated as equivalents 
thereof by those skilled in the art to which this invention pertains.