Abstract:
A sensor array is provided including transistors that are coupled together. The transistors are designed as sensors and wherein the sensor array has switching devices for selecting a transistor and wherein the selected transistor&#39;s condition may be detected. The sensor array is set up so that the selected transistor is driven as a source follower and at least some of the transistors are MOS field effect transistors that are configured so that at least some of the transistors are capable of detecting biological material.

Description:
BACKGROUND OF THE INVENTION 
   The invention concerns a sensor array and a method for detecting a condition of a transistor in a sensor array. 
   DESCRIPTION OF THE RELATED PRIOR ART 
   Such a sensor array and such a method are known from [5]. 
   In the sensor array known from [5], MOS field effect transistors are provided, which are arranged in a matrix having N rows and M columns, and coupled together via column connections and row connections respectively. The column connections and/or row connections are usually electrically conducting connections. In addition, a means of selection is provided for selecting a field effect transistor whose condition is to be detected. 
   In the sensor array known from [5], the field effect transistors are designed as sensors, i.e. they detect a signal to be detected, for example by means of a varying gate potential of the field effect transistor concerned. 
   When a field effect transistor is selected and its condition is read, the sensor array determines a non-linear characteristic of the curve of the voltage present between the source and drain of the respective field effect transistor. The curve of the voltage that is read is non-linear. 
   Considerable problems arise in the practical application of the known sensor array as a result of this non-linearity. 
   In particular, in sensor arrays that are meant to achieve a high spatial resolution, for example in sensor arrays having a very large number of several thousand to several million field effect transistors, each arranged with a mutual separation of 5 μm or less, huge reliability problems arise with regard to the condition to be detected for the selected transistor. In other words, the reliability problems are to be observed in particular when the ratio of the dimension of the sensor array to the number of field effect transistors held in the sensor array is very low. 
   This means that for an increasing spatial resolution of the sensor array for a constant or even increasing total area of the sensor array, or correspondingly for a constant spatial resolution and for an increasing total area of the sensor array, considerable reliability problems arise when detecting bioelectric signals. 
   In addition, the technology used in [5] is very complicated and expensive to manufacture, and is barely compatible with common standard industrial manufacturing processes. 
   In addition, it is known how to modify MOS field effect transistors so that they can be used as sensors. 
   In such a field effect transistor, the control of the channel, or rather the control of the density of the charge carriers in the channel region by means of the object or medium to be characterized by the sensor, is performed in such a way that the object or medium influences the potential on the surface of the dielectric lying above the channel region, and changes the condition of the field effect transistor concerned. The condition of the field effect transistor is read via the source contacts and drain contacts. 
   In normal, non-modified MOS field effect transistor circuit configurations, for example in a standard memory array of transistors in the form of a matrix (e.g. a random access memory (RAM)), the condition of a field effect transistor is modified and read via the source contacts and drain contacts. 
   An example of a field effect transistor modified in such a way is shown in FIG.  2 . 
   The field effect transistor  200  has a substrate  201 , a source region  202 , a drain region  203 , a channel region  204  and an insulator dielectric  205  specially adapted to applications of biosensor or bioelectronics technology. For biosensor or bioelectronics applications, a cell  206  made of biological material is arranged above the insulator dielectric  205 . 
   As has been described in [2] and [3], with the aid of such a field effect transistor  200 , neural signals from the cell  206  made of biological material, which manifest themselves in the form of changes in potential at the cell wall  207  of the cell  206 , can be detected and characterized. This is possible because the changes in potential at the cell wall  207  control and modulate the channel current made up of the charge carriers in the field effect transistor  200 , or rather the density of the charge carriers present in the channel region  204  of the field effect transistor  200 . 
   Such a field effect transistor  200  is set up so that metabolic products from the cell  206  do not damage the field effect transistor  200  and do not modify its properties. 
   Furthermore, the sensor materials used in the field effect transistor  200 , that lie in contact with the cell  206 , have no effect on the metabolism of the cell  206  nor its function, and have no toxic effect on the cell  206 . 
   A further field effect transistor  300 , shown in  FIG. 3 , known as an ion-sensitive field effect transistor, is normally used to find a pH value of a solution under investigation. In general, the ion-sensitive field effect transistor  300  can also be used in gas sensor technology. The field effect transistor  300  has a substrate  301 , a source region  302 , a drain region  303 , a channel region  304 , an insulator dielectric  305 , where the interface  306  of the insulator dielectric  305  not in contact with the substrate  301 , or the dielectric region  307  forming the detection area, contains a large number of what are known as interface states. Ions attach themselves to these interface states according to the concentration of the parameter under analysis of the medium  308  to be characterized; these would be H+ ions if the field effect transistor  300  were designed as a pH-value sensor. In other words, this means that an interaction takes place between the medium  308  under investigation and the field effect transistor  300 . The potential effect caused by the interaction acts in a deterministic way on the channel current made up of the charge carriers within the channel region  304 , or rather on the density of the charge carriers present in the channel region  304  of the field effect transistor  300 . 
   In particular in biosensor or bioelectronics technology, it is desirable to provide a large number of such field effect transistor sensors described above in a sensor array, in order to enable detection with precise spatial resolution and temporal resolution of one or more parameters to be detected of a given specimen, for example of a specimen containing cells  206  or of a gas specimen as medium  308 . 
   One possible application of such sensor arrays can be found in the characterization of the neural activities of a multiplicity of mutually coupled biological cells. In such a sensor array, the sensors, i.e. the field effect transistors, are meant to be arranged in a matrix containing several thousand field effect transistors along each row and each column, each sensor having a mutual separation that is less than 5 μm*5 μm. The signal range of the electrical signal to be detected that is available in such an application can have values here of the order of magnitude of several microvolts (μV). 
   In addition, an array having a multiplicity of transistors and a multiplicity of sensor elements is known from [4], each sensor element being connected in series with a transistor. The transistors share a common source-follower resistor connected to the common series output of the transistors. 
   BRIEF SUMMARY OF THE INVENTION 
   In [5], a method for the manufacture of micro-and nano-structured carbon layers, carbon electrodes and chemical field effect transistors is known. 
   The invention is based on the problem of defining a sensor array, which, for a very high spatial resolution, up to a spatial resolution at which the field effect transistors are spaced down to 5 μm *5 μm and below, having a very large number of field effect transistors, from up to several thousand up to several million, can be used to detect bioelectric signals. 
   In addition, the invention is based on the problem of defining a method for detecting a condition of a transistor in a sensor array having up to several million field effect transistors arranged with a spatial resolution of down to 5 μm*5 μm and below, where the field effect transistors can be used to detect bioelectric signals. 
   The problem is solved by the sensor array and by the method for detecting a condition of a transistor in a sensor array having the features claimed in the independent claims. 
   A sensor array contains transistors (transistor elements) that are coupled together. The transistors themselves, preferably field effect transistors, are designed as sensors. In addition, a means of selection is provided that is used for selecting a transistor whose condition is to be detected. The sensor array is set up so that the selected transistor itself can be driven as a source follower, at least when selection has been made. 
   In a method for detecting a condition of a transistor in a sensor array that contains transistors coupled together, the transistors are used as sensors. This means that the condition of a transistor depends on a signal to be detected, which is detected by the transistor. The transistor is selected, and the condition of the selected transistor is detected. The selected transistor is driven as a source follower, at least when selection has been made. 
   By means of the invention, it is possible for the first time to produce a sensor array containing a large number of transistors, up to several million transistors, as a sensor array with a high spatial resolution and high temporal resolution. 
   In particular, by driving the selected transistor as a source follower, i.e. at an operating point at which the source voltage of the selected field effect transistor has an essentially linear dependence on the gate voltage applied to the field effect transistors, which represents the potential of the specimen under investigation, the array having this high level of spatial and temporal resolution becomes essentially noise-free and thus stably possible. 
   The sensor array is robust to interference effects and it is ensured that the bioelectric signals, for instance from the cell  206  or the medium  308 , can be detected very precisely. 
   Preferred development of the invention arise from the dependent claims. 
   At least some of the transistors can be field effect transistors. According to an embodiment of the invention, at least some of the field effect transistors are MOS field effect transistors. 
   At least some of the MOS field effect transistors can be set up so that they can detect biological material. 
   According to a further embodiment of the invention, there is provision for at least some of the transistors to be ion-sensitive field effect transistors. In this way it is possible to use the sensor array in gas sensor technology or to find a pH value of solutions for instance. 
   The selected transistor can be driven as a source follower for example. The selected transistor can be driven at an operating point in inversion, at least when selection has been made. Alternatively, the selected transistor can be driven at an operating point in the sub-threshold region of the transistor, at least when selection has been made. 
   According to a further embodiment of the invention, there is provision to apply a voltage, equal to the operating voltage of the sensor array, in order to detect a condition of the selected transistor. This development makes it possible to implement the whole sensor array very compactly and easily, since the operating voltage can also be used to detect the condition of the selected transistor concerned. 
   The transistors can be arranged compactly in columns and in rows in the form of a matrix, and coupled together via column connections and row connections similar to the connecting structure of a matrix of a standard semiconductor memory. 
   According to a further embodiment of the invention, a current source is provided that can be coupled to the source contacts of the field effect transistors. A voltage source can also be provided that can be coupled to the drain contacts of the field effect transistors. The sensor array according to this embodiment is particularly suited to compensating for signal errors that may arise within the sensor array. 
   The means of selection may contain switches, by means of which a transistor can be selected. 
   In particular, in order to reduce possible parasitic effects, which can arise in the sensor array, particularly for reduced dimensions of the sensor array for a constant or growing number of field effect transistors, provision is made according to an embodiment of the invention to assign to each transistor a selection element, which can be used to couple the selected transistor conductively to the means of selection, and which can be used, when the transistor is not selected, to cut off the current flow through it electrically. 
   The selection element can be a diode or a transistor. 
   In addition, a buffer circuit, using an operational amplifier for instance, can be provided, which is coupled to the transistors, preferably to the row connections of the field effect transistors. Using the buffer circuit, the condition detected at the time is made available, via the row connections and the buffer element, at an output of the buffer circuit for further processing, where, for example, the signal representing the condition and made available at the output is of low impedance and capable of taking a load. Alternatively, within the scope of the invention, any electrical circuit that guarantees the functionality described above can be used as buffer circuit, i.e. that guarantees that an input signal present at the input of the buffer circuit is made available at low-impedance at the output of the buffer circuit. 
   Thus by means of the buffer circuit, the signals taken off at its output are prevented from having a loading effect on the sensor array. 
   It is preferred to make the signal present at the output of the buffer circuit available via column connections to the transistors of the sensor array, in order to provide at least some of the unselected transistors with a defined electrical potential equal to the potential present at the output of the buffer circuit. 
   Exemplary embodiments of the invention are depicted in the figures and explained in more detail below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the figures: 
       FIG. 1  shows a sensor array according to a first exemplary embodiment of the invention. 
       FIG. 2  shows a sketch of a field effect transistor with biological material. 
       FIG. 3  shows a sketch of an ion-sensitive field effect transistor. 
       FIG. 4  shows a sketch of a symbol of a transistor designed as a sensor, which is used in the description of the exemplary embodiments. 
       FIGS. 5   a  and  5   b  show electrical equivalent circuits of the sensor array shown in  FIG. 1  without buffer circuit ( FIG. 5   a ) and with buffer circuit ( FIG. 5   b ). 
       FIG. 6  shows a sensor array according to a second exemplary embodiment of the invention. 
       FIG. 7  shows a sensor array according to a third exemplary embodiment of the invention. 
       FIG. 8  shows the sensor array as shown in  FIG. 4 , taking into account in addition parasitic resistances in the connecting lines between the individual field effect transistors. 
       FIG. 9  shows a sensor array according to a fourth exemplary embodiment of the invention. 
       FIG. 10  shows a sensor array according to a fifth exemplary embodiment of the invention, in which each field effect transistor is assigned a selection element. 
       FIG. 11  shows the electrical equivalent circuit of the sensor array shown in FIG.  10 . 
       FIG. 12  shows a sensor array according to a sixth exemplary embodiment of the invention. 
       FIG. 13  shows a sensor array according to a seventh exemplary embodiment of the invention. 
       FIG. 14  shows a sensor array according to an eighth exemplary embodiment of the invention. 
       FIG. 15  shows an electrical equivalent circuit of the sensor array shown in FIG.  14 . 
       FIG. 16  shows a sensor array according to a ninth exemplary embodiment of the invention. 
       FIG. 17  shows an electrical equivalent circuit of the sensor array from FIG.  16 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Identical elements in the figures are labeled below with the same references. 
     FIG. 1  shows a sensor array  100  according to a first exemplary embodiment of the invention. 
   The sensor array  100  contains MOS field effect transistors  101  designed as a sensor. 
   The field effect transistor depicted in  FIG. 2  or in  FIG. 3  may be used, for instance, as such a field effect transistor. 
   In addition, the field effect transistors described in [1] and [2] may alternatively be used as sensor in the sensor array  100 . 
   Each field effect transistor  101  forms a sensor cell  102 . 
   The sensor array  100  contains M * N sensor cells  102 , the sensor cells  102 , and hence the field effect transistors  101 , being arranged in N columns and M rows in the form of a matrix  103 . This means that the sensor array  100  contains N sensor cells  102  in each row. The sensor array  100  contains M rows, i.e. M sensor cells  102  in each column of the sensor array  100 . 
     FIG. 4  shows the symbol  400  for a field effect transistor  101  as used in the description below. 
   Each of the source contacts  104  of each field effect transistor  101  is coupled to a row connection (an electrical conductor)  105 ,  106 ,  107 , so that each of the source contacts  104  of all the field effect transistors  101  in one row is coupled to a row connection  105 ,  106 ,  107 . 
   The drain contacts  108  of all the field effect transistors  101  are coupled to the column connections  109 ,  110 ,  111 , preferably electrical conductors, so that each of the drain contacts  108  of the field effect transistors  101  in one column is coupled to the corresponding column connection  109 ,  110 ,  111 . 
   A row-selection switch  112 ,  113 ,  114  is connected to each row connection  105 ,  106 ,  107  respectively as means of selection. If the row-selection switch  112 ,  113 ,  114  in question is in the open position, then no current flows through the corresponding row connection  105 ,  106 ,  107 . 
   If, however, the row-selection switch  112 ,  113 ,  114  in question is closed, then an injected current I IN  provided by a current source  115  flows through the corresponding row connection  105 ,  106 ,  107 . 
   In addition, a column-selection switch  116 ,  117 ,  118  is provided as means of selection for each column connection  109 ,  110 ,  111  respectively. 
   In a first switch position, which corresponds to selecting the relevant column connection  109 ,  110 ,  111 , i.e. in the case that a field effect transistor  101 , which is coupled to the column connection  109 ,  110 ,  111  selected at the time, in  FIG. 1  the second column connection  110 , is meant to be selected, the column connection  109 ,  110 ,  111  selected at the time is coupled to a first connecting line  119 , which is coupled to a voltage source  120 . The voltage source  120  supplies an operating voltage V RW , which is used to select the relevant field effect transistor  101  or  124 . 
   In the second switch position, the relevant column-selection switch  116 ,  117 ,  118  is connected to a second connecting line  121 , via which the switch concerned is coupled to the output  122  of a buffer circuit  123 . 
   The buffer circuit  123  may, as shown in the exemplary embodiments, be an operational amplifier for instance, whose non-inverting input can be coupled to the row connections  105 ,  106 ,  107 , and whose inverting input is coupled to the output of the operational amplifier. 
   It must be noted in this context that a different electrical circuit, which makes an input signal present at its input available at low impedance at its output, can be used directly instead of the buffer circuit  123 . 
   The electrical potential of the signal that acts on the channel region of the field effect transistor  101  concerned, i.e. the signal to be characterized by the field effect transistor as sensor, is referred to below by V char . 
   By selecting the row-selection switches  112 ,  113 ,  114  and the column-selection switches  116 ,  117 ,  118 , the injected current I IN  is injected into the field effect transistor  124  of the selected row, in  FIG. 1  the second row connection  106 . 
   A buffered measurement signal V sense,buf  present at the output  122  of the buffer circuit  123 , which in terms of its value is equivalent to the measurement signal V sense  amplified with a gain of 1, is fed to the sensor array  100  by means of the column-selection switches  116 ,  117 ,  118 , specifically to the column connections  109 ,  111  of the field effect transistors  101  that have not been selected. 
   This arrangement ensures that all the field effect transistors  101  that are coupled to an unselected column connection  109 ,  111  are driven with a potential difference of zero volts between drain and source of the corresponding field effect transistor  101 , and thus carry no current. 
   Hence this ensures that the injected current I IN  flows without loss through the selected sensor transistor  124  at the position (x, y), where x refers to the column containing the selected field effect transistor, and y refers to the row containing the selected field effect transistor. 
   It must be noted in this context that the unselected row connections  105 ,  107  may be connected in theory to any potential within the specific operating-voltage limits set by the technology employed for the corresponding sensor array. 
   Alternatively, the relevant row-selection switches can simply be opened. 
   In addition, it is possible alternatively to couple all unselected row connections  105 ,  107  to the output  122  of the buffer circuit  123 . 
   This coupling has advantages, particularly with regard to the access time to a field effect transistor  101  within the sensor array  100 , since the potential of the row connection just selected in each case is, at the time of its selection, already lying close to the potential value that is being determined by the sensor just selected. Thus a smaller amount of electrical charge needs to flow until the new potential is set up. 
     FIG. 5   a  and  FIG. 5   b  show the electrical equivalent circuit for the selected field effect transistor  124  of the sensor array  100  depicted in  FIG. 1 , where  FIG. 5   a  shows the electrical equivalent circuit  500  without buffer circuit  123 , and  FIG. 5   b  shows the electrical equivalent circuit  501  with buffer circuit  123 , which makes no difference, however, as regards the value of the output signal V OUT  of this circuit. 
   The value of the source voltage V S  of the selected field effect transistor  124 , i.e. of the selected sensor transistor, whose value is identical to the potential V sense  present at the input to the buffer circuit  123 , is a function of the potential V char  acting on the channel region of the selected field effect transistor  124 , of the current through the selected field effect transistor  124 , which is equal to the injected current I IN , and of the drain voltage at the selected field effect transistor  124 , this being the operating voltage V RW  in FIG.  1 . 
   The injected current I IN  is selected such that the selected transistor  124  adopts an operating point so that the selected field effect transistor  124  is driven as a source follower. 
   This is possible by the injected current I IN  being selected so that the selected field effect transistor  124  adopts an operating point in inversion, i.e. the following applies:
 
 V   char   −V   s   &gt;V   th ,  (1)
 
where V th  refers to the threshold voltage of the field effect transistor  124 , and by the drain voltage being selected to be greater than the difference
 
V char −(V th +V s ),  (2)
 
which equals what is known as the effective gate voltage of the selected field effect transistor  124 .
 
   In this way, an operating point of the field effect transistor  124  is set up in the saturation region, and the field effect transistor is driven as a source follower, as required. 
   This situation exploits the fact that under the cited conditions, the transistor current only exhibits a weak dependence on the drain voltage of the selected field effect transistor, and is mainly determined by the effective gate voltage. 
   Since the current is preset, however, and the voltage V char  is the variable to be characterized, i.e. the signal to be detected, this clearly results in an essentially linear mapping of the voltage V char  onto the source voltage V S  of the selected field effect transistor. 
   Since, as is apparent from  FIG. 1 , the following holds:
 
V sense =V s ,  (3)
 
then at the selected second row connection  106  there is
         a detected signal V char  that is changed by a constant amount. Changes ΔV char  in the electrical signal to be detected V char  hence lead to changes ΔV sense  at the selected row connection  106 , where the following is true to a good approximation:
 
 ΔV sense =ΔV char .  (4)
       

   The positive operating voltage V DD  used to drive the sensor array  100  is preferably chosen as the value for the voltage V RW . 
   Alternatively, the selected field effect transistor  124  concerned can also be driven at an operating point in what is known as the sub-threshold region, i.e. such that the following is true:
 
 V   char   −V   S   &lt;V   th .  (5)
 
   Such an operating point can be set if a very small current I IN  is injected. 
   Also in this case, the change in the source voltage of the selected field effect transistor is approximately linearly dependent on the change in the electrical signal V char  acting on the selected field effect transistor  124 . 
   The buffered measurement signal V sense,buf  made available at the output  122  of the buffer circuit  123  is used as output signal of the sensor array  100 , which can be employed in further signal-processing circuit components not shown, or alternatively evaluated directly. 
   The buffered measurement signal V sense,buf  made available at the output  122  of the buffer circuit  123  is of low impedance and therefore capable of taking a load, i.e. signal processing can take place without any fears of the sensor array  100  being loaded by the signal processing. 
   Provided the measurement signal is taken off with sufficiently high impedance, for example using an amplifier whose inputs are formed by the gates of MOS field effect transistors, the measurement signal V sense  can also be used directly as output signal. 
   In a sensor array  600  according to a second exemplary embodiment, which is shown in  FIG. 6 , extra row-selection switches  601 ,  602 ,  603  are provided in addition to the row-selection switches  112 ,  113 ,  114 , which are arranged on the opposite side of the sensor array  600  from the row-selection switches  112 ,  113 ,  114 . 
   According to the sensor array  600  shown in  FIG. 6 , the injected current I IN  is fed from the current source  115  via the row-selection switches  112 ,  113 ,  114  to the selected row connection  106 . The selected voltage signal V sense  is taken off via the extra row-selection switches  601 ,  602 ,  603  of the selected row connection  106 , and fed via connecting lines  604  to the input of the buffer circuit  123  as measurement signal V sense , a buffered output signal V sense,buf  thereby being generated from the buffer circuit  123 . 
   The other elements of the sensor array  600  according to the second exemplary embodiment correspond to the sensor array  100  according to the first exemplary embodiment. 
     FIG. 7  shows a sensor array  700  according to a third exemplary embodiment of the invention. 
   According to the third exemplary embodiment, extra column-selection switches  701 ,  702 ,  703  are provided compared with the sensor array  600  according to the second exemplary embodiment. 
   In addition, the buffered measurement signal V sense,buf  at the output  122  of the buffer circuit  123  is fed back to the extra row-selection switches  601 ,  602 ,  603  via feedback connections  704  in such a way that the unselected row connections  105 ,  107  are coupled to the buffered measurement signal V sense,buf  because of the corresponding switch position of the row-selection switches  601 ,  603 , in which they are coupled to the corresponding feedback connection  704 . 
     FIG. 8  shows a sensor array  800  in which parasitic effects are taken into account. 
   This sensor array  800  shows parasitic resistances R pix,RW , R pix,CL , which arise for each sensor cell  102  as a result of the row connections  104 ,  105 ,  106  and the column connections  109 ,  110 ,  111 . 
   For each sensor cell  102 , a given section of the corresponding column connection  109 ,  110 ,  111  or of the corresponding row connection  104 ,  105 ,  106  is taken into account by the parasitic resistance 
           R     pix   ,   RW       ⁡     (     =     2   ·     1   2     ·     R     pix   ,   RW           )       ⁢           ⁢   or   ⁢           ⁢         R     pix   ,   CL       ⁡     (     =     2   ·     1   2     ·     R     pix   ,   CL           )       .         
 
   The parasitic resistances R pix,CL  and R pix,RW  cause voltage drops on the row connections  105 ,  106 ,  107  and column connections  109 ,  110 ,  111  respectively, so that a relatively complex profile of the node potentials of all the nodes in the sensor array  800  is obtained within the sensor array  800  overall. 
   If the sensor array  800  is designed so that the dimensions, i.e. the distances between the individual sensor cells  102 , of the sensor array  800  are further reduced for a constant or increasing number of field effect transistors contained in the sensor array, then these parasitic resistances R pix,CL  and R pix,RW  should be taken into account, first because it is no longer absolutely guaranteed in this case that the injected current I IN  flows completely through the selected field effect transistor  124 . This is the case here, since drain-source voltages not equal to the value 0 arise across the unselected transistors  101  within a selected row, i.e. within a row that also contains the selected transistor  124 , as a result of the previously described complex voltage drop across the whole array, so that these transistors also carry current. 
   Secondly, the voltage drop on the row connection  105 ,  106 ,  107  or column connection  109 ,  110 ,  111  between the current source  115  and the source  104  of the selected field effect transistor  124 , means that the measurement voltage V sense  is no longer identical to the source voltage of the selected field effect transistor  124 . The difference between the measurement voltage V sense  and the source voltage V 8 , and the difference between the injected current I IN  and the current actually flowing through the selected field effect transistor  124 , are also dependent on the position of the selected field effect transistor  124  within the matrix  103  of the sensor array  800 . 
     FIG. 9  shows a sensor array  900  which is used to minimize the problems set out above when dimensions of the sensor array  800  are further reduced for a constant or increasing number of field effect transistors contained in the sensor array, i.e. to provide optimum compensation for the measurement errors resulting from the parasitic resistances R pix,CL  and R pix,RW . 
   Compensation is made possible in particular by the fact that the injection of the injected current I IN  by means of the current source  115  is performed in each case on the opposite side of the sensor array  900  from the detection of the measurement signal V sense , and that the buffered measurement voltage V sense,buf  at the output  122  of the buffer circuit  123  is applied not only to the column connections  116 ,  117 ,  118 , but also on both sides of the array to the unselected row connections  105 ,  107 . In addition, the column potentials are applied to both sides of the column connections  116 ,  117 ,  118  of the sensor array  900 . 
   This sensor array  900  has the effect that that section of the row connections  105 ,  106 ,  107  that couples the source of the selected field effect transistor  124  to the input  125  of the buffer circuit  123 , i.e. to the buffer circuit  123 , carries approximately no current. Thus there is also approximately no voltage drop on this section of the row connection, and the signal present at the source of the selected field effect transistor can be read from the sensor array  900  with almost no alteration. 
   In order for it also to be possible to operate without measurement errors a sensor array  900  having very many elements or having small geometrical dimensions per sensor cell  102 , and in order to improve the sensor array  900 , particularly with regard to driving the field effect transistors with relatively large currents in order to reduce the access time to the selected field effect transistor  124 , extra selection elements are provided in addition to the field effect transistors  101 , in the sensor cell  102  within the sensor array  900 , in order to decouple the corresponding field effect transistors  101 , by means of which selection elements, using control signals, a targeted selection of the desired field effect transistor is possible without any alteration of the signal to be characterized, detected by the selected field effect transistor. 
   Such sensor arrays  900  having extra selection elements are described below. 
   These exemplary embodiments share the common feature that the injection of the injected current I IN  is performed in each case on the opposite side of the sensor array from the detection of the measurement signal V sense , and meet the following two fundamental conditions:
         The injected current I IN  flows completely via the selected sensor transistor  124 , i.e. via the selected field effect transistor  124  that is designed as sensor.   A voltage drop arises only across that section of the row connections connected to the source of the selected sensor transistor  124  and running in the x-direction that lies between the source of the selected sensor transistor and the current source  115 . The section of this row connection that is arranged between source of the selected sensor transistor and the signal pick-up point of the measurement signal V sense , carries no current, so that no voltage drop arises on this section of the row connection, and the signal present at the source of the selected sensor transistor  124  can be read from the sensor array without alteration.       

   The following nomenclature is also used in the description below. The sensor array contains N columns  109 ,  110 ,  111 , where 1≦x≦N, and M rows  105 ,  106 ,  107 , where 1≦y≦M, the selected sensor element  124  being located at the position (x, y) within the sensor array. 
   Compliance with these two conditions cited above is ensured, in particular, by the fact that only the selection element at the column position of the selected sensor transistor  124 , i.e. the selection element in the sensor cell of the selected field effect transistor, is in the open state, or is driven in this way, whilst all other selection elements to be assigned to the same row at the positions (1, y) . . . , (x−1, y), (x+1, y), . . . , (N, y) are in the blocked state, or are driven in this way. 
     FIG. 10  shows a sensor array  1000  according to a fifth exemplary embodiment, which complies with the conditions cited above. The parasitic resistances R pix,CL  and R pix,RW  are drawn in the sensor cells  102 . The parasitic resistances also arising in the extra selection lines  1001 ,  1002 ,  1003  are not shown, as they do not cause any measurement errors during operation of the sensor array  1000 . 
   Each sensor cell  102  contains two active elements, namely the actual sensor transistor  101  and a selection transistor  1004  in each case. The selection transistors  1004  are driven via the extra selection connections  1001 ,  1002 ,  1003  running in the y-direction. 
   The positive operating voltage V DD  is, for example, applied to the extra selection line  1002  of the selected sensor transistor  124 , the column-selection switches  116 ,  118  being coupled to the negative operating voltage at the positions (x, 1), . . . (x, M). 
   A low level (i.e. a negative operating voltage V SS ) is applied to all the other control lines  1001 ,  1002 ,  1003 , so that all the selection transistors  1004  at these positions are in the non-conducting state. 
   Regarding the choice of the injected current I IN  and the voltage V RW , these parameters are chosen so that the selected sensor transistor  124  is driven at a suitable operating point in the saturation region or in the sub-threshold region, so that source-follower operation is possible. 
   It must be taken into account, that the drain voltage of the sensor transistor  101  of the corresponding sensor cell  102  is not determined solely by the value of the operating voltage V RW  and the voltage drops along the line running in the y-direction, i.e. column connection  109 ,  110 ,  111 , that is connected to the selection transistor  1004  of the selected sensor transistor  124 , but also by the voltage falling across the selection element itself through which the current is flowing. 
   Once again, the positive operating voltage V DD  can be selected for the voltage V RW . 
   According to this exemplary embodiment, the unselected row connections  105 ,  107  may also be connected in theory to any potential or connected to a potential provided by the sensor arrays  900 , by the corresponding row-selection switches  112 ,  114  simply being left in the open state, as depicted in FIG.  10 . 
   Alternatively, the unselected row connections  105 ,  107  can be set to the potential V RW . In this case the selection transistors  1004  and the sensor elements, i.e. the sensor transistors  101  at the positions (x, 1), . . . (x, y−1), (x, y+1), . . . (x, M), carry no current, and the voltage drops are minimized along the column connections  109 ,  110 ,  111  that couple the selection transistor  1004  of the selected sensor transistor  124  to the operating voltage V RW , since this column connection need not take, in addition to the current flowing through the selected sensor transistor  124 , any further currents flowing through unselected sensor transistors of the same column. 
   Alternatively, all unselected row connections  105 ,  107  can be coupled to the buffered measurement signal V sense,buf  provided via a buffer circuit. 
   This array can have advantages when changing rows of the sensor position read out, or for the access time, since the potential of a row connection just selected is already lying close to the value that is being determined by the sensor just selected, and thus less electrical charge needs to flow until the new potential is set up. 
   In this context it should be noted in particular that for this version the buffer circuit  123  is not absolutely necessary. 
     FIG. 11  shows the electrical equivalent circuit  1100  of the sensor array  1000  shown in FIG.  10 . 
     FIG. 11  also shows the values for the total parasitic resistances derived from R pix,RW  and R pix,CL  in the electrical equivalent circuit  1100  for a sensor cell  102  at the position (x, y). 
   As is apparent from  FIG. 11 , the selected sensor transistor  124  is again driven as source follower, and the output voltage V out  provides the unaltered measurement result, since no current is carried on that section of the row connections connected to the source  104  of the selected sensor transistor  124  and running in the x-direction that leads to the measurement-signal pick-up point. 
   As is apparent from  FIG. 10 , in  FIG. 10 , the voltage V RW  is applied in parallel to all lines connected to the selection transistors  1003 , i.e. to the extra selection connections RW d,1 , . . . , RW d,N , but only on one side of the sensor array  1000 . It can be advantageous to apply the voltage V RW  to these lines on both sides, because in this case the total effective parasitic resistance in the y-direction R tot,RW  
               R     tot   ,   RW       =           1   2     ·     R     pix   ,   RW         +       ∑     i   =   1       y   -   1       ⁢     R     pix   ,   RW           =       (     y   -     1   2       )     ⁢     R     pix   ,   RW                   (   6   )             
 
is reduced in the equivalent circuit  1100  from  FIG. 11  to 
                     R     tot   ,   RW       =             (     y   -     1   2       )     ·     [     M   -     (     y   -     1   2       )       ]           (     y   -     1   2       )     +     [     M   -     (     y   -     1   2       )       ]         ·     R     pix   ,   RW         =                 =         (     y   -     1   2       )     ⁡     [     1   -       y   -     1   2       M       ]       ·       R     pix   ,   RW       .                     (   7   )             
 
   The value in equation (7) is obtained from the parallel connection of the line section lying above and below the selected sensor transistor  124 . 
   In the sensor array  1000  shown in  FIG. 10 , there are twice as many column connections running in the y-direction compared with the sensor arrays shown in  FIG. 1 ,  FIG. 6 ,  FIG. 7 , FIG.  8  and FIG.  9 . In order to improve this possibly unfavorable arrangement, and in order to reduce the lines overhead compared with the sensor arrays shown in  FIG. 1 ,  FIG. 6 ,  FIG. 7 , FIG.  8  and  FIG. 9  to a factor of 1.5, the sensor array  1000  from  FIG. 10  is modified as shown in  FIG. 12 , resulting in a sensor array  1200  according to a sixth exemplary embodiment. 
   In the sensor array  1200  shown in  FIG. 12 , each pair of sensor cells  102  adjacent in the x-direction shares one selection line  1201 ,  1202  running in the y-direction. The column connections  109 ,  110 ,  111 ,  1203 , however, continue to be taken individually to each column. In addition, an additional column-selection switch  1204  is also shown in FIG.  12 . 
   In a further embodiment it is directly possible that more than two sensor cells  102  adjacent in the x-direction also share a supply line running in the y-direction. 
     FIG. 13  shows a sensor array  1300  according to a seventh exemplary embodiment of the invention, in which all lines running in the y-direction, i.e. both the column connections  109 ,  110 ,  111  and selection lines  1201 ,  1202 ,  1301  are used with extra switches  1302 ,  1303 ,  1304  for selecting the sensor cell. 
   In this way, the overhead in additionally required column connections can be avoided completely. All column connections leading to the drain nodes or gate nodes of the selection transistors are coupled in each row to the drains or the gates respectively of each pair of adjacent selection transistors  1003 . 
   Only the lines on the left and right edge of the sensor array  1300 , which may be both drain supply lines and two gate lines or else one drain supply line and one gate line each, are coupled in each row to just one drain or gate of a selection transistor  1003 . 
   In this way, the exact value for the factor specifying the overhead is equal to 
           (     M   +   1     )     M     ,       
 
which is very close to 1 for large values of M.
 
   The selection of a sensor cell  102  at the position y is made by the gate-selection line coupled to the corresponding selection transistor  1003  being taken to high level, i.e. to the positive operating voltage V DD , whilst a low level, i.e. the negative operating voltage V SS , is applied to all other gate lines to the left and right of this, so that all the selection transistors at these positions are in the non-conducting state. 
   In addition, the drain-selection line coupled to the corresponding selection transistor  1003  must be taken to operating voltage V DD , and the other drain-selection lines are short-circuited to the output of the buffer amplifier or coupled to the ground potential. 
     FIG. 14  shows a sensor array  1400  according to an eighth exemplary embodiment, in which a diode  1401  is provided for each sensor cell  102  as selection element. Unlike the sensor arrays  1000  and  1200 , which are shown in FIG.  10  and  FIG. 12 , this sensor array  1400  requires exactly the same number of supply lines in the x- and y-direction as the sensor arrays shown in  FIG. 1 ,  FIG. 6 ,  FIG. 7 , FIG.  8  and FIG.  9 . 
   The voltage V RW  is again applied to the selected column connection; either a sufficiently low voltage, for instance the ground potential, can be applied to the unselected column connections, so that the diodes  1401  in these columns are reverse biased, or alternatively no connection may be made to a potential provided by the sensor array  1400 , simply by leaving the corresponding column-selection switches in the open state. 
   This version is based on the principle that between the selected column connection and the selected row connection of the selected sensor transistor  124 ,
         no additional current path is created in which not at least one diode  1401  is in reverse bias.       

   The diodes  1401  and thus the assigned sensor transistors  101  in the unselected columns, i.e. in the unselected column connections, carry no current. 
   With regard to the choice of the potentials of the unselected row connections, or rather their drive potentials, the same applies as described in connection with FIG.  10 . 
     FIG. 15  shows the electrical equivalent circuit  1500  belonging to the sensor array  1400  from FIG.  14 . 
   Here the potentials can again also be fed to both sides of the supply lines running in the y-direction, which leads to the same results as have already been described above in connection with the sensor array  1000  from FIG.  10 . 
   The diode  1401  of a sensor cell  102  in FIG.  14  and  FIG. 15  can be implemented, for instance, by a pn-junction. 
   A MOS field effect transistor connected as a diode  1601 , i.e. a MOS field effect transistor in which the drain and gate are connected together, can also be used, however. 
   In this case this yields the sensor array  1600  shown in  FIG. 16  instead of the sensor array  1400  shown in  FIG. 14 , and the electrical equivalent circuit  1700  depicted in  FIG. 17  instead of the electrical equivalent circuit depicted in FIG.  15 . 
   The following publications are cited in this document:
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