the present involves a method of characterizing individual cells within a thin film transistor / liquid crystal display ( tft / lcd ) array . the method provides an indication of the state of each transistor along with information on the threshold voltage and transconductance of the transistor . in addition , the leakage current and c gs of each cell may be measured . the present method is useful for failure analysis , process variation and production line tests . in fig1 a thin film transistor ( tft ) 110 includes source contact 112 , drain contact 114 and gate contract 116 . source contact 112 supplies liquid crystal display ( lcd ) pad 120 which may be comprised of , for example , indium tin oxide ( ito ). drain contact 114 is connected to data line 118 . gate contact 116 is connected to gate line 122 . the combination described is referred to as a cell . a tft / lcd array comprises many such cells . each cell is addressable by means of a unique gate line / data line combination . thus , each cell may be individually accessed and tested . gate line 123 and data line 119 may be used to access a unique cell site not illustrated in fig1 . fig2 is a schematic of one preferred embodiment of the present invention attached to a typical tft / lcd cell . a tft / lcd cell , similar to the cell described with respect to fig1 is illustrated in fig2 in schematic form . in fig2 a thin film transistor ( tft ) 210 includes source contact 212 , drain contact 214 and gate contact 216 . source contact 212 supplies the liquid crystal display ( lcd ) pad which is represented schematically in fig2 by capacitor 220 . it will be noted that some tft / lcd cells include storage capacitance in parallel with the lcd pad ( see capacitor 721 in fig7 ). these storage capacitors may , therefore , contribute to the capacitance of capacitor 220 . drain contact 214 is connected to data line 218 . gate contact 216 is connected to gate line 222 . c gd in fig2 represents the gate - drain capacitance of transistor 210 . to test individual cell transistors without the presence of the front plate , it is necessary to detect the magnitude of charge stored on the parasitic capacitance of lcd pad 120 . consider the individual cell detail of a typical large display array as shown in fig1 . in the cell illustrated in fig1 lcd pad 120 is the only connection to the transistor source . parasitic capacitance from the pad to the adjacent gate and drain lines are included in the capacitor 220 as illustrated in fig2 . a sensing circuit 230 , according to the present invention , which is capable of measuring small change quantities with good resolution , is illustrated in fig2 . in this embodiment , operational amplifier 234 , capacitor 232 and switch s3 form a resettable integrator 200 . operational amplifier 234 includes inverting input 240 and non - inverting input 242 . non - inverting input 242 is connected to ground potential . inverting input 240 is connected to operational amplifier output 244 through integration capacitor 232 and switch s3 . output 244 is connected to output terminal 238 . switch s2 connects inverting input 240 to input terminal 236 . switch s1 connects input terminal 236 and switch s2 to supply voltage v d . input terminal 236 connects sensing circuit 230 to data line 218 . gate line 222 connects v g to gate 216 of transistor 210 . switches s1 and s2 are used to control the voltages applied to drain contact 214 . switch s3 is used to reset integrator 280 . r d represents the distributed resistance of data line 218 . c d represents the distributed capacitance of data line 218 . r g represents the distributed resistance of gate line 222 . c g represents the distributed capacitance of gate line 222 . c gd is the gate - drain capacitance of transistor 210 . it will be noted that the ground connection for capacitor 220 , which is necessary for the present invention , may be provided by , for example , grounding adjacent line 123 and / or adjacent data line 119 in fig1 . this is equally true for the embodiment illustrated in fig4 and 6 . in one embodiment of the method of testing a tft / lcd cell according to the present invention , the circuit of fig2 can be used to determine the functional quality of a cell , i . e . the ability of the cell to store charge . fig3 is a timing diagram for a method of testing a tft / lcd cell according to the present invention using the embodiment of the present invention illustrated in fig2 . fig3 illustrates a test sequence according to one embodiment of the present invention . the timing diagram illustrated in fig3 indicates the sequence of signals applied to the switches and voltage sources in fig2 . initially , that is prior to time t1 in fig3 switch s1 is open , switch s2 is open , switch s3 is open , v g is at approximately ground potential and v d is at approximately ground potential . at time t1 , s3 is closed , resetting the output of sense amplifier 230 to zero . further , at time t1 , v d is set to a value sufficient to charge capacitor 220 when transistor 210 is on ( e . g . the maximum data voltage which would be used to driven the data line during normal operation of the displays ). in conventional displays , the data line voltage may be approximately 10 volts . at time t2 , s1 is closed , connecting v d to data line 218 , and biasing drain 214 . at time t3 , v g is set to a value sufficient to turn on transistor 210 , for example a value which exceeds five times the threshold voltage of transistor 210 , ( e . g . 15 volts ), thus biasing gate contact 216 through gate line 222 and turning on transistor 210 . at time t4 , v g is reduced to a voltage below that required to turn on transistor 210 , reducing the voltage supplied to gate contact 216 through gate line 222 , and turning off transistor 210 . at time t5 , v d is reduced , for example , to approximately ground potential , thus discharging the charge on the distributed capacitance ( c . sub . d ) of the data line . at time t6 , switch s1 is opened , to disconnect voltage source v d from the data line . at time t7 , s2 is closed , connecting transistor 210 to operational amplifier 234 , thus setting sense amplifier 230 up for a sense operation . at time t8 , switch s3 is opened to allow the current from data line 218 to begin charging capacitor 232 . at time t9 , v g is increased to a voltage sufficient to turn on transistor 210 . at time t10 , v g is reduced to a voltage sufficient to turn off transistor 210 . in the embodiment of the invention illustrated in fig3 the period between time t9 and t10 is referred to as the integration period . in order to ensure accuracy of the test , this period will normally be at least four times the time constant of the cell . the cell time constant may be approximately by , for example , measuring the source to drain resistance r ds and cell capacitance ( capacitor 220 in fig2 ) of a test cell . during the integration period , s2 is closed and s1 is open , thus operational amplifier 234 holds the drain line at approximately ground potential due to virtual ground between inverting input 240 and non - inverting input 242 of operational amplifier 234 . therefore , during this period , the charge transferred to capacitor 232 is substantially equal to the charge removed from capacitor 220 . because inverting input 240 to operational amplifier 234 is a virtual ground , no charge is accumulated on drain line distributed capacitance c d . this capacitance , along with drain line distributed resistance r d , will , however , increase the time constant ( i . e . the time required to charge capacitor 232 and discharge capacitor 220 ) and , thus the integration period . during the integration period , the voltage at output terminal 238 changes due to the charge required to compensate for the rise in voltage across c gd and the charge removed from capacitor 232 when the gate is biased on at t9 . however , at t10 , transistor 210 is turned off by reducing voltage v g which removes the charge from c gd and reduces the voltage at output terminal 238 as illustrated in fig3 . therefore , after time t10 , the voltage at output terminal 238 will be substantially proportional to the charge stored on capacitor 220 at the end of the hold period ( t 9 ). the output voltage may be converted to a digital word using an analog to digital converter and used to analyze various parameters of the cell . this test may be repeated a number of times for each individual cell to average out any transient errors . in addition , by selecting the appropriate gate and data lines , any cell in an array may be tested by accessing the external pads of the display . it is not necessary to mechanically access individual cells . if a sense circuit is connected to every data line on an array , all of the transistors on a given gate line can be tested simultaneously . to check for basic function on each requires approximately 1 millisecond . therefore , using the method of the present invention , a 1000 by 1000 array can be tested in 1 second . it will be recognized that a portion of the voltage present at output terminal 238 after time t10 is attributable to parasitic capacitance in the test fixture . however , the effect of the parasitic capacitance may be measured and subtracted out of the voltage present at output terminal 238 to obtain a more accurate approximation of the charge on capacitor 220 . the effect of the parasitic capacitance may be measured , for example , by repeating the test described above with respect to fig3 omitting the steps required to turn on transistor 210 during the period between t3 to t4 . thus , at time t10 , the voltage at output node 238 would represent the charge stored on any parasitic capacitance in the test fixture . the test described above with respect to fig3 will provide an indication of the functionality of the cell being addressed . by varying some of the time periods and voltages , a number of other important parameters of the cell may be defined . for example , the tft leakage resistance is an important parameter which may be derived by plotting the measured output voltage as a function of the hold period . the period t4 to t9 is defined as the hold period and represents the period of time that the charge is held on an individual cell during a frame time in an actual display . it will be noted that the period is varied by extending the time between t4 and t9 . the times t4 to t5 , t4 to t6 , t4 to t7 and t4 to t8 remain fixed as does the time t9 to t10 . since the capacitance of the cell may be determined by , for example , measuring the capacitance of a test cell , a logarithmic plot of the output voltage as a function of the hold time will provide an indication of the leakage resistance . in a similar manner , the cell charging time may be determined by varying the period between time t3 and time t4 to obtain a plot of the output voltage as a function of the gate on time ( t3 to t4 ). since the cell capacitance may be determined as described above , a logarithmic plot of the output voltage after t10 as a function of the gate on time provides an indication of the cell charging time , which in turn , provides an indication of the drain to source resistance r ds of the cell transistor . similarly , by varying the voltage on the data line v d and measuring the output voltage over a number of iterations of the test , it is possible to derive the threshold voltage of the transistor in the cell from a plot of output voltage as a function of data line voltage . the circuit of fig2 may be susceptible to transient currents exiting when s3 is opened at time t8 . this effect may be overcome by sampling the voltage at output terminal 238 just prior to beginning the sense operation ( i . e . just prior to time t9 ) and subtracting that voltage from the voltage at the end of the sense time ( after t10 ), doubling the number of analog to digital ( a / d ) conversions required . in addition , the output range of the circuit illustrated in fig2 may not be compatible with some commercial a / d converters ( 0 to + 5 v ). that is , its output may drop below the input range of the analog to digital converter . to eliminate the uncertainty resulting from transients on s3 , and increase the dc bias at the output terminal 238 , a further embodiment of the present invention has been developed and is illustrated in fig4 . fig4 is a schematic of one preferred embodiment of the present invention attached to a typical tft / lcd cell . the tft / lcd cell , similar to the cell described with respect to fig1 is illustrated in fig4 in schematic form . in fig4 a thin film transistor ( tft ) 410 includes source contact 412 , drain contact 414 and gate contact 416 . source contact 412 supplies the liquid crystal display ( lcd ) pad which is represented schematically in fig4 by capacitor 420 . drain contact 414 is connected to data line 418 . gate contact 416 is connected to gate line 422 . c gd in fig4 represents the gate - source capacitance of transistor 410 . in the embodiment of the present invention illustrated in fig4 reset control 450 is used in place of switch s3 . the reset level for analog integrator 480 is controlled by reset control 450 which includes a rest reference voltage ( v ref ) and a comparison circuit . v ref provides sense range control . in this embodiment , switch s2 is replaced by resistor 460 and capacitor 462 . r d represents the distributed resistance of drain line 418 , while c d represents the distributed capacitance of drain line 418 . in a display comprising one million cells arrayed in rows and columns of 1000 cells , distributed capacitance may approach about 1 nf . in the embodiment of fig4 operational amplifier 434 , capacitor 432 and reset control 450 form a resettable integrator . operational amplifier 434 includes inverting input 440 and non - inverting input 442 . non - inverting input 442 is connected to ground potential . inverting input 440 is connected to operational amplifier output 444 through capacitor 432 and reset control 450 . operational amplifier output 44 is connected to output terminal 438 . resistor 460 connects inverting input 440 to input terminal 436 . switch s1 connects input terminal 436 , resistor 460 and capacitor 462 to a supply voltage v d . input terminal 436 , connects sensing circuit 430 to data line 418 . switch s1 is used to control the voltages applied to drain contact 414 . fig5 illustrates a timing diagram for one embodiment of the method of the present invention , using the apparatus illustrated in fig4 . all times referred to in the following description relate to the times illustrated in fig5 . prior to t1 , switch s1 is open , the reset signal is raised , v g is at approximately ground potential and v d is at approximately group potential . the first step in a single charge test is to stabilize the data voltage at the value desired for the test . thus , at t1 , the data voltage v d is set to a value sufficient to charge capacitor 420 . data voltage v d should , preferably , be adjustable over the full range of values used in display operation . for a typical tft / lcd , using a twisted nematic liquid crystal , this data voltage may range from approximately zero to eleven volts . in some displays the data voltage may reach approximately fourteen volts . at time t1 , transistor 410 is held off by the voltage at gate 416 ( v g ), switch s1 is open to disconnect the data line from a data voltage v d , and a switch ( not shown in fig4 ) controlled by the reset signals , is closed to reset output 444 of integrator 480 to a preselected level . this preselected level , defined by v ref , is approximately the center of the a / d range ( e . g . 4 volts ). at time t2 , switch s1 is closed , connecting data voltage ( v d ) to data line 418 . this state is held for at least a time constant τ dataline of data line 418 . the data line time constant is τ dataline ( τ dataline = r d c d ) where r d is the data line resistance and c d is the data line capacitance . at time t3 , gate voltage v g is turned on . the gate on voltage should be greater than the threshold voltage of the thin film transistor to insure that the transistor channel will turn on , conversely , the gate off voltage should be chose to be less than the threshold voltage of the transistor to insure that the transistor channel is biased off . the width of the gate pulse should be greater than the time constant of a gate line ( τ gateline = r g c g ). at time t4 , the end of the gate pulse , the tester enters a hold time ( t4 to t8 ), during this period the charge that has been transferred to the cell is stored on capacitor 420 . this period emulates an actual display , where charge is stored on the cell capacitance for at least a line time . for a display with n gate lines , and with m refresh cycles per second , the gate pulse width should be τ gatepulse = 1 /( n * m ), and the hold time would be approximately 1 / m . for a typical 1000 gate line display operating at 100 hertz refresh , the gate pulse width would be 10 microseconds , and the hold time would be 10 milliseconds . during the hold time , the data voltage v d is returned to zero volts at time t5 . at time t6 , the data line is disconnected from the data voltage source by opening switch s1 . in order to ensure proper operation , t6 is delayed from the trailing edge of the gate pulse by at least one gate line time constant τ gateline ). since data line 418 is connected to inverting input 440 of operational amplifier 480 , which is a virtual ground , the voltage on the data line relaxes to ground which requires at least 1 data line time constant ( τ dataline ). after the data line voltage has relaxed to ground , at time t7 , capacitor 432 is released from the reset voltage level maintained by reset control 450 . thus , at time t7 , integrator 480 will begin to integrate the current into the inverting input of operational amplifier 434 . at t8 , the end of the hold time , the gate voltage is turned on to a level greater than the threshold voltage of the thin film transistor , insuring that the transistor channel ohmically connects the cell capacitance to the data line . between t7 and t8 the voltage at output 438 drops to a level ( a in fig5 ), which is due to a transient caused by opening the switch controlled by the reset signal . the gate voltage is turned on at t8 and is held on until t9 for several cell discharge time constants to insure that all of the charge on the cell capacitance can return to the operational amplifier virtual ground and be integrated by capacitor 432 . at t9 , gate voltage v g is reduced , turning off transistor 410 . this voltage may be measured immediately . however , it is normally better to wait for at least a gate line time constant ( τ gateline ). the difference in voltage between the output just before time t8 and the output after time t9 of the analog to digital converter is substantially proportional ( q cell =( v a / d - v reset ) c 1 ) to the charge on the cell capacitance 420 . the gate on time ( t3 to t4 ) can be varied to determine the amount of charge that is transferred to the cell . this data can be used to calculate the charging time constant for a cell . reset circuit 450 is designed to hold the output of the operational amplifier at a fixed potential ( v reset ) during the period t1 to t7 . the output of operational amplifier 434 is controlled by the integrated circuit into the inverting input after t7 . fig6 is a schematic of a further preferred embodiment of the present invention . the data lines of the tft / lcd array to be tested are connected to the test circuit through connector 690 . analog multiplex switch s4 is used to select an individual data line to be connected to the test circuit through line 692 . analog multiplex switch s4 may be , for example a mux16 analog multiplex switch from precision monolithic integrated circuits . the data lines are further connected through connector 690 to lines 694 which are individually connected to switches s5 . switch s5 may be , for example a sw202 analog switch from precision monolithic integrated circuits . since the test circuit in fig6 is connected to only one data line at a time , switches s5 may be used to ground the nonselected switches , thus ensuring a good ground reference for the cell capacitance even when the backplate is removed . the data line selected by switch s4 is not grounded since its s5 switch is held open during the test period . the data line selected by s4 is connected to v d through switch s1 and to operational amplifier 634 through resistor 660 . resistor 660 may be , for example , 10 thousand ohms . operational amplifier 634 may be , for example , an op - 17 . the non - inverting input 642 of operational amplifier 634 is connected to ground potential , thus creating a virtual ground at inverting input 640 . inverting input 640 is connected to capacitor 632 which is also connected to operational amplifier output 644 and acts as an integrating capacitor . capacitor 632 may be , for example , on the order of 5 picofarads . however , its size will normally be determined by the desired amplitude of the integrator output voltage . operational amplifier 634 in combination with capacitor 632 comprise integrator 680 . since the voltage across capacitor 632 must be reset after each test , reset circuit 650 is provided . reset circuit 650 has two input signals , v ref and reset . since switch s3 is controlled by the reset signal , the level of the reset signal must be sufficient to drive s3 . s3 may be , for example , a sw202 from precision monolithic integrated circuits which requires normal ttl levels . v ref is a voltage level which supplies a reference signal to the inverting input of operational amplifier 652 . operational amplifier 652 may be , for example , an op - 16 . v ref is also connected to ground potential through capacitor 659 . capacitor 659 acts as a noise filter and may be , for example , 0 . 1 microfarads to effectively remove ac noise from the reference voltage . the non - inverting input of operational amplifier 652 is connected to one side of capacitor 632 and to output 644 of operational amplifier 634 . the output of operational amplifier 652 is connected to resistor 651 which in turn is connected to one terminal of switch s3 . resistor 651 may be , for example 4 . 99 thousand ohms . the other terminal of s3 is connected to ground through resistor 657 and to capacitor 632 through the parallel combination of resistor 654 and capacitor 655 . resistor 657 may be , for example 4 . 02 thousand ohms . resistor 654 may be , for example , 3 . 01 thousand ohms . capacitor 655 may be , for example , 3 picofarads . thus , the combination of reset circuit 650 and integrator 680 form a resettable integrator . it will be apparent to those of skill in the art that reset circuit 650 , described with respect to fig6 may be substituted for reset circuit 450 in fig4 as a means of implementing the embodiment of fig4 . in short , the method of the present invention is designed to store charge in the active area of a particular tft / lcd cell . the electrical characteristics of the cell may then be calculated by reading the charge stored on the active area after a period of time . more particularly , in a backplane tester according to one embodiment of the present invention charge is stored on the cell through a series of switches while an operational amplifier is employed in an inverting , integrating mode to detect the charge stored on the cell . when utilized according to the method of the present invention , this arrangement provides a means of reliably and accurately measuring the charge on the liquid crystal pad capacitance . in the method of the present invention , the measurement resolution may be improved by sequencing the test steps to minimize the effect of gate to source coupling through c gd coupling . the resolution may be further improved by subtracting out the parasitic capacitance introduced by the test fixture . this parasitic capacitance may be measured by turning of the transistor during the charge cycle and measuring the output voltage . since the output voltage is proportional to the parasitic capacitance of the test fixture , that voltage may be subtracted off the voltage measured during normal testing . in the method of the present invention , the resolution may be further improved by running the test several times and averaging the output voltages measured to remove random error from the measurements . in lcd displays which do not use storage capacitors ( as in fig1 ) the primary capacitance measured by the method according to the present invention is the pad to adjacent gate line capacitance . it is desirable to minimize this capacitance in the design of conventional tft / lcd &# 39 ; s to decrease the charging time . unfortunately , this reduces the accuracy of the test method according to the present invention . in order to increase the measured charges , utilizing the method of the present invention , the pel capacitance can be doubled by testing alternate columns of devices , thus utilizing the capacitance to the adjacent drain line as well as that of the adjacent gate line . for an 80 pixel per inch display and using a gap of 10 microns , the total parasitic capacitance may be expected to be approximately 0 . 05 pf , and for a 160 pixel per inch display and using a gap of ten microns , the capacitance would be expected to be approximately 0 . 025 pf . in one preferred embodiment of the present invention , a sixteen pole analog multiplexor is used to select one out of sixteen data lines to test . the other fifteen data lines are grounded using analog switches . thus the output voltage will be proportional to the charge stored on one cell along the selected data line . the cell is determined by selecting the gate line . in fig6 switch s3 may be an analog switch ( typically an sw202 from precision monolithic incorporated , pmi ) to either connect or disconnect the output node of an operational amplifier ( typically an op16 from pmi ) to the inverting input of the integrating operational amplifier ( typically an op17 from pmi ), the non - inverting output of the op16 operational amplifier is connected to the output of the op17 , the inverting input of the op16 is connected to the v ref . when s2 is closed the potential across the integrating capacitor , c1 , is held fixed at v reset , when s2 is open the integrating capacitor is allowed to integrate the current from the data line of the display . an analog method of testing tft arrays cell capacitance for use in lcd displays , has been described . this method provides significant advantages over conventional digital test methods . in addition , techniques have been described to minimize parasitic effects and noise induced errors . finally , test sequences have been described which utilize the method according to the present invention to test for go / nogo , threshold voltage , transconductance , leakage current , and c gs . it is known that , a photosensitive element such as an amorphous silicon resistor , could be added to the lcd cell to charge or discharge the lcd capacitor ( see u . s . pat . no . 4 , 819 , 038 ). it will be apparent that a circuit according to the present invention could be used to detect the change in charge on the capacitor . thus , the present invention could be used to construct an input device . while preferred embodiments of the present invention have been shown and described herein , it will be obvious to those skilled in the art that such embodiments are provided by way of example only . numerous variations , changes , and substitutions will now occur to those skilled in the art without departing from the invention . accordingly , it is intended that the invention be limited only by the spirit and scope of the appended claims .