Abstract:
The present invention is directed to touch sensors with arrays of switches (e.g., diodes or transistors) that can be used to selectively apply voltage gradients across a resistive touch regions of the touch sensor substrate. Touches on the touch sensor can then be sensed by measuring the voltage at the touch location on the resistive touch region. The switch arrays take the form of strips of switches that can be cut from a prefabricated reel or a sheet and applied to the touchscreen substrate.

Description:
FIELD OF THE INVENTION  
       [0001]     The field of the present invention relates to touch sensor technology, and more particularly to resistive and capacitive touch sensor technology.  
       BACKGROUND OF THE INVENTION  
       [0002]     Touch sensors are transparent or opaque input devices for computers and other electronic systems. As the name suggests, touch sensors are activated by touch, either from a user&#39;s finger, a stylus or some other device. Transparent touch sensors, and specifically touchscreens, are generally placed over display devices, such as cathode ray tube (CRT) monitors and liquid crystal displays, to create touch display systems. These systems are increasingly used in commercial applications such as restaurant order entry systems, industrial process control applications, interactive museum exhibits, public information kiosks, pagers, cellular phones, personal digital assistants, and video games.  
         [0003]     The dominant touch technologies presently in use are resistive, capacitive, infrared, and acoustic technologies. Touchscreens incorporating these technologies have delivered high standards of performance at competitive prices. All are transparent devices that respond to a touch by transmitting the touch position coordinates to a host computer. An important aspect of touchscreen performance is a close correspondence between true and measured touch positions at all locations within a touch sensitive area located on the touch sensor.  
         [0004]     Referring to  FIG. 1 , many resistive touchscreens  10  share the following mechanical components: a rigid insulative substrate  12  with a resistive coating  16  applied thereto; and a flexible membrane coversheet  14  with a conductive coating  18  applied thereto, wherein the flexible membrane is laid over the rigid substrate  12  with the two coatings opposed and separated by spacers  20  to avoid electrical contact between the two coatings until the membrane  14  is touched.  
         [0005]     Many resistive touchscreens on the market are referred to as “4-wire” touchscreens. In 4-wire touchscreens, both the cover sheet and the rigid substrate are required to have resistive coatings of uniform resistivity. A voltage gradient on one coating is used to measure x-coordinates of touches, and a gradient on the other coating is used to measure y-coordinates of touches. For example,  FIG. 2  illustrates a 4-wire touchscreen  30  that comprises a rigid substrate  32  and a flexible membrane coversheet  34 , which are shown separately for purposes of clarity. The touchscreen  30  further comprises a uniform resistive coating  36  that is applied to the rigid substrate  32 , and a uniform conductive coating  38  that is applied to the flexible cover sheet  34 . A pair of wires  40 ( 1 ) and  40 ( 2 ) are connected to resistive coating  38  at the left and right edges of the cover sheet  34  via respective electrodes  42 ( 1 ) and  42 ( 2 ), and a pair of wires  40 ( 3 ) and  40 ( 4 ) are connected to resistive coating  36  at the top and bottom edges of the cover rigid substrate  32  via respective electrodes  42 ( 3 ) and  42 ( 4 ).  
         [0006]     The x-coordinate of a touch can be measured by grounding wire  40 ( 1 ), supplying voltage to wire  40 ( 2 ), and connecting wires  40 ( 3 ) and  40 ( 4 ) to a voltage sensing circuit (not shown) that preferably has a high input impedance relative to the resistivity of the coatings  36  and  38 . In a similar manner, the y-coordinate of a touch can be measured by grounding wire  40 ( 3 ), supplying voltage to wire  40 ( 4 ), and connecting wires  40 ( 1 ) and  40 ( 2 ) to the voltage sensing circuit. Significantly, accurate measurements of the x- and y-coordinates of a touch require the resistivity of both coatings  36  and  38  to be uniform and stable over time. However, the formation of cover sheets over spherically curved resistive touchscreens and the mechanical flexing of the cover sheet for both flat and curved resistive touchscreens tend to degrade the uniform resistivity of the coating on the cover sheet. For example, small cracks may form in the resistive coating. Because styluses generally have sharper radii than that of fingers, thus hastening the degradation process, the resistive coating degradation problem is an even greater concern in stylus-input devices.  
         [0007]     Another type of commercially available resistive touchscreen is referred to as a “5-wire” touchscreen, which does not require the resistivity of the coating on the cover sheet to be uniform, since the x- and y-coordinates of touches are determined based on voltage gradients on the resistive coating of the rigid substrate. For example,  FIG. 3  illustrates a 5-wire touchscreen  50  that comprises a rigid substrate  52  and a flexible membrane coversheet  54 , which are shown separately for purposes of clarity. The touchscreen  50  further comprises a uniform resistive coating  56  that is laid over the rigid substrate  52 , and a uniform resistive coating  58  that is laid over the flexible cover sheet  54 . Four wires  60 ( 1 )-( 4 ) are connected to the coating  56  at the respective corners of the rigid substrate  52  via respective electrodes  62 ( 1 )-( 4 ), and a fifth wire  60 ( 5 ) is connected to the coating  58  on one edge of the cover sheet  54  via an electrode  62 ( 5 ). To ensure that a uniform voltage gradient is created along the coating  56  of rigid substrate  52 , the touchscreen  50  further comprises four resistive networks  64 ( 1 )-( 4 ) that are disposed on the coating  56  along the periphery of the rigid substrate  52 .  
         [0008]     The x-coordinate of a touch can be measured by grounding wires  60 ( 1 ) and  60 ( 2 ), and supplying voltage to wires  60 ( 3 ) and  60 ( 4 ). The voltage on the wire  60 ( 5 ) connected to the cover sheet  54  is sensed by a high impedance voltage sensing circuit to determine the x-coordinate of the touch. The y-coordinate of a touch can be measured by grounding wires  60 ( 2 ) and  60 ( 3 ), and supplying voltage to wires  60 ( 1 ) and  60 ( 4 ). The voltage on the wire  60 ( 5 ) is sensed by the voltage sensing circuit to determine the y-coordinate of the touch. Significantly, the resistivity of the coating  58  on the cover sheet  54  need not be uniform or stable with time and usage in order to obtain accurate measurements of the x- and y-coordinates of a touch. The coating  58  need only provide electrical continuity and have a resistance that is small compared to the input impedance of the voltage sensing circuit. Thus, the performance of 5-wire resistive touchscreens is generally not adversely affected by any degradation in the coating  58  of the cover sheet  54 , and is therefore more reliable than the 4-wire resistive touchscreens. This benefit, however, does not come without a price, since the resistive networks required for 5-wire designs add complexity to the resistive touchscreen design and manufacturing process.  
         [0009]     Another type of resistive touchscreen is referred to as a “3-wire” touchscreen, wherein voltage gradients are applied to the resistive coating of the rigid substrate using a network of diodes. For example,  FIG. 4  illustrates a 3-wire touchscreen  70  that comprises a rigid substrate  72  and a flexible membrane coversheet  74 , which are shown separately for purposes of clarity. The touchscreen  70  further comprises a uniform resistive coating  76  that is applied to the rigid substrate  72 , and a uniform conductive coating  78  that is applied to the flexible cover sheet  74 . A first wire  80 ( 1 ) is connected to the coating  76  at the left edge of the rigid substrate  72  via a first array of diodes  82 ( 1 ) and at the top edge of the rigid substrate  72  via a third array of diodes  82 ( 3 ). A second wire  80 ( 2 ) is connected to the coating  76  at the right edge of the rigid substrate  72  via a second array of diodes  82 ( 2 ) and at the bottom edge of the rigid substrate  72  via a fourth array of diodes  82 ( 4 ). A third wire  80 ( 3 ) is connected to the coating  78  of the flexible cover sheet  74  on one edge of the cover sheet  74  via an electrode  84 . The diodes  82  serve as switches that allow voltage gradients to be selectively applied to the coating  76  of the rigid substrate  72  in the x- and y-directions, depending on which of the wires  80  is energized.  
         [0010]     In particular, the x-coordinate of a touch can be measured by grounding the second wire  80 ( 2 ), and supplying a voltage to the first wire  80 ( 1 ) sufficient to forward bias the diodes of the diode arrays  82 ( 1 ) and  82 ( 2 ) and to apply the desired voltage gradient. Notably, when this occurs, both the first and second diode arrays  82 ( 1 ) and  82 ( 2 ) will become forward biased (closed switches), and both the third and fourth diode arrays  82 ( 3 ) and  82 ( 4 ) will become reverse biased (open switches). As a result, current will flow from the first wire  80 ( 1 ), through the forward biased diode array  82 ( 1 ), across the resistive coating  76  in the x-direction, through the forward biased diode array  82 ( 2 ), and to the second wire  80 ( 2 ). The reverse biased diode arrays  82 ( 3 ) and  82 ( 4 ) will prevent current from flowing in the y-direction, thereby resulting in a uniform voltage gradient in the x-direction. The voltage on the wire  80 ( 3 ) connected to the cover sheet  74  is sensed by a high impedance voltage sensing circuit to determine the x-coordinate of the touch.  
         [0011]     Similarly, the y-coordinate of a touch can be measured by grounding the first wire  80 ( 1 ), and supplying a voltage to the second wire  80 ( 2 ) sufficient to forward bias the diodes of the diode arrays  82 ( 3 ) and  82 ( 4 ) and to apply the desired voltage gradient. Notably, when this occurs, both the third and fourth diode arrays  82 ( 3 ) and  82 ( 4 ) will become forward biased (closed switches), and the first and second diode arrays  82 ( 1 ) and  82 ( 2 ) will become reverse biased (open switches). As a result, current will flow from the second wire  80 ( 2 ), through the forward biased diode array  82 ( 4 ), across the resistive coating  76  in the y-direction, through the forward biased diode array  82 ( 3 ), and to the first wire  80 ( 1 ). The reverse biased diode arrays  82 ( 1 ) and  82 ( 2 ) will prevent current from flowing in the x-direction, thereby resulting in a uniform voltage gradient in the y-direction. Again, the voltage on the wire  80 ( 3 ) is sensed by the voltage sensing circuit to determine the y-coordinate of the touch.  
         [0012]     As illustrated in  FIG. 4 , the touchscreen  70  may employ an additional set of four wires  86 ( 1 )- 86 ( 4 ) for sensing the temperature dependent voltage drops across the diodes. In particular, the wires  86 ( 1 )- 86 ( 4 ) are respectively connected to the diode arrays  82 ( 1 )- 82 ( 4 ) at the connection to the resistive coating  76  of the substrate  72 . The voltage sensing circuitry is connected to these wires  86 ( 1 )- 86 ( 4 ) to compensate for any abnormal voltage variances in the diodes. As long as the voltage drop on the diodes in a given array is the same, the voltage sensing circuitry can correct for temperature drifts in diode voltage drip, variations in excitation voltages, and any drift in the offset or gain of the analog-digital-converter (ADC) used to convert the measured analog voltages into digital signals. Such touchscreens have been referred to as “7-wire” touchscreens in the marketplace. We, however, reserve this term for the touchscreens described below.  
         [0013]     Still another type of resistive touchscreen is referred to as a “7-wire” touchscreen, wherein voltage gradients are applied to the resistive coating of the rigid substrate using a network of transistors. For example,  FIG. 5  illustrates a 7-wire touchscreen  90  that is similar to the previously described 3-wire touchscreen  70 , with the exception that the touchscreen  90  employs field-effect transistors (FETs), rather than diodes, as switches. In particular, a first wire  92 ( 1 ) is connected to the coating  76  at the left edge of the rigid substrate  72  via a first array of FETs  94 ( 1 ) and at the top edge of the rigid substrate  72  via a third array of FETs  94 ( 3 ). A second wire  92 ( 2 ) is connected to the coating  76  at the right edge of the rigid substrate  72  via a second array of FETs  94 ( 2 ) and at the bottom edge of the rigid substrate  72  via a fourth array of FETs  94 ( 4 ). Four control wires  96 ( 1 )- 96 ( 4 ) are respectively connected to the gates of the FET arrays  92 ( 1 )- 92 ( 4 ). The x- and y-coordinates of a touch can be measured by supplying a voltage to the first wire  92 ( 1 ) to allow current to flow in the FETs when the gates are energized and grounding the second wire  92 ( 2 ), while selectively energizing and grounding the wires  96 ( 1 )- 96 ( 4 ).  
         [0014]     In particular, the x-coordinate of a touch can be measured by supplying a sufficient voltage to the control wires  96 ( 1 ) and  96 ( 2 ) to “turn on” the FETs in arrays  94 ( 1 ) and  94 ( 2 ), and grounding the control wires  96 ( 3 ) and  96 ( 4 ) to “turn off” the FETs in arrays  94 ( 3 ) and  94 ( 4 ). As a result, current will flow from the first wire  92 ( 1 ), through the turned-on FET array  94 ( 1 ), across the resistive coating  76  in the x-direction, through the turned-on FET array  94 ( 2 ), and to the second wire  92 ( 2 ). The turned-off FET arrays  94 ( 3 ) and  94 ( 4 ) will prevent current from flowing in the y-direction, thereby resulting in a uniform voltage gradient in the x-direction. The voltage on the wire  80 ( 3 ) connected to the cover sheet  74  is sensed by a high impedance voltage sensing circuit to determine the x-coordinate of the touch.  
         [0015]     Similarly, the y-coordinate of a touch can be measured by supplying a sufficient voltage to the control wires  96 ( 3 ) and  96 ( 4 ) to “turn on” the FETs in arrays  94 ( 3 ) and  94 ( 4 ), and grounding the control wires  96 ( 1 ) and  96 ( 2 ) to “turn off” the FETs in arrays  94 ( 1 ) and  94 ( 2 ). As a result, current will flow from the first wire  92 ( 1 ), through the turned-on FET array  94 ( 3 ), across the resistive coating  76  in the y-direction, through the turned-on FET array  94 ( 4 ), and to the second wire  92 ( 2 ). The turned-off FET arrays  94 ( 1 ) and  94 ( 2 ) will prevent current from flowing in the x-direction, thereby resulting in a uniform voltage gradient in the y-direction. The voltage on the wire  80 ( 3 ) connected to the cover sheet  74  is sensed by a high impedance voltage sensing circuit to determine the y-coordinate of the touch.  
         [0016]     Significantly, the 3-wire and 7-wire resistive touchscreen designs are simplistic and do not require the resistivity of the coating  78  to be uniform or stable over time. In addition, the 3-wire and 7-wire resistive designs avoid the complex and carefully tuned resistor networks of the 5-wire resistive touchscreens. Thus, it can be appreciated that either of the 3-wire and 7-wire resistive designs combines the advantages of both the 4-wire and 5-wire resistive designs. At present, however, 3-wire and 7-wire resistive touchscreens have not gained commercial acceptance, mainly because no one has developed a low-cost means to mount the diodes or transistors onto the rigid substrate, which otherwise would involve hours of manual soldering of many discrete components onto the substrate.  
         [0017]     As such, there remains a need to provide an improved means for mounting arrays of solid state switches, such as diodes and transistors, onto touchscreen substrates.  
       SUMMARY OF THE INVENTION  
       [0018]     In accordance with a first aspect of the present invention, a method of manufacturing a touch sensor is provided. The method comprises providing a substrate having a resistive touch region. In the preferred embodiment, the substrate is rigid, although the substrate can also be flexible in some cases. The resistive touch region is preferably rectangular, although other types of geometries are contemplated by the present invention, depending upon the application of the touch sensor.  
         [0019]     The method further comprises providing a tape strip with a plurality of devices. Each of the devices has first and second terminals and is configured to allow electrical current conduction from the first terminal to the second terminal when in a first state, and prevent electrical current conduction from the second terminal to the first terminal when in a second state. Diodes and transistors are examples of devices that can perform this function. The method further comprises securing the tape strip along an edge of the resistive touch region, wherein one of the first and second terminals of the devices are in electrical contact with the resistive touch region. Preferably, the method comprises securing an electrically conductive lead to the other of the first and second terminals. In one preferred embodiment, the devices are surface mounted devices. In another preferred embodiment, the devices are thin-film devices, e.g., conductive polymer devices.  
         [0020]     In accordance with a second aspect of the present invention, another method of manufacturing a touch sensor is provided. The method comprises providing a substrate having a resistive touch region with first and second oppositely disposed edges and third and fourth oppositely disposed edges, and providing four tape strips. Each of the tape strips comprises a plurality of devices similar to those previously described. The method further comprises securing two of the tape strips along the respective first and third edges of the resistive touch region, and the other two strips along the respective second and fourth edges of the resistive touch region. The second terminals of the devices on the first two tape strips are in electrical contact with the resistive touch region, and the first terminals of the devices on the remaining two tape strips are in electrical contact with the resistive touch region. In the preferred embodiment, at least one electrically conductive lead is coupled to the first terminals of devices not connected to the touch region, and at least another electrically conductive lead is connected to the second terminals of devices not connected to the touch region. The tape strips may be advantageously supplied in a tape reel or as a sheet, in which case the tape strips can be cut therefrom.  
         [0021]     In accordance with a third aspect of the present invention, reversible diode tape is provided. The diode tape comprises a first electrically insulative layer, a layer of spaced apart anodes disposed on the first electrically insulative layer, a p-type semiconductor layer disposed on the anode layer, an n-type semiconductor layer disposed on the p-type semiconductor layer, a layer of spaced apart cathodes disposed on the n-type semiconductor layer, wherein the cathodes are substantially aligned with the anodes to discretely form diodes, and a second electrically insulative layer disposed on the cathode layer. In one embodiment, the semiconductor layers are composed of conductive polymer, such as doped polythiophene, poly (3,4-ethylenedioxythiophene)-poly(4-styrenesulfonate) and doped poly(2-methoxy,5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene).  
         [0022]     In the preferred embodiment, a layer of exposed anode terminals are respectively disposed on the anode layer, and a layer of exposed cathode terminals are respectively disposed on the layer of cathodes. For example, the anode and cathode terminals can respectively extend along the opposite edges of the tape. Thus, it can be appreciated that the reversible diode tape can be used to conduct current in a selected one of two directions, depending on which side of the diode tape is bonded to the touchscreen substrate. The diode tape may optionally comprise a first electrically conductive trace connecting the anodes, and a second electrically conductive trace connecting the cathodes. In this case, one of the conductive traces can be subsequently etched to either disconnect the cathodes from each other or disconnect the anodes from each other, when the diode tape is bonded to a touchscreen substrate. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     The drawings illustrate the design and utility of preferred embodiment(s) of the present invention, in which similar elements are referred to by common reference numerals. In order to better appreciate the advantages and objects of the present invention, reference should be made to the accompanying drawings that illustrate the preferred embodiment(s). The drawings depict only an embodiment(s) of the invention, and should not be taken as limiting its scope. With this caveat, the preferred embodiment(s) will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:  
         [0024]      FIG. 1  is a cross-section of a prior art touchscreen;  
         [0025]      FIG. 2  is a plan view of a prior art “4-wire” touchscreen;  
         [0026]      FIG. 3  is a plan view of a prior art “5-wire” touchscreen;  
         [0027]      FIG. 4  is a plan view of a prior art “3-wire” touchscreen;  
         [0028]      FIG. 5  is a plan view of a prior art “7-wire” touchscreen;  
         [0029]      FIG. 6  is a block diagram of a touchscreen system constructed in accordance with one embodiment of the present invention;  
         [0030]      FIG. 7  is a perspective view of a 3-wire touchscreen used in the touchscreen system of  FIG. 6 ;  
         [0031]      FIG. 8  is a perspective view of a surface mounted diode array strip used to fabricate the touchscreen of  FIG. 7 ;  
         [0032]      FIG. 9  is a perspective view of another surface mounted diode array strip that can be used to fabricate the touchscreen of  FIG. 7 ;  
         [0033]      FIG. 10  is a perspective view of a tape reel from which the diode array strip of  FIG. 8  can be cut;  
         [0034]      FIG. 11  is a plan view of a sheet from which the diode array strip of  FIG. 8  can be cut;  
         [0035]      FIGS. 12-18  are plan views illustrating a preferred method of fabricating a thin-film diode array strip that can alternatively be used in the touchscreen of  FIG. 7 ;  
         [0036]      FIG. 18   a  is a cross-sectional view of the diode array strip illustrated in  FIG. 18 , taken along the line  18   a - 18   a;    
         [0037]      FIG. 19  is a cross-sectional view showing the placement of the diode array strip of  FIG. 18  on a touchscreen substrate to create a touchscreen;  
         [0038]      FIGS. 20-23  are plan views illustrating another preferred method of fabricating a thin-film diode array strip that can alternatively be used in the touchscreen of  FIG. 7 ;  
         [0039]      FIG. 23   a  is a cross-sectional view of the diode array strip illustrated in  FIG. 23 , taken along the line  23   a - 23   a;    
         [0040]      FIG. 24  is a cross-sectional view showing the placement of the diode array strip of  FIG. 23  on a touchscreen substrate to create a touchscreen;  
         [0041]      FIGS. 25-31  are plan views illustrating a preferred method of fabricating reversible diode tape for use in the touchscreen of  FIG. 7 ;  
         [0042]      FIG. 28   a  is a cross-sectional view of the diode array strip illustrated in  FIG. 28 , taken along the line  28   a - 28   a;    
         [0043]      FIG. 29   a  is a cross-sectional view of the diode array strip illustrated in  FIG. 29 , taken along the line  29   a - 29   a;    
         [0044]      FIG. 30   a  is a cross-sectional view of the diode array strip illustrated in  FIG. 30 , taken along the line  30   a - 30   a;    
         [0045]      FIG. 31   a  is a cross-sectional view of the diode array strip illustrated in  FIG. 31 , taken along the line  31   a - 31   a;    
         [0046]      FIG. 32  is a plan view of a 7-wire touchscreen that can alternatively be used in the touchscreen system of  FIG. 6 ;  
         [0047]      FIG. 33  is a perspective view of a surface mounted transistor array strip that can be used to fabricate the touchscreen of  FIG. 32 ;  
         [0048]      FIGS. 34-41  are plan views illustrating a preferred method of fabricating a thin-film transistor array strip that can be used in the touchscreen of  FIG. 32 ;  
         [0049]      FIG. 41   a  is a cross-sectional view of the transistor array strip illustrated in  FIG. 41 , taken along the line  41   a - 41   a ; and  
         [0050]      FIG. 42  is a cross-sectional view showing the placement of the transistor array strip of  FIG. 41  on a touchscreen substrate to create a touchscreen. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0051]     Referring to  FIG. 6 , a resistive touchscreen system  200  constructed in accordance with a preferred embodiment of the present invention is described. The touchscreen system  200  generally comprises a touchscreen  205  (i.e., a touch sensor having a transparent substrate), controller electronics  210 , and a display (not shown). The touchscreen system  200  is typically coupled to a host computer  215 . Generally, the controller electronics  210  send excitation signals to the touchscreen  205  and receive analog signals carrying touch information from the touchscreen  205 . Specifically, the controller electronics  210  establish voltage gradients across the touchscreen  205 . The voltages at the point of contact are representative of the position touched. The controller electronics  210  digitize these voltages and transmit these digitized signals, or touch information in digital form based on these digitized signals, to the host computer  215  for processing.  
         [0052]     Referring now to  FIG. 7 , the touchscreen  205  comprises a rigid substrate  220  having a resistive touch region  230  that is formed by permanently applying a uniform resistive layer to one surface of the substrate  220 . The touchscreen  205  further comprises a plastic coversheet  225  having a conductive layer  235  applied thereto. Generally, orthogonal voltage gradients will be alternately applied over the resistive touch region  230  of the touchscreen  205  via diodes  245  arranged along the respective four edges of the touchscreen  205  as four diode arrays (a left diode array  240 ( 1 ), a right diode array  240 ( 2 ), a top diode array  240 ( 3 ), and a bottom diode  240 ( 4 )). The touchscreen system  200  employs a 3-wire architecture, and thus, a first electrically conductive lead  250 ( 1 ) connects the left and top diode arrays  240 ( 1 ) and  240 ( 3 ) to the controller electronics  210 , and a second electrically conductive lead  250 ( 2 ) connects the right and bottom diode arrays  240 ( 2 ) and  240 ( 4 ) to the controller electronics  210 . A third electrically conductive lead  250 ( 3 ) connects the conductive layer  235  of the coversheet  225  to the controller electronics  210  via an electrode  255 .  
         [0053]     When the touchscreen  205  is pressed, the conductive coating  235  of the cover sheet  225  makes direct electrical contact with the resistive touch region  230  on the substrate  220 . For a quasi-DC resistive touchscreen, commonly referred to as a “resistive touchscreen,” the cover sheet  225  can function as either a voltage sensing probe for sensing the voltage at the contacted area, or as a current injection source. As another option, the coversheet  225  may be replaced with a thin dielectric coating applied directly to resistive layer of the touch region  230 , in which case, the controller electronics  210  may support AC operation.  
         [0054]     The topology of the touchscreen  205  is similar to that of the touchscreen  70  previously described above. That is, the x-coordinate of a touch on the touchscreen  205  can be determined by applying a voltage to the first lead  250 ( 1 ), grounding the second lead  250 ( 2 ), and sensing the voltage on the third lead  250 ( 3 ). Likewise, the y-coordinate of a touch on the touchscreen  205  can be determined by grounding the first lead  250 ( 1 ), applying a voltage to the second lead  250 ( 2 ), and sensing the voltage on the third lead  250 ( 3 ). Here, the term “ground” refers to a low voltage or local ground at the touchscreen  105 , which may or may not correspond to other grounds of the system.  
         [0055]     As will be discussed in further detail below, the diode arrays  240  are applied to the touchscreen substrate  220  as tape strips that are suitably adhered to the resistive touch region  230  of the substrate  220 . During the fabrication process, it should be appreciated that the electrical connection of the anode and cathodes will depend on the particular location of the diode array  240  on the substrate  220 . In particular, the left diode array  240 ( 1 ) will be applied to the substrate  220 , such that the cathodes and anodes are in respective electrical contact with the resistive touch region  230  and first lead  250 ( 1 ) (see diode array  82 ( 1 ) in  FIG. 4 ). Similarly, the bottom diode array  240 ( 4 ) will be applied to the substrate  220 , such that cathodes and anodes are in respective electrical contact with the resistive touch region  230  and second lead  250 ( 2 ) (see diode array  82 ( 4 ) in  FIG. 4 ). In contrast, the right diode array  240 ( 2 ) will be applied to the substrate  220 , such that anodes and cathodes are in respective electrical contact with the resistive touch region  230  and the second lead  250 ( 2 ) (see diode array  82 ( 2 ) in  FIG. 4 ). Similarly, the top diode array  240 ( 3 ) will be applied to the substrate  220 , such that the anodes and cathodes are in respective electrical contact with the resistive touch region  230  and the first lead  250 ( 1 ) (see diode array  72 ( 3 ) in  FIG. 4 ). As a result of these specific connections, the current will flow across the resistive touch region  230  in the desired orthogonal directions, in the same manner described in the touchscreen  70  of  FIG. 4 , when the leads  250 ( 1 ) and  250 ( 2 ) are alternately energized and grounded.  
         [0056]     With further reference to  FIG. 8 , each diode strip  240  comprises an insulative tape strip  265  composed of a suitable material, such as polyester (e.g., Mylar®) or polyimide (e.g., Kapton®), and a plurality of diodes, and specifically standard surface mounted diodes  245 , mounted along the length of the tape strip  265 . Each diode  245  comprises an anode terminal  270  and a cathode terminal  285 . The diode strip  240  further comprises an electrically conductive trace  290  that extends off center along the length of tape strip  265  and electrically connects the diodes  245  together.  
         [0057]     In the diode strip  240  illustrated in  FIG. 8 , the anode terminal  270  of each diode  245  is soldered to the conductive trace  290 , and the cathode terminal  285  of each diode  245  is exposed, so that it can be soldered or glued to the resistive touch region  230  of the substrate  220 . The cathode terminals  285  extend over the edge of the tape strip  265  to provide clearance for mounting to the exposed touch region  230 . Alternatively, holes or vias  295  can be provided through the tape strip  265  (as illustrated in  FIG. 9 ), so that the cathode terminals  285  can be connected to the resistive touch region  230  through the holes or vias  295 . Advantageously, the use of holes or vias  295  may also prevent solder migration. Notably, either of the diode strips  240  illustrated in  FIGS. 8 and 9  can be applied to the substrate  220  along the left and bottom peripheral edges of the resistive touch region  230  to form the diode arrays  240 ( 1 ) and  240 ( 4 ). A diode strip similar to the diode strips  240  illustrated in  FIGS. 8 and 9 , with the exception that the anodes and cathodes are switched, can be applied to the substrate  220  along the right and top peripheral edges of the resistive touch region  230  to form the diode arrays  240 ( 2 ) and  240 ( 3 ). The diode strip  240  may optionally comprise additional electrically conductive traces (not shown), e.g., in order to sense temperature dependent voltage drops across the diodes (see  FIG. 4 ).  
         [0058]     It can be appreciated that the use of diode strips  240  simplifies the fabrication process, since the diode strips  240  may be manufactured separately using standard automated processes. The use of diode strips  240  also allows touchscreen designers to more easily introduce touch capability on non-conventional surfaces, such as ubiquitous computing applications.  
         [0059]     In the preferred embodiment, the diode strips  240  are supplied as a tape reel  296 , as illustrated in  FIG. 10 . The touchscreen designer need only cut the diode strips  240 , which are sized to the respective edges of the touchscreen  205 , from the tape reel  296 . Alternatively, the diode strips  240  may be supplied as a sheet  297 , as illustrated in  FIG. 11 . In this case, the touchscreen designer need only cut the sheet  297  (along the dashed lines) to provide the required diode strips  240 . Differently sized sheets  297  can be used, depending on the length of the edge on which the cut diode strip  240  will be mounted. Whether the diode strips  240  are cut from a tape reel or a sheet, the use of two different tape reels or sheets having different directions of current conduction (one for the diode arrays  240 ( 1 ) and  240 ( 4 ), and the other for the diode arrays  240 ( 2 ) and  240 ( 3 )) will be required for each fabricated touchscreen.  
         [0060]     After the diode strips  240  have been properly measured and cut, the diode strips  240  can then be bonded to the touchscreen substrate  220 , as illustrated in  FIG. 7 . Using a suitable electrically conductive adhesive, the cathodes  285  of the left and bottom diode arrays  240 ( 1 ) and  240 ( 4 ), and the anodes  270  of the right and top diode arrays  240 ( 2 ) and  240 ( 3 ), can be connected to the resistive touch region  230 . Electrically conductive leads  250 ( 1 ) and  250 ( 2 ) can then be respectively soldered to the electrical traces  290  of the left and bottom diode arrays  240 ( 1 ) and  240 ( 4 ) at the bottom left corner of the touchscreen  205 . A first jumper wire  260 ( 1 ) is used to connect the electrical traces  290  of the left and top diode arrays  240 ( 1 ) and  240 ( 3 ) together, and a second jumper wire  260 ( 2 ) is used to connect the electrical traces  290  of the right and bottom diode arrays  240 ( 2 ) and  240 ( 4 ) together.  
         [0061]     Although the diodes in the diode strips  240  of  FIG. 7  are illustrated and described as surface mounted diodes, diode strips with thin-film diodes can also be used. For example,  FIGS. 12-19  illustrate a process for fabricating and mounting a diode strip  340  onto a touchscreen substrate using conductive polymer technology.  
         [0062]     First, a layer of anode material  370 , e.g., copper, is disposed onto a flexible insulative layer  320 , such as polyester (e.g., Mylar®) or polyimide (e.g., Kapton®) ( FIGS. 12 and 18   a ). Next, a layer of p-type conductive polymer  375  is deposited over the anode layer  370  ( FIGS. 13 and 18   a ). In the preferred embodiment, the p-type conductive polymer layer  375  is composed of polythiophene, poly (3,4-ethylenedioxythiophene)-poly(4-styrenesulfonate) (PEDOT-PSS) that is coated onto the anode layer  370 . Alternatively, other electrically conductive polymers can be used, such as acetylenes, thiophenes, phenylenes, pyrroles, or a combination thereof. Next, a layer of n-type conductive polymer  380  is deposited over the p-type conductive polymer layer  375  ( FIGS. 14 and 18   a ). In the preferred embodiment, the n-type conductive polymer layer  380  is composed of poly(2-methoxy,5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) that is coated onto the p-type conductive polymer  375 . Next, a layer of cathode material  385 , e.g., aluminum, is deposited over the n-type conductive polymer  380  ( FIGS. 15 and 18   a ). As can be seen, the cathode layer  385  is segmented into an array of cathode elements to form discrete diodes. As with the diode strips  240  illustrated in  FIGS. 8 and 9 , this step can advantageously be performed separately from the touchscreen fabrication process using standard automated processes, with the resulting tape supplied in the form of a reel or a sheet.  
         [0063]     Next, an electrically conductive lead  350 , e.g., copper tape or wire, is soldered or otherwise bonded to the anode layer  370  ( FIGS. 16 and 18   a ). Then, another flexible insulative layer  325 , such as, e.g., polyimide, is applied over the subassembly ( FIGS. 17 and 18   a ). Alternatively, the subassembly can be encapsulated using a suitable material, such as electrical grade epoxy resin. In this case, the flexible insulative layer  325  serves as both an insulator and an encapsulator. As illustrated in  FIG. 17 , a portion of the cathode layer  385  is left exposed. Cathode terminals  390  can then be fabricated onto the exposed portions of the cathode layer  385  using a suitable electrically conductive material, such as copper tape, conductive tape/gel, or lead solder ( FIGS. 18 and 18   a ). Next, the diode strip  245  is mounted onto the resistive touch region  230  of the substrate  220  using a suitable adhesive ( FIG. 19 ), with the insulating layer  325  abutting the resistive touch region  230 . As can be seen in  FIG. 19 , the cathode layer  385  is electrically connected to the resistive touch region  230  of the substrate  220  via the cathode terminals  390  and the resistive layer  230 .  
         [0064]     Referring to  FIGS. 20-24 , diode strips  340  with the opposite current direction can be prepared simply by applying the anode layer  370  to the flexible insulating layer  320 , with the anode layer  370  arranged into strips to form discrete anode elements, and repeating the p-type conductive polymer  375 , n-type conductive polymer  380 , and then cathode layer  385  application steps ( FIGS. 20 and 23   a ). Next, an electrically conductive lead  350  is soldered or otherwise bonded to the cathode layer  385  ( FIG. 21 ). Then, the other flexible insulative layer  325  is applied over the subassembly ( FIGS. 22 and 23   a ), or alternatively, the subassembly can be encapsulated. Anode terminals  395  are then fabricated onto exposed portions of the anode layer  370  ( FIGS. 23 and 23   a ), and then the diode strip  340  is suitably mounted to the resistive touch region  230  of the substrate  220  ( FIG. 24 ).  
         [0065]     Alternatively, the diode strip  340  illustrated in  FIG. 18  can be fabricated by reversing the application order of the anode layer  370 , p-type conductive polymer layer  375 , n-type conductive polymer layer  380 , and cathode layer  385 , with the electrically conductive lead  350  coupled to the cathode layer  385  and anode terminals coupled to the anode layer  370 . The reverse order diode strip  340  can then be mounted to the resistive touch region  230  of the substrate  220 , with the anode terminals in contact with the resistive touch region  230 . Likewise, the diode strip  340  illustrated in  FIG. 23  can be fabricated by reversing the application order of the anode layer  370 , p-type conductive polymer layer  375 , n-type conductive polymer layer  380 , and cathode layer  385 , with the electrically conductive lead  350  coupled to the anode layer  370  and anode terminals coupled to the cathode layer  385 . The reverse order diode strip  340  can then be mounted to the resistive touch region  230  of the substrate  220 , with the cathode terminals in contact with the resistive touch region  230 .  
         [0066]     As previously mentioned, when using the diode strips  240  and  340  to fabricate touchscreens, two types are required. The first type conducts current in a first direction (for the left and bottom diode arrays), and the second type conducts current in a second direction (for the right and top diode arrays).  FIGS. 25-30  illustrate a fabrication process that produces a reversible diode strip  440  that can be used to conduct current in either of the directions, depending on how it is applied to the touchscreen substrate. In particular, an anode layer  470  (divided into anode elements) is first applied to a flexible insulative layer  420  ( FIGS. 25 and 28   a ). Next, a p-type conductive polymer  475  is applied over the anode layer  470 , and then an n-type conductive polymer  480  is applied over the p-type conductive polymer  475  ( FIGS. 26 and 28   a ). Then, a cathode layer  485  (divided into cathode elements that are aligned with the underlying anode elements) is applied to the n-type conductive polymer  480  ( FIGS. 27 and 28   a ). Next, another flexible insulative layer  425  is applied over the cathode layer  485  ( FIGS. 28 and 28   a ). Then, a portion of the insulative layer  420  adjacent one lateral edge of the strip, and a portion of the insulative layer  425  adjacent the other lateral edge of the strip, are both etched away to expose the respective edges of the anode and cathode layers  470  and  485  ( FIGS. 29 and 29   a ).  
         [0067]     Like the previously described diode strips  240  and  340 , the reversible diode strip  440  illustrated in  FIG. 29  can be supplied in reel or sheet form. The diode strips  440  can be cut to length, and then applied to the substrate  220  along the respective edges of the resistive touch region  230  (shown in  FIG. 7 ). The electrical connections between the diode strips  440  and the substrate  220  will depend on which edge of the resistive touch region  230  that respective diode strip  440  will be applied to. For example, if the diode strip  440  is designed to take the form of a left or bottom diode array, an electrically conductive lead  350  may be soldered across the exposed portions of the anode layer  470 , and cathode terminals  390  may be applied to the exposed portions of the cathode layer  485  ( FIGS. 30 and 30   a ). In contrast, if the diode strip  440  is designed to take the form of a right or top diode array, an electrically conductive lead  350  may be soldered across the exposed portions of the cathode layer  485 , and anode terminals  395  may be applied to the exposed portions of the anode layer  470  ( FIGS. 31 and 31   a ).  
         [0068]     In an alternative diode tape fabrication process, the anode and cathode elements of the respective anode and cathode layers  470  and  485  can be coupled together lithographically or using electrically conductive tape prior to placing the diode tape in reel or sheet form. When mounting the cut diode strips to the touchscreen substrate, the cathode elements can be electrically isolated by etching the connections between the elements, and the electrically conductive lead  350  can then be coupled to the anode layer (in the case of left and bottom diode arrays), or the anode elements can be electrically isolated by etching the connections between the elements, and the electrically conductive lead  350  can then be coupled to the cathode layer (in the case of right and top diode arrays). The diode strips can then be suitably bonded on the substrate along the respective edges of the resistive touch region.  
         [0069]     Further details regarding the fabrication of diode arrays using conductive polymer technology are set forth in further detail in U.S. patent application Ser. No. ______ (Attorney docket number ELG056 US1), which is expressly incorporated herein by reference.  
         [0070]     Although the diode arrays  240 ,  340 , and  440  have been described as comprising two semiconductor materials (a p-type semiconductor material and an n-type semiconductor material), it should be noted that diode arrays can be fabricated using a single type of semiconductor material. For example, diode arrays formed from Schottky diodes, which typically utilize one layer of a semiconductor material, can be used. For example, the previously described diode strips  340  and  440  can use a single conductive polymer layer between anode and cathode layers. Or the diode strip  240  can carry surface mounted Schottky diodes.  
         [0071]     It can be appreciated that the previously described diodes can be characterized as switching devices that can be switched between first and second states. In particular, each diode is configured to allow electrical current conduction from a first terminal (anode) to the second terminal (cathode) when in a first state (diode is forward biased), and prevent electrical current conduction from the second terminal to the first terminal when in a second state (diode is reverse biased).  
         [0072]     Other types of solid-state devices, such as field-effect transistors (FETs), can be used as switching devices instead. That is, each FET is configured to allow electrical current conduction from a first terminal (source) to the second terminal (drain) when in a first state (FET is on), and prevent electrical current conduction from the second terminal to the first terminal when in a second state (FET is off). For example,  FIG. 32  illustrates a touchscreen  605  that uses transistors, and specifically, field-effect transistors (FETs), as switches for applying the desired voltage gradients across the touchscreen. In particular, the touchscreen  605  comprises a rigid substrate  620  having a resistive touch region  630 , a coversheet  625  having a resistive layer  635 , and a plurality of transistors  645  arranged along the respective four edges of the touchscreen  605  as four transistor arrays  640  (a left transistor array  640 ( 1 ), a right transistor array  640 ( 2 ), a top transistor array  640 ( 3 ), and a bottom transistor array  640 ( 4 )).  
         [0073]     In this case, the touchscreen system  200  employs a 7-wire architecture, and thus, a first electrically conductive lead  650 ( 1 ) connects transistor arrays  640 ( 1 ) and  640 ( 3 ) to the controller electronics  210 , and a second electrically conductive lead  650 ( 2 ) connects the transistor arrays  640 ( 2 ) and  640 ( 4 ) to the controller electronics  210 . A third electrically conductive lead  650 ( 3 ) connects the resistive layer  635  of the coversheet  625  to the controller electronics  210  via an electrode  655 . Four electrically conductive control leads  660 ( 1 )- 660 ( 4 ) are also connected between the respective transistors arrays  640 ( 1 )- 640 ( 4 ) and the controller electronics  210  in order to turn the respective transistors on and off.  
         [0074]     The topology of the touchscreen  605  is similar to that of the touchscreen  90  previously described above. That is, the x-coordinate of a touch on the resistive touch region  630  can be determined by applying a voltage to the first lead  650 ( 1 ), grounding the second lead  650 ( 2 ), turning the left and right transistor arrays  640 ( 1 ) and  640 ( 2 ) on by applying a voltage to the first and second control leads  660 ( 1 ) and  660 ( 2 ), turning the top and bottom transistor arrays  640 ( 3 ) and  640 ( 4 ) off by grounding the third and fourth control leads  660 ( 3 ) and  660 ( 4 ), and sensing the voltage on the third lead  650 ( 3 ). Likewise, the y-coordinate of a touch on the resistive touch region  630  can be determined by applying a voltage to the first lead  650 ( 1 ), grounding the second lead  650 ( 2 ), turning the left and right transistor arrays  640 ( 1 ) and  640 ( 2 ) off by grounding the first and second control leads  660 ( 1 ) and  660 ( 2 ), turning the top and bottom transistor arrays  640 ( 3 ) and  640 ( 4 ) on by applying a voltage to the third and fourth control leads  660 ( 3 ) and  660 ( 4 ), and sensing the voltage on the third lead  650 ( 3 ).  
         [0075]     During the fabrication process, it should be appreciated that the electrical connection of the sources and drains of the transistors arrays  640  will depend on the particular location of the transistor array  640  on the substrate  620 . In particular, the left transistor array  640 ( 1 ) will be applied to the substrate  620 , such that the drains and sources are in respective electrical contact with the resistive touch region  630  and the first lead  650 ( 1 ) (see transistor array  94 ( 1 ) in  FIG. 5 ). Similarly, the top transistor array  640 ( 3 ) will be applied to the substrate  620 , such that the drains and sources are in respective electrical contact with the resistive touch region  630  and the first lead  650 ( 1 ) (see transistor array  92 ( 3 ) in  FIG. 5 ). In contrast, the right transistor array  640 ( 2 ) will be applied to the substrate  620 , such that the sources and drains are in respective electrical contact with the resistive touch region  630  and the second lead  650 ( 2 ) (see transistor array  92 ( 2 ) in  FIG. 5 ). Similarly, the bottom transistor array  640 ( 4 ) will be applied to the substrate  620 , such that the sources and drains are in respective electrical contact with the resistive touch region  630  and the second lead  650 ( 2 ) (see transistor array  92 ( 4 ) in  FIG. 5 ). As a result of these specific connections, the sources of the transistor arrays  640 ( 1 ) and  640 ( 3 ) will remain energized, and the drains of the transistor arrays  640 ( 2 ) and  640 ( 4 ) will remain grounded. The current will flow across the resistive touch region  630  in the desired orthogonal directions, in the same manner described in the touchscreen  90  of  FIG. 5 , when the control lead pair  660 ( 1 ) and  660 ( 2 ) and the control lead pair  660 ( 3 ) and  660 ( 4 ) are alternately energized and grounded.  
         [0076]     Like the diode arrays  240 , the transistor arrays  640  are applied to the touchscreen substrate  620  as transistor tape strips. For example,  FIG. 33  illustrates a transistor strip  640  that comprises an insulative tape strip  665  composed of a suitable material, such as polyester (e.g., Mylar®) or polyimide (e.g., Kapton®), and a plurality of transistors, and specifically standard surface mounted FETs  645 , mounted along the length of the tape strip  665 . Each transistor  645  comprises a source terminal  670 , drain terminal  685 , and a gate terminal  680 . The diode strip  640  further comprises a first and second electrically conductive traces  690  and  695  that extend along the length of the tape strip  665 .  
         [0077]     In the transistor strip  640  illustrated in  FIG. 33 , the source terminal  670  of each transistor  645  is soldered to the conductive trace  690 , the gate terminal  680  of each transistor  645  is soldered to the conductive trace  695 , and the drain terminal  670  of each transistor  645  is exposed, so that it can be soldered or glued to the resistive touch region  630  of the substrate  620 . The drain terminals  685  extend over the edge of the tape strip  665  to provide clearance for mounting to the exposed touch region  630 . Alternatively, holes or vias can be provided through the tape strip  665  in the same manner illustrated in the diode strip  240  of  FIG. 9 , so that the drain terminals  685  can be connected to the resistive touch region  630  through the holes or vias. The transistor strip  640  can be applied to the substrate  620  along the left and top peripheral edges of the resistive touch region  630  to form the left and top transistor arrays  640 ( 1 ) and  640 ( 3 ). A transistor strip similar to the transistor strip  640  illustrated in  FIG. 33 , with the exception that the source and drain terminals are switched, can be applied to the substrate  620  along the right and bottom peripheral edges of the resistive touch region  630  to form the right and bottom transistor arrays  640 ( 2 ) and  640 ( 4 ). The transistor strip  640  may optionally comprise additional electrically conductive traces (not shown), e.g., in order to sense temperature dependent voltage drops across the transistors in a similar manner accomplished in the diode arrays illustrated in  FIG. 4 .  
         [0078]     Although the transistors in the transistor strip  640  of  FIG. 33  are illustrated and described as surface mounted transistors, transistor strips with thin-film transistors can also be used. For example,  FIGS. 34-42  illustrate a process for fabricating and mounting a transistor strip  740  onto a touchscreen substrate using conductive polymer technology.  
         [0079]     First, an insulative layer  765 , such as, e.g., silicone, is deposited onto a flexible insulative layer  720 , such as polyester (e.g., Mylar®) or polyimide (e.g., Kapton®) ( FIGS. 34 and 41   a ). Next, a layer of metal, e.g., gold, is deposited on the insulative layer  765  to form an outer electrode  770  and inner electrodes  785  (shown in  FIGS. 35 and 41   a ). Next, a layer of conductive polymer  775  is deposited over the metal layer  770  ( FIGS. 36 and 41   a ). In the preferred embodiment, the conductive polymer layer  775  is composed of regio-regular poly(3-hexyl-thiophene). Then, another layer of insulative material  780  is deposited over the conductive polymer layer  775  ( FIGS. 37 and 41   a ), and another layer of metal  790 , e.g., gold, is deposited along the center of the insulative material  780  to serve as the gates of the transistor strip  740  ( FIGS. 38 and 41   a ).  
         [0080]     Next, electrically conductive leads  750  and  760 , e.g., copper tape or wire, are soldered or otherwise bonded to the respective outer electrode layer  770  and gate layer  790  ( FIGS. 39 and 41   a ). Then, another flexible insulative layer  725 , such as, polyimide, is applied over the subassembly ( FIGS. 40 and 41   a ). Alternatively, the subassembly can be encapsulated using a suitable material, such as electrical grade epoxy resin. In this case, the flexible insulative layer  725  serves as both an insulator and an encapsulator. As illustrated in  FIG. 39 , a portion of the inner electrode layer  785  is left exposed. Terminals  795  can then be fabricated onto the exposed portions of the inner electrode layer  785  using a suitable electrically conductive material, such as copper tape, conductive tape/gel, or lead solder ( FIGS. 41 and 41   a ). Next, the transistor strip  745  is mounted onto the resistive touch region  630  of the substrate  620  using a suitable adhesive ( FIG. 42 ).  
         [0081]     As can be seen in  FIG. 42 , the inner electrode layer  785  is electrically connected to the resistive touch region  630  via the terminals  795 . If the transistor strip  745  is used as a left or top transistor array, the terminals  795  will serve as drain terminals, and if the transistor strip  745  is used as a right or bottom transistor array, the terminals  790  will serve as source terminals.  
         [0082]     Further details regarding the fabrication of transistor arrays using conductive polymer technology are set forth in further detail in U.S. patent application Ser. No. ______ (Attorney docket number ELG056 US1), which is expressly incorporated herein by reference.  
         [0083]     Although the transistor arrays  640  and  740  have been described as comprising a single semiconductor material, it should be noted that transistor arrays can be fabricated using two types of semiconductor material (a p-type semiconductor material and an n-type semiconductor material.) For example, transistors arrays formed from bipolar transistors, which utilize two types of semiconductor material, can be used. For example, the previously described transistor array  740  can use two conductive polymer layers between collector and emitter terminals. Or the transistor strip  640  can carry surface mounted bipolar transistors.  
         [0084]     Although particular embodiments of the present invention have been shown and described, it should be understood that the above discussion is not intended to limit the present invention to these embodiments. Those of ordinary skill in the art will appreciate that various changes and modifications may be made without departing from the spirit and scope of the present invention. Thus, the present invention is intended to cover alternatives, modifications, and equivalents that may fall within the spirit and scope of the present invention as defined by the claims.