Abstract:
This invention relates to pixel driver circuits for active matrix optoelectronic devices, in particular OLED (organic light emitting diodes) displays. We describe an active matrix optoelectronic device having a plurality of active matrix pixels each said pixel including a pixel circuit comprising a thin film transistor (TFT) for driving the pixel and a pixel capacitor for storing a pixel value, wherein said TFT comprises a TFT with a floating gate.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to pixel driver circuits for active matrix optoelectronic devices, in particular OLED (organic light emitting diodes) displays. 
       BACKGROUND TO THE INVENTION 
       [0002]    Embodiments of the invention will be described while particularly useful in active matrix OLED displays although applications and embodiments of the invention are not limited to such displays and may be employed with other types of active matrix display and also, in embodiments, in active matrix sensor arrays. 
       Organic Light Emitting Diode Displays 
       [0003]    Organic light emitting diodes, which here include organometallic LEDs, may be fabricated using materials including polymers, small molecules and dendrimers, in a range of colours which depend upon the materials employed. Examples of polymer-based organic LEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of dendrimer-based materials are described in WO 99/21935 and WO 02/067343; and examples of so called small molecule based devices are described in U.S. Pat. No. 4,539,507. A typical OLED device comprises two layers of organic material, one of which is a layer of light emitting material such as a light emitting polymer (LEP), oligomer or a light emitting low molecular weight material, and the other of which is a layer of a hole transporting material such as a polythiophene derivative or a polyaniline derivative. 
         [0004]    Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-colour pixellated display. A multicoloured display may be constructed using groups of red, green, and blue emitting sub-pixels. So-called active matrix displays have a memory element, typically a storage capacitor, and a transistor, associated with each pixel (whereas passive matrix displays have no such memory element and instead are repetitively scanned to give the impression of a steady image). Examples of polymer and small-molecule active matrix display drivers can be found in WO 99/42983 and EP 0,717,446A respectively. 
         [0005]    It is common to provide a current-programmed drive to an OLED because the brightness of an OLED is determined by the current flowing through the device, this determining the number of photons it generates, whereas in a simple voltage-programmed configuration it can be difficult to predict how bright a pixel will appear when driven. 
         [0006]    Background prior art relating to voltage programmed active matrix pixel driver circuits can be found in Dawson et al, (1998), “The impact of the transient response of organic light emitting diodes on the design of active matrix OLED displays”, IEEE International Electron Device Meeting, San Francisco, Calif., 875-878. Background prior art relating to current programmed active matrix pixel driver circuits can be found in “Solution for Large-Area Full-Color OLED Television—Light Emitting Polymer and a-Si TFT Technologies”, T. Shirasaki, T. Ozaki, T. Toyama, M. Takei, M. Kumagai, K. Sato, S. Shimoda, T. Tano, K. Yamamoto, K. Morimoto, J. Ogura and R. Hattori of Casio Computer Co Ltd and Kyushu University, Invited paper AMD3/OLED5-1, 11 th  International Display Workshops, 8-10 Dec. 2004, IDW &#39;04 Conference Proceedings pp 275-278. Further background prior art can be found in U.S. Pat. No. 5,982,462 and in JP2003/271095. 
         [0007]      FIGS. 1   a  and  1   b , which are taken from the IDW &#39;04 paper, show an example current programmed active matrix pixel circuit and a corresponding timing diagram. In operation, in a first stage the data line is briefly grounded to discharge Cs and the junction capacitance of the OLED (Vselect, Vreset high; Vsource low). Then a data sink Idata is applied so that a corresponding current flows through T 3  and Cs stores the gate voltage required for this current (Vsource is low so that no current flows through the OLED, and T 1  is on so T 3  is diode connected). Finally the select line is de-asserted and Vsource is taken high so that the programmed current (as determined by the gate voltage stored on Cs) flows through the OLED (I OLED ). 
         [0008]    There is, however, a need for improved pixel driver circuits. 
       SUMMARY OF INVENTION 
       [0009]    According to a first aspect of the invention there is therefore provided an active matrix optoelectronic device having a plurality of active matrix pixels each said pixel including a pixel circuit comprising a thin film transistor (TFT) for driving the pixel and a pixel capacitor for storing a pixel value, wherein said TFT comprises a TFT with a floating gate. 
         [0010]    In embodiments the floating gate TFT has one or more capacitively coupled input terminals to the floating gate, coupled via input capacitors. In embodiments there are no other connections to the floating gate other than through the input capacitors (ie. no direct or resistive inputs). The floating gate and associated gate connection(s) may be integrated within the TFT structure or the floating gate may comprise a gate connection to the TFT which is substantially resistively isolated from the remainder of the pixel circuit—that is it has only one or more capacitative connection(s) to the remainder of the pixel circuit (“non-integrated”). In a non-integrated device the input capacitors therefore may be devices patterned separately to the floating gate TFT. 
         [0011]    The “non-integrated” configuration is particularly useful as it enables vias between gate and drain-source metal layers to be avoided. This is because one plate of a coupling capacitor may be patterned in the source-drain layer. Thus in embodiments where a floating gate device with non-integrated input capacitors is employed the use of a said Floating Gate (FG) device avoids the need for an additional via typically between a gate layer of the drive TFT and the drain-source layer of a control or switching TFT. 
         [0012]    In some particularly preferred embodiments the driver TFT has two inputs each with an associated capacitive connection to the FG of the device. One of these input capacitances may be employed for storing a voltage which modulates the threshold voltage of the drive TFT whilst the other may be used as the programming input, in an OLED display for controlling the brightness of an OLED pixel driven by the drive TFT. 
         [0013]    In embodiments with two capacitively coupled input terminals the additional flexibility afforded by the second input terminal facilitates the fabrication of pixel circuits with an increased operating efficiency and/or the ability for greater control of the operation of the circuit. Thus in embodiments one of the input terminals and its associated capacitance may be employed for compensation of pixel brightness and/or colour for one or more of aging, temperature and positional non-uniformity. An input terminal may be employed to tune one or more parameters of the pixel circuit and/or to programme the pixel circuit to set a pixel brightness (here brightness includes the brightness of a colour sub-pixel of a multicolour display). 
         [0014]    In still further embodiments the additional capacitively coupled input terminal may be employed to provide compensation for mis-match between devices, for example to compensate for variations due to device mis-match in a current mirror based pixel circuit. 
         [0015]    In still other pixel circuits the effective threshold voltage of a FG thin film transistor may be reduced to zero or even inverted by applying a voltage to one (or more) of the capacitively coupled input terminals of the FG transistor. This can reduce the input voltage required for a given drain-source current, thus reducing the required drain-source voltage (Vds), in particular if it is preferred that the device operates in saturation. This can therefore reduce power requirements and increase operating efficiency. 
         [0016]    Furthermore, ability to change the effective threshold voltage is beneficial for circuits that need tuning and programming, where mismatch needs to be corrected between adjacent transistors. 
         [0017]    As previously mentioned, in preferred embodiments the active matrix optoelectronic device comprises an OLED device and the pixel circuit includes an OLED driven by the TFT. In still other embodiments the active matrix device may comprise an active matrix sensor, or an active matrix sensor in combination with an active matrix display device. 
         [0018]    In some embodiments the pixel circuit comprises a voltage-programmed pixel circuit—that is a programming voltage applied to the pixel circuit controls the pixel brightness (or colour). The pixel value stored on input capacitor may then include a threshold offset voltage value to offset a threshold voltage of the TFT. Where the drive TFT has two capacitively coupled input terminals, an input terminal may be employed to set a programming voltage for the pixel. In some embodiments the pixel circuit may include opto-feedback, for example comprising a photodiode coupled to an input terminal of the FG drive TFT. In embodiments a control circuit for such a voltage-programmed pixel has two cycles, a first cycle in which the threshold offset voltage value is stored, and a second cycle in which the brightness of the OLED is set by a programming voltage adjusted or modulated by the threshold offset voltage value. 
         [0019]    In other embodiments the pixel circuit comprises a current programmed pixel circuit and a voltage stored on the input capacitor comprises a voltage programmed by a current applied to a current data line for the pixel circuit. Again, in embodiments, a second capacitively coupled input terminal to the FG of FG TFT may be employed to modulate a threshold voltage of the TFT. The skilled person will appreciate, however, that even where two separate capacitively coupled input terminals are provided a common floating gate within the TFT structure may be employed for both connections (one plate of the capacitor is common, and for the opposite plates each input is connected to a different plate). 
         [0020]    In embodiments of the current programmed pixel circuit in which the drive TFT has two input terminals capacitively coupled to the FG of drive TFT a first input terminal may be coupled to a source (or drain) connection of the drive TFT, either directly or indirectly via one or more switching or select transistors. Such a select transistor may be controlled (switched on) to enable current programming of the pixel circuit. In embodiments one select transistor may be provided for programming and another for diode connecting the drive TFT, or both functions may be implemented by a single select transistor. 
         [0021]    In embodiments another capacitively coupled input terminal of the drive TFT may also be coupled to a pixel select transistor (either one of the aforementioned select transistors, or a further select transistor). This select transistor may be coupled between the second capacitively coupled input terminal of the drive TFT and a drain connection of the drive TFT, or it may be coupled to a bias voltage connection for the pixel circuit, for example to enable application of a bias voltage to adjust the threshold voltage of the drive TFT (for example, increasing Vt so that it reverse biases the oled during programming time). 
         [0022]    Embodiments of the current programmed pixel circuit include a current data line which may be selectively connected to one of the capacitively coupled input terminals of the drive TFT, by a select transistor (either one of the aforementioned transistors or a further select transistor) to selectively provide programming current to the pixel circuit and to enable a gate voltage corresponding to the programming current to be stored on the input capacitor associated with a floating gate connection. Embodiments of the circuit may also include a disable transistor coupled between the drive TFT and the OLED for disabling illumination from the OLED during programming. 
         [0023]    In still other embodiments the pixel circuit comprises a current mirror or other current copier circuit in which case the drive TFT may comprise an input or an output transistor of the current mirror or current copier. Thus in embodiments one or more transistors in the current mirror or current copier circuit may have one or more FG devices with some of the input terminals used, for example, for tuning the characteristics of the devices to more closely match one another. 
         [0024]    In a related aspect the invention provides a method of driving an active matrix pixel circuit of an organic electroluminescent display, in particular as described above, said pixel circuit comprising a thin film transistor (TFT) for driving the pixel and a pixel capacitor for storing a pixel value, wherein said TFT comprises a TFT with a floating gate, wherein said floating gate has an associated floating gate capacitance, the method comprising programming said pixel circuit to store a voltage on said floating gate to source capacitor, wherein said stored voltage defines a brightness of said organic electroluminescent display element. 
         [0025]    As previously described, the floating gate TFT preferably has one or more capacitively coupled input terminals to the floating gate, coupled via one or more input capacitors. These may be integrated with the floating gate TFT or patterned separately to the floating gate TFT and with no other connections to the floating gate other than through these input capacitors. Thus the pixel capacitor may comprise such an input capacitor. 
         [0026]    In preferred embodiments the method further comprises setting the voltage defining the pixel brightness on an input capacitor coupled to one of the input connections and storing a voltage to modulate a threshold voltage of the TFT on an input capacitor coupled to a second input connection. The input capacitors may be integrated or non-integrated. 
         [0027]    In a still further aspect the invention provides a floating gate organic thin film transistor comprising at least one input terminal capacitively coupled to a floating gate of the thin film transistor. In embodiments the input terminal comprises a floating gate connection to an integrated floating gate capacitor. 
         [0028]    The skilled person will understand that in the above described aspects and embodiments of the invention the floating gate transistor may be either an n-channel or a p-channel transistor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures in which: 
           [0030]      FIGS. 1   a  to  1   g  show examples of pixel circuits according to the prior art and a corresponding timing diagram, and further examples of active matrix pixel driver circuits; 
           [0031]      FIG. 2  shows a schematic representation of a floating gate TFT (thin film transistor); 
           [0032]      FIGS. 3   a  to  3   c  show, respectively, examples of voltage programmed pixel circuits according to embodiments of an aspect of the invention; 
           [0033]      FIG. 4  shows a timing diagram illustrating the operation of a voltage programmed pixel circuit of the type shown in  FIG. 3 ; 
           [0034]      FIGS. 5   a  to  5   h  show examples of current programmed pixel circuits according to embodiments of an aspect of the invention; 
           [0035]      FIGS. 6   a  and  6   b  show, respectively, an example of a floating gate current mirror circuit for a pixel circuit, and an example of an active matrix sensor circuit incorporating a floating gate thin film transistor; and 
           [0036]      FIGS. 7   a  and  7   b  show, respectively, integrated and non-integrated floating gate device structures, and corresponding circuits, for an active matrix pixel circuit according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Active Matrix Pixel Circuits 
       [0037]      FIG. 1   c  shows an example of a voltage programmed OLED active matrix pixel circuit  150 . A circuit  150  is provided for each pixel of the display and Vdd  152 , Ground  154 , row select  124  and column data  126  busbars are provided interconnecting the pixels. Thus each pixel has a power and ground connection and each row of pixels has a common row select line  124  and each column of pixels has a common data line  126 . 
         [0038]    Each pixel has an OLED  152  connected in series with a driver transistor  158  between ground and power lines  152  and  154 . A gate connection  159  of driver transistor  158  is coupled to a storage capacitor  120  and a control transistor  122  couples gate  159  to column data line  126  under control of row select line  124 . Transistor  122  is a thin film field effect transistor (TFT) switch which connects column data line  126  to gate  159  and capacitor  120  when row select line  124  is activated. Thus when switch  122  is on a voltage on column data line  126  can be stored on a capacitor  120 . This voltage is retained on the capacitor for at least the frame refresh period because of the relatively high impedances of the gate connection to driver transistor  158  and of switch transistor  122  in its “off” state. 
         [0039]    Driver transistor  158  is typically a TFT and passes a (drain-source) current which is dependent upon the transistor&#39;s gate voltage less a threshold voltage. Thus the voltage at gate node  159  controls the current through OLED  152  and hence the brightness of the OLED. 
         [0040]    The voltage-programmed circuit of  FIG. 1   c  suffers from a number of drawbacks, in particular because the OLED emission depends non-linearly on the applied voltage, and current control is preferable since the light output from an OLED is proportional to the current it passes.  FIG. 1   d  (in which like elements to those of  FIG. 1   c  are indicated by like reference numerals) illustrates a variant of the circuit of  FIG. 1   c  which employs current control. More particularly a current on the (column) data line, set by current generator  166 , “programs” the current through thin film transistor (TFT)  160 , which in turn sets the current through OLED  152 , since when transistor  122   a  is on (matched) transistors  160  and  158  form a current mirror.  FIG. 1   e  illustrates a further variant, in which TFT  160  is replaced by a photodiode  162 , so that the current in the data line (when the pixel driver circuit is selected) programs a light output from the OLED by setting a current through the photodiode. 
         [0041]      FIG. 1   f , which is taken from our application WO03/038790, shows a further example of a current-programmed pixel driver circuit. In this circuit the current through an OLED  152  is set by setting a drain source current for OLED driver transistor  158  using a current generator  166 , for example a reference current sink, and memorizing the driver transistor gate voltage required for this drain-source current. Thus the brightness of OLED  152  is determined by the current, flowing into reference current sink  166 , which is preferably adjustable and set as desired for the pixel being addressed. In addition, a further switching transistor  164  is connected between drive transistor  158  and OLED  152  to prevent OLED illumination during the programming phase. In general one current sink  166  is provided for each column data line.  FIG. 1   g  shows a variant of the circuit of  FIG. 1   f.    
         [0042]    Referring to  FIG. 2  this shows a schematic diagram of a floating gate thin film transistor  200  with drain (D), source (S) and multiple  202  input terminals capacitively coupled to the FG  204  of the transistor each with a respective applied voltage V 1 , V 2 , . . . V N . The transistor  200  also incorporates a floating gate (FG)  204 .  FIG. 2  also illustrates how the multiple input terminals and floating gate of the transistor may be considered as a set of capacitors C 1 , C 2  . . . C N . This latter representation is employed in the later described pixel circuits. 
         [0043]    Referring now to  FIG. 3   a , this shows a first example of a voltage programmed pixel circuit  300  comprising a floating gate drive transistor  302  with multiple input terminals  304  each with an associated capacitive coupling to the floating gate of the TFT  302  (T 2 ). The inherent gate-source capacitance C gs  is also shown dashed (when T 2  is on this comprises a parasitic capacitance of the transistor plus a portion of the channel capacitance; in the off state this is solely parasitic). Typically this parasitic capacitance is increased through increasing the overlap area between the gate and source to provide the circuit storage capacitance. Drive transistor  302  drives an OLED  301 . A first select transistor  306  (T 1 ) selectively couples one of the input terminals of the floating gate driving TFT to a data line  308  bearing a programming voltage for the pixel circuit; and second select transistor  310  selectively couples the second input terminal of transistor  302  to the drain connection of transistor  302  in response to a signal on auto-zero line AZ. This provides an auto-zeroing function to compensate the pixel drive, for example for aging and/or non-uniformity. It will be understood that in the example circuit of  FIG. 3   a  transistor  302  (T 2 ) is a p-channel device. 
         [0044]      FIG. 3   b  shows the same circuit as  FIG. 3   a , but adopting slightly different representation. 
         [0045]      FIG. 3   c  shows a p-channel example of a variant of the circuit of  FIGS. 3   a  and  3   b , in which like elements are indicated by like reference numerals, the circuit of  FIG. 3   c  including a photodiode  350 , in a similar manner to the circuit of  FIG. 1   e  described previously. This provides optical feedback when OLED  301  is on and provides an advantage over the arrangement of  FIG. 1   e  in that the circuit corrects for differences or shifts in the threshold voltage Vt of transistor  302 . 
         [0046]    Referring now to  FIG. 4 , this shows a timing diagram illustrating operation of the circuits of  FIG. 3  in more detail. The stages A-G in the operation of the active matrix pixel circuit of  FIG. 3   a  are as described below: 
         [0047]    A—pixel circuit is in OFF state; Vdata is disconnected from the pixel circuit; C 1  and C 2  capacitors float at an indeterminate state. 
         [0048]    B—select switch is enabled and a reference data voltage (VHIGH) is applied to one input terminal (V 1 =VHIGH) of the floating gate TFT  302  so it does not cause current through the floating gate TFT  302  (|V FGS |&lt;|Vt|); VDD is high. 
         [0049]    C—AZ is low and T 3  is enabled; the V 2  input of drive TFT (T 2 ) is connected to the drain and so T 2   302  is diode connected. The V 1  input is still at VHIGH(V 1 =VHIGH). Current starts to conduct through T 2  and Vgs/Vds increases. Charge redistributes between capacitors C 1 , C 2  and Cgs. 
         [0050]    D—V DD  and V 1  (driven by the change in Vdata) go low by ΔV; V D (T 2 ) goes low and the OLED  301  is reverse biased. Current through T 2  is redirected through enabled T 3  into C 2 , charging the capacitance C 2 . The voltage V 2  goes high and transistor  302  switches OFF when the threshold voltage is reached at the floating gate of TFT  302  (and Vt is recorded on Cgs). 
         [0051]    E—AZ goes HIGH, T 3  goes OFF and V 2  disconnects. 
         [0052]    F—VDD and V 1  (through T 1  enabled) go HIGH again so that the OLED is in a forward biased state; and 
         [0053]    G—Data programmed onto T 2  is offset by the threshold voltage Vt. 
         [0054]    The skilled person will appreciate from the above description that the pixel circuits of  FIG. 3  enable threshold voltage compensation in a voltage programmed pixel driver without requiring a TFT switch to disconnect the OLED (because this can effectively be accomplished by controlling an input voltage to reverse bias the OLED). Further in embodiments all the capacitors used can be provided by an integrated floating gate TFT as device  302 . Alternatively if the circuits are constructed without integrated TFTs, then the design of the circuit layouts can avoid the need for vias between the gate and source/drain metal layers. The data voltage information programming the pixel is, in embodiments, stored by the capacitance C gs  and hence is determined by the parasitic capacitance of the drive TFT  302  (T 2 ). This is determined by the overlap area between the gate and the source, as well as by a portion of the channel capacitance of the drive TFT  302 . This overlap may typically be increased in order to provide sufficient storage capacitance, or an external capacitance provided. The capacitors C 1  and C 2  can be integrated capacitances of the floating gate transistor  302  (T 2 ), or separate components patterned next to the drive TFT, and comprise part of the circuit design; their values may be determined by choosing a geometric overlap area between the floating gate electrode and input terminal, regardless of being integrated or separated. 
         [0055]    Referring now to  FIG. 5   a , this shows a first example of a current programmed active matrix pixel circuit  500  incorporating a floating gate drive transistor  502 . The circuit of  FIG. 5   a  can be compared with the circuit of  FIG. 1   a . One input terminal  502   a  (G 1 ) of transistor  502  serves as a input connection for select transistor  504  (which corresponds to T 1  in  FIG. 1   a ). The other input terminal  502   b  (G 2 ) is used to store the gate-source voltage programmed by the current set on the current dataline Idata on the input capacitance of transistor  502  when the second select transistor  506  to which this input terminal is coupled is switched on. Thus, in operation, when the SEL line is asserted both transistors  504  and  506  are switched on and to programme the pixel the Vdd line is taken low and a current sink is applied to the Idata line to set the voltage corresponding to the programmed current on input terminal capacitor of transistor of  502 . The SEL line is then de-asserted and Vdd is taken high so that the programmed current flows through the OLED  508 . A reset transistor (not shown in  FIG. 5   a ) may be coupled to the Idata line to reset the voltage stored on input capacitor connected between input terminal G 2  and FG prior to programming the output current. 
         [0056]    The circuit of  FIG. 5   a  can be fabricated with a reduced number of vias; an integrated input capacitor results in a smaller physical size for the pixel circuit. Thus the circuit can be implemented with an integrated floating gate device (i.e. with integrated input capacitors) to provide with a smaller physical size at the expense of a more complex layer structure, or with non-integrated input capacitors a simpler layer structure with fewer or no vias can be achieved. 
         [0057]    The circuit of  FIG. 5   a  uses n-channel transistors but, as the skilled person would understand, p-channel transistors may alternatively be employed. Referring now to  FIG. 5   b  this shows a variant of the circuit of  FIG. 5   a  (in which like elements are indicated by like reference numerals, in which select transistor  504  is coupled to a bias line Vbias  510  rather than to Vdd. This bias line can be used to adjust the effective threshold voltage of the drive transistor by adjusting the voltage on an input terminal G 1 . In the case where the threshold voltage is non-zero, and therefore where, in programming a drive device through the use of diode connection, a larger drain-source voltage (than required to maintain saturation) would be produced, the threshold voltage for a floating gate device can be adjusted to zero thereby lowering the gate source voltage employed for the same OLED drive current. This in turn enables a lower Vdd to be employed, thus reducing the power consumption. The skilled person will understand that, in a similar way, rather than Vbias being adjusted in a positive direction to reduce Vt, Vbias may be adjusted in a negative direction to increase Vt. 
         [0058]    The arrangement of  FIG. 5   b  also facilitates an alternative mode of operation in which, during programming, rather than Vdd being sent to the lower voltage level to reverse bias the OLED the voltage on the Vbias line is controlled so that the OLED is not illuminated during current programming of the pixel circuit. This arrangement relies on adjusting Vbias in a positive direction to shift the programming voltage in a negative direction. After programming Vgs stays approximately constant (G 1  in  FIG. 5   b  essentially floats), as the source voltage rises and the OLED turns on. 
         [0059]    Referring now to  FIG. 5   c , this shows a further variant of the circuit of  FIG. 5   a  again in which like elements are indicated by like reference numerals, this variant including a disable transistor  512  coupled to an inverted version of SEL line so that the OLED  508  may be actively switched off during programming rather than the Vdd taken low. 
         [0060]    Referring next to  FIG. 5   d , this shows another example of a current programmed active matrix pixel circuit  520 , the circuit using p-channel rather than n-channel devices. In the circuit of  FIG. 5   d  drive transistor  522  has a first input terminal  522   a  (G 1 ) which stores on a corresponding input capacitor a gate voltage programmed by a current on the data line when select transistors  524 ,  526  are on, whilst a second input terminal  522   b  (G 2 ) serves as an additional input terminal for transistor  522  and is connected to the drain of the drive TFT—providing drive TFT is on and in saturation during programming. Again, during programming, select transistors  524 ,  526  are on and programming current flows from the Vdd line through drive transistor  522  to a programmable data sink (not shown) connected to the Idata line. When select transistors  524 ,  526  are switched off this current then flows through OLED  528  (during the programming phase the current through the OLED should be disabled). 
         [0061]      FIG. 5   e  illustrates a variant of the circuit of  FIG. 5   d  in which, rather than select transistors  524 ,  526  being series coupled between the Idata line and the drain connection of drive transistor  522 , one of the select transistors  526  is coupled between the drain terminal of drive transistor  522  and the second input terminal G 2   522   b  of this transistor whilst the second select transistor  524  couples the Idata line directly to the drain terminal of drive transistor  522 . This has the advantage that there is a single select transistor between the drive transistor output and the Idata line passing the programming current. 
         [0062]      FIG. 5   f  shows a further variant of this circuit, in which like elements of those in  FIG. 5   d  are indicated by like reference numerals, in which the input terminal G 1   522   a  is connected to a bias voltage line Vbias  530  to allow adjustment/control of the threshold voltage of drive transistor  522  in a broadly similar manner to that described with reference to  FIG. 5   b.    
         [0063]    Continuing to refer to an arrangement such as that illustrated in  FIG. 5   f , including a bias voltage line, if, in operation, one input terminal of the floating gate TFT is biased so as to increase the threshold voltage to a large value—which can be performed by biasing the bias voltage line positive (it is p-type)—the drain source voltage VDS across the drive TFT, when it is diode connected, can reverse bias the OLED and hence disable its operation during the programming cycle. Thus this provides a useful advantage since modulation (taking low) of the Vdd voltage is not required. In embodiments this can provide a power saving since there is generally a significant capacitance associated with this line. In embodiments the bias voltage in an active matrix display device may be shared between neighbouring pixels/lines of pixels. 
         [0064]      FIG. 5   g  illustrates a further alternative circuit in which the select transistor  526  coupled to the second input terminal G 2   522   b  of the drive transistor is directly coupled to the Idata line rather than to the drain terminal (or both as in  5   e ) of the drive transistor (so that the drain terminal is connected to the input terminal G 2  via the series connected select transistors  524 ,  526 ). 
         [0065]      FIG. 5   h  illustrates a still further variant of the current programmed circuit in which an additional OLED disable transistor  532  is provided so that the OLED can be actively switched off during programming (and hence Vdd need not be taken low during programming). 
         [0066]      FIG. 6   a  shows an example of a current mirror circuit which may be incorporated into an active matrix pixel driver circuit using one, or as illustrated two, floating gate transistors  602 ,  604 . In the example shown, one or both of the second input terminals may be coupled to a bias voltage Vb to adjust one or both threshold voltages of transistors  602 ,  604  for example to better match the characteristics of the two transistors. A similar arrangement may be used in a current copier circuit. A further advantage of using one or more floating gate devices is that the required power supply can be reduced by reducing the threshold voltage of the drive TFT through controlling the gate voltage on one of the input terminals. 
         [0067]      FIG. 6   b  shows an example of an active matrix pixel circuit for a sensor incorporating a floating gate TFT, again with threshold voltage adjustment as described above. 
         [0068]    Referring to  FIGS. 7   a  and  7   b , these show integrated and non-integrated floating gate device structures and circuits. Like elements to those of  FIG. 2  are indicated by like reference numerals. 
         [0069]      FIG. 7   a  shows an embodiment of a floating gate (FG) TFT  200   a  with an integrated floating gate  204 . In this integrated FG device the floating gate capacitor comprises a layer of gate metal  204   b  sandwiched between dielectric layers  204   a,c  to form a floating gate over semiconductor  206  and source and drain connections in source-drain metal  208 . A first capacitively coupled input  202   a  forms a first input capacitor with a first portion of floating gate  204   b , and a second capacitively coupled input  202   b  forms a second input capacitor with a second portion of floating gate  204   b.    
         [0070]      FIG. 7   b  shows an embodiment of a floating gate (FG) TFT  200   b  with a non-integrated floating gate, in which like elements to those of  FIG. 7   a  are indicated by like reference numerals. Again in this structure a first capacitively coupled input  202   a  forms a first input capacitor with a first portion of floating gate metal  204   b , and a second capacitively coupled input  202   b  forms a second input capacitor with a second portion of floating gate metal  204   b . However, rather than the device having a vertical structure, the first and second capacitively coupled inputs are laterally disposed to either side of the source-drain contacts. This enables one plate of each input capacitor to be formed using the source-drain metal layer, and this enables the number of vias in a pixel drive circuit to be reduced. Further, as can be seen by comparison with  FIG. 7   a , there is one less metal layer and one less dielectric layer. 
         [0071]    In preferred embodiments of the above circuits the transistors comprise MOS devices, for example fabricated from amorphous silicon. However, in other implementations one or more organic thin film transistors may be employed. 
         [0072]    As the skilled person will understand the above described circuits may be implemented in either n- or p-channel variants. The skilled person will further understand that many other variations are possible and that, for example, one or the more of the circuits illustrated in  FIGS. 1   c  to  1   g  may also be implemented using a floating gate drive transistor. More generally, virtually any pixel circuit described in the art may be configured to incorporate a floating gate TFT along the lines described above. 
         [0073]    No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.