Pixel current driver for organic light emitting diode displays

[0001] 1. Field of the Invention

[0002] The present invention relates to a an organic light emitting diode display, and more particularly to an a pixel current driver for an organic light emitting display (OLED), capable of minimizing parasitic couplings between the OLED and the transistor layers.

[0003] 2. Description of the Prior Art

[0004] OLED displays have gained significant interest recently in display applications in view of their faster response times, larger viewing angles, higher contrast, lighter weight, lower power, amenability to flexible substrates, as compared to liquid crystal displays (LCDs). Despite the OLED's demonstrated superiority over the LCD, there still remain several challenging issues related to encapsulation and lifetime, yield, color efficiency, and drive electronics, all of which are receiving considerable attention. Although passive matrix addressed OLED displays are already in the marketplace, they do not support the resolution needed in the next generation displays, since high information content (HIC) formats are only possible with the active matrix addressing scheme. Active matrix addressing involves a layer of backplane electronics, based on thin-film transistors (TFTS) fabricated using amorphous silicon (a-Si:H), polycrystalline silicon (poly-Si), or polymer technologies, to provide the bias voltage and drive current needed in each OLED pixel. Here, the voltage on each pixel is lower and the current throughout the entire frame period is a low constant value, thus avoiding the excessive peak driving and leakage currents associated with passive matrix addressing. This in turn increases the lifetime of the OLED.

[0005] In active matrix OLED (AMOLED) displays, it is important to ensure that the aperture ratio or fill factor (defined as the ratio of light emitting display area to the total pixel area) should be high enough to ensure display quality. Conventional AMOLED displays are based on light emission through an aperture on the glass substrate where the backplane electronics is integrated. Increasing the on-pixel density of TFT integration for stable drive current reduces the size of the aperture. The same happens when pixel sizes are scaled down. The solution to having an aperture ratio that is invariant on scaling or on-pixel integration density is to vertically stack the OLED layer on the backplane electronics, along with a transparent top electrode (see FIG. 2). In FIG. 2, reference numerals S and D denote a source and a drain respectively. This implies a continuous back electrode over the OLED pixel. However, this continuous back electrode can give rise to parasitic capacitance, whose effects become significant when the electrode runs over the switching and other thin film transistors (TFTs). Here, the presence of the back electrode can induce a parasitic channel in TFTs giving rise to high leakage current. The leakage current is the current that flows between source and drain of the TFT when the gate of the TFT is in its OFF state.

[0006] Accordingly, it is an object of the present invention to provide to a pixel current driver for an organic light emitting display (OLED), capable of minimizing parasitic couplings between the OLED and the transistor layers.

[0007] In order to achieve the above object, a pixel current driver for OLED layer for emitting light according to the present invention comprises a plurality of thin film transistors (TFTs) each having dual gates and for driving the OLED layer. A top gate of the dual gates is formed between a source and a drain of each of the thin film transistors, to thereby minimize parasitic capacitance.

[0008] Each of the thin film transistor may be an a-Si:H based thin film transistor or a polysilicon-based thin film transistor.

[0009] The pixel current driver is a current mirror based pixel current driver for automatically compensating for shifts in the Vth of each of the thin film transistor in a pixel and the pixel current driver is for monochrome displays or for full color displays.

[0010] The dual gates are fabricated in a normal inverted staggered TFT structure. A width of each of the TFTs is formed larger than a length of the same to provide enough spacing between the source and drain for the top gate. Preferably, the length is 30 m and the width is 1600 m. The length and width of the transistors may change depending on the maximum drive current required by the circuit and the fabrication technology used. The top gate is grounded or electrically tied to a bottom gate. The plurality of thin film transistors may be two thin film transistors formed in voltage-programmed manner or five thin film transistors formed in a current-programmed V_T-compensated manner, or four or The OLED layer is vertically stacked on the plurality of thin film transistors.

[0011] With the above structure of an a-Si:H current driver according to the present invention, the charge induced in the top channel of the TFT is minimized, and the leakage currents in the TFT is minimized so as to enhance circuit performance.

[0012] The above objects and features of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which:

[0013] FIG. 1 shows variation of required pixel areas with mobility for 2-T and 5-T pixel drivers;

[0014] FIG. 2 shows a pixel architecture for surface emissive a-Si:H AMOLED displays;

[0015] FIG. 3 shows a cross section of a dual-gate TFT structure;

[0016] FIG. 4 shows forward and reverse transfer characteristics of dual-gate TFT for various top gate biases;

[0017] FIG. 5A and FIG. 5B show an equivalent circuit for a 2-T pixel driver and its associated input-output timing diagrams;

[0018] FIG. 6A and FIG. 6B show an equivalent circuit for a 5-T pixel driver and its associated input-output timing diagrams;

[0019] FIG. 7 shows transient performance of the 5-T driver for three consecutive write cycles;

[0020] FIG. 8 shows input-output transfer characteristics for the 2-T pixel driver for different supply voltages;

[0021] FIG. 9 shows input-output transfer characteristics for the 5-T pixel driver for different supply voltages;

[0022] FIG. 10 shows variation in OLED current as a function of the normalized shift in threshold voltage;

[0023] FIG. 11 shows a 2-T polysilicon based pixel current driver having p-channel drive TFTs;

[0024] FIG. 12 shows a 4-T pixel current driver for OLED displays;

[0025] FIG. 13 shows a 4-T pixel current driver with a lower discharge time;

[0026] FIG. 14 shows a 4-T pixel current driver without non-linear gain;

[0027] FIG. 15 shows a 4-T pixel current driver that is the building block for the full color circuit; and

[0028] FIG. 16 shows a full color (RGB) pixel current driver for OLED displays.

[0029] Although amorphous Si does not enjoy equivalent electronic properties compared to poly-Si, it adequately meets many of the drive requirements for small area displays such as those needed in pagers, cell phones, and other mobile devices. Poly-Si TFTs have one key advantage in that they are able to provide better pixel drive capability because of their higher mobility, which can be of the order of _FE100 cm²/Vs. This makes poly-Si highly desirable for large area (e.g. laptop size) VGA and SVGA displays. The lower mobility associated with a-Si:H TFTs (_FE1 cm²/Vs) is not a limiting factor since the drive transistor in the pixel can be scaled up in area to provide the needed drive current. The OLED drive current density is typically 10 mA/cm² at 10V operation to provide a brightness of 100 cd/m²the required luminance for most displays. For example, with an a-Si:H TFT mobility of 0.5 cm²/Vs and channel length of 25 m, this drive current requirement translates into required pixel area of 300 m², which adequately meets the requirements of pixel resolution and speed for some 3 inch monochrome display applications. FIG. 1 illustrates simulation results for the variation of the required pixel size with device mobility calculated for two types of drivers, which will be elaborated later, the 2-T and the 5-T drivers, wherein ₀ denotes a reference mobility whose value is in the range 0.1 to 1 cm²/Vs. For instance, the area of the pixel for the 2-T driver (see FIG. 5A) comprises of the area of the switching transistors, area of the drive transistor, and the area occupied by interconnects, bias lines, etc. In FIG. 1, the drive current and frame rate are kept constant at 10 A and 50 Hz, respectively, for a 230230 array. It is clear that there is no significant savings in area between the 2-T and 5-T drivers but the savings are considerable with increasing mobility. This stems mainly from the reduction in the area of the drive transistor where there is a trade-off between _FE and TFT aspect ratio, W/L (Wide/Length).

[0030] In terms of threshold voltage (V_T) uniformity and stability, both poly-Si and a-Si:H share the same concerns, although in comparison, the latter provides for better spatial uniformity but not stability (V_T). Thus the inter-pixel variation in the drive current can be a concern in both cases, although clever circuit design techniques can be employed to compensate for V_T hence improving drive current uniformity. In terms of long term reliability, it is not quite clear with poly-Si technology, although there are already products based on a-Si:H technology for displays and imaging, although the reliability issues associated with OLEDs may yet be different. The fabrication processes associated with a-Si:H technology are standard and adapted from mainstream integrated circuit (IC) technology, but with capital equipment costs that are much lower. One of the main advantages of the a-Si:H technology is that it has become low cost and well-established technology, while poly-Si has yet to reach the stage of manufacturability. The technology also holds great promise for futuristic applications since good as-deposited a-Si:H, a-SiN_x:H, and TFT arrays can be achieved at low temperatures (120 C.) thus making it amenable to plastic substrates, which is a critical requirement for mechanically flexible displays.

[0031] To minimize the conduction induced in all TFTs in the pixel by the back electrode, an alternate TFT structure based on a dual-gate structure is employed. In a dual gate TFT (see FIG. 3), a top gate electrode is added to the TFT structure to prevent the OLED electrodes from biasing the a-Si:H channel area (refer to FIG. 2). The voltage on the top gate can be chosen such so as to minimize the charge induced in the (parasitic) top channel of the TFT. The objective underlying the choice of the voltage on the top gate is to minimize parasitic capacitance in the driver circuits and leakage currents in the TFTs so as to enhance circuit performance. In what follows, the operation of the dual-gate TFT is described, which will be central to surface emissive (100% aperture ratio) AMOLED displays based on a-Si:H backplane electronics.

[0032] FIG. 3 illustrates the structure of a dual-gate TFT fabricated for this purpose, wherein reference numerals S and D denote a source and a drain respectively. The fabrication steps are the same as of that of a normal inverted staggered TFT structure except that it requires a sixth mask for patterning the top gate. The length of the TFT is around 30 m to provide enough spacing between the source and drain for the top gate, and the width is made very large (1600 m) with four of these TFTs are interconnected in parallel to create a sizeable leakage current for measurement. A delay time is inserted in the measurement of the current to ensure that the measurement has passed the transient period created by defects in the a-Si:H active layer, which give rise to a time-dependent capacitance.

[0033] FIG. 4 shows results of static current measurements for four cases: first when the top gate is tied to 10V, second when the top gate is grounded, third when the top gate is floating, and lastly when the top gate is shorted to the bottom gate. With a floating top gate, the characteristics are almost similar to that of a normal single gate TFT. The leakage current is relatively high particularly when the top gate is biased with a negative voltage. The lowest values of leakage current are obtained when the top gate is pegged to either 0V or to the voltage of the bottom gate. In particular, with the latter the performance of the TFT in the (forward) sub-threshold regime of operation is significantly improved. This enhancement in sub-threshold performance can be explained by the forced shift of the effective conduction path away from the bottom interface to the bulk a-Si:H region due to the positive bias on the top gate. This in turn decreases the effect of the trap states at the bottom interface on the sub-threshold slope of the TFT.

[0034] It should be noted that although the addition of another metal contact as the top gate reduces the leakage current of the TFT, it can potentially-degrade pixel circuit performance by possible parasitic capacitances introduced by vertically stacking the OLED pixel. Thus the choice of top gate connection becomes extremely critical. For example, if the top gates in the pixel circuit are connected to the bottom gates of the associated TFTs, this gives rise to parasitic capacitances located between the gates and the cathode, which can lead to undesirable display operation (due to the charging up of the parasitic capacitance) when the multiplexer O/P drives the TFT switch. On the other hand, if the top gates are grounded, this results in the parasitic capacitance being grounded to yield reliable and stable circuit operation.

[0035] The OLED drive circuits considered here are the well-known voltage-programmed 2-T driver and the more sophisticated current-programmed V_T-compensated 5-T version (see FIGS. 5A and 6A). The latter is a significant variation of the previous designs, leading to reduced pixel area (<300 m), reduced leakage, lower supply voltage (20V), higher linearity (30 dB), and larger dynamic range (40 dB). Before dwelling on the operation of the 5-T driver, the operation of the relatively simple voltage-driven 2-T driver is described. FIG. 5B shows input-output timing diagrams of the 2-T pixel driver. When the address line is activated, the voltage on the data line starts charging capacitor C_s and the gate capacitance of the driver transistor T₂. Depending on the voltage on the data line, the capacitor charges up to turn the driver transistor T₂ on, which then starts conducting to drive the OLED with the appropriate level of current. When the address line is turned off, T₁ is turned off but the voltage at the gate of T₂ remains since the leakage current of T₁ is trivial in comparison. Hence, the current through the OLED remains unchanged after the turn off process. The OLED current changes only the next time around when a different voltage is written into the pixel.

[0036] Unlike the previous driver, the data that is written into the 5-T pixel in this case is a current (see FIG. 6A). FIG. 6B shows input-output timing diagrams of a 5-T pixel driver. The address line voltage, V_address and I_data are activated or deactivated simultaneously. When V_address is activated, it forces T₁ and T₂ to turn on. T₁ immediately starts conducting but T₂ does not since T₃ and T₄ are off. Therefore, the voltages at the drain and source of T₂ become equal. The current flow through T₁ starts charging the gate capacitor of transistors T₃ and T₅, very much like the 2-T driver. The current of these transistors start increasing and consequently T₂ starts to conduct current. Therefore, T₁'s share of I_data reduces and T₂'s share of I_data increases. This process continues until the gate capacitors of T₃ and T₅ charge (via T₁) to a voltage that forces the current of T₃ to be I_data. At this time, the current of T₁ is zero and the entire I_data goes through T₂ and T₃. At the same time, T₅ drives a current through the OLED, which is ideally equal to I_data*(W₅/W₃), which signifies a current gain. Now if I_data and V_address are deactivated, T₂ will turn off, but due to the presence of capacitances in T₃ and T₅, the current of these two devices cannot be changed easily, since the capacitances keep the bias voltages constant. This forces T₄ to conduct the same current as that of T₃, to enable the driver T₅ to drive the same current into the OLED even when the write period is over. Writing a new value into the pixel then changes the current driven into the OLED.

[0037] The result of transient simulation for the 5-T driver circuit is shown in FIG. 7. As can be seen, the circuit has a write time of <70 s, which is acceptable for most applications. The 5-T driver circuit does not increase the required pixel size significantly (see FIG. 1) since the sizes of T2, T3, and T4 are scaled down. This also provides an internal gain (W₅/W₃8), which reduces the required input current to <2 A for 10 A OLED current. The transfer characteristics for the 2-T and 5-T driver circuits are illustrated in FIGS. 8 and 9, respectively, generated using reliable physically-based TFT models for both forward and reverse regimes. A much improved linearity (30 dB) in the transfer characteristics (I_data/I_OLED) is observed for the 5-T driver circuit due to the geometrically-defined internal pixel gain as compared to similar designs. In addition, there are two components (OLED and T₅) in the high current path, which in turn decreases the required supply voltage and hence improves the dynamic range. According to FIG. 9, a good dynamic range (40 dB) is observed for supply voltage of 20V and drive currents in the range I_OLED10 A, which is realistic for high brightness. FIG. 10 illustrates variation in the OLED current with the shift in threshold voltage for the 2-T and 5-T driver circuits. The 5-T driver circuit compensates for the shift in threshold voltage particularly when the shift is smaller than 10% of the supply voltage. This is because the 5-T driver circuit is current-programmed. In contrast, the OLED current in the 2-T circuit changes significantly with a shift in threshold voltage. The 5-T driver circuit described here operates at much lower supply voltages, has a much larger drive current, and occupies less area.

[0038] The pixel architectures are compatible to surface (top) emissive AMOLED displays that enables high on-pixel TFT integration density for uniformity in OLED drive current and high aperture ratio. A 5-T driver circuit has been described that provides on-pixel gain, high linearity (30 dB), and high dynamic range (40 dB) at low supply voltages (15-20V) compared to the similar designs (27V). The results described here illustrate the feasibility of using a-Si:H for 3-inch mobile monochrome display applications on both glass and plastic substrates. With the latter, although the mobility of the TFT is lower, the size of the drive transistor can be scaled up yet meeting the requirements on pixel area as depicted in FIG. 1.

[0039] Polysilicon has higher electron and hole mobilities than amorphous silicon. The hole mobilities are large enough to allow the fabrication of p-channel TFTs.

[0040] The advantage of having p-channel TFTs is that bottom emissive OLEDs can be used along with a p-channel drive TFT to make a very good current source. One such circuit is shown in FIG. 11. In FIG. 11, the source of the p-type drive TFT is connected to Vdd. Therefore, Vgs, gate-to-source voltage, and hence the drive current of the p-type TFT is independent of OLED characteristics. In other words, the driver shown in FIG. 11 performs as a good current source. Hence, bottom emissive OLEDs are suitable for use with p-channel drive TFTs, and top emissive OLEDs are suitable for use with n-channel TFTs.

[0041] The trade-off with using polysilicon is that the process of making polysilicon TFTs requires much higher temperatures than that of amorphous silicon. This high temperature processing requirement greatly increases the cost, and is not amenable to plastic substrates. Moreover, polysilicon technology is not as mature and widely available as amorphous silicon. In contrast, amorphous silicon is a well-established technology currently used in liquid crystal displays (LCDs). It is due to these reasons that amorphous silicon combined with top emissive OLED based circuit designs is most promising for AMOLED displays.

[0042] Compared to polysilicon TFTs, amorphous silicon TFTs are n-type and thus are more suitable for top emission circuits as shown in FIG. 2. However, amorphous silicon TFTs have inherent stability problems due to the material structure. In amorphous silicon circuit design, the biggest hurdle is the increase in threshold voltage V_th after prolonged gate bias. This shift is particularly evident in the drive TFT of an OLED display pixel. This drive TFT is always in the ON state, in which there is a positive voltage at its gate. As a result, its V_th increases and the drive current decreases based on the current-voltage equation below:

[0043] In the display, this would mean that the brightness of the OLED would decrease over time, which is unacceptable. Hence, the 2-T circuits shown earlier are not practical for OLED displays as they do not compensate for any increase in V_th.

[0044] The first current mirror based pixel driver circuit is presented, which automatically compensated for shifts in the V_th of the drive TFT in a pixel. This circuit is the 5-T circuit shown in FIG. 6A.

[0045] Four more OLED pixel driver circuits are presented for monochrome displays, and one circuit for full colour displays. All these circuits have mechanisms that automatically compensate for V_th shift. The first circuit shown in FIG. 12 is a modification of the 5-T circuit of FIG. 6A. (Transistor T₄ has been removed from the 5-T circuit). This circuit occupies a smaller area than the 5-T circuit, and provides a higher dynamic range. The higher dynamic range allows for a larger signal swing at the input, which means that the OLED brightness can be adjusted over a larger range.

[0046] FIG. 12 shows a 4-T pixel driver circuit for OLED displays. The circuit shown in FIG. 13 is a 4-T pixel driver circuit based on a current mirror. The advantage of this circuit is that the discharge time of the capacitor Cs is substantially reduced. This is because the discharge path has two TFTs (as compared to three TFTs in the circuit of FIG. 12). The charging time remains the same. The other advantage is that there is an additional gain provided by this circuit because T₃ and T₄ do not have the same source voltages. However, this gain is non-linear and may not be desirable in some cases.

[0047] In FIG. 14, another 4-T circuit is shown. This circuit does not have the non-linear gain present in the previous circuit (FIG. 13) since the source terminals of T₃ and T₄ are at the same voltage. It still maintains the lower capacitance discharge time, along with the other features of the circuit of FIG. 8.

[0048] FIG. 15 shows another version of the 4-T circuit. This circuit is does not have good current mirror properties. However, this circuit forms the building block for the 3 colour RGB circuit shown in FIG. 16. It also has a low capacitance discharge time and high dynamic range.

[0049] The full colour circuit shown in FIG. 16 minimizes the area required by an RGB pixel on a display, while maintaining the desirable features like threshold voltage shift compensation, in-pixel current gain, low capacitance discharge time, and high dynamic range.

[0050] It is important to note that the dual-gate TFTs are used in the above-mentioned circuits to enable vertical integration of the OLED layers with minimum parasitic effects. But nevertheless the circuit compensates for the Vth shift even if the simple single-gate TFTs. In addition, these circuits use n-type amorphous silicon TFTs. However, the circuits are applicable to polysilicon technology using p-type or n-type TFTs. These circuits when made in polysilicon can compensate for the non-uniformity of the threshold voltage, which is a problem in this technology. The p-type circuits are conjugates of the above-mentioned circuits and are suitable for the bottom emissive pixels.