1. Field of the Invention
The present invention relates to a display device, and particularly to a display device including a light-emitting element, such as an organic EL (Electro Luminescence), which varies its light-emitting luminance according to a drive current, in each of pixels and executing gray-scale expression based on a digital signal.
2. Description of the Background Art
As a display device of a flat-panel type, attention has been given to a display device of a self-light-emitting type, in which each pixel is formed of a light-emitting element of a current drive type. The display device of the self-light-emitting type has high visibility as well as high moving picture quality. A light-emitting diode (LED) is well known as a kind of light-emitting element of the current drive type.
Generally, a display device includes a plurality of pixels, which are arranged in rows and columns, and are successively driven by dot-sequential scanning or line-sequential scanning to receive a display current. Each pixel element keeps brightness corresponding to the display current thus received until its next driving. The display current received by each pixel is usually formed of an analog current for achieving gray-scale expression. This analog current can be set to a level intermediate between maximum (white) and minimum (black) luminance levels of each light-emitting element so that each pixel can execute the gray-scale expression.
Therefore, the display device provided with the light-emitting elements of the current drive type requires a current supply circuit for accurately producing the analog current (which may also be referred to as the “data current” hereinafter) according to the display signal.
FIG. 21 is a circuit diagram showing a structure of a general current supply circuit.
Referring to FIG. 21, a general current supply circuit 300 includes an n-channel TFT (which will be referred to as an “n-type TFT” hereinafter) 301, which is used as a current drive element, a switch 303 and a capacitor 305. In the specification, the TFT (Thin Film Transistor) is described as a typical example of a field-effect transistor.
n-type TFT 301 has a source and a drain, which are electrically connected to a predetermined voltage Vss and an output node No, respectively. A gate of n-type TFT 301 is connected to a node Ng. When switch 303 is turned on, an input voltage Vin is transmitted to node Ng, i.e., a gate of n-type TFT 301. A capacitor 305 is connected between predetermined voltage Vss and the gate of n-type TFT 301, and holds voltage difference between a gate voltage and predetermined voltage Vss, i.e., a gate-source voltage (which will be merely referred to as a “gate voltage” hereinafter) of n-type TFT 301.
Capacitor 305 holds input voltage Vin, which is transmitted to the gate of n-type TFT 301 when switch 303 is turned on. Consequently, n-type TFT 301 keeps the gate voltage equal to input voltage Vin. As can be understood from a circuit structure, the current drive element may be formed of a p-type field-effect transistor instead of the n-type transistor. The typical example, which will now be described, uses a ground voltage as predetermined voltage Vss.
A drain current Id in a saturation region of a field-effect transistor such as a TFT can be generally represented by the following formula (1):Id=(β/2)·(Vgs−Vth)2  (1)where β is equal to μ·(W/L)·Cox (i.e., β=μ·(W/L)·Cox).
In the above formula, β represents a current coefficient, μ represents an average surface mobility (which may be merely referred to as a “mobility” hereinafter), L represents a gate channel length, W represents a gate channel width, Cox represents a gate channel capacitance (per unit area), and Vth represents a threshold voltage.
In current supply circuit 300, therefore, when output node No is driven by a voltage different from predetermined voltage Vss, an output current Io corresponding to input voltage Vin is provided on output node No.
In current supply circuit 300, however, output current characteristics significantly depend on the characteristics of the current drive element, i.e., n-type TFT 301. If manufacturing variations, i.e., variations due to manufacturing occur in the characteristics (e.g., threshold voltage Vth and mobility μ) of n-type TFT 301, the output current characteristics significantly change.
FIG. 22 is a diagram illustrating a relationship between an input voltage and an output current of the current supply circuit shown in FIG. 21.
FIG. 22 illustrates I-V characteristic lines 310 and 320 of circuits, which use two TFTs (i.e., TFTa and TFTb) having different characteristics as n-type TFT 301 shown in FIG. 21, respectively. Also, FIG. 22 illustrates examples, in which four levels V1–V4 are selected as the levels of input voltage Vin, respectively.
As can be seen from I-V characteristic line 310, when TFTa is used, output current Io attains levels of I1a–I4a corresponding to input voltage V1–V4, respectively. As can be seen from I-V characteristic line 320, when TFTb is used, output current Io attains levels of I1b–I4b corresponding to input voltages V1–V4, respectively. Thus, output current variations ΔI1–ΔI4 unpreferably occur corresponding to input voltages V1–V4 due to difference in transistor characteristics, respectively.
If output current variation ΔI4 (=|I4b–I4a|), which appears when voltage V4 achieving the highest gray level is input, is larger than output currents I1a and I1b corresponding to the input voltage level V1 achieving the lowest gray level, gray-scale shift occurs due to the inversion of the current levels when gray-scale expression is executed by output current Io.
When conventional current supply circuit 300 shown in FIG. 21 is used to supply a display current to the light-emitting element of the current drive type, manufacturing must be done to reduce variations in characteristics of the current drive elements (typically, TFTs) in the circuit. This results in severe requirement relating to the manufacturing variations, and thus deteriorates the manufacturing yield.
Meanwhile, Japanese Patent National Publication No. 2002-514320 has disclosed, in FIG. 7, a current supply circuit, in which compensation is made for certain characteristic variations of a transistor used as a power drive element, and particularly, current variations due to threshold voltage Vth.
FIG. 23 is a circuit diagram showing a structure of a current supply circuit 400 disclosed in the above publication. Although current supply circuit 400 is provided within each pixel according to the structure of the above publication, FIG. 7 shows, as current supply circuit 400, a circuit portion functioning as a current supply circuit.
Referring to FIG. 23, current supply circuit 400 includes a capacitor 350 and switches 355 and 360 in addition to the structures of current supply circuit 300 shown in FIG. 21. Capacitor 350 is arranged between an input node Ni and a node Ng, and transmits a voltage change, which is caused on node Ni by transmission of input voltage Vin in response to the turn-on of switch 303, to node Ng by capacitive coupling.
Switch 355 is arranged between nodes Nd and Ng corresponding to the drain and gate of n-type TFT 301, respectively. Switch 360 is arranged between output node No and node Nd.
Current supply circuit 400 performs the following calibration operation to compensate for variations in output current due to variations in threshold voltage.
In the calibration operation, switch 360 is turned off, and switch 355 is turned on for accumulating electric charges corresponding the threshold voltage of n-type TFT 301 in capacitor 305. Thereby, node Ng carries a voltage equal to threshold voltage Vth of n-type TFT 301. Further, in the calibration operation, switch 303 is turned on when a reset voltage Vr is being supplied as input voltage Vin so that for the purposes of preventing noises and resetting capacitor 350,
Assuming that capacitors 305 and 350 have capacitance values of C1 and C2, respectively, initial charges Q10 and Q20 accumulated in capacitors 305 and 350 in the calibration operation can be expressed by the following formulas (2) and (3), respectively:Q10=C1·Vth  (2)Q20=C2·(Vg−Vin)=C2·(Vth−Vr)  (3)
In the current output operation, input voltage Vin is set corresponding to a display signal. In response to the turn-on and turn-off of switch 303, the capacitive coupling of capacitors 305 and 350 change voltage Vg on node Ng in an AC fashion. Charges Q1 and Q2, which are accumulated in capacitors 305 and 350 in the above operation, are expressed by the following formulas (4) and (5), respectively.Q1=C1·Vg  (4)Q2=C2·(Vg−Vin)  (5)
Therefore, gate voltage Vg on node Ng is expressed by the following formula (6) according to charge conservation (Q10+Q20=Q1+Q2):C1·Vth+C2·(Vth−Vr)=C1·Vg+C2·(Vg−Vin)∴(C1+C2)·Vth−C2·Vr=(C1+C2)·Vg−C2·Vin ∴Vg=Vth+C2/(C1+C2)·(Vin−Vr)  (6)
By substituting gate voltage Vg obtained from the formula (6) into the formula (1), drain current Id of n-type TFT 301 and thus output current Io of current supply circuit 400 are expressed by the following formula (7):Io=(β/2)·{C2/(C1+C2)}2·(Vin−Vr)2  (7)
As can be understood from the formula (7), output current Io of current supply circuit 400 does not depend on threshold voltage Vth of the transistor (n-type TFT). Therefore, current supply circuit 400 in FIG. 23 has I-V characteristics, which are illustrated in FIG. 24 and are to be compared with those in FIG. 22.
Referring to FIG. 24, since current supply circuit 400 compensates for an error in the I-V characteristics corresponding to the variation ΔVth in threshold voltage illustrated in FIG. 22, a difference between I-V characteristic lines 310 and 320, which correspond to TFTa and TFTb, respectively, is smaller than the difference between I-V characteristic lines 310 and 320 illustrated in FIG. 22.
By using current supply circuit 400, it is possible to reduce the errors due to variations in characteristics of the transistors, and thus to produce accurately the data current for gray-scale expression.
However, as can be understood from I-V characteristics 310# and 320# illustrated in FIG. 24, compensation can be made for the variations in output current due to the variations in threshold voltage between transistors (TFTs), but compensation cannot be performed for the variations in output current due to influences, which are exerted by variations in characteristics such as mobility μ and others caused in the manufacturing process, and thus due to variations of β in the foregoing formula (1).
Therefore, according to current supply circuit 400, the variations in output current can be suppressed within a region, where gate voltage Vg is close to threshold voltage Vth, and thus a region of a small current, but the variations in output current is unavoidably large within a region of a large current. Consequently, if the number of gray levels is large, it is impossible to ignore the influence by the variations in output current within a region of high gray level (large output current), and gray-scale shift may occur.
In the structures, which employ conventional current supply circuits 300 or 400 for supplying the data current for gray-scale expression by the light-emitting elements of the current drive type, therefore, it is necessary to request severely the suppression of the characteristic variations of the transistors (TFTs) due to the manufacturing.
In particular, a low-temperature polycrystalline silicon TFT (low-temperature p-Si TFT), which is a kind of thin film transistor and can be manufactured by a low-temperature process, exhibits a higher electron mobility than amorphous silicon TFT. Therefore, a drive circuit employing the low-temperature p-Si TFTs can be formed integrally with a pixel matrix circuit on a glass substrate. Accordingly such drive circuits are being widely used in EL display devices, liquid crystal display devices and others.
However, the low-temperature polycrystalline silicon TFT is generally formed by laser anneal, and it is difficult to control uniformly a laser illumination intensity within a plane of a glass substrate. Therefore, the low-temperature p-Si TFT tends to exhibit larger manufacturing variations in transistor characteristics such as Vth (threshold voltage) and μ (mobility) than a single crystal silicon TFT. Accordingly, the display device using the low-temperature polycrystalline silicon TFTs cannot reliably have an intended data current accuracy for gray-scale expression without difficulty.