A LTPS TFT pixel circuit for active matrix organic light emitting diode based on improved current mirror

Accepted Manuscript A LTPS-TFT Pixel Circuit for Active Matrix Organic Light Emitting Diode Based on Improved Current Mirror Kun Sun, Liuyan Chen, Jianping Guo, Dongdong Teng, Lilin Liu PII: DOI: Reference: S0141-9382(16)30072-5 http://dx.doi.org/10.1016/j.displa.2016.05.005 DISPLA 1792 To appear in: Displays Received Date: Revised Date: Accepted Date: 20 August 2015 November 2015 22 May 2016 Please cite this article as: K Sun, L Chen, J Guo, D Teng, L Liu, A LTPS-TFT Pixel Circuit for Active Matrix Organic Light Emitting Diode Based on Improved Current Mirror, Displays (2016), doi: http://dx.doi.org/10.1016/ j.displa.2016.05.005 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain A LTPS-TFT Pixel Circuit for Active Matrix Organic Light Emitting Diode Based on Improved Current Mirror Kun Suna,b, LiuyanChena,b, JianpingGuoa,b, DongdongTenga,c*, LilinLiua,b* a State key laboratory of optoelectronic materials and technology, bSchool of Microelectronics, c School of Physics and Engineering, Sun Yat-Sen University, Guangzhou, P.R China *The corresponding authors: liullin@mail.sysu.edu.cn; tengdd@mail.sysu.edu.cn Abstract: In this paper, a voltage-driving and current compensation method for active matrix organic light emitting diode (AMOLED) displays is proposed An improved current mirror is introduced into the pixel circuit to overcome the channel length modulation effect of TFTs The SPICE simulation results show that the proposed pixel circuit not only effectively compensates for non-idealities related with deviations of μ and VT in TFTs, the OLED degradation, but also offers a less setting time and guarantees a good liner relationship between VDATA and IOLED Keywords: Active matrix organic light emitting diode (AMOLED); low-temperature poly-silicon thin-film transistor (LTPS-TFT); current programmed pixel circuit (CPPC); channel length modulation effect Introduction Organic light-emitting diode (OLED) displays are one important type of self light-emitting display techniques They offer many unique performances compared with liquid crystal displays (LCD), such as high contrast ratio, very fast response time (µs), wide diverging angle, lower power-consumption, etc These merits make OLED display panels have great potentials for developing glass-free three-dimensional (3D) display techniques At present, more and more television manufacturers begin to incorporate 3D display functions in their products Due to the compatibility with existing plane image systems based on flat two-dimensional (2D) display panels[1], multi-view 3D display technologies become prosperous Through presenting position dependent views of a 3D scene to viewers, the multi-view display can offer both stereo andmotion parallax depth cues Recently, taking advantages of wide diverging angles of OLED pixels，Teng et al.[2-4] solved the discontinuous motion parallax problem of multi-view 3D display systems In general, high-refresh-rate and high-resolution AMOLED display panels are crucial for obtaining better 3D effects Thus, low temperature poly-silicon thin film transistors (LTPS-TFT) have attracted wide attentions for AMOLEDs, due to their higher field-effect mobility and better electrical stability compared to other TFT technologies However, random distribution of grain boundaries in poly-silicon films cause the inevitable non-uniformity of LTPS-TFT parameters, including mobility (μ) and threshold voltage (VT)[5].The application of LTPS-TFTs in flexible displays poses an additional stress factor to AMOLEDs, which will also result in non-uniformity varied with deformation In addition, the threshold voltage of OLEDs (VT_OLED) degrades at 0.2mV/h during operation[6] The conventional pixel circuit of two transistors and one capacitor (2T1C), as shown in Fig 1(a), doesn’t work for the non-uniformity of AMOLED display panels As a result, some compensation methods have been proposed [5-14] In those methods, voltage compensation has a faster setting time, but at the price of less effective compensation for VTH shift than the current compensation scheme[10] However, the current compensation methods require inconvenient constant current sources[8], and ignore the channel length modulation effect of TFTs[15] A typical mirrored current programmed pixel circuit(CPPC) is shown in Fig 1(b)[7] In this work, an improved current mirror pixel circuit is proposed which uses voltage driving and provides current compensation The compensation circuit takes the advantage of current compensation and avoids the disadvantage of using constant current sources The improved current mirror can make the input voltage (VDD) and the output current (IOLED) exhibit a good linear relationship by overcoming the channel length modulation As a result, the pixel circuit not only can effective compensate μ and VT shifts of TFTs, the OLED degradation, but also offers a less setting time Proposed Pixel Circuit And Driving Method The proposed pixel circuit is schematically drawn in Fig 2(a), which includes six P-type TFTs (T1-T6) and one storage capacitor (CS) The column external driver circuit consists of an integral module and a current sensing module Signal communications between the pixel circuit and the external driver are carried out through the data line and the feedback line The signal timing diagram is given in Fig 2(b) The operation consists two phases: programming model and hold model During the programming model, VSEL is low to turn on T1 and T2, the output voltage of AMP1 (VFB) is given as: VFB  VDATA  I FB  RFB   G (1) where G is the gain of the amplifier (AMP1) The output voltage (VOUT)of the integral module is derived by integratingVFB: VOUT(t )   RC 1 t2  t1 VFBdt  VOUT(t ) (2) where VOUT(t1) and VOUT(t2) are the voltage values of VOUT at the start time of t1 and the end time of t 2, R1 and C1 denote the values of the used resistor and capacitor The channel length modulation effect of TFT will make the drain current (ID) keep changing with the variation of VDS in saturation region In the proposed pixel circuit, this effect of TFT is taken into consideration by using the MOS model[16], the drain current of T1 and T2 in saturation region can be expressed as I D   W Cox   (VGS  VT )2(1  VDS ) L (3) where COX denotes the gate capacitance, μ is the mobility, W/L denotes the aspect ratio of TFT The threshold voltage VT and the channel length modulation factor λ are introduced into the equation Hence, I D1 ( W / L)1 (1  VDS1 )   I D2 ( W / L)2 (1  VDS2 ) (4) Compared with the typical mirrored CPPC, the proposed pixel circuit adds two TFTs (T3 and T4) to cope with the channel length modulation effect In this way, VD1+VSG3=VD2+VSG4 If (W/L)3/(W/L)4=(W/L)1/(W/L)2, then VSG3=VSG4 and VD1=VD2 According to the device parameters summarized in Table 1, IOLED and IFB are equal At the end of programming mode, the improved current mirror pixel circuit takes the self-regulation voltage (VG) through voltage driving and current feedback operation, the negative feedback system will keep balance, leading to: I OLED  I FB  VDATA RFB (5) Since IOLED is determined by VDATA and RFB, the final stabilized IOLED will be insensitive to variations of the electrical properties of TFTs and OLEDs Thus, the proposed driving scheme has high immunity to the variation of LTPS-TFTs and OLED characteristics During the hold phase, VSEL is high to turn off T1 and T2 Consequently, CS continues to sustain VG Therefore, IOLED will keep a specific value, the same as that at the end time of programming model Simulation Setup To verify performances of the proposed driving scheme, circuit simulations are conducted by the Cadence-virtuoso software A p-type LTPS-TFT SPICE model (Level 62) is used, where μ and VT are set as 48cm2/V·s and -2.5V, respectively The output characteristics of the LTPS-TFTs are simulated, as shown in Fig 3(a), which presents obvious channel length effects The OLED is modeled as a diode-connected TFT and a capacitor The VT_OLED and size of OLED are set as 3V and 104um2, respectively The OLED capacitance takes a value of 25nF/cm2 The simulated current–voltage curve of this OLED model is drawn in Fig 3(b) The proposed external driving circuit is simulated with a 0.35μm 24V CMOS process in HHNEC The gain of AMP1 and AMP2 are set as 38 and 66 dB, respectively Their -3dB bandwidth are 540k and 39kHz Other simulation parameters are summarized in Table I Results And Discussion The proposed scheme has good stability to deviations of VT, μ and VT_OLED values The comparisons between the proposed scheme and the conventional 2T1C circuit are conducted at the nominal programming current of 1μA, results as shown in Fig 4(a-c) Within a VT deviation range of -1.0V~1.0V, the relative errors of the proposed scheme are all less than 0.5%, as indicated by Fig 4(a) The proposed scheme also provide a relative error of less than 0.3% as the μ deviation range of -20cm2/V.s ~ 20cm2/V.s, as indicated by Fig 4(b) A relative error less than 1.4% is obtained by the proposed driving scheme within a VT_OLED shift range of -1.0~1.0V related with OLED degradation, as indicated by Fig 4(c) The proposed scheme can offer better linearity through overcoming the channel length modulation effect of TFT In a typical CPPC, the two TFTs (T3 and T4 in Fig1.b) have the same VGS, but a different VDS in the saturation region In the improved current mirror, the added two TFTs (T3 and T4 in Fig2 b) will help to overcome the channel length modulation effect Thus the improved current mirror circuit can support a stronger loading capability than the typical mirrored CPPC, which is importance to resist the non-uniform electrical characteristics of OLED pixels Simulations are conducted under the proposed scheme and the typical mirrored CPPC scheme at the same operation situation Their input/output data relationship and the relative error as a function of input data are compared, as presented in Fig 5(a) and (b) The maximum relative error of the proposed circuit is 2%, much smaller than that of typical mirrored CPPC Compared with the typical mirrored CPPC, the proposed scheme provides less setting time, as shown in Fig The setting time for all ranges of data currents is smaller than 20 us with parasitic loads of Cp and Rp on both the data line and the feedback line in the proposed scheme In a word, above results clearly prove that the proposed pixel circuit not only has high immunity to non-idealities of poly-Si TFTs and OLED degradation, but also offer less setting time Conclusions A current compensation method for AMOLED pixel circuits based on voltage driving scheme is proposed.The improved current mirror can overcome the channel length modulation effect of TFTs by adding two TFTs, which guarantees a good linear relationship between VDATA and IOLED Simulation results based on LTPS-TFTs show that the proposed circuit can effectively compensates for non-idealities related with deviations of μ and VT in TFTs, the OLED degradation and achieves less setting time Acknowledgment This work was supported by the grants from the Natural Science Foundation of China (No U1201254), the National High Technology Research and development Program of China (No.2013AA03A106, No.2015AA03A101), the Guangzhou Technical Plan Project under No 201510010280 Reference [1] J.-Y Son, B Javidi, Three-dimensional imaging methods based on multiview images, Display Technology, Journal of, (2005) 125-140 [2] D Teng, L Liu, B Wang, Super multi-view three-dimensional display through spatial-spectrum time-multiplexing of planar aligned OLED microdisplays, Opt Express, 22 (2014) 31448-31457 [3] D Teng, Y Xiong, L Liu, B Wang, Multiview three-dimensional display with continuous motion parallax through planar aligned OLED microdisplays, Opt Express, 23 (2015) 6007-6019 [4] D Teng, L Liu, B Wang, Generation of 360° three-dimensional display using circular-aligned OLED microdisplays, Opt Express, 23 (2015) 2058-2069 [5] A Nathan, G.R Chaji, S.J Ashtiani, Driving schemes for a-Si and LTPS AMOLED displays, J Disp Technol, (2005) 267-277 [6] C Lin, Y Chen, A novel LTPS-TFT pixel circuit compensating for TFT threshold-voltage shift and OLED degradation for AMOLED, IEEE electron device letters, 28 (2007) 129 [7] A Nathan, A Kumar, K Sakariya, P Servati, S Sambandan, D Striakhilev, Amorphous silicon thin film transistor circuit integration for organic LED displays on glass and plastic, Solid-State Circuits, IEEE Journal of, 39 (2004) 1477-1486 [8] S.H Jung, W.J Nam, M.K Han, A New Voltage Modulated AMOLED Pixel Design Compensating Threshold Voltage Variation of Poly‐Si TFTs, IEEE Electron Device Letters, 25 (2004) 690-692 [9] C.-L Fan, F.-P Tseng, H.-L Lai, B.-J Sun, K.-C Chao, Y.-C Chen, A Novel LTPS-TFT Pixel Circuit to Compensate the Electronic Degradation for Active-Matrix Organic Light-Emitting Diode Displays, Int J Photoenergy, 2013 (2013) [10] S.J Ashtiani, A Nathan, A driving scheme for active-matrix organic light-emitting diode displays based on current feedback, J Disp Technol, (2009) 257-264 [11] W Liu, G Yao, C Jiang, Q Cui, X Guo, A New Voltage Driving Scheme to Suppress Non-Idealities of Polycrystalline Thin-Film Transistors for AMOLED Displays, J Disp Technol, 10 (2014) 991-994 [12] R.-H Yao, L.-R Zhang, L Zhou, W.-J Wu, A new compensation pixel circuit with all-p-type TFTs for AMOLED displays, Displays, 34 (2013) 187-191 [13] E Song, H Nam, Novel voltage programming n-channel TFT pixel circuit for low power and high performance AMOLED displays, Displays, 35 (2014) 118-125 [14] W Wei-Jing, Z Lei, X Miao, Z Li-Rong, Y Ruo-He, P Jun-Biao, An AC Driving Pixel Circuit Compensating for TFTs Threshold-Voltage Shift and OLED Degradation for AMOLED, Display Technology, Journal of, (2013) 572-576 [15] C.C Li, K Ikeda, T Inoue, P.K Ko, A physical poly-silicon thin film transistor model for circuit simulations, in: Electron Devices Meeting, 1993 IEDM'93 Technical Digest., International, IEEE, 1993, pp 497-500 [16] A Kumar, A Nathan, G.E Jabbour, Does TFT mobility impact pixel size in AMOLED backplanes?, Ieee T Electron Dev, 52 (2005) 2386-2394 Table and Figure captions Table 1: Device parameters used in the simulation Fig.1 (a)The conventional 2T1C pixel circuit; (b)the typical mirrored current programmed pixel circuit (CPPC) Fig.2 (a)Equivalent circuit of the proposed driving scheme; (b)signal timing diagram Fig.3 (a)Output characteristics of the LTPS-TFTs; (b)current–voltage curve of the OLED device Fig.4 The comparisons between the proposed scheme and the conventional 2T1C circuit at the nominal programming current of 1μA on their resistances to deviations of (a)VT; (b)μ and (c)VT_OLED values Fig.5 Comparisons on the input/output relationship and the relative erroras a function of input data between (a) the proposed scheme and (b) the typical mirrored CPPC Fig.6 The evolutions of the setting time with IOLED under the proposed driving scheme and the typical mirrored CPPC VDD VSEL VSEL CS VDATA IDATA T1 T2 VDD T1 T3 T2 CS OLED OLED (a) (b) Fig.1 (a) The conventional 2T1C pixel circuit (b) the typical mirrored current programmed pixel circuit (CPPC) AMOLED Pixel External Driver Integral Model VDD C1 R1 CC VFB R2 AMP2 Data line CP1 RP1 T5 VSEL CP2 RP2 G T3 T6 OLED Vss Current Sensing Module (a) T2 T4 IOLED IFB Feedback line AMP1 VDATA RFB Cs T1 T4 ① ② VSEL VDATA XXX XXXXXX XXX ① Programming Mode ② Hold Mode (b) Fig (a) Equivalent circuit of the proposed driving scheme (b) signal timing diagram 60 (a) W/L=15m/5m VGS=-7V 50 ISD(A) 40 VGS=-6V 30 20 VGS=-5V 10 VGS=-4V -10 -8 -6 -4 -2 VDS(V) (b) 10 IOLED(A) COLED OLED -1 10 11 VOLED(V) Fig (a) Output characteristics of the LTPS-TFTs (b) current–voltage curve of the OLED device 150 a Error  Relative Error() 100 IOLED(VT )  IOLED(VT  0) IOLED(VT  0) 50 -50 -100 Conventional 2T1C Proposed Scheme -1.0 -0.5 0.0 0.5 1.0 VT(V) 20 16 (b) Error  Relative Error() 12 IOLED() IOLED(  0) IOLED(  0) -4 -8 -12 -16 Conventional 2T1C Proposed Scheme -20 -24 -20 -10 10 20 (cm /Vs) 40 Relative Error() 30 (c) Error  20 IOLED(VT _ OLED )  IOLED(VT _ OLED  0) IOLED(VT _ OLED  0) 10 -10 -20 Conventional 2T1C Proposed Scheme -30 -40 -1.0 -0.5 0.0 0.5 1.0 VT_OLED(V) Fig The comparisons between the proposed scheme and the conventional 2T1C circuit at the nominal programming current of 1μA on their resistances to deviations of (a) VT, (b)μ and (c) VT_OLED values 2.0 (a) Error  IOLED(A) 2.5 IOLED  (VDATA / RFB ) (VDATA / RFB ) 1.5 1.0 0.5 2.0 0.0 1.5 -0.5 1.0 -1.0 0.5 -1.5 0.0 Relative Error(%) 3.0 -2.0 10 15 20 25 30 VDATA(mV) (b) Error  IOLED(A) 2.5 35 IOLED  I DATA I DATA 30 25 20 2.0 15 10 1.5 1.0 -5 0.5 0.0 0.0 -10 Relative Error() 3.0 -15 0.5 1.0 1.5 2.0 IDATA(A) 2.5 3.0 Fig 5.Comparisons on the input/output relationship and the relative error as a function of input data between (a) the proposed scheme and (b) the typical mirrored CPPC Setting Time(s) 80 Typical CPPC Proposed Scheme 70 60 50 40 30 20 10 0.0 0.5 1.0 1.5 2.0 IOLED(A) 2.5 3.0 Fig The evolutions of the setting time with IOLED under the proposed driving scheme and the typical mirrored CPPC Table 1: Device parameters used in the simulation TFT W/L Value Capacitor Value Resistor Value T1 15µm/5µm C1 10pF R1 10kΩ T2 15µm/5µm CC 5pF R2 1kΩ T3 5µm/5µm CS 5pF RFB 10kΩ T4 5µm/5µm CP1 100pF RP1 1.5kΩ T5 5µm/5µm CP2 100pF RP2 1.5kΩ T6 5µm/5µm Highlights 1) Introducing an improved current mirror into the pixel circuit to overcome the channel length modulation effect of TFTs 2) The designed pixel circuit can support a stronger loading capability than the typical mirrored CPPC, which is importance to resist the non-uniform electrical characteristics of OLED pixels 3) The proposed pixel circuit not only effectively compensates for non-idealities related with deviations of μ and VT in TFTs, the OLED degradation, but also offers a less setting time and guarantees a good liner relationship between VDATA and IOLED 4) The proposed pixel circuit uses voltage driving and provides current compensation The compensation circuit takes the advantage of current compensation and avoids the disadvantage of using constant current sources  .. .A LTPS- TFT Pixel Circuit for Active Matrix Organic Light Emitting Diode Based on Improved Current Mirror Kun Suna,b, LiuyanChena,b, JianpingGuoa,b, DongdongTenga,c*, LilinLiua,b* a State... compensation circuit takes the advantage of current compensation and avoids the disadvantage of using constant current sources The improved current mirror can make the input voltage (VDD) and the... channel length modulation effect of TFTs by adding two TFTs, which guarantees a good linear relationship between VDATA and IOLED Simulation results based on LTPS- TFTs show that the proposed circuit