A Simple Pixel Circuit using LTPS TFTs with Mirror
Compensation for AMOLED Displays
Po-Syun Chen
, Yeng-Ting Liu, Fu-Hsing Chen, Chih-Lung Lin
Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan Keywords: LTPS TFT, AMOLED, Threshold voltage
Abstract
This work presents a new pixel circuit using low-temperature polycrystalline-silicon (LTPS) thin-film transistors (TFTs) for active-matrix organic light-emitting diode (AMOLED) displays. The proposed circuit can effectively compensate for the threshold voltage variation of TFTs and power line I-R drop by a simple mirror structure.
Introduction
Active-matrix organic light-emitting diode (AMOLED) is regarded as the candidate of the next-generation display technologies for the fast response time, high contrast ratio, low power consumption, and wide viewing angle [1]-[3]. However, the image non-uniformity caused by characteristics variation of the driving devices, such as the threshold voltage (VTH) variation of the low-temperature
polycrystalline-silicon (LTPS) thin-film transistors (TFTs), is a big problem which must be solved [4]. Many researches have been reported to enhance the image uniformity by compensating for the variations of LTPS TFT VTH [5]-[8], but they require many additional TFTs to
achieve the compensation. These pixel circuits with complicated structures are difficult to meet the demand of high-resolution mobile devices because of the large layout area of single pixel. Furthermore, the complicated structures would also increase the power consumption and thereby shorten the life time of panel. To simplify the pixel structure, both Kwon et al and Lin et al proposed the external compensation methods with extra detecting circuits in 2009 and 2011 [9], [10], respectively. They sense the characteristics variations of the TFTs and OLEDs by adding external detecting circuits and algorithm. Nevertheless, the external compensation methods would increase the fabrication cost of the panel. In addition to characteristics variations of TFTs, the power line I-R drop caused by the parasitic resistance of the bus line also leads to the current fluctuation of the pixels. Therefore, this effect should be compensated for AMOLED pixel circuits [11].
This work presents a new pixel circuit with internal mirror compensation to maintain the OLED current uniformity. The proposed pixel circuit can compensate for the VTH variation of LTPS TFTs and the power line I-R drop.
Because only one scan line is required for the simple 3-T structure, the aperture ratio can be improved and thus the proposed pixel circuit is suitable for the high-resolution displays. Simulation results show that the proposed pixel circuit successfully suppresses the current error rates
caused by TFT VTH variations below 1% over the data
voltage range. Furthermore, the power line I-R drop is also considered in the simulations, and the simulated currents of the proposed pixel circuit show a good stability.
2. Circuit Operation
Fig.1 shows the schematic of the proposed pixel circuit and the corresponding timing diagram. The proposed pixel circuit is composed of a switching TFT (T2), a mirror TFT (T3), a driving TFT (T1), and a storage capacitor (CS). SCAN is the control line of each row, and
VDD and VSS are the power line and ground line of the
panel, respectively. The driving operation of the proposed pixel circuit can be divided into three phases, including reset phase, compensation phase, and emission phase.
1). Reset phase: First, SCAN goes low to turn on T2. VDD
stays in the high level (VDD_high). Therefore, node A is
reset to a reference voltage (VREF).
2). Compensation phase: In the second phase, SCAN goes high to turn off T2, and VDD goes to the VDD_low.
V
DATAOLED
V
REFV
SSV
DDT1
T2
T3
SCAN
C
SA
SCAN VDD VDATA VDD_high VDD_low Vdata (1) (2) (3)Because of the coupling effect of CS, the voltage of
node A (VA) goes to the much lower voltage as
VDD_low+VREF-VDD_high, and T3 is turned on.
Simultaneously, the data line applies a data voltage (Vdata) to the proposed pixel circuit. Therefore, node A
starts being charged through T3 until T3 is turned off. In the end of compensation step, VA and the voltage
across CS (VCs) become, _ 3 A data TH T
V
=
V
−
V
(1) _ _ 3 Cs DD low data TH TV
=
V
−
V
+
V
(2) where VTH_T3 is the threshold voltage of T3.3). Emission phase: In the emission phase, VDD goes to
the VDD_high. Based on the charge conversation, VA is
bootstrapped to VDD_high-VDD_low+Vdata-|VTH_T3|, which is
much higher than the data voltage range, and therefore T3 can be turned off without any control signal. Consequently, the OLED current determined by the VCs is, 2 1 _ 1 _ 1 2 1 _ _ 3 _ 1
(
|
|)
2
(
|
|
|
|)
2
T OLED SG T TH T T DD low data TH T TH Tk
I
V
V
k
V
V
V
V
=
−
=
−
+
−
2 1 _(
)
2
T DD low datak
V
V
=
−
(3) where VTH_T1 is the threshold voltage of T1. For theproposed pixel circuit, the electrical characteristics of T3 are assumed to be identical to that of T1 because T3 is adjacent to T1. Therefore, in the saturation current of T1, VTH_T1 and VTH_T3 can be eliminated, as
shown in Eq. (3). In addition, because the VDD_high is
also eliminated in Eq. (3), the power line I-R drop can be compensated.
3. Simulation Results
To verify the feasibility of the proposed pixel circuit, HSPICE software is used in the simulation with the LTPS TFT models which are established by fabricated TFTs. The aspect ratio of T2 is 3 µm /3 µm, and T1 and T3 are both 3 µm in width and 15 µm in length. CS is set to 0.1 pF.
SCAN is from -10 V to 10 V, and VDD is from 1 V to 7 V. The data voltage ranges from 1 V to -1 V. The OLED is modeled by a 4 µm /5 µm diode-connected LTPS TFT with a parallel capacitance of 1 pF.
Fig. 2 shows the transient waveform of node A during the driving operation of the proposed pixel circuit when the VTH_T1 and VTH_T3 vary ± 0.5 V. The reset phase and
compensation phase are set to 1.7 µs and 13.3 µs respectively to investigate the performance of the proposed pixel circuit for the resolution of full high-definition (FHD, 1920×1080) at 60 Hz. As can be seen, the VTH variation of T3can be successfully sensed
in compensation phase. In emission phase, the proposed
15 20 25 30 35 40 45
Time (
µs)
-8 -4 0 4 8V
o
lt
a
g
e
(V
)
VTH = 0 V VTH = 0.5 V VTH = -0.5 V (1) (2) (3)Fig. 2. Simulated waveform of node A of proposed
pixel circuit with TFT VTH variations.
-1.2 -0.8 -0.4 0 0.4 0.8 1.2
Data Voltage (V)
0 1000 2000 3000 4000O
L
E
D
C
u
rr
en
t
(n
A
)
0 2 4 6 8 10R
el
a
ti
v
e
C
u
rr
en
t
E
rr
o
r
R
a
te
(
%
)
I (VTH = 0 V) I (VTH = 0.5 V) I (VTH = - 0.5 V) Error (VTH = 0.5 V) Error (VTH = -0.5 V)Fig. 3. OLED currents and relative current error rates versus data voltage.
0 0.2 0.4 0.6
V
DDDrop (V)
0 1000 2000 3000 4000O
L
E
D
C
u
rr
en
t
(n
A
)
Vdata = 0.5 V, IOLED = 175 nA Vdata = -0.25 V, IOLED = 1272 nA Vdata = -0.75 V, IOLED = 2693 nAFig. 4. OLED currents of different gray levels when VDD drops.
pixel circuit can also maintain the sensed voltage in order to produce uniform OLED current. Fig. 3 shows the OLED currents and the corresponding current error rates versus data voltage with the TFT VTH variations. According to Fig.
3, although the VTH variations reach ± 0.5 V, the OLED
current of the proposed pixel circuit is still uniform. In detail, the current error rates are under 1% over the whole data voltage range. Fig. 4 shows the OLED currents versus the VDD I-R drop of the proposed pixel circuit. Different data
voltages are input to investigate the effect of VDD drop at
high, middle, and low gray levels. Based on the simulation results, the proposed pixel circuit can maintain the stable currents at different gray levels even VDD drops 0.5V.
4. Conclusion
This work presents a new pixel circuit adopting LTPS TFTs with the mirror compensation method. The proposed pixel circuit can compensate for the VTH variation of LTPS
TFT as well as the power line I-R drop. Furthermore, the simple three-TFT and one-scan-line pixel structure can enhance the aperture ratio. The simulation results show that the proposed pixel circuit successfully suppresses the current fluctuations caused by TFT VTH variations below
1%. Moreover, the proposed pixel can effectively maintain the OLED current at different gray levels when VDD drops
0.5 V.
REFERENCES
[1] R. Dawson, Z. Shen, D. A. Furest, S. Connor, J. Hsu, M. G. Kane, R. G. Stewart, A. Ipri, C. N. King, P. J. Green, R. T. Flegal, S. Pearson, W. A. Tang, S. Van Slyke, F. Chen, J. Shi, M. H. Lu, and J. C. Sturm, “The impact of the transient response of organic light emitting diodes on the design of active matrix OLED displays,” in IEDM Tech. Dig., 1998, pp. 875–878.
[2] A. Nathan, G. R. Chaji, and S. J. Ashtiani, “Driving schemes for a-Si and LTPS AMOLED displays,” J. Display Technol., vol. 1, no. 2, pp. 267–277, Dec. 2005.
[3] C. L. Lin and Y. C. Chen, “A novel LTPS-TFT pixel circuit compensating for TFT threshold-voltage shift and OLED degradation for AMOLED,” IEEE Electron Device Lett., vol. 28, no. 2, pp. 129–131, Feb. 2007.
[4] S. H. Jung, W. J. Nam, and M. K. Han, “A new voltage-modulated AMOLED pixel design compensating for threshold voltage variation in poly-Si TFTs,” IEEE Electron Device Lett., vol. 25, no. 10, pp. 690–692, Oct. 2004.
[5] H. Y. Lu, P. T. Liu, T. C. Chang, and S. Chi, “Enhancement of brightness uniformity by a new voltage-modulated pixel design for AMOLED displays,” IEEE Electron Device Lett., vol. 27, no. 9, pp. 743–745, Sep. 2006.
[6] S. H. Jung, W. J. Nam, and M. K. Han, “A new voltage-modulated AMOLED pixel design compensating for threshold voltage variation in poly-Si TFTs,” IEEE Electron Device Lett., vol. 25, no. 10, pp. 690–692, Oct. 2004.
[7] Y. H. Tai, B. T. Chen, Y. J. Kuo, C. C. Tsai, K. Y. Chiang, Y. J. Wei, and H. C. Cheng, “A new pixel
circuit for driving organic light-emitting diode with low temperature polycrystalline silicon thin-film transistors,” J. Display Technol., vol. 1, no. 1, pp. 100–104, Sep. 2005.
[8] S. H. Jung, H. S. Shin, J. H. Lee, and M. K. Han, “An AMOLED pixel circuit for the VT compensation
of TFT and a p-type LTPS shift register by employing 1 phase clock signal,” in SID Tech. Dig., 2005, pp.300–303.
[9] H. J. In and O. K. Kwon, “External compensation of nonuniform electrical characteristics of thin-film transistors and degradation of OLED devices in AMOLED displays,” IEEE Electron Device Lett., vol. 30, no. 4, pp. 377–379, Apr. 2009.
[10] C. L. Lin, C. C. Hung, W. Y. Chang, K. W. Chou,
and C. Y. Chuang, “Novel a-Si:H AMOLED pixel circuit to ameliorate OLED luminance degradation by external detection,” IEEE Electron Device Lett., vol. 32, no. 12, pp. 1716–1718, Dec. 2011.
[11] C. L. Lin, W. Y. Chang, C. C. Hung, and C. D. Tu,
“LTPS-TFT pixel circuit to compensate for OLED luminance degradation in three-dimensional AMOLED display,” IEEE Electron Device Lett., vol. 33, no. 5, pp. 700–702, May 2011.
