Fig 1
Source publication
A new accurate voltage-programmed pixel circuit for active matrix organic light-emitting diode (AMOLED) displays is presented. Composed of three TFTs and one storage capacitor, the proposed pixel circuit is implemented both in a-Si and a-IGZO TFT technologies for the same pixel size for fair comparison. The simulation result for the a-Si-based desi...
Contexts in source publication
Context 1
... is well known that the threshold voltage shift has a direct impact on ciruit performance. For example, consider the simple 2−TFT pixel circuit shown in Fig. 1. The data line provides the required programming voltage for the drive TFT, 1 Mainly because the deposition method that is used for fabricating a-IGZO TFTs is Physical Vapour Deposition (PVD) rather than Chemical Vapour Deposition (CVD). while the scan line determines the running state of the switch TFT, i.e. ON or OFF. The voltage ...
Context 2
... compensation time. The most accurate method, however, is to use a trial-and- error approach through simulation with the real circuit models. Using the extracted model parameters of an a-Si TFT and for an OLED capacitor of C OLED = 1pF and α between 0.5 and 1, this optimum time as well as the corresponding current error is obtained and plotted in Fig. 10. The same trend can be followed for an a-IGZO ...
Context 3
... listed in Table I has been implemented with both a-Si and a-IGZO TFTs, and simulated with Verilog-AMS extracted model based on real measurement data [13][14][15][16]. A list of important parameters used in the models is summarized in Table II. A sample transient waveform of the drain and gate voltage of the drive a-Si TFT is illustrated in Fig. 11 are similar for a-IGZO model. The total programming times are 91µs and 26µs for the a-Si and the a-IGZO implemen- tation, respectively. Fig. 12 shows the current error for a 3V shift in the threshold voltage of T 1 as well as the relative error in a conventional 2−TFT pixel (Fig. (1)) with the same size of the drive and the switch TFTs ...
Context 4
... data [13][14][15][16]. A list of important parameters used in the models is summarized in Table II. A sample transient waveform of the drain and gate voltage of the drive a-Si TFT is illustrated in Fig. 11 are similar for a-IGZO model. The total programming times are 91µs and 26µs for the a-Si and the a-IGZO implemen- tation, respectively. Fig. 12 shows the current error for a 3V shift in the threshold voltage of T 1 as well as the relative error in a conventional 2−TFT pixel (Fig. (1)) with the same size of the drive and the switch TFTs used in the proposed circuit. As can be seen, the maximum error is almost zero for the a-Si circuit, while this is around 8% for the a-IGZO ...
Context 5
... drain and gate voltage of the drive a-Si TFT is illustrated in Fig. 11 are similar for a-IGZO model. The total programming times are 91µs and 26µs for the a-Si and the a-IGZO implemen- tation, respectively. Fig. 12 shows the current error for a 3V shift in the threshold voltage of T 1 as well as the relative error in a conventional 2−TFT pixel (Fig. (1)) with the same size of the drive and the switch TFTs used in the proposed circuit. As can be seen, the maximum error is almost zero for the a-Si circuit, while this is around 8% for the a-IGZO implementation. The larger error of a-IGZO circuit is because of the low value of overdrive voltage during the compensation phase (V ov = 5V ). ...
Context 6
... of the low value of overdrive voltage during the compensation phase (V ov = 5V ). A larger value, as explained in section III(B), would result in high susceptibility to any timing error of the pixel. The overdrive voltage for a-Si circuit during the compensation period is 14V. The profiles of the OLED current versus V data are also depicted in Fig. 13 and Fig. 14 is near zero for the a-Si implementation when experiencing a 3V shif of the threshold voltage. The a-IGZO circuit, however, shows an error of around 8% while being 3.5 times faster than its equivalent a-Si circuit. This demonstrates that the accuracy- speed trade-off of transistor-based circuits holds here as ...
Similar publications
Low-voltage-drive InGaZnO (IGZO) thin-film transistor (TFT) with SiO2/HfO2/SiO2 (SHS) multilayer gate insulator is deposited by radio frequency sputtering at room temperature. The fabricated device performed a threshold voltage of 1.4 V, a field effect mobility of 8.6 cm²/VS, an Ion/Ioff ratio of 2.9 × 10⁵, and a sub-threshold voltage swing of 0.67...
Citations
... In addition, V TH should be effectively controlled to greater or less than 0. For example, when configuring a compensation circuit within a pixel in a display, the compensation error may increase if V TH is too high or low [9,10]. In addition, a positive V TH is required to reduce circuit design complexity, as well as power consumption, when it is used as a gate driver circuit [11,12]. ...
The initial electrical characteristics and bias stabilities of thin-film transistors (TFTs) are vital factors regarding the practical use of electronic devices. In this study, the dependence of positive bias stress (PBS) instability on an initial threshold voltage (VTH) and its origin were analyzed by understanding the roles of slow and fast traps in solution-processed oxide TFTs. To control the initial VTH of oxide TFTs, the indium oxide (InOx) semiconductor was doped with aluminum (Al), which functioned as a carrier suppressor. The concentration of oxygen vacancies decreased as the Al doping concentration increased, causing a positive VTH shift in the InOx TFTs. The VTH shift (∆VTH) caused by PBS increased exponentially when VTH was increased, and a distinct tendency was observed as the gate bias stress increased due to a high vertical electric field in the oxide dielectric. In addition, the recovery behavior was analyzed to reveal the influence of fast and slow traps on ∆VTH by PBS. Results revealed that the effect of the slow trap increased as the VTH moved in the positive direction; this occured because the main electron trap location moved away from the interface as the Fermi level approached the conduction band minimum. Understanding the correlation between VTH and PBS instability can contribute to optimizing the fabrication of oxide TFT-based circuits for electronic applications.
... However, there are high-density sub-gap defects located in the bandgap of a-IGZO, which cause a change in threshold voltage (∆V th ) and the degradation of subthreshold swing (∆SS) under electrical and light stress, respectively [3,4]. Correspondingly, the uniformity of pixel-to-pixel brightness are affected in active-matrix display applications [5,6]. As a result, electrical stability modeling is required for the accurate stability prediction of a-IGZO TFTs under external stress conditions. ...
In this work, an electrical stability model based on surface potential is presented for amorphous In-Ga-Zn-O (a-IGZO) thin film transistors (TFTs) under positive-gate-bias stress (PBS) and light stress. In this model, the sub-gap density of states (DOSs) are depicted by exponential band tails and Gaussian deep states within the band gap of a-IGZO. Meanwhile, the surface potential solution is developed with the stretched exponential distribution relationship between the created defects and PBS time, and the Boltzmann distribution relationship between the generated traps and incident photon energy, respectively. The proposed model is verified using both the calculation results and experimental data of a-IGZO TFTs with various distribution of DOSs, and a consistent and accurate expression of the evolution of transfer curves is achieved under PBS and light illumination.
... For the mirrored current programming scheme [11], the current mirror TFTs must be matched accurately to maintain current uniformity in the display, whereas non-mirrored circuits [12] require higher supply voltages and more power consumption. Moreover, the current programming circuits possess a higher settling time [13] which becomes a bottleneck for high switching operations. ...
This article presents a novel voltage-programmed pixel circuit using a-IGZO TFTs to effectively compensate threshold voltage (VTH) variations of driving TFT. The compensation is very important to maintain the pixel brightness of active-matrix organic light-emitting diodes (AMOLED) displays. The proposed pixel design uses four-phase clocking schemes: reset period, VTH detection period, data input period and emission period. To maintain uniformity in phases, the duration of all the phases is taken to be same which results in easier design of the multi-phase clock. Moreover, the switching speed of control signals is as high as 25 kHz, ensuring a high frame rate. The circuit has been simulated extensively, and the operation has been verified using SPICE models of a-IGZO TFT and OLED on Cadence Spectre. The circuit also achieves high contrast as OLED does not emit light in any periods except the emission period. The OLED current error in the proposed design for a VTH variation of 170% is below 0.1%. This circuit also provides a high aperture ratio of 51.6%, which ensures high resolution of AMOLED displays.
... Moreover, ZnO based TFTs possess illumination instability, which results in negative threshold voltage shifts [9]. OLED current is dependent on the threshold voltage of driver thin film transistor [10]. So, the change in threshold voltage leads to change in OLED current, which brings non-uniformity in display brightness and this is undesirable. ...
This work proposes a novel amorphous-indium-gallium-zinc-oxide thin film transistor (a-IGZO TFT) based pixel driver circuitry to compensate for threshold voltage variations of TFTs and OLED current degradations for AMOLED (Active Matrix Organic Light-Emitting Diodes) displays. The proposed circuit uses a three-phase clocking scheme i.e. threshold voltage detection phase, data input phase and emission phase for compensating threshold voltage variations. The circuit has been simulated and verified with cadence spectre using a-IGZO TFT and OLED SPICE models. The proposed circuit is able to maintain the OLED current with a maximum error of 0.68% when the threshold voltage is varied from −3.5 V to 3.5 V.
... On the contrary, the negative values of V represent the positive shift of V th . The mechanism of the proposed circuit is the diode-connecting way [21]; the positive shift of V th is not within the scope of compensation. The results demonstrate that the drain current remains consistent for the cases where V are positive and collapses for the cases with negative V . ...
In this article, we proposed a low-temperature poly-silicon (LTPS) active pixel sensor (APS) circuit with threshold voltage (V
<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub>
) compensation. LTPS thin-film transistors (TFTs) are known to degrade under long-term irradiation and this is quantified. Simulation results and experimental data confirm that the proposed APS circuit compensates the instability of LTPS TFTs and is a promising, high-performance alternative to current amorphous silicon (a-Si) pixel-based X-ray detectors.
... Finally, we summarized key factors of the proposed pixel circuit in comparison to other pixel circuits in literatures [17]- [19] as shown in Table II. The summary confirms that our proposed pixel circuit can provide the full compensation for the threshold voltage shift and the OLED degradation with less number of signal lines and TFTs. ...
... At the same time, by reducing the number of signal lines from the circuit, we could achieve an improved aperture ratio on the single pixel circuit to be applied for low-power high-resolution AMOLEDs. Ref. [17] Ref. [18] Ref. [19] This work Fig. 6. OLED current variations of the proposed circuit as a function of COLED varying from -50 % to +50% of the initial value (= 3 nF). ...
In this paper, we propose a novel voltage programmed pixel circuit based on amorphous-indium-gallium-zinc-oxide thin-film transistors (a-IGZO TFTs) for active-matrix organic light-emitting displays (AMOLEDs). Through the extensive simulation work based on a-IGZO TFT and OLED models, we confirm that the proposed pixel circuit can compensate for threshold voltage variations of TFTs and OLED degradation over wide dynamic range (∼10⁴) of OLED current as well as achieve a high pixel aperture ratio with the suppressed OLED current error rate below 9%.
... However, other issues such as stability, temperature sensitivity and process variations can also limit the overall performance and may even affect the functionality of the circuit. There has been significant effort devoted to the study and modeling of bias induced V T -shift [11], [15], [16] and V T -shift compensation in AMOLED pixel circuits [32]- [35]. We will analyze the sensitivity of drain current on V T -stability and temperature and process variations with the aim of establishing guidelines on the level of accuracy that can be achieved without applying compensation methods (i.e. the intrinsic performance) as well as identify the parameters that contribute most to error and how this can be improved with processing. ...
... The basic working principles of these have been reviewed in [37], [53]. Recent progress in voltage programming has been reported in [35], [54]. Here, the working principle is slightly different from the original idea in that the technique does not capture the cut-off point of a diode-connected transistor. ...
This paper reviews the importance of device-circuit interactions (DCI) and its consideration when designing thin film transistor circuits and systems. We examine temperature- and process-induced variations and propose a way to evaluate the maximum achievable intrinsic performance of the TFT. This is aimed at determining when DCI becomes crucial for a specific application. Compensation methods are then reviewed to show examples of how DCI is considered in the design of AMOLED displays. Other designs such as analog front-end and image sensors are also discussed, where alternate circuits should be designed to overcome the limitations of the intrinsic device properties.
Nowadays, the display industry is a driving force of consumer electronics, including various gadgets like mobile phones, laptops, and HD-TVs. This chapter focuses on thin film transistor (TFT)-based circuits, mainly used to drive an organic light-emitting diode (OLED) present in a pixel of an active-matrix organic light-emitting diode (AMOLED) display. TFTs are much cheaper transistors than silicon-based MOSFETs. However, they face low electron mobility, threshold voltage shift, low on-to-off ratio, and subthreshold slope. An increase in threshold voltage over time may reduce OLED current and can result in a dark pixel. However, the TFT offers a significant advantage in lower cost/cheaper electronics if the challenges can be addressed. TFTs do not require costly processes and costly wafers. One can make TFT-based circuits and devices on a plastic substrate using printable electronics. However, before the product goes to the market, the performance, yield, and lifetime of pixel circuits in particular or displays in general need to be substantially increased. Therefore, as a circuit designer, it becomes crucial to address the challenging issues of threshold voltage shift over time for the display’s uniform brightness. This chapter will focus on design strategy and challenges for designing voltage-programmable pixel circuit.
Since the invention of amorphous indium-gallium-zinc-oxide (IGZO) based thin-film transistors (TFTs) by Hideo Hosono in 2004, investigations on the topic of IGZO TFTs have been rapidly expanded thanks to their high electrical performance, large-area uniformity, and low processing temperature. This article reviews the recent progress and major trends in the field of IGZO-based TFTs. After a brief introduction of the history of IGZO and the main advantages of IGZO-based TFTs, an overview of IGZO materials and IGZO-based TFTs is given. In this part, IGZO material electron travelling orbitals and deposition methods are introduced, and the specific device structures and electrical performance are also presented. Afterwards, the recent advances of IGZO-based TFT applications are summarized, including flat panel display drivers, novel sensors, and emerging neuromorphic systems. In particular, the realization of flexible electronic systems is discussed. The last part of this review consists of the conclusions and gives an outlook over the field with a prediction for the future.
https://ieeexplore.ieee.org/document/9190545
