Flexible IGZO TFT SPICE Model and Design of Active Strain-Compensation Circuits for Bendable Active Matrix Arrays

Abstract

The detailed measurement and characterization of strain induced performance variations in flexible InGaZnO thinfilm transistors (TFTs) resulted in a Spice TFT model able to simulate tensile and compressive bending. This model was used to evaluate a new concept, namely the active compensation of strain induced performance variations in pixel driving circuits for bendable active matrix arrays. The designed circuits can compensate the mobility and threshold voltage shifts in IGZO TFTs induced by bending. In a single TFT, a drain current of 1 mA varies by 83 µA per percent of mechanical strain. The most effective compensation circuit design, comprising one additional TFT and two resistors, reduces the driving current variation to 1.1 µA per percent of strain. The compensation circuit requires no additional control signals, and increases the power consumption by only 235 µW (corresponds to 4.7 %). Finally, switching operation is possible for frequencies up to 200 kHz. This opens a way towards the fabrication of flexible displays with constant brightness even when bent.

A 6-TFT Charge-Transfer Self-Compensating Pixel Circuit for Flexible Displays

“…8(b). All three cases with the pixel circuit being laid flat, and under tensile strain and compressive strain showed a slight variation in the initial output current values due to mobility change under bending [18], [20]. After 24-hour bias stress, all the pixel circuits demonstrated the correct compensation behavior with less than ±1% variation (Fig.…”

“…Previous reports [13], [18] have shown that, under tensile strain, the TFTs have slightly higher carrier mobility and much slower bias-induced degradation. On the other hand, when the TFTs are bent under compressive strain, they have slightly lower mobility and relatively faster bias-induced degradation.…”

“…The mobility of the TFTs was also slightly adjusted due to bending. When placed under tensile strain, 0.3% was added to the TFT mobility, and when under compressive strain, 0.3% was subtracted from the original value [18], [19]. In order to maintain the desired correlation between V T,2 and V T,0 , both T 2 and T 0 were assumed to have the same orientation and placed in close proximity in the layout.…”

Design of bendable high-frequency circuits based on short-channel InGaZnO TFTs

“…Although the tolerable tensile strain and minimum bending radius can be improved to 1.7% and 25 μm by using encapsulation layers [10], thin substrates [11], or ductile materials [12], circuits have to consider bending effects. Therefore a SPICE level 61 transistor model was modified to simulate the influence of strain [13].…”

Research Progress on Flexible Oxide-Based Thin Film Transistors

“…OLEDs are considered as one of the most promising next generation display technologies due to their superior performance such as high efficiency, wide viewing angle, low power and small thickness [20][21][22][23][24][25][26][27][28][29][30][31][32][33]. Recently, the oxide-based TFTs driving OLED have become the most promising technology to realize really flexible displays [34][35][36][37][38][39][40][41]. The technological advance in TFTs is a significant factor for promoting the development of such displays [42][43][44][45][46][47][48][49][50].…”