Bias-stress

FIGURE 9.2 Schematic diagram of (a) 1- and (b) 2-a-Si H TFTs pixel electrode circuits, and (c) bias stress result for circuit (b). Control signal waveforms for 2-a-Si H TFTs pixel electrode circuits are also included in (b). 

Fig. 5.1. Example of successive transfer curves obtained if bias stress is due to charge trapping in immobile states (a) and if bias stress is due to the formation of shallow donor-like states...

If bias stress occurs on time-scales typical of device characterization ( 0.1 s to a few tens of seconds), it causes the I-V curves to display hysteresis and nonideal shapes [7]. As a result, device properties often used to characterize the semi-... 

As with crystalline organic semiconductors, there are only few systematic and quantitative studies of bias stress in polymeric TFTs. For example, it is still unclear whether in all polymers a positive VG leads to positive Vj shifts [7, 18]. Here we will concentrate on negative AVT after application of a negative Vg, which is nearly universally observed in p-type devices with a number of semiconductor-dielectric combinations. In the next section of this chapter we will focus on studies of poly-fluorene and polythiophene TFTs, because these are the polymer devices for which bias stress has been most thoroughly characterized in recent years. 

Fig. 5.3. Amount of bias stress, measured as fractional current decay after a 100 ms gate pulse (Vc = -20 V), as a function of temperature in PQT-12.

Dependence of Bias Stress on Operating Conditions Lifetime Predictions... 

The recovery rate of the reversible traps determines the duty-cycle cut-off below which only long-lived traps contribute to bias stress. At higher duty cycle, the current decay is the result of the simultaneous interplay of fast and slow trapping. 

Because the electrophoretic display effect that is used is multi-stable, the row drivers are only operational during an image update. This is different from the commonly used LC display effects that need to be driven continuously during use of the display. In view of bias stress effects, this makes the electrophoretic display effect ideal for integration of row drivers in organic electronics. 

Understanding and controlling stability is a critical issue for display applications. A simple bias stress test (Vg = —40 V, Vos = —40 V) was conducted on pentacene TFTs for different times. Figure 15.14 shows the TFT characteristics before and after the bias stress. A few volts shift is observed in the threshold potential after a 20-min bias stress. 

Fig. 15.14. Bias stress test of a drive TFT. A constant bias of Vg = Vds = —40 V was applied for different times. Threshold potential shifts of a few volts are seen after 20 min continuous...

T. Sekitani, S. Iba, Y. Kato, Y. Noguchi, T. Someya, T. Sakurai, Suppression of DC bias stress-induced degradation of organic field-effect transistors using postannealing effects, Appl. Phys. Lett. 2005, 87, 073505. 

A. Salleo, R. A. Street, Light-induced bias stress reversal in polyfluorene fhin-film transistors, ... 

R. A. Street, A. Salleo, M. L. Chabinyc, Bipolaron mechanism for bias-stress effects in polymer transistors, Phys. Rev. B 2003, 68, 085316. 

S. J. Zilker, C. Detcheverry, E. Cantatore, D. M. d. Leeuw, Bias stress in organic fhin-film transistors and logic gates, Appl. Phys. Lett. 2001, 79, 1124-1126. 

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