Subthreshold slope

FIGURE 9.8 (a) An example of the BTS data for driver a-Si H TFT. The inset shows the threshold voltage and subthreshold slope shift versus the stress time. (From Kim, J.-H., Lee, D., and Kanicki, J., Proc. 22nd Int. Display Res. Conf.,601,2002. With permission.) (b) The output current density changes versus threshold voltage shifts of a-Si H TFT driver for our pixel electrode circuit. (From Kim, J.-H., Lee, D., and Kanicki, J., Proc. 22ndInt. Display Res. Conf., 601, 2002. With permission.)... 

Fig. 15.6. Characteristic of a PVA-patterned pentacene TFT (W = 240 pm, L = 10 pm), showing relatively poor subthreshold slope, but relatively good on/off ratio in the...

Nunes et al. wanted to find out whether transistors made with PHS and other organic dielectrics could show the excellent performance already reported for the pentacene transistors. This will help to determine whether other polymers with improved characteristics can be found. They investigated the effect of PHS on important device characteristics, threshold voltage and subthreshold slope. The dielectrics they investigated and their dielectric properties are shown in Fig. 6.22. Measured mobilities in transistors fabricated... 

There were not remarkable differences of the electrical performances of a-Si H TFTs as planarization layers of SOG/SiNx and SOP/ SiNx. For both TFTs, the off current is about 10-A and the on current at gate voltage of 20 V is about lO A at a drain voltage of 10 V, resulting in an on-off current ratio of lOT We obtain a subthreshold slope 0.62 0.64 V/decade, demonstrating a sharp device turn-on. We also obtain a threshold voltage and mobility of 0.3 0.6 V and 0.27 0.45 cm /Vs, respectively, in the saturated regime. 

Mobility (cmV Vs) Threshold Voltage(V) Subthreshold Slope(V/dec.) Off Current(A) ... 

Fig. 8. Relative field-effect mobility, subthreshold slope and threshold voltage plotted as functions of stress time. TFTs were measured as bended except 0-hour stress time. The data at 0-hour stress time were measured from the flattened-TFTs before bending.

TFTs in the bended condition (R = 5 mm) were measured at arbitrary intervals as shown in Fig. 11 (left) and 12. A zero hour duration time meant that TFTs were measured as flattened before bending them. The mobility change ( ife/nfeo) of 0.92, the subthreshold slope change (SS/SSo) of 1.04, and the threshold voltage shift (AVth) of 1.03 are almost same as those of TFT employing the single acryl passivation. As the 50 nm-thick SiNx and 3 )can-thick acrylic polymer were employed as the passivation, the position of the neutral plane may shift from mid-surface toward the TFT-films. These results are similar to that of a single acrylic polymer passivation. 

One concern about any such treatment is that the donating or withdrawing group will form a new energy level (or distribution of levels) in the semiconductor. These energy levels may impact the device performance by scattering carriers or degrading the subthreshold slope. 

Fig. 6.4. A schematic showing the subthreshold voltage/current characteristic and the subthreshold slope extraction. The threshold voltage can be taken as the point at which the current deviates from the exponential character expected from purely diffusive transport.

The subthreshold slope is the inverse slope of the log//) vs Vqs measured below threshold. It is typically reported in base 10 logarithmic units of mV/decade (i.e. mV of gate bias for decade of drain current modulation). Smaller values in these units correspond to a larger slope, which is generally more desirable. Fig. 6.4 shows the extraction of subthreshold slope. 

Current in the channel in subthreshold is a function of charge carrier concentration in the channel. The best subthreshold slope which can be observed in an FET device at room temperature is 60mV/decade, which is the slope at the edge of a Fermi distribution when it is convolved with an abrupt density of states. Because organic semiconductors exhibit a gradual rise in the density of states at the channel edge and not all carriers are equally mobile, the convolution produces a shallower rise in the carrier density and the observed subthreshold slope is worse than this value. 

Measurement of the subthreshold slope is simple for a Vos of interest, the slope can be taken at the steepest point of the log-linear //j vs Vqs curve. 

The poor stability of some semiconducting polymers and their sensitivity to external factors often make it difficult to deduce true intrinsic properties of a certain material [101,102]. Most commonly, the off-currents in a TFT are governed by extrinsic factors and it also has an impact on subthreshold slope, threshold voltages, and bias stress. In fact, this sensitivity prompted some researchers to use OSC materials as gas and chemical sensors in TFT devices [103,104]. The observed selective sensitivity is related either to the chemical nature of the semiconductor functionalities or the interaction of the grain... 

A major problem in the use of OTFTs is the instability of organic semiconductors to light, atmospheric oxygen, and humidity, or a combination of these stress factors, which limits the shelf life of these components. The exposure of OTFT devices to the environment typically results in devices with high off currents, large and unstable onset voltages, and large subthreshold slopes. Encapsulation of the devices can prevent the penetration of air and humidity and thus effectively enhances the environmental stability of OTFTs. 

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