Threshold voltage temperature dependence

Fig. 11. The time dependence of the threshold voltage shift for a 20 V bias applied to an a-Si H based metal-insulator-semiconductor device for various temperatures (Jackson et al., 1989a).

The next step was the introduction of the temperature dependence of the relevant parameters. A linear approximation was chosen for the temperature dependence of the threshold voltage ... 

Figure 7. Temperature dependence comparison of threshold voltage drift and dc conduction data. Conditions area = 0.01 cm AH = 0.7 eV I —A> PI-2 and , PI-3 I/pp—AVrjpjc data.

Yoshida et al. recently disclosed an alternative method that allowed them to produce stable suspensions of gold nanoparticles (1-2 nm in diameter) in nematic liquid crystals [315]. They used a simple sputter deposition process, which allowed them to prepare thin liquid crystal films of well-dispersed gold nanoparticles in both 5CB and E47 (available from Merck) with a nanoparticle size depending on the used nematic liquid crystal. Unfortunately, the authors did not provide any details on whether the nanoparticles were capped with a ligand or bare, non-coated particles, which makes it difficult to assess and compare the reported thermal as well as electro-optic data. However, very similar effects were found as a result of nanoparticle doping, including lower nematic-to-isotropic phase transition temperatures compared to the used pure nematics as well as 10% lower threshold voltages at nanoparticle concentrations below 1 wt% [315]. 

The electro-optical characteristics of multiplexed STN-LCDs exhibit a significant dependence on temperature. This has to be compensated in order to avoid variations of the optical performance of the display with temperatures. This can be achieved electronically. However, this problem can also be solved by the use of optically active, chiral dopants. The capacitive threshold voltage of a chiral nematic mixture depends on the pitch of the mixture ... 

Figure 6. Typical ZnO-TFT characteristics with the channel layer deposited at room temperature by rf magnetron sputtering, for a TFT with a width-to-length ratio of 1.4. (a) Transfer characteristics for Vds = 20 V. The on/off ratio is 2x10. The ZnO-TFT operates in the enhancement mode with a threshold voltage of 21 V and a saturation mobility of 20 cmVVs. (b) Output characteristics for a ZnO-TFT. The saturation was about 230 pA under a gate bias of 40 V. The ZnO-TFT exhibits hard saturation, evidenced by the flatness of slope of each Ids curve, for large Vds- The dashed line represents the saturation drain current that follows an exponential dependence on the voltage.

To investigate moisture permeation phenomenon dependent on the passivation materials, a humidity-temperature test was carried out at 65 °C and 90 % relative humidity (RH). We compared the electrical performances of the SiNx-passivated and the acryl-passivated TFTs after 3-hour humidity-temperature test. Fig. 9 shows transfer curves before and after the humidity-temperature test. We could not find any significant shift of the threshold voltage in the SiNx-passivated TFT. The SiNx passivation did not allow moisture to p>enetrate into TFT. On the other hand, threshold voltage shifted as large as 28 V for the aayl-passivated TFT. The acrylic polymer could not keep the moisture from permeating. Other researchers have reported the effect of moisture on a-Si FI TFT. 

The rapid drop of p at T < 160 K and the vanishing of the anisotropy indicate a crossover to the trap-dominated regime, where p < p. Note that, for the device whose conductivity is shown in Figure 2.1.18, the density of shallow traps, which can be estimated from the linear temperature dependence of the threshold voltage, Vg (7), is relatively low 10 ° cm ). For devices with higher the crossover between the intrinsic and trap-dominated regimes occurs at higher temperatures. As a result, devices with > 10" cm demonstrate an activated temperature dependence of p ff even at room temperature, and the effective mobility is smaller than p [29],... 

The temperature dependence of the threshold voltage of p-type air-gap rubrene single-crystal OFETs is shown in Eigure 2.1.28 [38]. At room temperature, the threshold voltage is small it corresponds to the deep-trap density on the pristine crystal surface 7 x 10 cm-. The concentration of deep traps increases quasilinearly with cooling up to 2 x 10 ° cm- at 150 K. Assuming that the thickness of the conduction channel does not exceed one or two molecular layers, the three-dimensional density of traps near the surface at 300 K can be estimated as 2 x 10 cm-°. 

FIGURE 2.1.28 Temperature dependence of the threshold voltage of a rubrene OFET measured along a and b axes in the basal plane of an orthorhombic single crystal. (From Podzorov, V. et ah, Phys. Rev. Lett., 93, 086602, 2004.)... 

The composition C3S is an organic semiconductor which follows Ohm s law over many decades of current up to a temperature-dependent threshold voltage (1-lOV), where the current surges, independent of the applied voltage [123]. The compound is prepared by passing CS2 vapor over an incandescent tungsten filament [123]. 

From this equation, Ey (T)/E- -(0) increases monotonically from unity at T=0K to 1,33 at T=Tc. This behaviour differs from one observed in the CDW materials, where Ey exhibits a divergence at T=Tc and a minimum slightly below Tq, which results from an increase in Ey at low temperatures, due to phase fluctuations. Hence, one should expect to observe similar properties of the SDW current-carrying state to ones of the CDW nonlinear current-voltage characteristics, accompanied by broad and narrow band noise, with sharp threshold fields, frequency-dependent conductivity, interference effects between the ac voltage generated in the sample, and an external rf field, hysteresis and memory effects etc. 

FIGURE 16.14 Temperature dependence of the threshold voltage. Inset power exponent, f vs. T for the R-hel PA fiber and for conventional iodine doped PA fiber (diameter 16 nm).(From Park, J.G., et al. Synth. Met, 135-136, 299, 2003.)... 

Fig. 4. Typical dependence of threshold voltages on temperature and viewing angle.

For a display without temperature compensation if we assume 9V/3T/ave 0.2%/ C over the range 0 C to 50 C a value representative of LC mixtures with low temperature dependence of voltage threshold,then Mx 0.9, M=0.73 and from Eq. (6) the multiplexing limit for a direct view display without temperature compensation will be N=ll. 

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