Pentacene threshold voltage

Fig. 6.10. Statistical distribution of (a) mobility, (b) threshold voltage, and (c) subthreshold swing of 32 pentacene TFTs on 32 individually and manually processed substrates.

Fig. 13.9. Distribution of the threshold voltage VT for 60 pentacene FETs with L = 20 pm spread uniformly over a 150-mm wafer (one FET per process evaluation module).

A 0.5-pm film of parylene-C was deposited on pentacene TFTs at room temperature. TFT characteristics were measured and compared before and after the deposition without patterning of the parylene. The bottom-contact TFT (W/L = 400/80 pm) characteristics before and after parylene passivation are compared in Fig. 15.9. Field-effect mobility decreased by 50% from 1.6 cm2 V-1 s-1 to 0.80 cm2 V 1 s 1 whereas the threshold voltage stays the same. 

Transfer characteristics of TC pentacene TFTs have also been measured at different temperatures in the range between 205 and 300 K (see Fig. 6.12). As shown in Fig. 6.13, the field effect mobility and threshold voltage, evaluated from the slope of the linear portion of the transfer characteristics, monotonically increase with temperature. 

Fig. 6.13. Field effect mobility, and threshold voltage Vfh versus temperature of TC pentacene TFT [154].

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... 

The device with interdigitated drain and source contacts of 46.8 cm channel width and of 20 pm channel length, characterised at ambient atmosphere, could drive drain currents of about -6.8 mA directly after the pentacene deposition under ambient atmosphere = 23400). The on-off ratio reached a value of 10" and the threshold voltage was 12.3 V. After that the sample was encapsulated by a 1.5 pm thick Teflon layer. To avoid damage of the organic semiconductor, a low deposition rate of 21 nm/min was selected by a thickness gradient of less than 10% [19]. 

Figure 24.14 (a) Drain current as a function of Fp for a UV modified pentacene OFET in the electron accumulation mode. Inset threshold voltage shift between the and 8 successive measurement cycles at Fg = 0V. (h) Electron and hole accumulation mode for a UV modified OFET during the 1 and after the 8 measurement cycle. The OFETs were reahsed using Ca electrodes. 

Surface treatment has also been used to modify the threshold voltage as well as measured mobility in pentacene and Cgo TFTs [60]. These transistors have a heavily doped silicon/silicon oxide gate dielectric structure where alkyl, aUcylamine, and fluoroalkyl silanes are used to modify the Si02. Evaporated pentacene and Cgo form the active p- and n-type semiconductors. The experimental effect of these monolayer treatments is to alter Vj and effective mobility dramatically (see Table 3.2.3). For pentacene, the mobility decreases from -F, -CH3, untreated, -NH2, with a similar shift in Vj from 17 to -11V. The opposite trend is observed for Cgo, in which mobility is largest for the untreated material and smallest for the fluorinated SAM. In the case of Vji the alkylamine SAM shows the lowest VjOi 5.3 V. The underlying reasons for these trends are not completely understood. What is intriguing is how dramatic... 

Effect of SAMs on Mobility and Threshold Voltage for Pentacene (p-Type) and C(,o (n-Type) Organic Semiconductor TFTs... 

FIGURE 6.4.2 Measured linear-region drain-current data (cnrved line, = -0.1 V) and the tangent at the point of maximnm slope for the same pentacene OTFT as in Fignre 6.4.1. The tangent allows the threshold voltage to be calcnlated as +6.5 V. 

FIGURE 6.4.8 (See color insert following page 468.) Threshold voltage and mobility uniformity maps for a 1- x 1-cm 200-OTFT array using pentacene devices with a SiOj gate dielectric. 

Fig. 5.7. (a) shows the QSCV of a pentacene OFET doped using a surface dipole method [68]. (b) shows the stationary and mobile charges involved in the process. The same general principle also applies to the use of electronegative/electropositive silanes and other SAMs on gate dielectric layers for manipulating the threshold voltage. 

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