Pentacene OFETs With Bottom Contacts

Figure 20.1 Sample geometries (a) Schematic contact layout for bottom-contacted pentacene OFETs, and (b) for top-contacted pentacene OFETs, with sample holder and electrical contacts.

Figure 20.4 (a) Results of a two-dimensional simulation of a pentacene OFET with geometric parameters close to the bottom-contacted device investigated with potentiometry, for // = 0.014 cm V s and an effective injection barrier of 0.42 eV, and (b) construction of an injection barrier of 0.73 eV out of the effective barrier of 0.42 eV and the electric field close to the source contact, as obtained in the simulation for a gate voltage of Uq = -30 V. 

Comparing the potentiometry measurements in Figure 20.8 obtained on top-contacted pentacene OFETs with the data for the bottom-contacted sample in Figure 20.2, the most striking difference is the absence of a substantial potential drop close to the source contact. The reduction of the contact resistance... 

The OFETs using inorganie gate dieleetries were prepared on a heavily doped silieon substrate, whieh also serves as the eommon gate eleetrode for all fabricated devices. The semiconducting film consists of evaporated pentacene, which is commercially available. In Figure 18.1 the transistor architecture with bottom gate and bottom drain and source contacts is shown. 

In the following, we present the results of charge transient spectroscopy performed on the bottom contacted pentacene OFETs, a variant of DLTS where the current transient is integrated, yielding a charge transient [43, 44]. In combination with capacitance DLTS, this technique can also provide information on the depth profile of the trap distribution [45]. 

If the density of states (DOS) of the trap states were broadened, the charge transient Q(t) would not follow a single exponential rise as in Eq. (6), resulting in turn in broadened QTS traces with respect to the fit based on Eqs. (5), (6). Such a behaviour was observed for polymer-based diodes [20] and for phthalocyanines [46], but for our bottom-contacted pentacene OFETs, we found no evidence of a broadened DOS of the trap states with a corresponding distribution of de-trapping rates. 

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