Electron-accumulation mode, OFET

Illustrated in Figure 24.4 is the output characteristic of a pentacene OFET with Au drain-source electrodes and a 200 nm Si02 dielectric [32]. The OFET exhibits unipolar p-type behaviour with a hole mobility = 0.165 cmWs, a threshold of = -4.5 V as well as an On/Off ratio of >10. These parameters have been derived from the respective transfer characteristics. The absence of an s-shaped feature in the linear range of the characteristic indicates ohmic contacts between the Au electrodes and the pentacene active layer. This is attributed to the good matching of the ionisation potential of the organic semiconductor and the Au work frmction. However, employing a Ca drain-soirrce metallisation, with an otherwise identical OFET device structure, the transistor did not exhibit any current in the electron accumulation mode. This is unexpected, since the metal work frmction is well matched to the electron affinity of pentacene. 

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. 

In Eq. (6) L is the channel length while Mi,2 re either the mobility of electrons (1 —> n) and holes (2 —> p), respectively, or vice versa that of holes (1 p) and electrons (2 —> n). The former holds for OFETs driven in electron accumulation mode ( electron accumulation mode we define as Vg > OV following the... 

Fig. 17 Transfer characteristics of pentacene-based OFETs differing by the used source/drain metallization only. Device structure p++-Si/200nm SiO2/120nm PMMA/50nmpentacene/50nm source/drain metal. Vg (open squares, filled squares) is the gate voltage for the n-type and the ambipoltir transistor measured in electron accumulation mode at Vj = 80 V Vg (open circles) is the gate voltage for the p-type transistor measured in hole accumulation mode at V

Contemporary OFETs are based on undoped organic semiconductors, and mobile charges in these devices must be injected from the metallic contacts. These devices can potentially operate in the electron- and the ftote-accumulation modes, depending on the polarity of the gate voltage (the so-called ambipolar operation). Often, however, the injection barrier at the contact or the held-effect threshold for either n- or... 

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