OFETs unipolar

As compared with film growth at room temperature, an inereased mobility is found for the higher substrate temperature and a lurther inerease is realised by lowering the surface energy with OTS. This increase of mobility was reported for unipolar OFETs [43] and is also valid for these blends. Interestingly, for all treatments balaneed mobilities are found at about 25% C o eontent. 

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. 

The demonstrated change in OFET polarity can be utilized to integrate unipolar p- and n-type pentaeene transistors on a single substrate, without altering the organic semiconductor, or even the device cross section. 

This section will therefore examine several unipolar blend systems that have been employed to improve OFET mobility, to enhance processability especially with a view to printing devices, to study the links between morphology and charge transport, and finally to enable novel fabrication techniques such as gate dielectric self-assembly. We will broadly divide the section into blends based on oligothiophenes and those based on acenes since this covers a large fraction of the materials and concepts investigated to date. 

OFETs. On the other hand, unipolar n- or p-type transistors can be seen as am-bipolar OFETs where the fragile balance in the hole and electron injection and/or transport properties is sufficiently disturbed. Hence, a controlled engineering of the transport and injection properties starting from ambipolar acting OFETs appears to be a promising route to obtain complementary OFETs for CMOS devices. Whether a field-effect transistor acts ambipolar, unipolar p-type or unipolar n-type depends beside the applied voltages on ... 

Fig. 18 Calculated output (a) and transfer (b) characteristics of an unipolar OFET considering constant contact resistances at the source Rg and the drain /Jd terminal. The assumed normalized resistances Rs/i -C VixlL were 0.1V ...

Fig. 19 Circuit of an unipolar inverter (a) and an inverter based on a CMOS-like circuit (b). Electrical losses in the ON state of the unipolar inverter exist, since the OFET is open and hence, a substantial current flows due to the finite resistivity of the resistor. For the CMOS inverter on the other hand only little current can flow at each logical state since one of the employed OFETs is always closed. The resistivities of the involved unipolar p- and n-type OFETs are named R and / 2 respectively...

Fig. 20 Simulated inverter characteristics of a CMOS inverter for different field-effect mobility ratios. In Eq. (11), and )tn are set to zero and thus truly unipolar OFETs have been considered...

Apparently, for Vm = 0 V the entire potential Vdd determines Vout and Vout = OV is calculated for Vjn = Vaj. This is only possible as long as truly unipolar OFETs have been employed with a rather low off current. Then, at the logical 1 and the logical 0, evanescent voltage losses prevail. The situation changes if the used OFETs show weak ambipolar character since the field-effect transistors lose their ability to close completely. This is when ju and/or jUn are not zero. In Fig. 21 simulated characteristics from an inverter are depicted, where ambipolar transistors have been... 

From Fig. 28 it can be concluded, that the voltage losses in the final logical states are rather low, since truly unipolar complementary OFETs have been used (see Fig. 27). This is an advantage if compared to CMOS-like inverters based on ambipolar OFETs because those disclose in the contrary substantial voltage losses. 

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