Ambipolar operation

Figure 17.1 Sketch for the unipolar and ambipolar operation regimes of an organic field-effect transistor.

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... 

For several important applications in plastic optoelectronics, including the possibility of electrically pumped organic lasers, it would be very important to achieve an ambipolar operation in OFETs, with high electron and hole mobilities. Gate-controlled electroluminescence from organic small-molecule thin-film transistors... 

Kim D, Huang J, Rao BK et al (2006) Pseudo Y-junction single-walled carbon nanotube based ambipolar transistor operating at room temperature. IEEE Trans Nanotechnol 5 731-736... 

The principle of operation of ambipolar drift field imagers is presented in GB-A-1488258. 

Depending on the details of the carrier distributions and densities, majority and minority carrier parameters have different importance. Under ambipolar transport conditions, which often hold in solar cell operation, the ambipolar urproduct is the relevant transport parameter. This has a somewhat complicated dependence on the minority- and majority-carrier parameters, but in many cases it can be approximated by the minority-carrier uz product. However, when the absorber material is not sufficiently doped, or if the absorber layer is depleted, a distinction between minority and majority carriers becomes difficult, and a more in-depth analysis may be needed, as discussed by Bube (1992). 

Figure 17.12 (a) Electrical circuit for an ambipolar inverter, (b) The two operation regimes (positive and negative supply voltage) for ambipolar and complementary inverters. 

Here I, represents the drain current and ju, jUp the respective electron and hole mobility. C defines the area capacitance of the insulator. The channel geometry is defined by the channel width W and length L. The ambipolar range, described by Eq. (3), is only valid as long as both electrons and holes can be injected and further transported in the active layer of the transistor. However, in most cases the injection and/or the transport in the transistor channel are suppressed for one charge carrier type. In that case, the FET operates only in the unipolar and saturation range as described by Eqs. (1) and (2). 

Electron Loss Processes. Ambipolar diffusion to the walls is an important mechanism for loss of electrons from the discharge. It is appropriate to enumerate other electron loss mechanisms and then balance these against the various production mechanisms operative in a sustained discharge. 

Ambipolar behavior has also been observed in BBL and poly(thiophene-3-propionic acid, anunoninm salt).[297] 51a showed n-channel mobihties of 0.04-0.06 cm V s and 0.02-0.03 cm V s for p-channel operation. 51c showed valnes of 0.5-0.7 cm V s for n channel and 1.2-1.7 cm V s for p-channel with on/off ratios between 2 and 50 for devices operated in air. A similar mechanism involving ion-modulated electrochemical conduction, as described above for water soluble phthalocyanines was proposed by the authors. 

Although Eqs. (12.4) to (12.6) elucidate the nature of the driving force operative during creep, they do not shed any light on how the process occurs at the atomic level. To do that, one has to go one step further and explore the effect of applied stresses on vacancy concentrations. For the sake of simplicity, the following discussion assumes creep is occurring in a pure elemental solid. The complications that arise from ambipolar diffusion in ionic compounds are discussed later. The equilibrium concentration of vacancies Cq under a flat and stress-free surface is given by (Chap. 6)... 

Figure 1.1a shows schematically the operation of a membrane that is permeable to hydrogen molecules (corresponding to a porous membrane or a dense material in which molecules dissolve and diffuse) or to neutral hydrogen atoms (corresponding to a material in which hydrogen dissolves dissociatively, as in a metal). Figure 1.1b shows schematically how a mixed proton-electron conductor performs the same process by so-caUed ambipolar diffusion of both protons and electrons in the same direction to maintain electroneutrality and zero net current. 

The differenf operating regimes of an ambipolar tiansisfor are shown in Figure 13.8a. Under cerfain biasing conditions, fhe charmel current in an ambipolar transistor can be approximated by an equivalent circuit comprising of two unipolar, i.e., a p-charmel and an n-channel, transistors connected in parallel as depicted in Figure 13.8b. 

Smits et al. have derived analytical equations that describe transistor operation under ambipolar and unipolar regimes [110]. The model has been successfully employed for fhe sfudy of ambipolar organic transistors based on a narrow bandgap conjugated molecule, and more recently for the description of light-emitting organic transistors [117]. 

Figure 13.2. Relationship for ambipolar diffusion in an MX compound effective diffusion coefficient versus grain size when both lattice and grain boundary diffusion are operative.

This chapter concentrates on organic bipolar transistors, including details about the basic operation principles, device configurations, and processing methods, and describing the various strategies that have been applied to achieve ambipolar transport. Touching upon small molecule-based FETs and the hybrid approach, the main focus will be on polymer-based bipolar transistors since they can provide one of the ultimate solutions for simple, low-cost fabrication of flexible bipolar FETs. 

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