Bipolar FETs

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. 

Figure 16.2 Illustration of all operating regimes for a bipolar FET. (Reprinted with permission from Ref [20]).

The transfer and output curves of a bipolar FET are more complex [16]. The output curve, illustrated in Figure 16.4a, shows the superposition of the standard saturation behavior for one of the charge carriers at higher Vg and a superlinear current increase at lower Vg and high Vjs due to the injection of the opposite charge carriers. The transfer curve (Figure 16.4b) has a unique V-shape where the left arm represents the hole transport and the right arm corresponds to the electron transport. 

The first bipolar FETs based on a blend were fabricated by Tada et al. [53, 54] by mixing the electron conducting dye N,N-bis(2,5-di-terf-butylphenyl)-3,4,9,10- pery-lene dicarhoximide, whose chemical structure is similar to PTCDI-C13H27 (shown in Figure 16.11a), with p-type poly(3-dodecylthiophene) in chloroform and spin casting this mixture on a Si/Si02 substrate with prepatterned Ti/Au electrodes. Although bipolar behavior was observed, the effective mobilities were extremely low (10 cm (V s ) for holes and 10 cm (V s ) for electrons). 

Another example of all-polymer bulk heterojunction bipolar FETs was demonstrated recently by Szendrei et al. [15]. The limited number of polymer blend bipolar FETs is due to the scarcity of high-performing n-type polymers. The recent... 

Figure 16.18a and b shows the transfer characteristics of these polymer bipolar FETs for positive and negative Vjs, respectively. In both cases the transfer shows a symmetric shape indicating the presence of balanced electron and hole populations in the channel. The saturation field-effect electron and hole mobilities were calculated to be 4xl0 cm at Vds = +30V and a p-type mobility of... 

X 10 cm (V s ) at Vds= 30 V. These mobilities are the highest balanced mobilities reported so far for solution processed all polymer bulk heterojunction bipolar FETs. The balanced FET mobilities indicate the presence of sufficient percolation pathways for both charge carriers in these polymer blends. 

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