## 3 DNA as Functional Layers for OTFTs  
The basic structure of OTFTs is composed of three functional layers: an insulating layer (gate dielectric), a semiconducting organic layer (\(p\)-type, \(n\)-type, or ambipolar), and three conducting electrodes (source, drain, and gate electrodes) [20]. The charge transport OTFTs is turned on when the gate field increases so that carriers accumulate at the interface between semiconductor and insulators. Hence, the performance of the OTFT critically relies on the physical and chemical performance of the dielectrics. When the dielectric layer has tunable polarity, the carriers can be depleted or accumulated more at the interface, allowing a memory effect and thus the OTFT memory device. The dielectric materials in OTFT device can be classified into three categories: self-assembled monolayers (SAMs), inorganic, and organic materials [21]. Biocompatible and biodegradable dielectrics are applied to make the OTFTs sustainable and environmentally friendly. For instance, [PERSON] et al. developed the synthetic polypeptide poly (\(\gamma\)-methyl-L-glutamate) (PMLG), as the gate dielectric to prepare the organic field effect transistor memory as early as 2001 [22]. [PERSON] et al. [23] and [PERSON] et al. [24] demonstrated OTFTs based on chicken albumen as the gate dielectric. Recently, base pairs of DNA have also been applied as gate insulators in OTFTs as presented below. The repeating units of DNA are nucleotides and each nucleotide is composed of nucleobases, including cytosine (referred by C), and thymine (T), adenine (A), and guanine (G). Among these base pairs, the adenine (A) and guanine (G) are found to have high dielectric constants, low dielectric losses, and high breakdown strengths, which are promising for being the dielectric of OTFTs. As shown in Figure 3, [PERSON] et al. [2] found that the base pair A and G based OTFTs exhibited the carrier mobility, operation voltage and the ON/OFF current ratio as 0.012 cm\({}^{2}\cdot\)V\({}^{-1}\cdot\)s\({}^{-1}\), 15 V, 10\({}^{2}\), respectively, which needs to be the improved with a further optimized structure. Such a demonstration in applying the base pairs in OTFTs indicated a possibility to create 'all green' electronic devices. Besides the application in basic OTFTs, bio-organic polymers derived from DNA have been widely applied to prepare photo-induced write-once read many-times memory devices [25], memristive memory devices [26] and OTFT memory devices [27; 28]. The mechanism of the memorizing effect  
Figure 3: **(a)** Schematic configuration of the Bio-TFT device and chemical structures of natural materials; **(b,c)** Transfer and output characteristics of OTFTs based on four alternating layers of guanine and adenine as gate dielectric; **(d,e)** Transfer and output characteristics of OTFTs based on six inorganic layers of adenine and guanine as gate dielectric (Reproduced with permission from [1], [PERSON], 2010). caused by the DNA gate dielectrics remains poorly understood. Some of the researchers attribute the hysteresis behavior to the mobile ion impurities [29; 30]. However, [PERSON] et al. [19] demonstrated the memory window still exists even in a much lower temperature region, as shown in Figure 4. Since movement of ions may not explain such hysteresis, the memorizing effect could be attributed to the quasi-ferroelectric polarization originates from the orientation of the dipoles existed in the DNA complex. Besides acting as the gate dielectric, DNA derivatives have also been employed as the insertion layer in the preparation of OTFTs. Such an insertion layer is developed to enhance the charge injection and the carrier mobility of OTFTs. In fabrications, the renewable, biodegradable polyelectrolyte DNA or DNA-CTMA complexes are easily deposited by solution processes using aqueous solvents or organic solvents, respectively [31; 32]. For example, [PERSON] et al.