Polarization and charge transport in polymer and 2-D material based field-effect transistors
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This work investigates new semiconductors that are currently being considered for use in flexible electronics. It is critical to understand charge transport physics in field-effect transistors (FETs) to get better device performance and integrate them into commercial products. The charge transport studies include understanding of effect of wide-range of applying voltage (sub-threshold and above threshold operation), dominant charge transport mechanisms under specific conditions (band, band-like and hopping transport), and of polarization effect (polaronic and non-polaronic transport). Charge transport in high-mobility diketopyrrolopyrrole (DPP) co-polymers FETs is investigated. Sub-threshold regime conduction, including diffusion- and drift-limited regimes, are accurately modeled with above threshold regime. Based on modeling results, the realistic density of states (DOS) curve of polymer FETs in a wide range of gate voltages is calculated. From modeled sub- and above- threshold regime mobility data, dominant charge transport mechanisms under specific conditions are investigated. Shallow states charge transport is well-described by multiple trap and release (MTR) transport, while hopping transport models such as variable range hopping (VRH) or Gaussian disorder based model (GDM) can describe deep states charge transport. The transition between the conduction regimes is a function of temperature and carrier density. In addition, a polarization effect from polar molecules in both atmosphere and dielectric on the electronic properties of polymer FETs is demonstrated. High-k and low-k surface dielectric devices are measured in polar and non-polar atmospheres. Dipoles in both conditions affect conduction in polymer FETs, but have different aspects, such as uniformly shifted DOS or only shallow states shifted DOS. The improved electrical characteristics of graphene monolayer sheet FETs with fluoropolymer capping is explained via measurements under polar vapor flow, which is reversible and non-destructive. It is found that the higher dipole moment of polar molecules corresponds to better improvement of electrical properties of graphene FETs, including the Dirac voltage shift, mobility and residual carrier concentration. In addition, a similar experiment is applied to graphene nano-ribbon (GNR) FETs. GNR FETs, which have high on/off ratio but some degraded electrical characteristics due to a considerable number of edge defects, also show a highly improved performance under polar vapor.