Device physics and transistor modelling:
A TFT compact model needs to be developed that allow expedient simulation of analog and digital circuits for the design of an all integrated system along the lines discussed in the next Section. The models have to be physically based so as to minimise the number of fitting coefficients and simple device parameter extraction. As we know, oxide TFTs, e.g. IGZO TFTs, can exhibit a high mobility even when fabricated at room temperature. This is mainly due to ionic bonding structure which is insensitive to bonding angle disorder. However, this class of channel materials has a compositional disorder. This makes a potential barrier above the conduction band minima (Em), suggesting percolation conduction when electrons are released into the conduction band. Moreover, there are localized tail states within the gap states, implying trap-limited conduction (TLC). In particular, the oxide semiconductor has a shallow slope of the tail states (kTt) ~ 20meV, smaller than the thermal energy (kT) at 300K, leading to different mobility behaviour.
Based on percolation and trap-limited conduction, the field effect mobility model needs to be modeled. For the large signal and small signal behaviours, we incorporated resistors and capacitors into an equivalent model for TFTs. Here, a RC product can explain a channel formation time. So far, this has been used in an empirical way. However, this approach fails to express ‘Non-Quasi-Static (NQS) dynamic behaviour’ while it is enough to reproduce a ‘Quasi-Static (QS) dynamic behaviour’. At the same time, it is not able to capture physical/geometrical mechanisms (e.g. trapping on localised traps and related channel-length dependent channel formations) and temperature dependency on Quasi-Static dynamic behaviour either. So, it is now required to develop a fully physically-based dynamic model for oxide TFTs while addressing both NQS and QS behaviours. If we got this, it would be breakthrough in this area. So, I would like to make this breakthrough.
Related key publications:
- Sungsik Lee, Arokia Nathan, Sanghun Jeon, John Robertson, “Oxygen Defect-Induced Metastability in Oxide Semiconductors Probed by Gate Pulse Spectroscopy,” Scientific Reports 5, 14902 (2015).
- Sungsik Lee and Arokia Nathan, “Conduction Threshold in Accumulation-Mode InGaZnO Thin Film Transistors,” Scientific Reports 6, 22567 (2016).
- Sungsik Lee, Khashayar Ghaffarzadeh, Arokia Nathan, John Robertson, Sanghun Jeon, Changjung Kim, I-Hun Song, and U-In Chung, “Trap-limited and Percolation Conduction Mechanisms in Amorphous Oxide Semiconductor TFTs,” Applied Physics Letters 98, 20, 203508-1 ~ 3 (May 2011).
- Sungsik Lee and Arokia Nathan, “Localized Tail State Distribution in Amorphous Oxide Transistors Deduced from Low Temperature Measurements,” Applied Physics Letters 101, 11, 113502-1 ~ 5 (2012).