Carbon NanoTubes (CNTs) have been extensively studied in recent years due to their exceptional electronic, opto-electronic, and mechanical properties. Some of the interesting electronic properties of CNTs are quasi-ballistic carrier transport, the suppression of short-channel effects due to one-dimensional electron transport, and the nearly symmetric structure of the conduction and valence bands, which is advantageous for complementary circuits. Moreover, owing to the excellent optical properties of CNTs, it is possible to envision an all-CNT electronic and opto-electronic circuit. The direct band gap and the tunability of the band gap with the CNT diameter render them suitable candidates for opto-electronic devices, especially for Infra-Red (IR) applications, which have a relatively narrow band gap. Many groups have reported on IR photo detectors based on Carbon NanoTube Field Effect Transistors (CNT-FETs). Self-consistent quantum mechanical simulations have been employed in the exploration of the physics of such devices. The performance of IR photo detectors based on CNT-FETs is analyzed numerically, employing the Non-Equilibrium Green's Function (NEGF) formalism.
The NEGF formalism initiated by Schwinger, Kadanoff, and Baym allows the study of the time evolution of a many-particle quantum system. Knowledge of the single-particle Green's functions for a given system enables the evaluation of single-particle quantities such as carrier density and current. The many-particle information about the system is cast into self-energies, which are part of the equations of motion for the Green's functions. A perturbation expansion of the Green's functions is the key to approximating the self-energies. Green's functions are a powerful technique for the evaluation of the properties of a many-body system in both thermodynamic equilibrium and non-equilibrium situations.
The NEGF method has been successfully utilized to investigate the characteristics of CNT-FETs. We employed the NEGF method based on the tight-binding model to study quantum transport in IR photo detectors based on CNT-FETs and to investigate methods to improve the performance of such devices.