States In Thin-film Transistors Research Articles

Analytical Field-Effect Method for Extraction of Subgap States in Thin-Film Transistors

We present an analytical field-effect method to extract the density of subgap states (subgap DOS) in amorphous semiconductor thin-film transistors (TFTs), using a closed-form relationship between surface potential and gate voltage. By accounting the interface states in the subthreshold characteristics, the subgap DOS is retrieved, leading to a reasonably accurate description of field-effect mobility and its gate voltage dependence. The method proposed here is very useful not only in extracting device performance but also in physically based compact TFT modeling for circuit simulation.

Extraction of the Density of Interface Trap States in Thin-Film Transistors

In the present study, we propose a new extraction method for the density of interface trap states at the Si/SiO2 interface from thin-film transistors with doped active layers. In developing the proposed method, we derive an equation that describes the interface trap charge density as functions of the surface potentials. Whereas the front surface potential is obtained from the low-frequency CV characteristic, the back surface potential is calculated using the depletion approximation. We confirm the validity of the extraction method using a two-dimensional device simulator.

Extraction of the Density of Interface Trap States in Thin-Film Transistors

not Available.

Analysis of defects in microcrystalline islands in amorphous silicon films with a Scanning Charge-Transient Microscope

The performance of a novel instrument, the Scanning Charge-Transient Microscope, is demonstrated by analysis of defect states in thin-film transistors fabricated in location-controlled single-grain crystallised amorphous silicon. In the instrument, the principle of Isothermal Charge-Transient Spectroscopy is implemented. Its heart is a sensitive charge-to-voltage converter. It integrates the minute charge emitted from traps, periodically filled by voltage pulses. The sensitivity of the converter is in the attocoulomb range. The frequency of filling pulses is swept from a few hundred kHz to tens of Hz and the charge transients are transformed into a spectrum by suitable sampling and integration. In contrast to earlier investigations of defects in micron-sized field effect transistors, the sensitivity of the instrument makes it possible to investigate the defects in simple capacitors without the need to make use of the gain of the transistor. Thus, it can be combined with a scanning force or tunnelling microscope and perform local analysis of selected areas of the sample.

Method for the determination of bulk and interface density of states in thin-film transistors

In this article we present a method for the accurate determination of interface and bulk density of states (DOS) in thin-film transistors (TFTs), based on the combined analysis of transfer (ID–VGS) and capacitance–voltage characteristics. This analysis has achieved a number of results, eliminating sources of inaccuracies that are known to be present in other methods. A procedure for the determination of the electron and hole flatband conductances and bulk Fermi energy is demonstrated. A recursive procedure is employed to extract the bulk DOS directly from Poisson’s equation. The advantages of this method are the greater immunity to noise from the original data, the use of the complete Fermi function (no 0 K approximation), and the applicability to thin active layers. This method yields the interface state density spectrum as well as the bulk DOS. This information is very important for device design, process characterization, and modeling of TFTs.

Density of states in thin film transistors from the modulated photocurrent technique: application to the study of metastabilities

We demonstrate the possibility of probing the density of states (DOS) of the active a-Si:H layer of Thin Film Transistors (TFT) by means of the modulated photocurrent technique. The DOS deduced from this technique on Top Nitride TFTs is compared with values deduced from the subthreshold slopes of the transfer characteristics. We also study the consequences of gate bias stresses on the TFTs. For gate bias stresses up to ±30V, we observe a shift in the transfer characteristics along the gate voltage axis, but did not measure any significant change in the DOS above midgap. These results are discussed and compared with those recently published in the literature by other authors.

Trapping states in thin film transistors measured by thermally stimulated currents

Thermally stimulated current measurements were made on MIM capacitors of sputtered SiO 2 and on SiO 2/CdSe thin film transistors. The peaks in the spectra were interpreted in terms of trapping states in the insulator and defect levels in the semiconductor. Fast interface and grain boundary states, if present, have densities below the detection level of 10 19 m -3.