Enhancement-mode Thin-film Transistors Research Articles

Study of geometry effect on the performance of thin-film transistors fabricated with ZnGa2O4 epilayer grown by metalorganic chemical vapor deposition for high voltage applications

Enhancement mode thin film transistors (TFTs) having different geometries were fabricated on the Zinc gallium oxide (ZnGa2O4) epilayer grown on a c-plane sapphire substrate by metalorganic chemical vapor deposition (MOCVD). A field emission scanning electron microscope, atomic force microscopy, and X-ray diffraction were employed to conduct a detailed analysis of the film composition, morphology, and crystal structure. The distance between the source-to-gate and gate length was kept to be 7 μm and 3 μm in all the devices with varying gate-to-drain lengths of 10, 20, and 30 μm. Moreover, neutral beam etching technology was employed to isolate the individual devices and reduce the defects induced by plasma etching. It was found that the threshold voltage changes from 6 V to 7.6 V with the increase in source to drain length from 20 μm to 40 μm. The On/Off ratio of the devices was found to be 105, with a maximum current around 1 μA/mm. Additionally, the device breakdown voltage increased from 240 V to about 656 V with the increase in the device length. The results demonstrate the fabrication of the high breakdown voltage ZnGa2O4-based TFT and establish the co-relation of the source to drain length with the device characteristics.

A Delay-Cell-Controlled VCO Design for Unipolar Single-Gate Enhancement-Mode TFT Technologies.

This work outperforms the previous literatures by proposing a delay-cell-controlled voltage control oscillator (VCO) design for common unipolar, single-gate, and enhancement-mode thin-film transistor (TFT) technologies. A design example with InZnO TFTs is simulated to verify the proposed design. The design example has a 500 μW power consumption, 0.7 mm2 area, 3.8 kHz-8 kHz output frequency range, 600 Hz/V tuning sensitivity, and 4% maximum linear error. This design may have the potential to be used for flexible, low cost, and moderate speed sensor readout interfaces.

Electrode-Adaptive Thin-Film Integrated Logic Circuits

Amorphous oxide-based thin-film logic circuits have been fabricated using only an n-type amorphous silicon–zinc–tin–oxide (a-SZTO) channel layer with different source/drain electrodes. Enhancement-mode thin-film transistors (TFTs) were fabricated with oxide electrodes. Depletion-mode TFTs were fabricated with metal electrode. Work functions were measured by Kelvin probemicroscopy. The barrier heights ( $\Phi _{\textsf {B}}$ ) between the a-SZTO channel layer and the electrode were measured to be 1.831, 2.341, and 2.339 eV for the Ti/Al electrode, Indium–silicon–oxide (ISO) electrode, and indium–tin–oxide (ITO) electrode, respectively. The physicalmechanism on the variation of sheet and contact resistances was investigated using a transmission line method, and the change in the resistances is closely related to $\Phi _{\textsf {B}}$ . Inverters were fabricated, with different $V_{\textsf {TH}}$ values adopted simply by using different contact characteristics between various electrodes and the semiconductor channel. High values of voltage gains in inverters were obtained: 12.33 (ISO) and 11.75 (ITO) at $V_{\textsf {DD}}= 5$ V. Itwas also confirmed that more complicated n-type-based NAND and NOR thin-film circuits, implementedwith different electrodes, functioned as conventional logic circuits. This simple fabrication method of thin-film logic circuits opens the possibility of implementing next-generation stacked integrated circuit technology.

Toward Adequate Operation of Amorphous Oxide Thin-Film Transistors for Low-Concentration Gas Detection.

We suggest the use of a thin-film transistor (TFT) composed of amorphous InGaZnO (a-IGZO) as a channel and a sensing layer for low-concentration NO2 gas detection. Although amorphous oxide layers have a restricted surface area when reacting with NO2 gas, such TFT sensors have incomparable advantages in the aspects of electrical stability, large-scale uniformity, and the possibility of miniaturization. The a-IGZO thin films do not possess typical reactive sites and grain boundaries, so that the variation in drain current of the TFTs strictly originates from oxidation reaction between channel surface and NO2 gas. Especially, the sensing data obtained from the variation rate of drain current makes it possible to monitor efficiently and quickly the variation of the NO2 concentration. Interestingly, we found that enhancement-mode TFT (EM-TFT) allows discrimination of the drain current variation rate at NO2 concentrations ≤10 ppm, whereas a depletion-mode TFT is adequate for discriminating NO2 concentrations ≥10 ppm. This discrepancy is attributed to the ratio of charge carriers contributing to gas capture with respect to total carriers. This capacity for the excellent detection of low-concentration NO2 gas can be realized through (i) three-terminal TFT gas sensors using amorphous oxide, (ii) measurement of the drain current variation rate for high selectivity, and (iii) an EM mode driven by tuning the electrical conductivity of channel layers.

High-performance enhancement-mode thin-film transistors based on Mg-doped In2O3 nanofiber networks

Although In2O3 nanofibers (NFs) are well-known candidates as active materials for next-generation, low-cost electronics, these NF based devices still suffer from high leakage current, insufficient on–off current ratios (Ion/Ioff), and large, negative threshold voltages (VTH), leading to poor device performance, parasitic energy consumption, and rather complicated circuit design. Here, instead of the conventional surface modification of In2O3 NFs, we present a one-step electrospinning process (i.e., without hot-press) to obtain controllable Mg-doped In2O3 NF networks to achieve high-performance enhancement-mode thin-film transistors (TFTs). By simply adjusting the Mg doping concentration, the device performance can be manipulated precisely. For the optimal doping concentration of 2 mol%, the devices exhibit a small VTH (3.2 V), high saturation current (1.1 × 10–4 A), large on/off current ratio (>108), and respectable peak carrier mobility (2.04 cm2/(V·s)), corresponding to one of the best device performances among all 1D metal-oxide NFs based devices reported so far. When high-κ HfOx thin films are employed as the gate dielectric, their electron mobility and VTH can be further improved to 5.30 cm2/(V·s) and 0.9 V, respectively, which demonstrates the promising prospect of these Mg-doped In2O3 NF networks for highperformance, large-scale, and low-power electronics.

Threshold voltage manipulation of ZnO-graphene oxide hybrid thin film transistors via Au nanoparticles doping

In order to fabricate a complementary inverter, precise control of the threshold voltages for n-type semiconductor based thin film transistors (TFTs) is highly required. Here we provided a facile methodology for controlling the threshold voltage of ZnO-based TFTs. Chemically-derived graphene oxide (GO) and Au-decorated GO (Au-GO) flakes were hybridized with solution-processed ZnO thin films to control electron injection determined by the workfunction difference between ZnO and GO or Au-GO. As a result, the threshold voltages for the ZnO, GO/ZnO, and Au-GO/ZnO TFTs were 24 ± 3 V, −11 ± 4 V, and 63 ± 5 V, respectively, which determine depletion or enhancement mode TFTs without any significant change in the field effect mobility and on/off ratio.

A Multi-$V_{\mathrm {th}}$ a-IGZO TFT Technology Using Anodization to Selectively Reduce Oxygen Vacancy Concentration in Channel Regions

A multithreshold voltage amorphous indium- gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT) technology based on the anodic oxidation (anodization) technique is demonstrated. It is shown that the characteristics of the a-IGZO channel layer can be considerably tailored by the anodization treatment. As a result, the threshold voltage of the a-IGZO TFTs depends on the anodization voltage, making it possible to fabricate both depletion and enhancement mode TFTs on the same substrate. The secondary ion mass spectrometry result shows that the oxygen content is noticeably increased in the anodized a-IGZO film, suggesting that the dependence of the threshold voltage on the anodization treatment is attributed to the anodization-induced reduction of oxygen vacancy concentration in the channel region, which also leads to an alleviated Vth shift under negative bias illumination stress in the anodized devices.

Effect of NH 3 plasma treatment on the device performance of ZnO based thin film transistors

We fabricated an enhancement-mode thin film transistor (TFT) using ZnO as an active channel layer deposited by radio frequency (rf) magnetron sputtering. The NH 3 plasma passivation was performed in order to improve the electrical properties of the ZnO TFTs. We observed that the NH 3 plasma treated ZnO TFTs revealed improved device performances, which include the field effect mobility of 34 cm 2/Vs, threshold voltage of 14 V, subthreshold swing of 0.44 V/dec, off-current of 10 −11 A and on to off ratio higher than 10 5. These results demonstrate that NH 3 plasma treatment could effectively enhance the performance of the ZnO based TFT device.

Electrical properties of ZnO-based bottom-gate thin film transistors fabricated by using radio frequency magnetron sputtering

We report on enhancement-mode thin film transistors (TFTs) using ZnO as an active channel layer deposited by radio frequency (rf) magnetron sputtering at 300 °C. The TFT structure consisted of ZnO as a channel, SiN x as a gate insulator and indium tin oxide (ITO) as a gate which were deposited onto a Corning glass substrate. X-ray diffraction pattern revealed that dense columnar structure of closely packed ZnO nano grains along the c-axis. The transfer characteristics of a typical ZnO TFT exhibited a field effect mobility of 31 cm 2/V s, a drain current on/off ratio of 10 4, the low off-current value in the order of 10 −10 A, and a threshold voltage of 1.7 V. The transparent ZnO TFT exhibited n-channel enhancement mode behavior.

Enhancement-mode thin film transistor with nitrogen-doped ZnO channel layer deposited by laser molecular beam epitaxy

The thin film transistors (TFTs) based on nitrogen doped zinc oxide (ZnO) were investigated by laser molecular beam epitaxy. The increase of ZnO films' resistivity by nitrogen doping was found and applied in enhancement mode ZnO–TFTs. The ZnO–TFTs with a conventional bottom-gate structure were fabricated on thermally oxidized p-type silicon substrate. Electrical measurement has revealed that the devices operate as an n-channel enhancement mode and exhibit an on/off ratio of 10 4. The threshold voltage is 5.15 V. The channel mobility on the order of 2.66 cm 2 V − 1 s − 1 has been determined.

Dopant and defect interactions in polycrystalline silicon thin-film transistors

The influence of channel doping on the threshold voltage of polycrystalline silicon thin-film transistors (TFTs) has been investigated using both boron and phosphorus implants. For each dopant type, asymmetric TFT behavior was seen between enhancement and depletion mode TFTs, with the enhancement mode TFTs always showing a greater shift in threshold voltage for a given dose. From a detailed analysis of the boron-doped samples, using a two-dimensional device simulator, it was demonstrated that the results could be best explained in terms of an asymmetric density of states, in which the deep trapping state density in the lower half of the band gap increased with increasing boron concentration, and the effect of the dopant on threshold voltage was itself determined by this local defect concentration.

Channel doping effects in poly-Si thin film transistors

The influence of channel doping on the threshold voltage of poly-Si thin film transistors (TFTs) has been investigated using both boron and phosphorus implants. For each dopant type, asymmetric TFT behaviour was seen between enhancement and depletion mode TFTs, with the enhancement mode TFTs always showing a greater shift in threshold voltage for a given dose. From a detailed analysis of the boron-doped samples, using a two-dimensional device simulator, it was demonstrated that the results could be best explained in terms of a non-symmetric density of states, in which the deep trapping state density in the lower half of the band-gap increased with increasing boron concentration. This is the first reported observation in poly-Si of shallow level dopants also introducing deep levels in the majority carrier half band-gap.