Min–Koo Han

Hydrogen Passivation on the Grain Boundary and Intragranular Defects in Various Polysilicon Thin-Film Transistors

We investigated the hydrogenation effects on two types of trap states in poly-Si thin-film transistors (TFTs), where the gate oxide thickness was varied from 1000 A to 4000 A and the active poly-Si layers were altered by different annealing methods. The decrease of threshold voltage, which is related to the density of deep states, has shown a similar tendency irrespective of device structures. However, the other device parameters, such as field-effect mobility and minimum leakage current, which are influenced by the tail states, are improved considerably in the poly-Si TFTs with a thick gate oxide or with postannealed active layers. It was verified by the comparison of trap state distributions that the tail states were decreased significantly in the above TFTs with the thick gate oxide and postannealed poly-Si layers.

Different Hydrogen Passivation Mechanisms between Low-Temperature and High-Temperature Poly-Si TFT's

We have investigated the different hydrogen passivation effects on low-temperature (LT) processed and high-temperature (HT) processed poly-Si thin-film transistors (TFTs). The hydrogen passivation on LT poly-Si TFTs results in the increase of the field-effect mobility and the decrease of the threshold voltage, while the hydrogenation increases the field-effect mobility and decreases the leakage current in HT poly-Si TFTs. The effective trap-state densities of LT poly-Si TFT before and after 5 h of hydrogenation are estimated at about 4 × 10 12 /cm 2 and 1.5 × 10 12 /cm 2 , while those of HT poly-Si TFT are about 1.5 × 10 12 /cm 2 and 1.2 × 10 12 /cm 2 , respectively. The activation energy of HT poly-Si TFT has been increased significantly in the off state with hydrogen passivation, whereas that of LT device has been decreased remarkably in the on state

Novel offset gated poly-Si TFTs with subgate

We have fabricated a new offset gated poly-Si TFT by employing photoresist reflow, have measured various experimental data of the new device, such as hydrogenation results and high-frequency characteristics, and have analyzed device characteristics as a function of driving frequency. Our devices have a unique gate pattern and the hydrogenation effect is somewhat different from the previous results. Our experimental results suggest that with the same offset length, the device with a wider space between the maingate and the subgate is more advantageous for hydrogenation. Experimental results show that the leakage current of the new device is two orders of magnitude lower than that of the nonoffset gated device, while the ON current of the new device is almost identical to the nonoffset gated device in the typically used frequency range (10-100 kHz).

High-Voltage Schottky Barrier Diode on Silicon Substrate

New GaN Schottky barrier diodes (SBDs) on Si substrates are proposed to achieve a high-breakdown voltage. We have fabricated GaN SBDs using doped GaN/unintentionally doped (UID) GaN because doped GaN with the thickness of 200 nm is suitable for high-current operation. The 1-µm-deep mesa and low-temperature annealing of ohmic contacts suppress the leakage current of GaN SBDs. Annealing of Schottky contacts also improves the interface between a Schottky contact and GaN. Annealing of ohmic contacts at 670 °C yields the low leakage current of 2.8 nA through the surface and the buffer. When the anode–cathode distance is 5 µm, the fabricated GaN SBD successfully achieves a low forward voltage drop of 1.3 V at 100 A/cm2, low on-resistance of 4.00 mΩ cm2, and the low leakage current of 0.6 A/cm2 at -100 V. The measured breakdown voltage of GaN SBDs is approximately 400 V.

Excimer Laser Induced Crystallization of Polycrystalline Silicon Films by Adding Oxygen

We have investigated the behaviors of the grain-growth and the electrical properties in excimer laser-induced polycrystalline silicon films by adding oxygen (poly-SiO x ). The melting and solidification times in laser-induced crystallization were increased by adding oxygen into amorphous silicon films which promoted the grain-growth. The dark conductivity of poly-SiO x films was decreased with the increase of oxygen concentration up to 6.32 x 10 20 cm -3 . However, the degradation of dark conductivity by light stress was improved in poly-SiO x films probably due to the stable silicon networking. We have also observed that the electrical stability was enhanced in the poly-SiO x films with oxygen concentration of 9.1 x 10 19 cm -3 without much decrease of dark conductivity.

AlGaN/GaN Schottky Barrier Diode on Si Substrate Employing NiOx/Ni/Au Contact

We proposed and fabricated an AlGaN/GaN lateral Schottky barrier diode (SBD) employing nickel oxide (NiOx)-based double metal contacts, which showed a highly stable reverse blocking capability, by improving the reliability in the high-temperature reverse bias condition. The leakage current of the proposed device was decreased by three orders of magnitude. The measured leakage current of the diode, fabricated using a NiOx (oxidized at 500 °C)–Ni/Au contact as an anode was 236 pA at room temperature while that of the conventional diode was 2.6 µA. The barrier height of the proposed device measured at 200 °C was 0.84 eV while that of conventional one was 0.69 eV. We have also obtained a high breakdown voltage of 1200 V without any additional structure to mitigate the field concentrated at the anode. The highly rectifying contact of the proposed device was attributed to the increment in the barrier height owing to a stoichiometric change at the surface of the device.

CHARACTERISTICS OF NEW POLY-SI THIN FILM TRANSISTOR WITH A-SI CHANNEL REGION NEAR THE SOURCE/DRAIN

We propose the new poly-Si TFT structure which reduces the leakage current effectively employing highly resistive a-Si region in the channel. This new device has selectively crystallized active layer, where both edges of channel region adjacent to source and drain are not crystallized and remain a-Si. In the proposed device, the amorphous silicon region behaves like an offset to reduce the leakage current and also acts as the conduction channel of carriers under the ON state, so that the ON-current can be maintained. The experimental results show that the ON/OFF current ratio of proposed device is 2.71×106 while that of conventional one is 2.67×105. In the fabrication of the proposed device, there are not any additional photomasking steps and mis-align problem.

A Novel LDD-Structured Poly-Si Thin Film Transistors with High ON/OFF Ratio

We have fabricated a new polycrystalline silicon thin film transistor (poly-Si TFT), entitled a gate-overlapped lightly doped drain (GO-LDD) TFT, which may reduce the leakage current without sacrificing the ON current. Both the source/drain and LDD region of GO-LDD TFT are formed simultaneously by employing the buffer oxide without any additional LDD doping. Experimental results show that the leakage current of the proposed TFTs is decreased by the magnitude of two orders without sacrificing the ON current. The low-temperature ion shower doping is also employed to fabricate both the source/drain and LDD in the GO-LDD TFTs without a troublesome ion implantation.

Gate-Overlapped Lightly Doped Drain Poly-Si Thin Film Transistors by Employing Low-Temperature Doping Techniques

We have fabricated a gate-overlapped lightly doped drain (GO-LDD) polycrystalline silicon thin film transistor (poly-Si TFT) applicable for large area AMLCD by employing the large area- and low-temperature-doping techniques, such as ion shower doping and in-situ doping. Experimental results show that the leakage current of the proposed TFTs is reduced by more than the magnitude of two orders, compared with that of conventional non-offset TFT, while the ON current is scarcely decreased. The degradation phenomena after gate bias stress in GO-LDD TFTs with in-situ doping has not been found because the electron trapping into the overlayer may be suppressed effectively by the high quality TEOS interlayer.

The field modulation effect of fluoride plasma treatment on blocking characteristics of AlGaN/GaN HEMT

We proposed and fabricated AlGaN/GaN HEMTs with highly stable reverse blocking characteristics by employing fluoride plasma treatment using CF4 gas. The plasma treatment with various RF power was performed selectively on drain side gate edge region where electric field was concentrated. Unlike a normally-off process, fluoride plasma treatment with attenuated RF power expanded gate depletion region in the direction of drain electrode. Expansion of depletion was confirmed by simulated energy band structure and the change of off-state gate drain capacitance. Expanded gate depletion spread E-field more uniformly with reducing field intensity and prevented surface potential from dropping drastically at the gate edge under reverse bias condition. By the mitigation of field concentration and gradual potential change due to plasma treatment, was leakage current reduced and high breakdown voltage achieved. The breakdown voltage of plasma treated device with optimized RF power was1400 V while that of untreated sample was 900 V. The leakage current of plasma treated device was 9.5 nA.