Mobility Enhancement and Abnormal Humps in Top-Gate Self-Aligned Double-Layer Amorphous InGaZnO TFTs

Abstract

The mobility enhancement and the positive bias stress of top-gate self-aligned TFTs using the a-IGZO channel with a front barrier are investigated. The a-IGZO front barrier can keep electrons in the a-IGZO channel away from the top-gate oxide to significantly enhance the electron mobility at the top gate operation. The parasitic channel induces a hump in the transfer characteristics. The positive bias stress shifts the hump to the negative voltage abnormally. The H 2 O in the polymer film on array layer is responsible for the abnormal shift. The H 2 O diffuses into the top-gate insulator and is electrolyzed to create H + , which forms a parasitic channel with a negative shift of threshold voltage, leading to the abnormal hump. The abnormal humps are increasingly significant with the increasing channel width and the decreasing channel length. The channel width dependence on positive bias stress is due to the inverse narrow width effect caused by the fringe electric field. The channel length dependence on positive bias stress is due to the H + diffusion toward the center of the parasitic channel from both the source and drain sides.INDEX TERMS Positive bias stress (PBS), a-IGZO, hump, polymer film on array (PFA).

Enhancements of electrical properties and positive bias instability in self-aligned top-gate a-IGZO TFTs by hydrogen incorporation

“…The evolutions of transfer characteristics of the SSF-TFTs (figure 3(a)) and the HSF-TFTs (figure 3(b)) under PBS were measured. There is no hump in the transfer characteristics of both TFTs under PBS due to the passivation of SiN x cap layer [17]. The HSF-TFT exhibit much smaller positive V th shift than the SSF-TFT under PBS.…”

“…For the cap layer, 150 nm-thick SiN x was deposited by PECVD at 200 • C. Finally, polymer film on array and SiN x layer were deposited. Noted that a SiN x cap layer under the polymer film on array stops the moisture permeation into the TGI [15,16], and prevents the hump phenomenon [17].…”

Dependence of a Hydrogen Buffer Layer on the Properties of Top-Gate IGZO TFT

“…However, the dielectric in a top-gate IGZO TFT can serve as a gas permeation barrier [ 8 ]. In addition, a top-gate IGZO TFT is considered to be the most suitable structure for large high-resolution panel displays because it can provide better process controllability [ 7 , 9 , 10 ]. So, top-gate IGZO TFTs are receiving more and more attention from industry and academia [ 5 , 7 , 9 , 10 , 11 ].…”

“…In addition, a top-gate IGZO TFT is considered to be the most suitable structure for large high-resolution panel displays because it can provide better process controllability [ 7 , 9 , 10 ]. So, top-gate IGZO TFTs are receiving more and more attention from industry and academia [ 5 , 7 , 9 , 10 , 11 ]. In addition, IGZO films have become the most promising semiconductor materials in the flexible display field.…”

“…Some previous studies have shown that the hydrogen content of the buffer layer can be changed by changing how the buffer layers are stacked, the annealing temperature, or the type of substrate [ 2 , 13 , 14 , 15 ]. In addition, they have also put forward the idea that hydrogen can act as an impurity with a shallow donor state, and an appropriate hydrogen concentration in the buffer layer can improve the performance of devices such that they meet the requirements of display driving [ 7 , 9 , 13 , 14 , 15 , 16 , 17 , 18 ]. However, few studies have explored the preparation process for the precise control of the hydrogen content of the buffer layer, which affects the performance of a device.…”