Low turn-on voltage due to conduction band lowering effect in crystalline indium gallium zinc oxide transistors

Abstract

We found out that in an indium gallium zinc oxide (IGZO) transistor, the energy barrier in the channel region, i.e., the conduction band energy relative to the Fermi energy is lowered by electrons flowing from n + regions under source and drain electrodes. We have named this phenomenon "conduction band lowering (CBL) effect". Owing to this effect, even when the Fermi energy of an IGZO film gets closer to the mid-gap, a transistor formed using the film in the channel region is always turned on around a gate voltage of 0 V. In other words, by sufficiently reducing the donor concentration of the channel region, such IGZO transistors are turned on at a certain low gate voltage determined by the CBL effect and their characteristics variation can be suppressed.

21.4: Deposition Conditions and HRTEM Characterization of CAAC IGZO

“…As TFT properties are determined by the structure at the channel/gate oxide interface, bottom-gate TFTs fabricated from CAAC IGZO may exhibit transport properties that are very similar to a-IGZO despite the nanocrystal structure in the bulk of the film. Essentially all CAAC IGZO literature presents electrical characteristics, such as extremely low leakage currents, measured with top-gate TFTs [15][16]. These results hint that access to novel electronic properties offered by CAAC IGZO may require carrier transport in the nanocrystallized bulk far from the substrate interface.…”

“…While a-IGZO has dominated research efforts, Yamazaki [14] demonstrated that IGZO can also be structured into a partially nanocrystalline state with strong crystallographic texturing; this structure has been termed c-axis aligned crystal (CAAC) IGZO. Devices fabricated with CAAC IGZO exhibit similar uniformity to a-IGZO, but show extremely low leakage currents [15], making it potentially useful for non-volatile RAM and novel sensors [16][17].…”

“…Specific conditions for formation of CAAC and quantitative structural characteristics are seldom specified. For example, CAAC formation by DC sputtering and annealing films at 400 o C is reported by Matsubayashi et al [16], but specific conditions are omitted. This same group later reported CAAC deposition at substrate temperatures of 300 o C in O2 on crystalline substrates with a Zn-rich composition [18].…”

Characterization of reactively sputtered c-axis aligned nanocrystalline InGaZnO4

“…13 These characteristics make CAAC IGZO an ideal candidate for applications beyond traditional displays, including low-power displays, non-volatile RAM, and sensors. 14,15 However, little is known about CAAC formation conditions. Yamazaki's initial reports did not identify specific deposition and annealing conditions.…”

3.3: Invited Paper: Future Possibility of C‐Axis Aligned Crystalline Oxide Semiconductors Comparison with Low‐Temperature Polysilicon

“…Owing to a conduction band lowering effect, the I d -V g curve rises at approximately V g = 0 [7]. 5) Adding hydrogen (H) to an OS film can produce a transparent conductor [8].…”