Influence of Carrier Concentration at Front- and Back-Channel on Transfer Characteristics of Bottom-Gate Ιn-Ga-Zn-O Thin-Film Transistors
Authors: Koretomo Daichi , Matsuda Tokiyoshi , Kimura Mutsumi , Furuta Mamoru 
Published: February 2020

High drain current (I
DS) is required for thin-film transistors (TFTs) to drive next-generation high-definition and large-screen flat panel displays (FPDs). An In-Ga-Zn-O (IGZO) TFT has received considerable attention for the next-generation FPDs because it has high electron mobility (µ >10 cm2V-1s-1) and large area uniformity [1]. Previously, we reported that the I
DS and an apparent field-effect mobility (µ
FE) of IGZO TFT was enhanced owing to a reduction of an effective channel length (L
eff) when the sheet carrier concentration (N
s) at back-channel interface increased [2]. However, it has not clarified the influence of carrier concentration at either front- or back-channel on transfer characteristics of the IGZO TFT.

In this research, an influence of N
s at front- and back-channel on transfer characteristics of bottom-gate IGZO TFT was analysed by device simulator (ATLAS). First, physical parameters of the IGZO TFT was defined to reproduce the transfer characteristics of actual fabricated TFT which was used as reference [3]　(denoted hereafter as ref.-TFT). Next, a thin layer, that was modulated the N
s from 6.8×1011 to 2.5×1014 cm-2, was set at the back-channel region of the TFT as shown in Fig.1 (a) (denoted hereafter as MD-TFTs). Fig.1 (b) shows the comparison of transfer characteristics of the TFTs. From Fig.1 (b), the I
DS of MD-TFTs drastically increased as compared with the ref.-TFT. In addition, it can be seen that the I
DS enhancement was increased with increasing N
s at back-channel. From these I
DS enhancement, an apparent µ
FE calculated from transfer characteristics enhanced for the MD-TFTs as shown in Fig1 (c). For the case of the N
s was 6.8×1011 cm-2, an apparent µ
FE was increased to 29.8 cm2V-1s-1 for sub-threshold region, whereas an apparent µ
FE was almost the same as ref.-TFT at on (V
GS=+20 V) region. When the N
s further increased to 2.5×1013 cm-2, the overestimation of an apparent µ
FE was observed for both sub-threshold and on (V
GS=+20 V) region. To investigate the reason for changing apparent µ
FE of MD-TFT with 6.8×1011 cm-2 with gate-voltage, current density in MD-TFT with N
s of 6.8×1011 cm-2 was simulated as shown Figs.2 (a) and 2 (b). In case of sub-threshold region (V
GS=+3 V), the L
eff at front-channel interface drastically reduce as compared with the mask-defined channel length (L
m). On the other hand, the L
eff at the on (V
GS=+20 V) region was unchanged to the L
m. As a consequence of reduction of the L
eff, it was increased to apparent µ
FE of MD-TFT with 6.8×1011 cm-2 at the sub-threshold region. These results suggested that N
s difference between back- and front-channel interfaces are strongly related to the carrier transport mechanism in MD-TFTs, because the carrier transport path was changed by increasing N
s at front-channel with gate-voltage. As a results of further analysis by device simulator, it was revealed that the enhancement of I
DS and an apparent µ
FE were induced owing to the difference of N
s between back- and front-channel interfaces. Detail of the results will be presented at the conference.

[1] T. Kamiya, K. Nomura and H. Hosono, “Present status of amorphous In-Ga-Zn-O thin-film transistors” Sci. Technol. Adv. Mater, Vol.11, pp.044305-044328, Sept. 2010.

[2] D. Koretomo, T. Toda, D. Wang and M.Furuta, “Anomalous Increase of Field-Effect Mobility in In-Ga-Zn-O Thin-Film Transistors Caused by Dry-Etching Damage Through Etching-Stopper” presented at 35-4, 15th Int. Conf. IMID, 2015.

[3] Mutsumi Kimura, “Practical Guidance of Parameter Extraction for Device Simulation of Thin-Film Transistors”, J. Appl. Phys., vol.51, pp.054302-1-054302-5, April 2012

Figure 1