Rearrangement of Fringing Field by Sidewall Passivated Metal Gate in MIS Tunnel Diode
Authors: Chou Chia-Ju , Hwu Jenn-Gwo 
Published: February 2020

It is known that the dielectric constant (ϵ value) of Al2O3 is larger than SiO2. Based on the equation Ｑ= ϵＥ, it is predicted that it is more difficult for electric field to pass through Al2O3 passivated layer with respect to SiO2. A method to rearrange and block fringing field in MIS tunnel diode was demonstrated. After the gate electrodes were defined and patterned by lithography and wet etching, the photoresist on the top of aluminum gate was retained. And then, the sample was immersed in nitric acid solution by the mixture of nitric acid and water in a ratio of 2:1. After partial nitric acid oxidation for 2 mins, the aluminum oxide layer was formed on the edge side of aluminum gate.
There are two batches of MIS tunnel diodes, which are denoted by MIS and Al2O3SP-MIS. The cross-section of these two devices is shown in Fig. 1. For previous study [1-2], the deep depletion behavior could be described with several periods. Initially, the minority carriers could balance the gate bias in weak inversion, so the capacitance would hold for a period. If the amount of minority carrier is sufficient, the value of capacitance will rise. Subsequently, when a larger bias is applied, the edge region due to fringing field would make the device enter into deep depletion at edges while the bulk region is still in depletion. At this moment, the minority carrier would leak from substrate to gate, so the value of capacitance would drops rapidly. Therefore, the arch would appear in C-V curve. Finally, both the edge and the bulk regions would enter into deep depletion, and the value of capacitance drop continually.
From Fig. 2(a), it is found that the arch of Al2O3SP-MIS is in front of the arch of MIS. It is implied that Al2O3SP-MIS enters into deep depletion much quickly. For Al2O3SP-MIS, fringing field at the edge of gate electrode is stronger, so it would make the device enter into deep depletion earlier. However, the arch of Al2O3SP-MIS is higher than MIS, and Al2O3SP-MIS has higher capacitance values than those of MIS. It means that there are more carriers under the gate electrode for Al2O3SP-MIS. The TCAD simulation in Fig. 3 confirms this experimental result. Under weak inversion, the carrier is more concentrative in Al2O3SP-MIS, so more carriers could response to the gate bias.
In order to generate more hole and electron carriers, incandescent light was applied. From Fig. 2(b), it is also found that the arch of Al2O3SP-MIS is in front of the arch of MIS. In contrast to w/o light, the arch of MIS is higher than Al2O3SP-MIS. Because fringing field in MIS extends more distance away than Al2O3SP-MIS, so MIS would seize more carriers which were generated by incandescent light. Fig. 2(c) shows that the difference between w/o light and w/ light for Al2O3SP-MIS is less evident since fringing field seizes fewer carriers under incandescent light. It is further confirmed that fringing field was blocked by Al2O3passivated layer.
In order to confirm the experimental results, TCAD simulations were employed to discuss the rearrangement of fringing field. Fig. 4 shows the magnitude of electric field extracted from TCAD simulations. It is observed that there are two peak values in Al2O3SP-MIS while MIS only one. The first peak of Al2O3SP-MIS is lower than MIS, which implies that fringing field is reduced by Al2O3 passivated layer. The second peak of Al2O3SP-MIS is higher than MIS, which means that fringing field would be enhanced at the edge of gate electrode, so it would seize more minority carrier to leakage from substrate to gate and make the device enter into deep depletion earlier.
This work was supported by the Ministry of Science and Technology of Taiwan, ROC, under Contract No. MOST 103-2221-E-002-252-MY3 and MOST 105-2622-8-002-001.

References

[1] J. Y Cheng, IEEE Trans. Electron Device, vol. 49, no. 3, pp.565-572, March 2012
[2] K. M. Chen, J.Appl. Phys., vol.110, issue 11, pp.114104 -114104-4, December 2011

Figure 1