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