Enhancement of Transient Current in MIS(p) Tunneling Diode with Reduced Reversed Bias Current by Oxide Removal at the Gate Edge
Authors: Lin Hsuan-Yi , Hwu Jenn-Gwo 
Published: September 2023

Transient behavior plays a critical role in electronic devices since it represents the potential to serve as memory applications. Recently, some transient current behavior of metal-insulator-semiconductor tunnel diode (MISTD) has been demonstrated with unconventional structures [1], [2]. However, due to the usage of multiple photomasks and the process complexity at the gate edge, these devices may have scale-down issues in advanced CMOS-compatible processes. In this work, a structure of edge-removed (ER) MISTD has been proposed. Compared to previous work [2], which removes surrounding oxide and Si substrate by trench forming, the ER MISTD improves the device performance by not damaging the Si substrate. The ER MISTD can remain the same transient current window (CW) while being operated at smaller write voltage and device area. With the advantages of a smaller device area, energy-efficient operations, and the simplicity of the fabrication process, the ER MISTD is promising to serve as dynamic memory applications.
The schematics of the conventional co-planar and ER MISTD are shown in Fig. 1. With the removal of the surrounding gate oxide, ER MISTD demonstrates not only lower reverse bias current but also hysteresis displacement current in Fig. 2(a) compared to planar MISTD. The leakage current of ER MISTD is 3 orders of magnitude smaller than that of planar MISTD at VG = 2V. Fig. 2(b) shows the high-frequency capacitance-voltage measurement, which demonstrates enlarged deep depletion region of the ER MISTD. We further investigated the phenomenon by Sentaurus TCAD as shown in Fig. 3. The existence of oxide charges was also adopted in simulation. The deeper depletion phenomenon of ER MISTD can be attributed to the lack of minority carriers and oxide charges without the existence of surrounding gate oxide. Another evidence of the lack of minority carriers is shown in Fig. 4. The saturation current of ER MISTD remains lower while that of planar MISTD increases and eventually induces oxide breakdown around VG = 5V. It is also found that ER MISTD isn't in oxide breakdown when VG is over 40 V as shown in the inset of Fig. 4.
Next, the enhanced two-state transient current behavior has been demonstrated in Fig. 5. The specific measurement steps are shown in the inset of Fig. 5(a) and 5(b). ER MISTD has better CW in both operations compared to planar MISTD. We further investigated the mechanism of the two operations.
When VG is switched from 1V to 0V, the minority carriers (𝑄𝑖𝑛𝑣) would become excess carriers since no enough time for them to be recombined. Because ER MISTD has an insufficient supply of minority carriers due to the absence of gate edge fringing field, the electron tunnel current generated by excess carriers is smaller compared to planar MISTD. With a smaller electron tunnel current to compensate for the transient discharging current, the total transient read current of ER MISTD is larger than that of planar MISTD.
When VG is switched from -0.5V to 0V, the depletion region is suddenly expanded and therefore generates extra displacement current. Since the change of depletion width of ER MISTD is larger, the positive displacement current is larger as well. Because the transient current is the summation of the positive discharging current and displacement current, the transient current of ER MISTD is larger.
The endurance measurement of ER MISTD has been examined as shown in Fig. 6. Fig. 6(a) shows the endurance can be tested up to 5000 cycles between 1V and 0V without much degradation. Fig. 6(b) shows the endurance between 1V and -0.5V with 102 cycles.
We further investigated the CW of two operations corresponding to oxide thickness. As shown in Fig. 7, when oxide becomes thicker, the transient behavior becomes larger due to the decrease of the tunneling probability. However, the CW would eventually decay when the oxide is too thick. This is because tunnel current will disappear at thicker oxide, the transient behavior is therefore reduced. The peak value of the CW is around EOT = 3.1 nm.
In conclusion, ER MISTD shows enhanced transient current and reduced leakage current due to the enlarged deep depletion. We also proposed two-state memory operations and endurance measurement of ER MISTD. The correspondence of CW and oxide thickness has been investigated as well. Because of the above characteristics, ER MISTD shows its potential to serve as a dynamic memory application.
Reference:
[1] Huang S -W and Hwu J -G 2021 IEEE Trans. Electron Devices
68 6580-6585
[2] Lin J -Y and Hwu J -G 2021 IEEE Trans. on Electron Devices
68 4189-4194

Figure 1