Roles of Inner and Outer Fringe and Asymmetric Coupling Effect in Concentric Double-MIS(p) Tunneling Diodes
Authors: Chen Yu-Hsuan , Hwu Jenn-Gwo 
Published: February 2020

It has been known that the saturated current of MIS(p) tunnel diode (TD) is periphery-dependent because of magnifying of fringing-field effect [1]. Furthermore, due to its high sensitivity to the change of minority carriers from surroundings, MIS(p) shows noticeable results towards light or bias-added neighboring gate in a concentric structure [2]. Nonetheless, it is lack of discussion on the amount of coupling contribution from the inner and outer fringe of the outer ring-shaped MIS TD. In this work, two sets of ring-shaped and concentric MIS TD patterns are designed. The first set is made with the outer fringe ROo fixed and inner fringe ROi varying, and the second set is designed contrarily. The schematics of the top view, cross section and parameter table of ring-shaped and concentric double-MIS TD are shown in Fig. 1 (a) and (b), respectively. The radius of inner and outer fringe of the outer ring are named as ROi and ROo. Besides, “Vring” denotes the bias of the ring-shaped MIS TD in Fig. 1 (a) and the notation of “VI” and “VO” represent bias of inner and outer electrodes in the concentric MIS TD in Fig. 1 (b).
The distinct difference lies in the saturated light current part of Fig. 2 where the result of set 1 (Fig. 2 (a)) merge perfectly while that of set 2 (Fig. 2 (b)) do not. We further divide the currents in Fig. 2 (a) with their area (JA,ring) and obtain merged currents in the accumulation region in Fig. 3 (a), indicating they are uniform TDs with same oxide thickness. Then, we divide Fig. 2 (a) with their respective total perimeter (JP,ring) and find the saturated dark currents merged better now in Fig. 3 (b). This can be attributed to the similar perimeter-dependent phenomenon as in single MIS TD case [1]. Combining the results above, it is inferred that both of the inner and outer fringes contribute to saturation current of a ring-shaped MIS TD at dark. However, with light added the dominant term in saturated current becomes that amount of charges grabbed by the outer fringe through a diverging field so that the light current shows great consistency for devices with same ROo.
In the following analysis, we examine the concentric double-MIS TD structure with a 30μm gap from two operating situations. One is the case where the inner circle functions as the gate and the outer ring plays as the sensor [i.e., inner gate outer sensor (IGOS)]. In the opposite case, the inner circle serves as sensor while the outer ring works as the control gate [i.e., inner sensor outer gate (ISOG)]. It is found in our recent work [3] that the extent of the coupling effect between the inner device and the outer ring is unequal (shown in the inset of Fig. 4). By utilizing this asymmetric coupling effect, a higher sensitivity can be achieved at a lower voltage bias, therefore become more power-efficient in IGOS. Fig. 4 (a) and (b) show the I-V curves of sample A in IGOS and ISOG operations. It is observed that in the IGOS situation, the light currents under different VIG are nearly fixed. This can be interpreted as a combined result of asymmetric coupling effect and the above-mentioned inference that the main role in grabbing carriers under illumination is the outer fringe of the ring. Also, similar results are found among samples with 6 different outer ring-to-inner circle area ratios ranging from 48 to 4. Contrarily, in the ISOG situation, the extent of light current influenced by outer control bias will be relatively larger owing to the strongly besieging field.
Finally, Fig. 5 shows the I-V curves of the concentric MIS TD with ROo fixed and the total power consumption of (a) IGOS and (b) ISOG operations under a control bias of 1.0V. It is noted that as the outer ring-to-inner circle area ratio increases (from ratio of C : 0.9 to A : 8), the light-to-dark current ratio rises while total power consumption becomes lower and therefore more efficient. It is concluded that the roles of inner and outer fringe are different and the coupling effect is of importance to high-density devices.
This work is supported by the Ministry of Science and Technology of Taiwan, ROC, under Contract No. MOST 105-2221-E-002-180-MY3 and MOST 107-2622-8-002-018.
[1] Y. K. Lin and J. G. Hwu, IEEE TED, 61, 9, 2014
[2] W. T. Hou and J. G. Hwu, ECSJ, 6, 10, 2017
[3] Y. H. Chen and J. G. Hwu, IEEE TED, 65, 11, 2018

Figure 1