In previous work, current coupling phenomena have been studied. It was reported that, when oxide is thick enough, the saturation current of the inner circle matches well with that of outer concentric ring among two neighboring metal-insulator-semiconductor(p) (MIS) tunnel diodes (TD) [1]. Besides, based on the lateral diffusion (LD) current as well as Schottky barrier height lowering, more source of aggregating electrons under surrounding floating gate makes saturation tunnel current greater [2]. In this work, three different metal patterns of MIS tunnel diodes are used to pore over current coupling phenomena, which mainly come from the LD of minority carrier. Figure 1 and figure 2 show schematic diagrams of the top views and cross sections of the fabricated devices.

I-V curves of drain current versus drain voltage with and without floating aluminum rings are shown in figure 3; red curves are those under an irradiance of 4mW/cm². For three I-V curves in black, with more aluminum rings, the saturation currents are greater due to more aggregating electron source [2]. For another three in red, the similar phenomenon reappears: light-induced e-h pairs are supplements to aggregating electrons below the floating ring and they enhance the LD current as well as the lowering of Schottky barrier height, leading to the increase of saturation tunneling current. Thus, saturation photocurrent is increased compared to that with less or even no rings.

Figure 4(a) and 4(b) show current in dark and under irradiance respectively versus total perimeter (TP) extracted from figure 3 when V_D is 1V. It is clear that current values are proportional to TP; thus, we assume the saturation I_D increasing with longer perimeter is exactly due to extra neighboring aluminum gate ring, which accounts for the major impact of LD current.

Figure 5 shows I-V characteristics of devices with electrode pattern shown in figure 1(a). The drain current versus drain voltage, the gate1 current versus gate1 voltage, and the gate2 current versus gate2 voltage are separately exhibited. Three hollow circle curves stand for those under irradiance. Despite the variance of perimeter and area among drain, gate1, and gate2, the saturation dark or light current of those three shows no difference. However, in previous work [3], [4], it was indicated, the saturation current increases with greater perimeter for single MIS(p) TD due to edge fringing field. Accordingly, we consider again the LD current of minority carrier a dominating role in such phenomenon. Since the LD current works for drain current in figure 4, where drain current is elevated by neighboring gate1 and gate2, then it works for gate1 and gate2 currents as well. In view of central drain, applied 1V and thin SiO₂ make electrons beneath drain fewer than those beneath gate1 and gate2; therefore, the LD currents are established. Also, in view of gate1, applied 1V and thin SiO₂ make electrons beneath gate1 fewer than those beneath drain and gate2. The same phenomenon happens in view of gate2; thus, the mutual coupling effect makes three saturation current values all equal. Photocurrents (three hollow curves) have the same value in saturation region because light-induced e-h pairs supply the minority carrier and it enhances the mutual coupling effect; therefore, photocurrents share the same value just like the dark currents.

Under an irradiance of 4mW/cm², light still plays a role of the supplement to minority carrier. Thus, saturation current of central drain increases with more floating surrounding gate rings no matter whether there is applied light on devices or not. And saturation current increases exactly with TP, which implies the LD as one of potential coupling effects. It is worth attention that the distance between drain and gate1 is only 5um but the coupling effect is clearly observed. What if the distance is minimized to commercially-used nanometer scale and it is believed the coupling effect would be even strengthened impacting ready-made devices. Last but not least, for neighboring MIS(p) structures, the mutual coupling effect makes their saturation currents equal to each other in spite of their different structure perimeter. Therefore it deserves our further exploration in nowadays trend of gate length shrinkage due to the miniature distance among devices.

This work was supported by the Ministry of Science and Technology of Taiwan, ROC, under Contract No. MOST 105-2221-E-002-180-MY3 and NTU-ERP-105R89081.

[1]C. S. Liao and J. G. Hwu, ECS Trans., 75, 77 (2016).

[2]M. H. Yang and J. G. Hwu, IEDMS., 29, PD-3 (2016).

[3]T. Y. Chen and J. G. Hwu, ECS Trans., 58, 79 (2013).

[4]H. W. Lu and J. G. Hwu, ECS Trans., 58, 339 (2013).