Current Coupling Effect in MIS Tunnel Diode with Coupled Open-Gated MIS Structure

Abstract

It was found that the saturation current of metal-insulator-semiconductor (MIS) tunnel diode (TD) is proportional to the device perimeter due to the fringing field effect [1]. And, it was believed that the saturation current of MIS TD increases with the peripheral lateral flux of minority carriers if the majority carriers would encounter the effective Schottky barrier as tunneling through the oxide to the semiconductor [2]. The mechanism is that the saturation current of MIS TD exponentially increases with the decrease of Schottky barrier height, and the effective Schottky barrier height decreases with the increase of lateral flux of minority carriers due to the increase of oxide voltage. In this work, it was found that the saturation current of MIS TD would be the same as that of another MIS TD nearby while the oxide thickness is thick enough. We called the phenomenon as current coupling effect. Figure 1 shows the schematic diagrams of the top views and cross sections of the fabricated devices. The current-voltage (I-V) characteristics of the MIS TD’s are shown in figure 2. The device area of the circle MIS TD is 2.2×10- 4 cm2, and the device area of the concentric ring MIS TD is 1.0×10- 2 cm2. The I-V curves of the inner circle MIS TD and concentric ring MIS TD were measured independently, i.e., IG -TD -VG -TD ’s were measured as VG was open, and vice versa. Note that the I-V curves shows rectifying characteristics since the oxide thicknesses are so thin that quantum tunneling happens, i.e., the majority carriers direct tunneling through the oxide and encounter the effective Schottky barrier at reverse bias (V> 0) of MIS TD. The oxide can be considered as a effective Schottky barrier height modulation layer. The effective Schottky barrier is lower with thicker oxide due to the larger oxide voltage, which explains that the saturation current increases with the oxide thickness. Figure 3(a) shows the currents at 3 V versus the oxide thickness extracted from figure 2. The I-V characteristics of the MIS TD’s with S = 17.8 and 27.5 μm were also measured, and the currents at 3 V were also extracted and shown in figure 3(b) and (c). It can be seen that the saturation currents of the inner circle and concentric ring MIS TD’s matches well with larger oxide thickness, and the saturation currents of the inner circle and single MIS TD’s matches well with thinner oxide thickness. The mechanism can be explained as follows. The MIS TD, Al/SiO2/Si(p), is in inversion region while the voltage is open. The inversion charges supply the lateral electron flux to the MIS TD biased in saturation region. Since the saturation current of MIS TD with thicker oxide is more sensitive to the lateral electron flux, the saturation current is dominated by the lateral electron flux affected term for thicker oxide. Therefore, the saturation currents of the inner circle and the concentric ring MIS TD’s are the same due to the same lateral electron flux at the space between them. However, for thinner oxide, the sensitivity of oxide voltage drop modulation mechanism is weaker than the thicker oxide device. Therefore, the saturation current is proportional to each device perimeter for thinner oxide due to weaker coupling. The oxide thicknesses where the current switching from uncoupled to strongly coupled condition are 2.3-2.7 nm, 2.5-2.7 nm, and 2.6-2.8 nm for S = 7.6, 17.8, and 27.5 μm devices, respectively. This work was supported by the Ministry of Science and Technology of Taiwan, ROC, under Contract No. NSC 102-2221-E-002-183-MY3 and MOST 103-2622-E-002-031.