Negative Gate Transconductance in MIS Tunnel Diode Induced by Peripheral Minority Carrier Control Mechanism

Abstract

Negative gate transconductance (NGT) has been demonstrated in various devices like gated resonant tunneling devices, modulation doped FETs, and gate/source overlapped heterojunction tunnel FETs, and has been used for ultra-high frequency filters, multi-level logics, and phase encoder applications. [1] In this work, NGT is also observed in a metal-insulator-semiconductor (MIS) tunnel diode. The current of an MIS tunnel diode is very sensitive to surrounding signals. Therefore, MIS tunnel diodes have been used as solar cells and sensing devices. It was found that the saturation current of the MIS tunnel diode is mainly flowing along the edge of the device due to the fringing field effect. [2] Also, it was found that the saturation current increases with oxide thickness. [3] It was believed that the saturation current is dominated by the Schottky diode reverse­­­-bias current, the thicker the oxide, the lower the effective Schottky barrier height, and therefore the current is larger. In previous studies [3], it was also observed that the saturation current of the MIS tunnel diode may increase with increasing light intensity or temperature. It was attributed to that the light or temperature would change the minority carriers distribution profile at the fringe of the device. The lateral diffusion current of minority carriers is therefore changed, which affects the effective Schottky barrier height and the current at device edge. In this work, the edge Schottky diode current of an MIS tunnel diode, Al/2.2 nm SiO2/Si(p), controlled by the gate voltage of a nearby MIS tunnel gate capacitor (with the same structure as the MIS tunnel diode, but it functions as a gate capacitor with ultra-thin oxide) is demonstrated. Figure 1 shows the schematic diagrams of the fabricated MIS tunnel diode (the inner circle device) with an MIS tunnel gate capacitor (the ring device) nearby. Figure 2(a) shows the measured MIS tunnel diode saturation currents (ID ) versus the gate voltages of the MIS tunnel gate capacitor (VG ). NGT in ID is observed. Note that the valley of the kink matches the flat-band region of the MIS tunnel gate capacitor. Figure 2(b) shows the TCAD simulation of the electron concentration at the gate edge, ngap (x=0), versus VG while VD =3 V. Similar behavior to NGT is also observed, which evokes the correlation between the ngap and ID . Figure 3 shows the schematic diagrams of the mechanism of how VG affects ID . Note that larger gradient of ngap at drain depletion region edge (x=S *) leads to larger lateral diffusion flux of electrons, F e-, which lowers the effective Schottky barrier height of holes, φ h , at the edge of MIS tunnel diode, and increases ID (≈Ih , the Schottky diode current of holes). In short, ID increases with the gradient of ngap (x=S *). The gradient of ngap (x=S *) is changed as follows: (a) While VG is in accumulation region, the gate electrons (ninj ) inject into the Si substrate due to the quantum tunneling effect, which increases the ngap (x=0) and the gradient of ngap . (b) While VG is in flat-band region, the electron injection decreases due to the lower electric field, which decreases the ngap (x=0) and the gradient of ngap . (c) While VG is in depletion region, the depletion region comes out and the inversion charge, ninv , rises, which increases the gradient of ngap . (d) While biasing VG further positive, the electrons at the gap are attracted toward the gate by the substrate electric field at gate edge, ε s,edge , and then the electrons tunnel to the gate metal, which decreases the ngap (x=Wdep ) and the gradient of ngap . The NGT mechanism in ID is therefore concluded. 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. Figure Captions: Figure 1. Schematic diagrams of the fabricated device structure. Figure 2. (a) MIS tunnel diode saturation currents (ID ) versus the gate voltages of the MIS tunnel gate capacitor (VG ). (b) TCAD simulation of the electron concentration at the gate edge, ngap (x=0), versus VG while VD =3 V. Figure 3. Schematic diagrams of the mechanism of how VG affects ID (≈Ih ), as VG is in (a) accumulation, (b) flat-band, (c) depletion, and (d) deep depletion regions. The values of VFB , VG 1, and VG 2are shown in Figure 2(a).