The negative differential resistance (NDR) characteristic of the gated-MIS (p) structure was proposed in the previous study [1]. By varying the biasing procedure of the three terminals, the NDR behavior would change. In this work, the double NDR characteristic was formed by a special biasing technique.

The schematic top view and cross section of the substrate grounded MISIM device is shown in Figure 1. The inner circle is the main tunnel diode (I) electrode. The outer ring is the ring (R) electrode. The back contact (B) was grounded and biases of inner TD (I) and outer ring TD (R) electrodes were swept simultaneously with respect to ground with an offset of V_IR. By this biasing method, the double NDR characteristic can be seen at the inner-MIS current (I_I) and would vary with different offset voltages (Figure 2).

Figure 3 (a) shows the currents of the three terminals of inner, ring and back contact at V_IR= 1.3 V. The whole I_I-V_IB curve can be divided into four regions by several particular V_IB’s, i.e., PDR1, NDR1, PDR2 and NDR2 (Figure 3 (b)). Start from the PDR1, the four regions can be explained by V_IB status as follows. The total I_I current can be divided in two components which are the vertical current (J_V) from inner-MIS to back contact and the lateral current (J_L) from inner-MIS to ring-MIS. At PDR1, the current increases as V_IB becomes less negative (Figure 4 (a)). For the vertical current, the electron would inject from the inner-MIS metal which results in the negative current (J_V<0) and this injection would decrease as V_IB become less negative. On the perspective of the lateral current, since the voltage of inner-MIS is positive relative to ring-MIS with V_IR= 1.3 V, the electrons therefore flow from ring to inner which is in the opposite direction of vertical current (J_L>0). Because of the decreasing of the vertical current, the net current (J) of the inner-MIS becomes less negative and even more positive as V_IB increases in the positive bias sweep direction which exhibits the PDR characteristic. When V_IB exceeds the voltage near zero and becomes positive, the vertical current would remain small because of the saturated off state current characteristic of the vertical tunnel diode. Hence, the vertical current can be neglected for V_IB>V_peak1. Figure 4 (b)-(d) shows the band diagrams of the lateral directions from inner to ring of NDR1, PDR2 and NDR2 regions. The voltage bias between inner and ring is kept constant with V_IR= 1.3 V, but the voltage drops of the two MIS which are determined by V_IB and V_RB would change as V_IB changes. This brings the non-constant current of I_I. At NDR1, as shown in the band diagram of Figure 4 (b), because of the large negative voltage drop of ring, the ring-MIS is in accumulation region and the electric field in silicon have strong ability to attract the electron from the ring metal. Therefore, the lateral current in NDR1 is dominated by the electrons injection from ring. As V_IB increases, V_RB increases also (be more positive) at the same time and results in the decrease of electron injection. The current in inner therefore decreases which is the first NDR characteristic. At PDR2, V_RB exceeds the flat band voltage (≈ -0.9 V) and the direction of the electric field under ring converts (Figure 4 (c)). The inversion charge emerges under ring and increases as V_RB increase. Also, in this region, by Figure 3 (a), the electron injection from ring mainly flows to the back contact because of the equal large currents of back contact and the ring as observed in Figure 3 (a) as |I_R| ≈ |I_B|. The electron injection flow on the lateral direction is therefore small. In this situation, the lateral current in PDR2 is dominated by the coupling of the inversion charge under ring that can overcome the barrier (Ф_S) and reach to the inner side. Because of the increasing of the inversion charge, the lateral current increases and results in the PDR characteristics. At last, at NDR2, the current decreases again. Although the inversion charge increases as V_RB increases, the barrier height (Ф_S) for inversion charges to flow from ring to inner also increases (Figure 4 (d)). The total electron flow would therefore decreases when the barrier is high enough.

This work was supported by the Ministry of Science and Technology of Taiwan, ROC, under Contract No. MOST 105-2221-E-002-252-MY3 and MOST 105-2622-8-002-001.

[1] C.S.Liao and J.G.Hwu, ECS Trans., 69, 229,(2015).