Two states tunneling current for MIS (Al/SiO₂/p-Si) coupling tunneling diodes, i.e., read and gate diodes, were observed with ultra-thin oxide. For the gate MIS tunnel diode, after the electrical treatments of positive/negative voltage stressing for sufficient time, high and low current states can be achieved with electron trapping/de-trapping in the device edge [1]. With the aid of separated read diode, the two states could be enhanced by coupling effect. It is known that the tunnel current of MIS(p) structure is strongly dependent on the effective Schottky barrier height of majority carriers, a slight change of the effective Schottky barrier height by the surrounding signal would induce a considerable change of the tunneling current. It was proposed that the increase of lateral minority carrier diffusion current results in the decrease of the effective Schottky barrier height at the device edge, which leads to the increase of the current of MIS tunnel diode [2].

In this work, two states phenomenon of read tunneling current behavior induced by constant voltage stress on nearby MIS gate capacitor was demonstrated. Fig. 1 shows the device structures of top view and cross section. The radius of the inner circle of read diode, R_in is 125 µm. The space width between inner read diode circle and outer gate diode ring, S, is 30µm. The outside radius of ring, R_out is 576 µm. The thickness of oxide is 2.4 nm.

Fig. 2 shows the currents of the inner circle MIS read tunnel diode I_TD versus V_TD, after applying two different electrical treatments of constant voltage stress on outer the ring MIS gate tunnel diode, V_G. The set state is defined as the I-V curve after the stressing voltage V_G of -2 V for 300 s, whereas the reset state is 2 V for 200 s. The fresh I-V curve is without applying positive/negative voltage stressing on the gate device. It is clear that the I-V curves of read diode are diverged out at V_G > -0.5 V. The current decreases after applying V_G of -2 V for 300 s. The mechanism is due to the decreasing of lateral diffusion current. While applying negative voltage stress V_G of -2 V for 300 s on gate diode, the electrons trap in oxide layer under the outer ring region. Hence, the minority carrier density under the ring region decreases after negative voltage stress V_G, and the gradient of carrier concentration also decreases. It leads to lower lateral diffusion current and change the Schottky barrier at the edge of the circle read device. Due to Schottky barrier height increasing, the tunneling current is lower than fresh I-V curve.

Fig. 3 shows retention characteristics with various read voltage. It is noted that the negative current is caused by negative read voltage. Fig. 3(a) shows current-time (I-t) curves after V_G of -2 V for 300 s and Fig. 3(b) shows I-t characteristic after V_G of 2 V for 200 s. Since the electrons are de-trapped after the positive voltage stressing, the reset currents in Fig. 3(b) remain at constant values. It is obvious that the restored reset state currents in Fig. 3(a) are consistent the currents in Fig. 3(b).

Fig. 4(a) shows normalized curves of Fig. 3(a). Fig. 4(b) is for MIS diode without ring structure, the retention degrades with the increasing of read voltage. Nevertheless, for the structure in this work with the electrical operators of stressing on outer ring and read on circle, the performances of retention are not declined with the change of read voltage from -0.2 V to 1.8 V. It might exist an optimal read voltage which gives rise to the best retention performance.

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.

References

[1]  T. Y. Chen and J. G. Hwu, Appl. Phys. Lett., 101, 073506 (2012).

[2]  C. S. Liao and J. G. Hwu, IEEE Trans Electron devices, 61, 2061 (2015).