MIS tunnel diodes have been used as sensing devices since MIS tunnel currents are very sensitive to surrounding signals. It is found that MIS tunnel currents are perimeter-dependent due to magnifying of finging-field effect. And, it is shown that MIS saturation currents have a strong relationship with Schottky barrier height which indicates that a slight change of Schottky barrier height would make a big move of MIS tunnel currents. In this work, a structure of concentric circle is used. Photo response is examined by light-to-dark current ratio I_light/I_dark. It is found that MIS tunnel currents can be controlled by the voltage bias of the nearby MOS capacitor since the voltage bias of the nearby MOS capacitor(V_G) determines the amount of minority carriers underneath the gate and therefore the diffusion currents to the MIS sensing tunnel diode. As a result, by well controlling V_G, I_light/I_dark ratio can be effectively enhanced. It is also shown that under different intensities of illumination, the maximum of I_light/I_dark occurred at different value of V_G. The underlying mechanism will be explained later.

Fig.1(a),(b) show the top and schematic view of the concentric circle device with radius of inner ring R₁=170um, and outer ring R₂=675um. Fig.2 shows the MIS tunnel currents I_TD versus V_TD with V_G floating. It is found there are different mechanisms dominating when V_TD is biased at negative and positive voltage. When V_TD is biased at negative voltage, I_TD increases with oxide thickness decreasing. This can be explained by gate-injection theory. However, when V_TD is biased at positive voltage, I_TD increases with oxide thickness increasing. This can be explained by Schottky barrier theory, that is when oxide thickness increases, the voltage drop on the oxide will increase so the Schottky barrier height will decrease causing the increase of the tunnel current I_TDshown in Fig.2.

Fig.3(a) shows the MIS tunnel currents I_TD of different oxide thicknesses at dark and with light versus V_TD with V_G floating. Solid line and dash line represent the dark current and light current respectively. Note that the illumination intensity is 7mW/cm².Three different oxide thicknesses were extracted out. Fig.3(b) shows the I_light/I_dark versus V_TD with V_G floating. From the figure, it is shown that the I_light/I_dark is of one order. Fig.3(c) shows the MIS tunnel currents I_TD of different oxide thicknesses at dark and with light versus V_G while V_TD fixed at 2.5V. Fig.3(d) shows the I_light/I_dark versus V_G with V_TD fixed at 2.5V. Surprisingly, it is found that when V_G is biased around -0.5V, I_light/I_dark can be enhanced to two orders. The underlying mechanism is that when V_G is biased near the flat-band, there are nearly no charges underneath the gate, so there are few charges accumulating underneath the tunnel diode causing the low tunnel current and hence the enhancement of I_light/I_darkratio.

In the following, the intensity of light was enhanced to 15mW/cm². Fig.4(a) shows the MIS tunnel currents I_TD of different oxide thicknesses at dark and with light versus V_TD with V_G floating. Also, three different oxide thicknesses were extracted out. Fig.4(b) shows the corresponding I_light/I_dark versus V_TD with V_G floating. It is shown that the I_light/I_dark is of two orders. Fig.4(c) shows the MIS tunnel currents I_TD of different oxide thicknesses at dark and with light versus V_G while V_TD fixed at 2.5V. Fig. 4(d) shows the I_light/I_dark versus V_G with V_TD fixed at 2.5V. Amazingly, it is found that when V_G is biased near -0.9V, the I_light/I_darkratio can be effectively enhanced to four orders.

What’s interesting is that the maximum of I_light/I_dark ratio occurred at different value of V_G compared to the light intensity is 7mW/cm². The reason is that when implementing weak illumination which is 7mW/cm², the number of light-induced carriers is relatively low which means the tunnel currents are still affected by the charges from the gate. As a result, it can be inferred that light currents change relatively high while sweeping V_G. However, when implementing stronger illumination which is 15 mW/cm², the number of light-induced carriers will increase, so the coupling effect from the gate will reduce causing light currents move relatively slightly while sweeping V_G. Consequently, the maximum of I_light/I_darkratio will occur at where the dark currents is lowest.

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] W.C. Kao and J.G Hwu, ECS Trans.,72(2), 223 (2016)