Enhancement of Light-to-Dark Current Ratio Via Coupling Effect for MIS (p) Tunnel Diode Photo Sensors
Authors: Hou Wei-Tzu , Kao Wei-Chih , Hwu Jenn-Gwo 
Published: February 2020

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 Ilight/Idark. 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(VG) 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 VG, Ilight/Idark ratio can be effectively enhanced. It is also shown that under different intensities of illumination, the maximum of Ilight/Idark occurred at different value of VG. 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 ITD versus VTD with VG floating. It is found there are different mechanisms dominating when VTD is biased at negative and positive voltage. When VTD is biased at negative voltage, ITD increases with oxide thickness decreasing. This can be explained by gate-injection theory. However, when VTD is biased at positive voltage, ITD 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 ITDshown in Fig.2.
Fig.3(a) shows the MIS tunnel currents ITD of different oxide thicknesses at dark and with light versus VTD with VG 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 Ilight/Idark versus VTD with VG floating. From the figure, it is shown that the Ilight/Idark is of one order. Fig.3(c) shows the MIS tunnel currents ITD of different oxide thicknesses at dark and with light versus VG while VTD fixed at 2.5V. Fig.3(d) shows the Ilight/Idark versus VG with VTD fixed at 2.5V. Surprisingly, it is found that when VG is biased around -0.5V, Ilight/Idark can be enhanced to two orders. The underlying mechanism is that when VG 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 Ilight/Idarkratio.
In the following, the intensity of light was enhanced to 15mW/cm2. Fig.4(a) shows the MIS tunnel currents ITD of different oxide thicknesses at dark and with light versus VTD with VG floating. Also, three different oxide thicknesses were extracted out. Fig.4(b) shows the corresponding Ilight/Idark versus VTD with VG floating. It is shown that the Ilight/Idark is of two orders. Fig.4(c) shows the MIS tunnel currents ITD of different oxide thicknesses at dark and with light versus VG while VTD fixed at 2.5V. Fig. 4(d) shows the Ilight/Idark versus VG with VTD fixed at 2.5V. Amazingly, it is found that when VG is biased near -0.9V, the Ilight/Idarkratio can be effectively enhanced to four orders.
What’s interesting is that the maximum of Ilight/Idark ratio occurred at different value of VG 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 VG. 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 VG. Consequently, the maximum of Ilight/Idarkratio 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
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Figure 1