Oxide semiconductor

Figure on this right shows the change according to the stress time of the I-V characteristic curve tested by the forward voltage and the I-V characteristic curve measured by the reverse mode. When conducting I-V sensing based on an electrode to which a high voltage pulse was artificially applied for stress application, when a high voltage was applied to the same electrode, “Forward” was marked, and when applied in the opposite direction, “Reverse” was marked. The part where many changes are observed in the I-V characteristic curve is the thresholder area divided into the line and saturation areas. In the I-V curve of the Figure, a large change is shown when measuring Vds = 10V and 0.1V according to the stress time. In the forward sensing data, the I-V characteristic curve of Vds = 10V showed a very insignificant change according to the stress time, but at Vds = 0.1V, a hump characteristic and a very large decrease in current amount were observed near the gate voltage of 5V. In reverse sensing, current reduction was confirmed for both Vds 10V and 0.1V.

When pulse-based stress was applied to the drain electrode, asymmetrical electrical characteristics were observed in the Forward and the Reverse measurements. Through TCAD device simulation, the simulation was conducted considering the case where an additional defect was generated in a local area among the active channel of the oxide semiconductor. The local defect state implemented an additional defect state with a width of 0.2um from the source electrode edge to the drain edge. When a defect state in a local area was created under the drain edge, it was confirmed that an asymmetric I-V characteristic appeared, as shown in the measured data. The fact that the asymmetric characteristic curve was shown in the Forward and the Reverse I-V measurements confirmed that an asymmetrical local defect state (ALDS) was additionally created at the bottom of the drain edge in the active channel region.

The I-V characteristics secured by the Forward sensing show different behaviors depending on the Vds voltage. In case of Vds 10V data, current decrease is not seen near the thresholder (gate voltage is near 3~5V), but current decrease is clearly observed when Vds voltage is lowered to 0.1V. This has a close relationship with the occurrence of ALDS. As confirmed in the band-diagram, ALDS present at the edge of the drain electrode hinders the flow of electrons injected from the drain. When the Vds voltage is low, the electron flow disturbance phenomenon of ALDS is equally applied in both the Forward mode and the Reverse mode. However, when a high voltage such as Vds voltage of 10V is applied, the electron blocking phenomenon by ALDS is not observed. This is the same as the drain-induced barrier lowering (DIBL) phenomenon that occurs in short channels. It was confirmed that the DIBL phenomenon occurred in the long channel length due to ALDS. It was confirmed that the cause of the different behavior of the I-V characteristic curve according to Vds in forward sensing was caused by the DIBL phenomenon caused by ALDS.

When the device is driven for a long time, abnormal electrical characteristics may be expressed due to accumulation of fatigue of the device. A representative example is the hump phenomenon confirmed in the I-V characteristics. The Hump phenomenon is a phenomenon in which the flow of current changes near the threshold, and is generally caused by a dual current path. The dual current path is a structural problem of the device or a problem that occurs in the process.

In the case of a hump occurring in the reliability evaluation of the device, it may occur by generating a defect state rather than the aforementioned dual current path. Among the defect states generated inside the bandgap, a hump occurs if a defect state with a very narrow energy width (called needle defect states in this study) exists.

It was confirmed that the needle defect state can cause a hump phenomenon both when distributed throughout the channel layer and when distributed only in the local area.