Chapter 3 Results and discussion
3.1 The influence of leakage current on performance curve
Leakage current can have a significant impact on performance of OTFTs. In this section, we had adequately explained the origin of leakage current on OTFTs, and rigorously analyzed the distortion on performance curve. Additionally, methods of measurement, and the systematic comparison of the patterned active region and the lack one on leakage current had mentioned.
3.1.1 The adverse impact on output characteristics curve
Fig. 3-1(a) shows an output characteristics curve of our device which without patterning the active layer (W/L = 1600/200µm). It is observed that all the ID curves didn’t approximate zero at VD = 0V. In this respect, we showed an expanded figure in Fig. 3-1(b) to obtain a clear observation. In ideal, if the VD = 0V, it should not be appeared current due to the lack of electric field between source/drain electrodes. However, we found that the ID curves increased toward the positive as the |VG| increased while |VD|→0. The origin of ID offset was attributed to the electric field between drain electrode and gate electrode. When VD = 0V, drain electrode had been the highest potential in this system and then would induce a leakage current (ID leakage) from drain electrode toward gate electrode, and caused a positive shift in ID
curve. Because the ID leakage was following to the gate electrode, so we could use the IG to monitor the influence of ID leakage on output characteristics curve.
As can be seen in Fig. 3-1(c), IG (The negative quantity indicated that the current flowed toward the gate electrode) exhibited a dramatic increase while VD closed to 0V. In other words, the influence of gate current on output characteristics curve was more violent while VD
closed to 0V. But on the other hand, the curve of VG = -20V indicated IG = -1E-8A at VD = -20V. It was attributed to the leakage current (IS leakage) due to the electric field between source
source/drain electrodes and gate electrode could be given by
IS ISleakageIDS (3-1) ID IDS IDleakage (3-2) IG ISleakage ( IDleakage) (3-3) where the minus means that the current flows into electrode.
In order to obtain clear observation in IS leakage, we took the minus sign in IS, and illustrated together with the ID in the Fig. 3-1(d). As the figure shows, IS did not approach ID in the entire curve. In ideal, the IS must be identical with ID. This is because parts of current, which flowed from source electrode, flowed into the gate electrode.
Fig. 3-2(a) shows a transfer characteristics curve which measured as the same device in Fig. 3-1. In ideal, the channel should not accumulate charges at VG = 0V, but it is observed that the ID curves had a negative increment while VG approached 0V. The origin of negative shift in ID curve is attributed to the following reasons:
1. The leakage current (ID leakage) due to the electric field between drain electrode and gate electrode.
2. The leakage current (IDS leakage) due to the electric field between source electrode and drain electrode.
Comparing the ID leakage with ID leakage that mentioned in Fig. 3-1, the two leakages flow had opposite direction, and this direction of current flow, which flowing into drain electrode, led to a negative shift in ID curve. Secondly, the IDS leakage most likely correlated with the quality of organic active layer.
On the other hand, the shift of ID curve while VG approached 0V would attract adverse impact in device characteristics. In general, the ID numeral must be modulated with modulus, and illustrate transfer curve with logarithm scale while we characterizes the performance of a device. As can be seen in Fig. 3-2(b), if the ID curve had negative increase while VG
approached 0V, the off state current (ID current when VG approached 0V) would increase.
If the off-state current had positive shift, on/off ratio would decreased.
Additionally, in contrast with output characteristics curve, we also illustrated the IG and ID + IS curve in Fig. 3-2(c) / (d). As can be seen in Fig. 3-2(c), the IG decreased as the |VG| increased. In other words, the influence of ID leakage on transfer characteristics curve was more violent while VG closed to 0V. But on the other hand, we found that IG had a negative quantity while |VG| around surmounted 6V. This is attributed to the IS leakage, which flowed into gate electrode, and the two opposite direction of current in gate electrode caused positive, negative numerals that varied according to |VG|. In this respect, we had illustrated the current of source + drain electrodes in Fig. 3-2(d). The difference of each ID/IS agreed with the existence of IS leakage.
3.1.2 The methods of measurement
Whereas the influence of leakage current on performance curve, we suggested several methods to measure the leakage current as follows:
1. The measurement of IDS leakage: As can be seen in Fig. 3-3, the gate electrode was floated and the drain electrode was applied sweeping voltage (0~-20V), and then we measured the ID as IDS leakage.
2. The measurement of IG leakage: As shown in Fig. 3-4, the source/drain electrodes was grounded and the gate electrode was applied sweeping voltage (0~-20V), and then we measured the IG as IG leakage.
3. The measurement of ID leakage/IS leakage: As illustration in Fig. 3-5(a), the drain electrode was grounded and source electrode was floated, and after applying the sweeping voltage (0~-20V) at gate electrode, we measured the ID as ID leakage. At the opposite steps of measurement, the IS leakage was measured and illustrated in Fig. 3-5(b). Although the IS leakage did not have direct impact on performance curve, it could be used to critically analyze the uniformity of one transistor on a die.
patterning active layer and without
As the origin of leakage current, which had mentioned in section 3.1.1, the device would compare with the device that had patterned active layer. Fig. 3-6(a) shows the output characteristics curve of a device which had the same W/L ratio as the device in Fig. 3-1, and the organic active layer had been patterned. Apparently, the phenomenon of distortion was reduced, and we could obtain a clear observation in the expand illustration in Fig. 3-6(b). In regard to the reduction of distorted phenomenon on curve, it was attributed to the reduction of disordered leakage path after patterning the organic active layer, and it was confirmed in Fig.
3-6(d), which we used the third method of measurement in section 3.1.2. Additionally, for comparison with Fig. 3-1(a) in Fig. 3-6(c), it is observed that the on-current of the patterned device was less than the without patterning one. The gate electrode of device was not patterned, so the active layer would be induced charges everywhere, and the on-current would reduced as the area of organic active layer decreased.
In contrast to Fig. 3-1(c), the IG-VD curve was shown in Fig. 3-7(a). As the illustration, IG
had been compressed with patterning the active layer. Furthermore, we compared the gate current with Fig. 3-1(c), and illustrated in Fig. 3-7(b). It is clearly observed that IG was dramatically reduced wherever VG = 0V or -20V, and finally it was confirmed in Fig. 3-7(c), which we used the second method of measurement in section 3.1.2.
In contrast to Fig. 3-1(d), the ID + IS-VD curve was shown in Fig. 3-8(a). As the illustration, the difference between ID and IS was decreased. It is attributed to the reduction of ID leakage and IS leakage. As mention in Eq. (3-1) and Eq. (3-2), the difference of ID and IS were reduced as the ID leakage and IS leakage decreased. Furthermore, we compared the source current with Fig. 3-1(d), and illustrated in Fig. 3-8(b). It is clearly observed that IS was dramatically reduced wherever VG = 0V or -20V, and finally it was confirmed in Fig. 3-8(c), which we used the third method of measurement in section 3.1.2.
As shown in Fig. 3-9(a), a transfer characteristics curve with the same device as in Fig.
3-6 was observed that the negative increment while VG approached 0V was reduced. That is
to say that on/off ratio would be increased due to the reduction of off-current. As can be seen in Fig. 3-9(c), it is clearly observed, the on/off ratio had been conspicuously increased due to the reduction of disordered path of leakage. In contrast to Fig. 3-2(c), the IG-VG curve was shown in Fig. 3-9(d), Fig. 3-9(e). As the illustration, the IG had been considerably decreased in comparison to without pattern. Finally we used the first method of measurement, which had been mentioned in section 3.1.2, to confirm it in Fig. 3-9(f).