Light Illuminated Experiment

4.3.1 Motivation and experiment Steps

Because of equipment limitation, only top-light illuminated experiment at lowe temperature is performed. It’s to distinguish the difference of photo leakage current between top-light and bottom-light illuminated.

To investigate electrical characteristics under top-light and bottom-light illuminated at room temperature. We have two types TFTs with different [SiF4] / [ SiH4] ratio:

(1) Standard TFT with active layer a-Si:H

(2) SiF4(9sccm) TFT with active layer a-Si:H(:F)

Fig. 4.12 shows the device was illuminated under two direction ：

(a) under front-side illumination(Top-Light) and (b) gate-side illumination(Bottom-Light). The fluorine incorporated amorphous silicon [a-Si:H(:F)] and amorphous silicon (a-Si:H) were illuminated with top-light and bottom-light. The backlight module’s light intensity was 3500 nits.

4.3.2 A discussion on experiment results

Fig. 4.13 shows the comparison of VG-ID characteristics between top light illuminated and bottom light illuminated, (a)STD (b)9sccm.

When the light illuminated the threshold voltage decreases and field-effect mobility increases slightly. The decrease in the threshold voltage is due to the TFT

active layer illuminated by light, causes a large number of electron–hole pairs created to lead the shift of the Fermi level in the band gap toward the conduction bang edge. The Fermi level in the band gap toward the conduction bang edge lead the threshold voltage decreases and the transistor turn on more easily.

For photo leakage current under top-light illuminated, is much higher than that under bottom-light illuminated of inverted-staggered a-Si:H TFT. Top-light illuminate directly into channel but bottom-light illuminated the PN junction of source & drain due to light covered by gate metal layer under bottom-light illuminated.

Fig. 4.14 shows VG-ID transfer curves of STD and SiF4-9sccm a-Si TFTs under the different light illuminated intensity；On current does not increase along with light illuminated intensity is raising, due to turn on operation of TFT devices located at Saturation area. But off current is increasing along with light illuminated intensity raising. As S.S variation as Vg= -5V~ 0V, photo leakage current by Vg=

-20V~ -5V is increasing along with light illuminated intensity raising and VG-ID transfer curve is vertically lifted.

On the other hand, VG-ID transfer curve is vertically falling down by light illuminated intensity descending and it’s helpful to compare with the VG-ID transfer curve measured of bottom light illuminated and top light illuminated.

To adjust light illuminated intensity and have photo leakage current value of top light illuminated is closed to bottom light illuminated. The comparison of VG-ID characteristics between top light illuminated and bottom light illuminated show at Fig. 4.15. As Vg<-10V, it’s detected the different current transfer characteristics：

Bottom light illuminated：As Vg<-10V, photo leakage current is increased, and, photo leakage current of SiF4-9sccm a-Si TFT is higher than that of the STD a-Si

TFT. But, current transfer characteristics as Vg>-10V is opposite of that. The off-state dark leakage current of a-Si:H TFT mainly originates from the photo-induced hole current at the interface between a-Si:H and gate SiN layers. In contrast, electrons are the majority carriers of off-state current for the a-Si:H TFT under light illumination, since electron mobility is much higher than that of hole.

[4-3]

Due to higher vertical electrical field at source and drain separated the electron-hole pairs in channel and hole current occurring, photo leakage current is increasing along with gate bias raising.[4-10] Also, due to the SiF4-9sccm a-Si TFT increase in the defect density by F incorporation and due to shift of the Fermi level in the band gap toward the valence band edge[4-11]. Photo Leakage Current of the SiF4-9sccm a-Si TFTs is higher than that of STD a-Si TFTs shown in Fig.

4.16(b). Under bottom illumination, photo-leakage-current of the a-Si(:F) TFT is higher than STD TFT at larger negative gate votage(-15V~-20V) due to the hole accumulation.

Top light illuminated ： By way of ID-VG transfer characteristics measured, photo leakage current not increasing as Vg<-10V is mainly due to no vertical electrical field as electron-hole pairs produced by light illuminated in channel. As hole attracted as VG<-10V, electron is recombined with hole by state shown as Fig.

4.16(a). For the SiF4-9sccm a-Si TFT, due to shift of the Fermi level in the band gap toward the valence band edge[4-12]. Thus, current is slightly increasing along with gate bias raising but the whole ID-VG transfer characteristics is not effected.

Current value, device width, active layer, DOS is related with light illuminated intensity.

4.3.3 Summary

The off-state photo leakage current of a-Si:H(:F) TFT is smaller than those of conventional a-Si:H TFT in the density of states (DOS) limited region. It’s also because the a-Si:H TFT under back light illumination was in the non-equilibrium state( pn>ni^2). As a result, the trap states played the role of recombination centers.

As VG= -5V~-20V, off-state current transfer curve up and down along with light illuminated intensity raising or descending but the trend shifts horizontally due to the off-state photo leakage current conducted by electron–hole pairs

produced during light illuminated. The off current just raised without any leakage behavior change.

The photo leakage current of a-Si:H(:F) TFT operated in the small negative gate voltage (VG＞-10 V) and 1V drain voltage is less than conventional a-Si:H TFTs. Because the electric field is not large enough to separate the photo induced electron-hole pairs, the increased density of states serving as recombination centers in a-Si:H(:F) channel material has resulted in the lower photo leakage current.

However, with the hole conduction region (VG＜-10 V), the larger photo leakage current was observed. According to previous study,[4-6] The larger off-state photo leakage current is due to the faster hole channel accumulation in the larger negative gate voltage. The smaller hole accumulated voltage also indicated that the undoped a-Si:H(:F) shows near p-type-like behavior.

There is no high electrical field to separate the electron-hole pairs and the hole conduction region is as VG＜-10 V. Thus, photo leakage current is not increasing. In other words, photo leakage current under top light illuminated is not caused by the hole conducted as gate bias raising

Typical a-Si:H is n-type material even though the Fermi level lies near the midgap of undoped a-Si:H because the electron mobility is at least 10 times higher than that of hole.[4-4]

The undoped a-Si:H(:F) TFTs have shown larger threshold voltage and smaller field effect mobility.