Extraction Methods of Device Parameters - 低溫多晶矽薄膜電晶體元件特性應用於光感測器之研究

Chapter 2 Experimental

2.3 Extraction Methods of Device Parameters

Three important device parameters are extracted and studied in this work: the threshold voltage VTH, the sub-threshold swing S.S., and the field effect mobility μFE. Plenty methods are used to determine VTH, which may be the most important parameter in application. In most of the researches on TFT, the constant current method is adopted. In this work the threshold voltage is determined by this method, which extract VTH from the gate voltage at the normalized drain current ID=10 nA for VD=0.1V.

Sub-threshold swing S.S. (V/dec), is also a typical parameter to describe the control ability of gate toward channel. The sub-threshold swing should be ideally independent of drain voltage and gate voltage. However, in reality, the sub-threshold swing might increase with drain voltage due to short-channel effects. It might as well be affected by the serial resistance and interface traps and therefore become related to the gate voltage. In this work, it is defined as the minimum amount of gate voltage required to increase drain current by one order of magnitude.

The field effect mobility,

μ

FE, is determined from the maximum transconductance gm at low drain voltage, which in this work 0.1 V is used. The transfer characteristics of poly-Si TFTs are similar to those of conventional MOSFETs, so the first order the first order I-V relation in the bulk Si MOSFETs can be applied to the poly-Si TFTs,

which can be expressed as ]

gate oxide capacitance per unit area, W is channel width, L is channel length, VTH is the threshold voltage. If the drain voltage VD is much smaller compared with (VG-VTH), then the ID can be approximated as _D _FE _ox V_G V_TH V_D

L C W

I =μ ( − ) Therefore,

the electron field effect mobility can be expressed as _m

transconductance is defined as _V _const ^ox ^FE _D

Figure 2-1 The cross-section views of n-channel LTPS TFTs with LDD

structure.

Figure 2-2 The photo leakage current and the power variation spectrum of

the light source.

Figure 2-3 Emission spectrum of a phosphor-based white LED.

Chapter 3

Photosensitivity Analysis of Low-Temperature Poly-Si Thin Film Transistor Based on Unit-Lux-Current

3.1 Introduction

Low temperature polycrystalline silicon (LTPS) thin film transistors (TFTs) have attracted much attention for Active Matrix Liquid Crystal Display (AMLCD) and Active Matrix Organic Light Emitting Diode (AMOLED) applications due to the high mobility and the capability of realizing integrated circuits on glass. It could reduce the difficulties of the connection of the surrounding circuits and the cost of the panel [3.1].

The photosensitivity of LTPS TFTs is a significant design consideration for achieving high image quality display panels since it will affect the leakage current. Furthermore, several ambient light sensors using the off current of LTPS TFTs have been reported [3.2-3.7]. Thus, the photosensitive behavior of LTPS TFT off current is of great interest. However, this photo-induced leakage current behavior is not included in the present SPICE device model. In this work, a new parameter is used to analyze the effects of illumination on LTPS TFTs. It’s dependence on the gate, drain bias and temperature. An equation is provided to properly describe ULC under various bias and temperature conditions for further exploration of photo leakage behaviors. A qualitative deduction is developed to account for the photo leakage mechanism. In addition, since LTPS TFTs suffer from huge variation owing to the diverse and complicated grain distribution in the poly-Si film [3.8], the ULC variation will also be discussed.

3.2 Classification and Characterization of Photosensitivity 3.2.1 Sensing Area Consideration

Before the mechanism of photosensitivity is discussed explicitly, it should be first examined where is the most sensitive to the illumination inside LTPS TFTs. A special layout of the TFT with U-shaped source and drain electrodes configuration is adopted in this work, as shown in Fig. 3-1. Twenty-five TFTs are arranged in parallel and separated into two groups. The upper group consists of twelve TFTs and the lower one contains thirteen TFTs. The inner electrodes (about distance 33um) of the TFTs in these two groups are shorted together and so are the outer electrodes (about distance 59um) to form the U-shaped TFT.

An irradiation optical beam with 25µm light spot radius has been used to directly shine on the device. By scanning the beam along the channel direction of U-shaped TFT, the leakage currents of the LTPS TFT are measured in two cases with the inner or the outer electrodes as drain. As shown in Fig. 3-2, anomalous two peaks of the off current are observed. When the measurement is performed with outer electrodes as drain, the distance is larger, about 66um. On the other hand, when the inner electrodes are used as the drain, the distance is shorter, about 32um. The distance between the pair peaks is consistent with device’s real junction distance. It reveals the photo-induced current happens only at the drain side. Therefore, the following discussion of the photosensitivity mechanism will focus only on the drain region.

3.2.2 Definition of the index for photosensitivity and analysis

The typical ID-VG transfer characteristics of the LTPS TFT under illumination from dark to 31320 lux are shown in Fig. 3-3. It can be seen that the off current increases with the intensity of the incident light and it has weak gate bias dependence under

higher ambient light intensity. There are several parameters can be use to describe this behavior of the photo-induced off current. To discuss photo effect of TFTs, the previous study [3.9] used an index RL/D defined as the ratio of the current under illumination (ITotal) to the current in the dark (IDark). RL/D is suitable to evaluate the performance of light sensors. However, it may not be proper to be used to analyze photo leakage mechanism. As shown in Fig. 3-4, because ITotal is less dependent on the gate voltage, RL/D is mostly determined by the behavior of IDark. It can not reflect photo behaviors of TFTs. Therefore, for our discussion, it may be necessary to find another index which can eliminate the influence of IDark.

Fig. 3-5 shows the relationships between drain current and illumination intensity for several bias conditions in the off region. It can be seen that all drain currents are proportional to the amount of radiant illumination. Thus, it can be taken that the total leakage current under illumination (ITotal) is composed of two components: One is the leakage current that is not caused by photo illumination (IDark) which is measured under dark state. And the other part is illumination induced leakage current (IIllum) which means the component induced by illumination. In this paper, we will offset IDark

and only consider IIllum which is defined to be the difference between ITotal and IDark. To analyze the photosensitivity of the LTPS TFTs in detail, we further define the slope of the curve in which the current versus illumination intensity as Unit-Lux-Current (ULC in abbreviation) to be a new index. The physical meaning of ULC is the photo leakage current induced “per unit-photo flux” and independent of the dark current.

Therewith, the total off current Itotal of LTPS TFT can be expressed as

⋅L

where L is the illumination intensity in lux.

3.3 Insight of Photosensitivity

3.3.1 Field Effects on Unit-Lux-Current

Fig. 3-6 shows gate bias dependence of ULC under different drain biases. It is obvious that ULC is change severely under higher drain voltage. Fig. 3-7 shows drain bias dependence of ULC under different gate biases. When drain voltage Vd is lower than 8V, ULC increases linearly with drain bias [3.10], and gate bias effect is negligible. However, when Vd is large enough, ULC increases with drain bias more rapidly and gate bias effect becomes significant. As shown by the arrow line in Fig.

3-7, the linear ULC curve at low drain bias can be fit, and this is one of the two components of the total ULC. This component which increases with drain bias linearly and independent of gate bias is defined as ULC1. Then, the second component which subtracts ULC1 from the total ULC curve is called ULC2.

Furthermore, we plot ULC2 in Fig. 3-8. It is apparent that the log [ULC2] increases with drain bias linearly, indicating that ULC2 increases with drain bias exponentially when Vd is large enough. The parallel curves of log [ULC2] at different gate biases indicate that the dependence of gate bias is also exponential. Thus, ULC can be expressed by a linear combination of these two components as

2 drain voltage dependence and the zero drain bias offset of ULC1, respectively. γ is the scaling factor of ULC2, while η1 and η2 are the parameters about the exponential dependence on drain bias and negative gate bias of ULC2. As shown in Fig.3-9, the calculated results agree with our experiment data very well. The values of fitting factors α, β, γ, η1 and η2 are listed in Table. 3-1.

Moreover, the mechanisms of two ULC components will be further discussed. The ULC can be taken into account both the leakage current induced in the gate-drain overlap depletion and in the lateral depletion regions [3.11]. When device is operating at the low drain voltage, the linear increase with drain bias of ULC1 is attributed to lateral depletion region by the channel-drain junction in reverse bias. In this region, ULC2 is negligible. When drain voltage is large enough, ULC2 increase with drain exponentially and gate bias effect becomes significant. Several mechanisms of leakage current were discussed in previous report [3.12-3.15]. It considered that the reverse lateral depletion at drain region extends and causes gate induced drain leakage (GIDL) in gate-drain overlap depletion junction. The amount of the photo current should be associated with the carrier generation in the space charge region. By the junction reverse saturation current and GIDL, the drain current owing to the GIDL effect is also in an exponential relation. This phenomenon is similar to our case of ULC2 component. The voltage difference between the drain and gate biases corresponds to the magnitude of electric field across the depletion region. A more negative gate bias means that the electric field would get stronger, as the same as a more positive drain bias. It suggests that larger electric field across the drain depletion region causes larger photo effect. Both drain and gate bias affects the electric field strength in the depletion region in a slightly different ways.

3.3.2 Temperature Effects on Unit-Lux-Current

We further take into account the temperature effect of ULC. Fig. 3-10 shows the illumination effect on photo leakage current at different temperatures of 25, 40, and 60^oC under a certain bias condition of (Vd ,Vg) = (10V, -5V). The correlation between ITotal and illumination intensity is still linear at various temperatures.

Drain bias dependences of ULC at different temperatures are shown in Fig. 3-11.

ULC in the range of low drain bias is significantly affected by temperature. While in the higher drain bias range, the temperature effect reduces gradually. From the discussion above, we have separated the ULC intoULC1 and ULC2. It can be seen that ULC1 is actually the term subject to the temperature effect. On the other hand, as shown in Fig. 3-12, ULC2 is totally temperature independent, which means ULC2 is the term purely induced by electric field. Therefore, ULC1 may be induced by mechanism like excess carrier diffusion or thermionic emission and thus it has weak dependence on the electric field, especially gate bias. ULC2 may be induced by mechanism like excess carrier drift or field emission and thus it has strong dependence on the drain and gate biases.

Since the lateral electric field is relatively small, the photo-induced current is a thermally generated current dominantly. The temperature effect on IDark can identify constant activation energy [3.16-3.17], which hints us to add the fitting factors α and β and in eq. (3-3) in the Arrhenius plot. Fig. 3-13 shows the relationship between α and β and 1/kT. These two factors increase with 1/kT exponentially and can be expressed by corresponding fitting parameters. The fitting values of EaA, EaB, A and B are listed in the inset. By the temperature effect discussed above, it is confirmative to separate ULC into two components. ULC1 is thermally activated and might be corresponding to the thermionic emission or carrier diffusion, while ULC2 is independent of temperature and possible owing to field emission or carrier drift.

3.3.3 Mechanism of Unit-Lux-Current

Based on the experimental results of bias, temperature, and sensing location, a more complete mechanism of ULC is proposed to explain photosensitive effect on the leakage current of LTPS TFT. Fig. 3-14 illustrates the band diagrams under the condition of Vg<0 along the channel direction near the drain region at low and high drain biases. In the figure, Wd indicates the length of depletion region at the drain electrode side where electron-hole pairs can be generated under illumination in the poly-Si film. The generated electrons flow toward the drain electrode and the holes flow in the opposite direction. Wd consists of two regions. One is the high hole concentration region in the channel induced by the negative gate bias, the other is in the LDD region. The channel area is shielded by the gate metal, while the LDD region can be shined by the illumination. Based on the Poole-Frenkel effect lowering of a Coulombic barrier and phonon-assisted tunneling due to the electric field applied to a semiconductor [3.18], which enhances thermal emission and the trap-to-band field emission rate, the two components of ULC will be discuss. For the case at low drain bias with light irradiation, when the gate bias is changed, similarly to the abrupt p⁺n junction, the electric field of the other part in LDD region is invariable. Thus, the gate voltage independence of the ULC1 can be explained. As for the Vd effect, the lateral depletion region increases linearly with drain bias, corresponding to the parameter α in eq. (3). With extremely low drain bias, there is still a depletion region in LDD, in accordance with the parameter β in eq. (3). The conduction mechanism of the leakage current in the low drain field is thermal emission [3.19]. Consequently, the parameters α and of β of ULC1 are temperature dependent.

On the other hand, for the high drain bias with light irradiation, the electric field across the lateral depletion region is large enough to fully deplete the LDD region.

Therefore, the increase of drain voltage will increase the electric field within the limited LDD length pinched by the n⁺ region. In such case, the more negative gate bias will also result in the larger field with the same depletion width of the LDD region. The conduction mechanism of the leakage current at the high drain voltage is field enhanced emission in the space charge region [3.20]. The electric field dependence of ULC2 is reflected by the slightly different values of the fitting parametersη1 and η2 in eq. (4).

3.3.4 Devices variation

The uniformity of LTPS TFT is always an important issue. Different devices even fabricated by the same process suffer from serious device variation, especially for the off current. For this consideration, the photo leakage currents would also vary among devices. Fig. 3-15 shows the photo leakage currents with respect to the illumination intensity of several devices on the same glass. It verifies that there is still serious device variation in the aspect of photo leakage current. The results further confirm that the mechanisms of the photosensitivity for the LTPS TFT are closely related to the different defect distribution or density in the grain boundary, alike the case of the dark off current. This issue needs to be overcome before LTPS TFTs can be practically used as the photo sensing device.

3.4 Conclusion

In this chapter, we present detail studies on the factors that affect the photo leakage current like bias condition, temperature, and defect states of the LTPS TFTs. It is found that photo leakage current always exhibits good linear dependence on

illumination intensity. Thus, a new index ULC characterizing the slope of the curve is introduced to discuss the photosensitivity. Furthermore, the mechanism of the photosensitivity for the LTPS TFTs is proposed. It relates to the width and electric field in the lateral depletion region near drain. It is also shown that ULC variation is also related to defects in the depletion region. The empirical equation of ULC provides a potential modeling for simulation of LTPS TFT circuitry considering the photo effect.

Figure 3-1 Photograph of the special U-shaped TFT with an arrow

indicating the scanning path of the illumination beam.

Figure 3-2 The drain current of the U-shaped TFT with the distance that

the illumination beam scanning along the channel direction.

Figure 3-3 The I

-V

transfer characteristics under illumination from

dark to 31320 lux.

Figure 3-4 Gate bias dependence of LTPS TFT photo currents and dark

currents in off region.

Figure 3-5 The relationship between leakage current and illumination

intensity under different bias conditions.

Figure 3-6 Gate bias effect on Unit-Lux-Current at different drain biases.

Figure 3-7 Drain bias effect on Unit-Lux-Current at different gate biases.

Figure 3-8 The second component of Unit-Lux-Current (ULC

₂

) versus

drain bias at different gate voltages.

Figure 3-9 The calculated and experimental data of drain bias effect on

Unit-Lux Current at different gate biases.

Fitting

Factors Value Unit

α 4.02x10^-17 A/(V．Lux)

β 8.83x10^-16 A/(Lux) γ 1.61x10^-18 A/(Lux)

η1 0.42 1/V

η2 0.14 1/V

Table 3-1 The values of fitting factors under front light illumination.

Figure 3-10 Temperature effect on photo leakage current of LTPS TFTs.

Figure 3-11 Drain bias dependence of Unit-Lux-Current at different

temperatures.

Figure 3-12 The second component of Unit-Lux-Current (ULC2) versus

drain bias with different temperatures.

Figure 3-13 Dependence of factors α and β on temperature.

Figure 3-14 A proposed model of ULC mechanism for LTPS TFTs.

Figure 3-15 Photo leakage current variation among different devices.

Chapter 4

Dependence of Photosensitive Effect on the Defects Created by DC Stress for LTPS TFTs

4.1 Introduction

Low temperature polycrystalline silicon (LTPS) thin film transistors (TFTs) have attracted much attention for Active Matrix Liquid Crystal Display (AMLCD) and Active Matrix Organic Light Emitting Diode (AMOLED) applications due to the high mobility and the capability of realizing integrated circuits on the same glass [4.1]. For its application, several ambient light sensors using poly-Si TFTs, which is one of value-added functions for high-end flat panel display, have been reported [4.2-4.7].

The photosensitivity is a significant design consideration for achieving high image quality LCDs. However, it was reported that poly-Si TFTs suffer from several degradation mechanisms, such as hot carrier and self-heating effects [4.8]. Hot carrier effect, which was found that the degradation is related to the increase of strain bond tail states in the band gap of the poly-Si film, and damaged region, is near the drain.

Self heating effect is reported interface states near the source region and the deep states in the poly-Si film near drain can be created [4.9]. It like as the SOI (Silicon on Insulator) devices, originated from the poor dissipation behavior of the substrate. As the operation voltage for VDS and VGS are high, the current conducting in the channel is high and the joule heat, which can be rough calculated as P=IDS*VDS, would become large and if the heat can’t be dissipated in time it will be accumulated in the active region, as shown in Fig. 4-1 [4.10]. As for the main application field for poly-Si

TFTs, the devices are fabricated on glass substrates and the heat transfer coefficients for the films in the device structure are shown in Fig. 4-2 [4.11]. As can be observed in the figure, if joule heat is generated in the poly-Si film during operation, the films surrounding poly-Si film with much smaller heat transfer coefficients than poly-Si film would in turn hinder the active region from dissipating heat. Due to such degradation, the photo-induced leakage current is strongly influenced which is difficultly designed for sensing circuits. In this work, we apply both stress conditions deliberately to manipulate the defect-related photo behaviors and modify Unit-Lux-Current (ULC) [4.12] equations in TFTs. Comparatively, this work focused on how additional non-uniform defects and the photo leakage mechanism influence both lateral and gate-drain overlap depletion.

In this study, the TFTs are measured under different illumination conditions before and after bias stress. One of the stress conditions is that the drain voltage is equal to 20V and the gate voltage is 3V, which is corresponding to the hot carrier effect. The

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