Chapter 4: Conclusion and Future Work
4.2 Future Work
First, the feasibility of using poly-Si devices for sensing many various types of
gases should be investigated, such as the gas with (i.e., carbon monoxide) or without
(i.e., methane) chemical polarity. The difference between the measurement of gas with
or without chemical polarity would reveal useful information. Certain surface
treatments should also be explored to see if they are useful for enhancing the
sensitivity.
Some unclear points remained in the thesis need further efforts to address. For
example, in Sec. 3.4, the recovery of the electrical characteristics after the ammonia
was dispersed away. Passivation of defects with ammonia is obviously not stable. To
further investigate this phenomenon, some instruments such as Fourier transform
infrared transmission spectroscopy (FTIR) would be useful to understand the surface
composition of poly-silicon channel as the device is exposed to the gas. Thermal
desorption spectroscopy (TDS) technique is also suggested to explore the desorbed
species from the surface.
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Figure 1-1: Three types of solid-electrolyte gas sensors [9].
Figure 1-2: Pellistor: catalytic bead detector element.
Figure 1-3: Structure of a silicon planar pellistor.
Figure 2-1: Definition of the S/D
Figure 2-2: Structure of back-gate thin film transistor as gas sensor
Figure 2-3: NWTFT (a) Top and (b) cross-sectional view of poly-Si NWFET with the stacked dielectric nitride layer. These NWTFT devices were offered by Jo-Fen Wei.
Figure 2-4: The measurement system for probing electrical characteristics of the test devices:
The steam (vaporized H2O) is stored in the sampling bag. The nitrogen and ammonia are stored in the steel cylinder. Flow of the gases is controlled by a valve. Figure 2-4 was offered by Yuan-ren Lo.
Gate Voltage(V) 300A channel thickness TFT 500A channel thickness TFT 1000A channel thickness TFT NW FET
Figure 3-1: Transfer characteristics of planar devices with different channel thickness and a NW device.
Figure 3-2: The paths for carriers to go through the channel.
Gate Voltage(V)
Gate Voltage(V)
Figure 3-3: Transfer curves of poly-Si TFTs measured at atmosphere, vacuum, N2, and atmosphere again. The channel thickness is (a) 70Å (b) 300Å, (c) 500Å and (d) 1000Å.
Figure 3-4: Schematic illustration showing the outgas of hydrogen from the grain boundaries of poly-Si due to the pressure difference.
(a)
(b)
Figure 3-5: (a) When returning to the normal air from vacuum, the H-related species diffuse from the moisture in the air to the poly-Si through the grain boundaries and may passivate the defects therein. (b) As the poly-Si film is thick, the deeper part of the film receives less passivation due to diffusion mechanism.
Figure 3-6: Id-time measurement performed on a device with 300 Å channel. Vg = 2.9V and Vd
= 0.5V in the test.
Gate Voltage(V)
0 1 2 3 4 5
Drain Current(A)
1e-12 1e-11 1e-10 1e-9 1e-8
Before Id-time measurement After Id-time measurement
Figure 3-7: Transfer curves of the device tested in Fig.3-6 before and after 60 seconds Id-time test.
Gate Voltage(V)
0 1 2 3 4 5
Drain Current(A)
1e-12 1e-11 1e-10
1e-9 (1):before Id-time measurement
(2):after Id-time measurement(Vg=3.7V, Vd=0.5V) (3):after (2) and Id-Vg(0~-5V) measurement
(4):after (2),(3)and Id-time measurement(Vg=-3.7V, Vd=0.5V)
Figure 3-8: Transfer curves of a device measured after different stages of the designed experimental conditions.
Figure 3-9: Id-time measurements performed on the TFT tested in Fig. 3-8.
Gate Voltage(V)
-5 -4 -3 -2 -1 0
Drain Current(A)
1e-12 1e-11 1e-10
Figure 3-10: Id-Vg characteristics of the device tested in Fig. 3-8 measured with Vg varied from 0 to -5.5 V.
Figure 3-11: The mobile ions (mainly sodium) in the gate oxide were affected by gate bias.
Positive gate bias expels ions away from the gate and made threshold voltage shift negatively, while negative gate bias attracts the ions toward the gate and results in opposite trend in threshold voltage shift.
Gate Voltage(V)
Gate Voltage(V)
Figure 3-12: Transfer curves of poly-Si TFTs measured at atmosphere, vacuum, pure N2, and N2 containing 1.5 ppm ammonia. The channel thickness is (a) 70 Å (b) 300 Å, (c) 500 Å and (d) 1000 Å.
Figure 3-13: Id-time measurement result of a device with 300 Å-thick channel.
Figure 3-14: Schematic illustration for the reaction between ammonia and poly-Si.
Gate Voltage(V)
nitrogen with 43.2% humidity/1.5 ppm ammonia
(a)
nitrogen with 43.2% humidity/1.5 ppm ammonia
(b)
Gate Voltage(V)
nitrogen with 43.2% humidity/1.5 ppm ammonia
(c)
nitrogen with 43.2% humidity/1.5 ppm ammonia
(d)
Figure 3-15: Transfer curves of poly-Si TFTs measured at atmosphere, vacuum, pure N2, N2 with 43.2%
humidity, and N2 with 43.2% humidity/1.5 ppm ammonia. The channel thickness is (a) 70 Å (b) 300 Å, (c) 500 Å and (d) 1000 Å.
Figure 3-16: Schematic illustration for the reaction between humidity and poly-Si.
Figure 3-17: Schematic illustration for the reaction between ammonia/ moisture mixture and poly-Si
Gate Voltage(V)
Gate Voltage(V) Figure 3-18: Transfer curves of poly-Si TFTs measured at atmosphere, atmosphere with 1.5 ppm ammonia, and atmosphere again. The channel thickness is (a) 70 Å (b) 300 Å, (c) 500 Å and (d) 1000 Å.
Gate Voltage(V)
Figure 3-19: The Id-Vg characteristics measured under different ambient conditions. .
Drain Curren
Vita
姓名：施維濤
性別：男
生日：西元 1985 年 6 月 19 日
出生地：台南市
學歷：
國立台南第一高級中學．．．．．．．．．．．．．．．．．．．．．1998.09~2001.06
國立交通大學電機資訊學院學士班．．．．．．．．．．．．．．．．2003.09~2007.06
國立交通大學電子研究所．．．．．．．．．．．．．．．．．．2007.09~2009.06
論文題目：多晶矽薄膜電晶體與奈米線場效電晶體氣體感測器特性比較之研究
A Comparative Study of the Electrical Characteristics between Poly-silicon Thin Film Transistors and Nano-wire Field Effect Transistors
for Gas Sensors