Chapter 3 Experimental Procedure
3.2 Measurements
3.2.2 Magnetic Instrumentation
A gaussmeter measures magnetic flux density (B) at a given point in space.
Most gaussmeters employ Hall effect sensor elements as the magnetic probe [4]. In its simplest form, a gaussmeter is a linear Hall effect sensor with a meter readout. A few of the features to look for in a gaussmeter are：
(1) range – How small, and how large a field can it measure?
(2) accuracy – To what degree does the reading reflect reality?
(3) interface options – In addition to a front-panel display, can it communicate with PCs or other instruments?
Range is important because there are times when you will want to measure fields of a few gauss, and others where you will want to measure fields of several
kilogausses. The need for accuracy needs little, if any, elaboration. Inaccurate instruments can make your life vastly more difficult. Accurate instruments, regularly calibrated, can make development work go more smoothly by reducing one potential source of errors.
We place our devices between two magnets and magnetic field is perpendicular to our devices. We control the distance of the magnets to maintain magnetic field of 0.2 Tesla and use gaussmeter to measure the magnetic flux density (B).
Reference
[1] Yishay Netzer, A Very Linear Noncontact Displancement Measurement with a Hall Element Magnetic Sensor, Proceeding of the IEEE, vol, 69,No. 4, p.491, 1981
[2] Major, R.V. “Current measurement with magnetic sensors” Magnetic Materials for Sensors and Actuators (Digest No. 1994/183), IEE Colloquium on 11 Oct. 1994 Page(s):5/1 - 5/3
[3] R. S. Popovic, "Hall effect devices," Sens. Actuators 17, 39 (1989).
[4] Sensors : a comprehensive survey : magnetic sensors/Gopel, W. & Hesse, J. &
Zemel, J. N. ed. New York/VCH/c1989
Chapter 4
Results and Discussion
In this chapter, we will discuss the device performance of our proposed magnetic sensor. We measured the I-V characteristics by HP4156 semiconductor parameter analyzer. All the Hall effect measurements are made at a magnetic field of 0.5 Tesla and VH is extracted from a mean of three measurements.
4.1 Transistors Characteristics
4.1.1 The output characteristics of our proposed magnetic sensor Fig. 4-1, 4-8, 4-12 show Ids-Vds output characteristics of our proposed magnetic sensor based on TFT structure with varies Vgs following the well square low of a standard thin film transistor under general measuring environment (without magnetic field applied), And we can find that Ids is not obvious until the Vgs arise to 20 V. Obviously, there is no kink effect when Vds arise to 30 V. It is because that the kink current in TFT devices is basically due to the avalanche or impact ionization in the device and is strongly influenced by grain boundary traps. The grain boundary traps can prevent the channel carriers from gaining higher energy, and therefore the impact ionization probability can be reduced as the grain trap density is increased [1].
It is well know that the grain boundary trap density of SPC is usually higher than that of ELA or other recrystallization methods. Therefore, the TFT device with SPC method has higher carrier mobility and less kink effect.
4.1.2 The transfer characteristics of our proposed magnetic sensor Fig. 4-2, 4-9, 4-13 show the transfer characteristics of our proposed magnetic sensors based on TFT structure under general measuring environment (without magnetic field applied). Here we can find that the off leakage current Ids increase with the arising bias voltage Vds. There is less difference on the saturation currents regime on device 1 but much difference on device 3 and device 4. The main reason for lower off-state leakage current of the TFT device with low drain voltage is that the drain electric field is lower and hot carrier effect is less serious [2]. The best on/off current
ratio on device 1 is more than 7 orders in logarithm scale and 6 orders in logarithm on device 3 and device 4. The on-off current ratio is defined as that ratio of the maximum turn-on current to the minimum off-state current. Besides, the four kinds of device have different turn-on current due to different W/L value.
4.2 Sensors Characteristics
4.2.1 The comparison of the Hall voltage versus Vds
Fig. 4-3 shows the comparison of the Hall voltage versus Vds for varies Vgs at 10V to 30 V in steps with W/L = 80µm/150µm and sensing pad is closed to source with the 35µm. In this figure, we find there is a peak value of each characteristic curves. The first peak value happened in the curve of Vgs at 10V, corresponding to the Vds at 1.3V. The peak positions corresponding to the Vds shift to be larger as the Vgs
increasing. The maximum peak value happened at the Vgs with 15V, and then decrease with the Vgs increasing. The peak values appear at some gate and drain voltage. In the linear region, the Hall voltage arises with the Vds increasing and this is because the Hall voltage is dependent of drain current. The Hall voltage decreases with the Vds increasing in the saturation region of the drain current. We think the carrier accumulating pad may be attracted due to electric field between sensing pad and drain. The amount of the attracted carriers is more than that of the accumulated carriers. So the Hall voltage decreases deeply. Due to the sensing pad is closed to source, the voltage gap between sensing pad and drain is larger to absorb carriers accumulating pad easily.
Fig. 4-6 shows comparison of the Hall voltage versus Vds for Vgs = 15V, 20V, 25V, 30V, W/L = 80µm/150µm, and the sensing pad is closed to drain with the 35µm. In this figure, the characteristics are quasi linear for smaller drain voltages and saturate for higher drain voltage. There are smaller peaks in this condition. In other words, when the sensing pad is closed to drain, there are smaller peaks. The peak positions corresponding to the Vds shift to be larger as the Vgs increasing. The smaller peaks may arise from smaller voltage gap between sensing pad and drain due to the sensing pad is closed to drain. In the linear region, VH decrease with gate voltage increasing and drain voltage arriving saturation is higher for higher gate
voltage. The carrier concentration will increase as gate voltage increases. Thus, VH will decrease in this condition. In other words, the carrier concentration is dominate factor influenced VH in this device.
Fig. 4-10 shows comparison of the Hall voltage versus Vds for Vgs =
10V, 15V, 20V, 25V, 30V with W/L = 40µm/100µm and the sensing pad is closed to source with the 20µm. Like Fig. 4-3, it also has peak Hall voltage values when the sensing pad is closed to source. But the peaks are smaller than those in Fig. 4-3. This device is shorter so the voltage gap between sensing pad and grain is smaller. Thus, the attractive ability is smaller than device 1. Besides, we can find the Hall voltage versus Vds presents oscillation. The voltage gap between sensing pad and drain is smaller so the accumulating carrier can compensate the lost attracting carriers. The carrier mobility is proportion to gate voltage and then the current will increase as gate voltage increases. So we can find the Hall voltage is higher as gate voltage increases.
We think the Hall voltage arriving saturation is higher for higher drain voltage and drain current arriving saturation is also higher for higher gate voltage. Drain current arriving saturation shows no more carriers run to sensing pad and directly transport to drain.
Fig. 4-14 shows comparison of the Hall voltage versus Vds for Vgs = 10, 15, 20, 25, 30 V and W/L = 40µm/100µm and the sensing pad is closed to drain with the 20µm. From this figure, there is no peak in lower gate voltage. We think this is related to the channel length of this device. This device is shorter so the voltage gap between sensing pad and drain is smaller to keep the carriers at the sensing pad.
So we can find that there are two kinds of type in Hall voltage variation accompany with the increasing of drain voltage. The first one appears large peak type of Hall voltage when sensing pad is closed to source. The second type of Hall voltage appears rising with the increasing of drain voltage Vds initially, and the following is smaller oscillation after some critical Vds when sensing pad is closed to drain.
4.2.2 The voltage of sensing electrode pad varies with time
Fig.4-4, 4-5 show the voltage of sensing electrode pad varies with time with/without magnetic field bias and Hall voltage versus time for some Vgs and Vds.
We find VH decreases with time increasing and then saturates subsequently. Vsr is voltage with/without magnetic field and VH is Hall voltage. The relation between Vsr and VH is [3]
VH = Vsr (with B) – Vsr (without B) Where B is magnetic field
Fig. 4-5 can demonstrate the Hall voltage decreases with the Vds increasing in the saturation region of the drain current in Fig. 4-3. We apply 100 seconds to observe the change of the Hall voltage and we find the Hall voltage is not constant. The Hall voltage is higher initially and lower eventually due to the accumulating carriers are attracted by electric field between sensing pad and drain. In addition, we also find the time of arriving saturation is longer at higher gate voltage.
Fig.4-7 shows the voltage of sensing electrode pad varies with time with/without magnetic field bias and Hall voltage versus time for Vgs = 25V ,Vds = 15V. This figure also can explain why there are peaks in Fig. 4-6.
Fig. 4-11 shows the voltage of sensing electrode pad varies with time with/without magnetic field bias and Hall voltage versus time for different drain and gate voltage with W/L = 40µm/100µm and the sensing pad is closed to source with the 20µm . We can easily find the Hall voltage increases with the gate and drain voltage increasing in the linear region.
.
4.2.3 The offset voltage Voff
We take Fig. 4-15, 4-16, 4-17, 4-18, and 4-19 examples to explain how we get
the Hall voltage as the offset voltage exists.
Fig. 4-15 shows the voltage difference of sensing electrode pad varies with time with/without magnetic field bias and Hall voltage versus Vds for Vgs = 10,15,20,25,30 V and W/L = 40µm/100µm and sensing pad is closed to drain with the 20µm. There are five groups having different gate voltage in this figure and every group has four curves. Vsensing is the voltage difference of the sensing pad with and without magnetic field and Vsensing is larger with gate voltage increasing.
Fig. 4-16 shows the partial region of the Fig. 4-16 as Vgs = 20V. The black curve is voltage of the first sensing pad without magnetic field and blue curve is that of the second pad without magnetic field. We can find there is voltage gap between the two Hall probes and the gap increases with drain voltage increasing. And we call
this offset voltage Voff [4], [5]. Fig. 4-18 shows comparison of the offset voltage versus Vds for Vgs = 10,15,20,25,30 V. The red curve is voltage of the first contact pad with magnetic field and the green curve is that of the second pad. The red curve shifts upward and the green one does downward. The shift degree increases with drain voltage increasing. From this figure, we know the second pad is accumulated by electron carriers. Thus, the voltage of the second pad with magnetic field is lower than that without magnetic field. Fig. 4-14 shows the voltage difference between the black and red curves and Fig. 4-17 shows the voltage difference between the blue and green curves. Then, the Hall voltage is the mean of the two VH values. If we neglect the offset voltage, we would get wrong Hall voltage as Fig. 4-19.
Reference
[1] Kow Ming Chang, Yuan Hung Chung, Gin Ming Lin, Jian Hong Lin and Chi Gun Deng, “ A novel high-performance poly-silicon thin film transistor with a self-aligned thicker sub-gate oxide near the drain/source regions,” IEEE Electron Device Letters, vol. 22, no. 10, p. 472, 2001.
[2] M. Koyanagi, H. Kurino, T. Hashimoto, H. Mori, K. Hata, Y. Hiruma, T. Fujimori, I-Wei Wu and A. G. Lewis, “Relation between hot-carrier light emission and kink effect in poly-Si thin film transistors” IEEE 1991
[3] R. S. Popovic, "Hall effect devices," Sens. Actuators 17, 39 (1989)
[4] Carvou, E.; Le Bihan, F, “Hall effect magnetic sensors based on polysilicon TFTs”, Sensors Journal, IEEE, Volume: 4, Issue: 5, pp.597 – 602, 2004.
[5] Doyle, J, “High sensitivity silicon magnetic field detector”, Custom Integrated Circuits, IEEE Conference, pp.105 – 108, 2001.
Chapter 5
Conclusions and future work
5.1 Conclusion
The thin film transistor magnetic sensors were fabricated completely with several kinds of geometric structure patterns and different dimensions. Characteristics of sensor devices with different measuring environments are analyzed and discussed.
We can get effective Hall voltage under the magnetic field bias. Magnetic field strength changes the carriers’ behavior in the semiconductor and induces the semiconductor devices appear different electrical characteristics [1].
Different geometric structures appear different magnetic sensitivity under the same space field strength [2]. We also find that the Hall voltage of the sensor pads in the thin film transistor responds with individual operation conditions. Gate bias and Drain voltage are the main two parameters in these experiments. There are two kinds of type in Hall voltage variation accompany with the increasing of Drain voltage. The first one appears the peak type of Hall voltage. The second type of Hall voltage appears rising with the increasing of drain voltage Vds initially, and the following is smaller oscillation after some critical Vds.
Before the Gate bias is less than the threshold voltage, the Hall voltage appears the oscillation wave type with the increasing of Drain voltage. It is due to the carrier electrons accumulated and discharged in the sensing pad alternatively. The accumulated charge will increase the Hall voltage, but the increasing drain voltage will attract the accumulated charge as the drain current. Then, the Hall voltage will turn down immediately.
5.2 Future work
The Semiconductor magnetometers fabricated embedded with thin film transistor by our design mentioned above could provide well sensing analyzability. We will
design different shapes of the Hall sensing layer and study the relation between sensitivity and shape. Thus, we can choose the suitable structures to the application of Hall sensors to every life. Furthermore characteristics of TFT magnetic sensor are not well known, especially how do the magnetic field affect the electrical variety in TFT.
Semiconductor sensors can be applied in many kinds of family and industry applications. Relative process technology should be developed completely. We are deep interesting in optical, electrical and magnetic interactions in semiconductor micro structures. We believe that these technologies would extend and change our life mode in the future. How to integrate the magnetic sensor with other application circuits including some micro electro-mechanical systems in a unit chip is the most difficult challenge.
Reference
[1] “Hall effect in semiconductors” Safa Kasap, Department of Electrical Engineering Universuty of Saskatchewan, Canada
[2] R. S. Popovic, "Hall effect devices," Sens. Actuators 17, 39 (1989) .
Fig. 2-1 The schema of the Hall effect in bulk
Fig. 2-2 the profile structure of Hall sensor
2 VH
V +
2 VH
V−
Fig. 2-3 (a)
Fig. 2-3 (b)
Fig. 2-3 (c)
Fig. 2-3 (d)
Fig. 2-3 (e)
Fig. 2-3 (f)
Fig. 2-3 the different electrode designs of structure
(a) Thermal oxidation
(b) LPCVD a-Si,
recrystallization, and define active layer
(c) Deposit SiO
2dielectric by
PECVD
Fig. 3-1 Process flow of fabricating proposed magnetic field sensor
(d) Deposit poly-Si by
LPCVD and define gate
(e) Ion implantation
(self-align) and
dopant activation
Probe
Probe
Source
Drain
VH+
VH-Source
Drain
Silicon substrate Thermal oxide
Poly silicon
Poly Gate
N N
Silicon substrate Thermal oxide
Poly silicon
Poly Gate
N N
Oxide
Silicon substrate Thermal oxide
Poly silicon
Poly Gate
N N
Fig. 3-2 Profile schematic of the Hall sensor
Fig. 3-3 The schematic of the measurement
Fig. 3-4 The schematic of applied voltage
N S magnet
device
magnetic field
0 5 10 15 20
Fig. 4-1 Ids-Vds output characteristics of our proposed magnetic sensor based on TFT structure with versus Vgs；W/L = 80µm/150µm；sensing pad is closed to source with the 35µm
Fig. 4-2 Ids-Vgs transfer characteristics of our proposed magnetic sensor based on TFT structure for Vds = 5,10,15,20,25 V；W/L = 80µm/150µ m；sensing pad is closed to source with the 35µm
0 5 10 15 20 25 30 0.000
0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040
VH (V)
Vds (V)
Vgs (V) 10 15 20 25 30
Fig. 4-3 Comparison of the Hall voltage versus Vds for Vgs = 10,15,20,25,30 V；W/L = 80µm/150µm；sensing pad is closed to source with the 35µm
0 20 40 60 80 100 with/without magnetic field bias and Hall voltage versus time for Vgs = 15V ,Vds = 3.5V；W/L =
Fig. 4-5 The voltage of sensing electrode pad varies with time with/without magnetic field bias and Hall voltage versus time Vgs = 20V ,Vds = 7.5V；W/L = 80µm/150µm；
sensing pad is closed to source with the 35µm
0 5 10 15 20 25 30 0
3 6 9 12 15
VH (mV)
Vds (V) Vgs (V)
15 20 25 30
Fig. 4-6 Comparison of the Hall voltage versus Vds for Vgs = 15,20,25,30 V；W/L = 80µm/150µm；sensing pad is closed to drain with the 35µm
0 20 40 60 80 100 4.60
4.61 4.62
2 4 6 8 10 12 14 16
VH (mV)
Vsr (V)
Time (s)
Vgs=25V Vds=15V
Vsr W/O B Vsr W B VH
Fig. 4-7 The voltage of sensing electrode pad varies with time with/without magnetic field bias and Hall voltage versus time for Vgs = 25V ,Vds = 15V；W/L = 80µm/150µm；
sensing pad is closed to drain with the 35µm
0 5 10 15 20 25 30
Fig. 4-8 Ids-Vds output characteristics of our proposed magnetic sensor based on TFT structure with versus Vgs；W/L = 40µm/100µm；sensing pad is closed to source with the 20µm
Fig. 4-9 Ids-Vgs transfer characteristics of our proposed magnetic sensor based on TFT structure for Vds = 2,6,10,14,18 V；
W/L = 40µm/100µm；sensing pad is closed to source with µ
0 5 10 15 20 25 30 0
5 10 15 20 25 30 35 40 45 50
VH (mV)
Vds (V) Vgs (V)
10 15 20 25 30
Fig. 4-10 Comparison of the Hall voltage versus Vds for Vgs = 10,15,20,25,30 V；W/L = 40µm/100µm；sensing pad is closed to source with the 20µm
0 10 20 30 40 50 60 70 80 90 100
Fig. 4-11 The voltage of sensing electrode pad varies with time with/without magnetic field bias and Hall voltage versus time for Vgs = 25V ,Vds = 20V；W/L = 40µm/100µm；
sensing pad is closed to source with the 20µm
0 5 10 15 20 25 30
119 with closed to drain position
Fig. 4-12 Ids-Vds output characteristics of device 1 versus Vgs；
W/L = 40m/100m；sensing pad is closed to drain with the 20m 119 with closed to drain position
Fig. 4-13 Ids-Vds output characteristics of device3 versus Vgs；W/L
= 40m/100m；sensing pad is closed to drain with the 20m
0 5 10 15 20 25 30 0
10 20 30 40 50 60 70 80 90 100
VH (mV)
Vds (V) Vgs (V)
10 15 20 25 30
S1 B and NB
Fig. 4-14 Comparison of the Hall voltage versus Vds for Vgs = 10,15,20,25,30 V；W/L = 40µm/100µm；sensing pad is closed to drain with the 20µm；S1 is one of the sensing pads
0 5 10 15 20 25 30
Fig. 4-15 The voltage difference of sensing electrode pad varies with time with/without magnetic field bias and Hall voltage versus Vds for Vgs = 10,15,20,25,30 V；W/L = 40µm/100µm；sensing pad is closed to drain with the 20µm
0 5 10 15 20 25 30 0
20 40 60 80 100 120 140 160 180
VH (mV)
Vds (V) Vgs (V)
10 15 20 25 30
S2 B and NB
Fig. 4-17 Comparison of the Hall voltage versus Vds for Vgs = 10,15,20,25,30 V；W/L = 40µm/100µm；sensing pad is closed to drain with the 20µm；S2 is another pad
0 5 10 15 20 25 30
Fig. 4-18 Comparison of the offset voltage versus Vds for Vgs = 10,15,20,25,30 V；W/L = 40µm/100µm；sensing pad is
Fig. 4-19 The voltage gap between two pads with magnetic field versus Vds for Vgs = 10,15,20,25,30 V；W/L =
40µm/100µm；sensing pad is closed to drain with the 20µ
簡歷
姓 名：蕭宇盛 性 別：男
出生日期：民國 69 年 03 月 16 日 出 生 地：台灣省嘉義縣
住 址：高雄市苓雅區福德二路 75 巷 1 號
學 歷：國立成功大學水利系 （民國 87 年 9 月~92 年 6 月）
國立交通大學電子工程所碩士班 （民國 92 年 9 月~94 年 6 月）
碩士論文：薄膜電晶體磁場感測器之研究
A study of Hall effect magnetic sensors based on polysilicon TFTs