TCAD Modelling of Magnetic Hall Effect Sensors
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe authors report on the simulation and experimental validation of magnetic Hall effect sensors based on GaN. While the work appears to be of interest to relevant researchers, the current presentation and the lack of clear reasoning and scientific explanation make it unsuitable for publication at this time. The authors should pay more attention to the illustrations and figures. Therefore, I regret to say that I cannot recommend this work for publication in its present form. The following comments may be helpful for the authors to improve their manuscript
1. **Equation (2)**: The subscript “H” does not appear as a subscript. Please correct this.
2. **Line 107**: The statement “The source width is 20 μm and its two drain widths are 20 μm” is not marked and does not seem to be correct either in the figure or in the text.
3. **Figure 1**: This figure should be improved. Please mark all the important parameters, including the red dotted line.
4. **Figure 3**: There is no discussion provided for this figure. Why is the L/W ratio of 4 considered optimal, and not 3 or 5? Additionally, the axis labels and unit font size should be increased.
5. **Line 183**: The abbreviation for “transmission line model” should be provided before using it.
6. **Figure 4**: The legends are missing. Authors should include a detailed discussion about this figure. Clarify the meaning of the red and black circular markings. Remove ‘room temperature’ if you are mentioning the specific temperature. Please use cm\(^{-3}\) for the left-side axis unit.
7. **Experimental Details**: Provide detailed information about the experimental sensors used. Currently, this information is missing.
8. **Figure 10 (b)**: Surprisingly, the sensitivity remains constant with increasing field. How can the magnetic field be measured under these conditions?
Comments on the Quality of English Language
good
Author Response
Title of the paper: TCAD Modelling of Magnetic Hall Effect Sensors
Author: Vartika Pandey, Vlad Marsic, Petar Igic, Soroush Faramehr
We would like to thank the reviewer for the useful comments to the manuscript. We have revised the paper and addressed every point raised by the reviewer in great details.
Comment 1: **Equation (2)**: The subscript “H” does not appear as a subscript. Please correct this.
Response 1: Thank you for pointing this out. However we wish to bring this to your notice that the subscript “H” is correctly written in the equation format. This probably is the glitch in the template that the subscript doesn’t appear to be the subscript.
Comment 2: **Line 107**: The statement “The source width is 20 μm and its two drain widths are 20 μm” is not marked and does not seem to be correct either in the figure or in the text.
Response 2: Thank you for pointing this out. We have accordingly revised the figure as well as in the text “both the drain widths as 7.5 µm, source length of LS= 4.5 µm and drain length of LD= 4.5 µm and the drain to source distance as 26 µm. This change can be found on page number 3, line 134, 135 and 136, last paragraph.
The updated text in the manuscript is “The simulated sensor has a source length of LS= 4.5 µm and drain length of LD= 4.5 µm. The source width is 20 µm and both the drain widths are 7.5 µm. The drain to source distance is 26 µm”.
Comment 3: **Figure 1**: This figure should be improved. Please mark all the important parameters, including the red dotted line.
Response 3: Thank you for pointing this out. We have revised the figure. This change can be found on page number 4, line 144, 145.
Comment 4: **Figure 3**: There is no discussion provided for this figure. Why is the L/W ratio of 4 considered optimal, and not 3 or 5? Additionally, the axis labels and unit font size should be increased.
Response 4: Thank you for pointing this out. We have accordingly revised this point and therefore have provided the discussion for the figure 3. Additionally we have increased the axis labels and unit font size. The L/W ratio of 4 is considered optimal also from [1]. This change can be found on page number 13, from line 365 to 369.
The updated text in the manuscript is “To get the ideal value for the L/W ratio, Matlab is used to visualise Equation 5. Since sensitivity is dependent on the L/W ratio, finding the ideal value is necessary for sensor optimisation. The 3-D plot for G, L/W, and . Where G being the geometrical correction factor varies from 0 to 1, ranges from 0 to 0.45 radians. Plotting G against L/W shows that L/W begins to increase and becomes a constant line when L/W approaches 4. Thus, the graph illustrates that 4 is the ideal L/W ratio.”
Comment 5: **Line 183**: The abbreviation for “transmission line model” should be provided before using it.
Response 5: Thank you for pointing this out. We have accordingly revised the abbreviation for “transmission line model” before using it. This change can be found on page number 6, line 208, Section 3.1.
The updated text in the manuscript is “Simulation Results for a 2D GaN Transmission Line Model (TLM)”.
Comment 6: **Figure 4**: The legends are missing. Authors should include a detailed discussion about this figure. Clarify the meaning of the red and black circular markings. Remove ‘room temperature’ if you are mentioning the specific temperature. Please use cm\(^{-3}\) for the left-side axis unit.
Response 6: Thank you for pointing this out. We have accordingly revised this point and have provided the detailed discussion about the figure. We have clarified the meaning of the red and black circular markings. Have removed ‘room temperature’ and have used cm\(^{-3}\) for the left-side axis unit. This change can be found on page number 6, from line 225 to 229, Section 3.1. Page number 13, Line 370 to 388. Section 4.
The updated text in the manuscript is “The electron density is shown on the right axis, and energy is defined on the left. The black line shows the conduction energy band, the red line shows the valance energy band while the black dotted line depicts the fermi level. The 2 dimensional electron gas layer formed at the interface of GaN buffer and AlGaN barrier is shown by orange peak in the middle.”
Discussion section 4
“A transmission line model of GaN in 2D is simulated before the 3D model, keeping the dimensions same as that of 3D GaN Hall sensor. A channel is formed on the interface of GaN buffer and AlGaN barrier. Figure 4 shows the energy band diagram and electron density of this 2D GaN TLM model. EC is the conduction band energy which refers to the energy at the bottom of the conduction band in the semiconductor. While ET is the trap energy level which refers to the energy of a trap state within the band of the semiconductor. The surface states are placed at EC-ET = 0.67 eV with a density of Dsurface = 4.5*1019 cm-3 to define the surface potential. The capture cross section of electrons and holes are set to 1*10-14 cm2. The GaN sensor is composed of a passivation layer of Si, a GaN cap, an AlGaN barrier and a GaN buffer. In the GaN cap region the conduction band and valance band show band bending due to the presence of electric field. In the AlGaN layer a significant bending is observed due to the polarization effects and formation of two dimensional electron gas layer at the interface of AlGaN barrier and GaN buffer. Then the bands in the GaN buffer region become flat indicating the equilibrium conditions with no significant band bending. The band bending of and is important for confinement of electrons in 2DEG. In the AlGaN/GaN region, a high electron concentration is indicated by the Fermi level located above the conduction band edge. The electron density is illustrated on the right axis with orange circular marking and energy is defined on the left with black circular marking.”
Comment 7: **Experimental Details**: Provide detailed information about the experimental sensors used. Currently, this information is missing.
Response 7: Thank you for pointing this out. We have accordingly revised this point and have provided the detailed information about the experimental sensors used. This change can be found on page number 3, from line 119 to line 133, section 2.1.
The updated text in the manuscript is “Our GaN sensors are developed on a silicon substrate with step-graded AlGaN intermediary layers, resulting in inadvertent doping of GaN/ Al0.25Ga0.75N /GaN heterostructures. The thicknesses of the GaN buffer, AlGaN barrier, and GaN cap are 0.002 µm, 0.025 µm and 1.8 µm, respectively [2]. A four-inch-diameter GaN wafer on a silicon substrate was divided into smaller wafer pieces measuring three centimetres by three centimetres. A specially designed three-mask method was then employed to build various devices onto the tiny wafers. The first mask made it possible to dry etch the wafers and produce mesas, or isolated active zones, using inductively coupled plasma (ICP) [3].
Using physical vapour deposition, the second mask was utilised to create Ohmic contacts by sputter depositing a Ti(20 nm)/Al(100 nm)/Ti(30 nm)/Au(100 nm) metal stack. This was followed by a lift-off procedure and a brief, fast annealing operation at 800 °C in a N2 environment. Using plasma-enhanced chemical vapour deposition, a conventional SiO2 passivation layer measuring 100 nm was produced. Lastly, an ICP etch based on fluorine may be used to remove passivation from the Ohmic contact locations thanks to the third mask [3].”.
Comment 8: **Figure 10 (b)**: Surprisingly, the sensitivity remains constant with increasing field. How can the magnetic field be measured under these conditions?
Response 8: Thank you for pointing this out. We have accordingly revised this part. The sensitivity remains constant with the increasing magnetic field, the experimental data from TCAD simulation showing a good agreement with the previous work’s experimental data [4], Figure 7. The Hall sensor’s sensitivity should not be misinterpreted for the sensor’s current or voltage differential output measurement which linearly varies with the magnetic field density until reaching the saturation zones for North and symmetrically for South pole, such as presented in [5] Figure 4.
This change can be found in the manuscript on the page number 9, from the line 293 to line 298.
The updated text in the manuscript is “The sensitivity remains constant with the increasing magnetic field, the experimental data from TCAD simulation showing a good agreement with the previous work’s experimental data [4]. The Hall sensor’s sensitivity should not be misinterpreted for the sensor’s current or voltage differential output measurement which linearly varies with the magnetic field density until reaching the saturation zones for North and symmetrically for South pole, such as presented in [5].”
[1] S. Helkman, S. Keller, Y. Wu, J. S. Speck, S. P. DenBaars, and U. K. Mishra, “Polarization effects in AlGaN/GaN and GaN/AlGaN/GaN heterostructures,” J. Appl. Phys., vol. 93, no. 12, pp. 10114–10118, 2003, doi: 10.1063/1.1577222.
[2] R. Rodríguez-Torres, E. A. Gutiérrez-Domínguez, R. Klima, and S. Selberherr, “Analysis of split-drain MAGFETs,” IEEE Trans. Electron Devices, vol. 51, no. 12, pp. 2237–2245, 2004, doi: 10.1109/TED.2004.839869.
[3] B. R. Thomas, S. Faramehr, D. C. Moody, J. E. Evans, M. P. Elwin, and P. Igic, “Study of GaN dual-drain magnetic sensor performance at elevated temperatures,” IEEE Trans. Electron Devices, vol. 66, no. 4, pp. 1937–1941, 2019, doi: 10.1109/TED.2019.2901203.
[4] V. Marsic, S. Faramehr, J. Fleming, R. Bhagat, and P. Igic, “Understanding the limits of a Hall sensor sensitivity for integration on a GaN power transistor chip: experiments with market available components,” IET Conf. Power Electron., Mach. Drives (PEMD), pp. 350–356, 2023, doi: 10.1049/icp.2023.2022.
[5] V. Marsic, S. Faramehr, J. Fleming, R. Bhagat, and P. Igic, “GaN Transistors’ Radiated Switching Noise Source Evidenced by Hall Sensor Experiments Toward Integration,” IEEE Access, vol. 12, pp. 13783–13794, 2024, doi: 10.1109/ACCESS.2024.3357239.
[Kindly note that the reference sequence number is different than the manuscript document]
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsComments for author File: Comments.pdf
Author Response
Title of the paper: TCAD Modelling of Magnetic Hall Effect Sensors
Author: Vartika Pandey, Vlad Marsic, Petar Igic, Soroush Faramehr
We would like to thank the reviewer for the useful comments to the manuscript. We have revised the paper and addressed every point raised by the reviewer in great details.
Comment 1: A variety of research studies have employed TCAD simulations to explore the performance and optimization of GaN-based Hall sensors, focusing on aspects such as performance optimization, radiation effects, and device calibration. The authors of this study also emphasize device performance optimization. Please add a paragraph to elaborate on the difference from research focuses on literature and the significance of this work, as well as its potential contribution to the public.
Response 1: Thank you for pointing this out. We agree on this comment and have accordingly revised the introduction. This change can be found on page number 2, from line 66 to line 80.
The updated text in the manuscript is “This work highlights a comprehensive study for optimising device performance by combining several elements, such as temperature stability and sensitivity of the GaN sensor by varying its geometry and biasing parameters. This all-encompassing approach adds-up to the fundamental qualities of GaN-based Hall sensors. The possibility for creating enhanced Hall sensors that can be extensively used in crucial applications involving harsh environments, such as extreme industrial automation, aerospace and automotive systems, is what makes this work significant for the electronics research community. The study benefits also the large public providing a viable path for designing cutting-edge sensing technologies that can enhance sustainability, efficiency and safety across a range of industries by optimising these devices holistically.
This work may deliver a significant contribution for the future integration on the GaN power transistor chip technology [1]. An effective progressive approach in the energy system promising the integration of the sensors for in-situ monitoring [2], into the rechargeable batteries used for e-mobility in order to minimise charging times and boost battery longevity [3]”.
Comment 2: The geometry expression of “its two drain widths are 20 um” (Sentence 108) is not accurate. Please review and correct it accordingly to reflect the correct dimensions or context for the split drain widths.
Response 2: Thank you for pointing this out. We have accordingly revised the figure as well as in the text “both the drain widths as 7.5 µm, source length of LS= 4.5 µm and drain length of LD= 4.5 µm and the drain to source distance as 26 µm. This change can be found on page number 3, line 134, 135 and 136, last paragraph.
The updated text in the manuscript is “The simulated sensor has a source length of LS= 4.5 µm and drain length of LD= 4.5 µm. The source width is 20 µm and both the drain widths are 7.5 µm. The drain to source distance is 26 µm”.
Comment 3: No definitions of EC and ET was provided when they first appeared. Please define them upon their occurrence for clarity.
Response 3: Thank you for pointing this out. We have accordingly revised this section by defining EC and ET as EC is the conduction band energy which refers to the energy at the bottom of the conduction band in the semiconductor. While ET is the trap energy level which refers to the energy of a trap state within the band of the semiconductor.
This change can be found on page number 6, from line 215 to line 218 in the section 3.1.
The updated text in the manuscript is “EC is the conduction band energy which refers to the energy at the bottom of the conduction band in the semiconductor. While ET is the trap energy level which refers to the energy of a trap state within the band of the semiconductor.”
Comment 4: Figure 3 is difficult to read due to low-quality labels and scale bars. Please enhance the clarity and resolution of this figure to meet high-quality publication standards.
Response 4: Thank you for pointing this out. We have accordingly revised the figure by enhancing the clarity of the figure 3. The axis labels and unit font size is increased and resolution is improved. This change can be found on page number 5, line 181 182 183, 184.
Comment 5: Valence and conduction energies are defined as negative relative to the vacuum level. Please review the precision of the y-axis plotted in Figure 4 to ensure it accurately reflects this convention.
Response 5: Thank you for pointing this out. The energy plot from simulation is in good agreement with the other work [4][5] where the Valence and conduction energies are defined as negative relative to the vacuum level.
Comment 6: The discussion of the valence and conduction energy trends was not consistent with the trends shown in Figure 4. For example, the valence energy decreases in the SiO2 layer, then increases in the GaN cap, and finally decreases again in the AlGaN layer. Please review the discussion and correct it to accurately reflect the trends depicted in the figure.
Response 6: Thank you for pointing this out. We have accordingly revised this section. This can be found on page 13 from line 370 to 388 in the section 4.
The updated text in the manuscript is “A transmission line model of GaN in 2D is simulated before the 3D model, keeping the dimensions same as that of 3D GaN Hall sensor. A channel is formed on the interface of GaN buffer and AlGaN barrier. Figure 4 shows the energy band diagram and electron density of this 2D GaN TLM model. EC is the conduction band energy which refers to the energy at the bottom of the conduction band in the semiconductor. While ET is the trap energy level which refers to the energy of a trap state within the band of the semiconductor. The surface states are placed at EC-ET = 0.67 eV with a density of Dsurface = 4.5*1019 cm-3 to define the surface potential. The capture cross section of electrons and holes are set to 1*10-14 cm2. The GaN sensor is composed of a passivation layer of Si, a GaN cap, an AlGaN barrier and a GaN buffer. In the GaN cap region the conduction band and valance band show band bending due to the presence of electric field. In the AlGaN layer a significant bending is observed due to the polarization effects and formation of two dimensional electron gas layer at the interface of AlGaN barrier and GaN buffer. Then the bands in the GaN buffer region become flat indicating the equilibrium conditions with no significant band bending. The band bending of and is important for confinement of electrons in 2DEG. In the AlGaN/GaN region, a high electron concentration is indicated by the Fermi level located above the conduction band edge. The electron density is illustrated on the right axis with orange circular marking and energy is defined on the left with black circular marking.”
Comment 7: Please ensure that the terminology referring to total current and current imbalance is consistent with the corresponding labels in Figures 11, 12, 14 and 15, as they were found to be inconsistent with the labels used in Figures 8 and 9.
Response 7: Thank you for pointing this out. We have accordingly revised this and have ensured that the terminology referring to total current and current imbalance is consistent with the corresponding labels in Figures 11, 12, 14 and 15, with the labels used in Figures 8 and 9. This change can be found on page number 9, line 276, 277 for figure 8. And page number 9, line 279 280 for figure 9.
Comment 8: The discussion section lacks substantial insights into the underlying reasons or mechanisms behind the findings presented in the results section, somewhat redundantly reiterating the descriptive findings from the results.
Response 8: Thank you for pointing this out. We have accordingly improved the discussion section by giving the underlying reasons behind the findings. This change can be found on page number 13, from line 364 to 422, section 4.
The updated text in the manuscript is “To get the ideal value for the L/W ratio, Matlab is used to visualise Equation 5. Since sensitivity is dependent on the L/W ratio, finding the ideal value is necessary for sensor optimisation. The 3-D plot for G, L/W, and. Where G being the geometrical correction factor varies from 0 to 1, ranges from 0 to 0.45 radians. Plotting G against L/W shows that L/W begins to increase and becomes a constant line when L/W approaches 4. Thus, the graphic illustrates that 4 is the ideal L/W ratio.
A transmission line model (TLM) of GaN in 2D is simulated before the 3D model, keeping the dimensions same as that of 3D GaN Hall sensor. A channel is formed on the interface of GaN buffer and AlGaN barrier. Figure 4 shows the energy band diagram and electron density of this 2D GaN TLM model. EC is the conduction band energy which refers to the energy at the bottom of the conduction band in the semiconductor. While ET is the trap energy level which refers to the energy of a trap state within the band of the semiconductor. The surface states are placed at EC-ET = 0.67 eV with a density of Dsurface = 4.5*1019 cm-3 to define the surface potential. The capture cross section of electrons and holes are set to 1*10-14 cm2. The GaN sensor is composed of a passivation layer of Si, a GaN cap, an AlGaN barrier and a GaN buffer. In the GaN cap region the conduction band and valance band show band bending due to the presence of electric field. In the AlGaN layer a significant bending is observed due to the polarization effects and formation of two dimensional electron gas layer at the interface of AlGaN barrier and GaN buffer. Then the bands in the GaN buffer region become flat indicating the equilibrium conditions with no significant band bending. The band bending of and is important for confinement of electrons in 2DEG. In the AlGaN/GaN region, a high electron concentration is indicated by the Fermi level located above the conduction band edge. The electron density is illustrated on the right axis with orange circular marking and energy is defined on the left with black circular marking.
Figure 5 simulates the drain current against the drain voltage of the GaN TLM at a voltage sweeping from 0 to 1V. The drain current grows in tandem with the drain voltage as the voltage progressively climbs from 0 to 1V. Higher the drain voltage, the Lorentz force and electric field will be stronger, increasing the flow of electrons in the 2 DEG channel.
Figure 7(a) presents the simulated total current from Drain 1 & Drain 2 against drain source voltage at different temperature 300 K, 400 K, 500 K at applied voltage sweeping from 0 to 1V. This suggests that when the temperature rises, the combined current from both drains decreases. The primary cause of this is the reduction in mobility brought on by the sensor's increased number of scattering mechanisms. Figure 7 (b) mimics current imbalance vs. temperature. At 300 K, 400 K, and 500 K, the drain voltage is 1V as opposed to a rising magnetic field intensity (B = 0 to 30 mT). As the intensity of the magnetic field increases, so does the current imbalance measured between the two drain contacts. However, when temperature rises, both the total current and the current imbalance drop.
The 3D GaN simulations are validated in Figure 8 & 9 where the sensor is simulated for current imbalance against magnetic field sweeping from 0 to 30 mT, and sensor output current against drain source voltage sweeping from 0 to 0.5V. For simulation and experiment, both are verified at temperatures of 300 K, 373 K, and 448 K. As previously mentioned, an increase in the magnetic field and drain source voltage will cause the current imbalance and total current to rise; however, an increase in temperature will cause both to drop because of a decrease in mobility and saturation velocity. GaN sensors are subject to a variety of scattering phenomena, including phonon scattering [6], dislocation scatterings [7], ionized impurity scatterings [8], and interface roughness between the AlGaN top layer and the 2DEG channel. Mobility has been demonstrated to be impacted by ionized impurity scattering at low temperatures, but when temperatures rise over 300 K, phonon scattering takes over as the primary source of scattering [9] [6][10]. Furthermore, both high and low temperatures may be influenced by surface roughness [6]. Since all of the temperatures examined in this work were higher than 300 K, surface roughness and phonon scattering are thought to have had a role in the decreasing current that was seen [6][11].
When the temperature rises from 300 K to 448 K, the sensitivity decreases from 12.27 % to 9.6 % as illustrated in Figure 10 (a) and (b). The drop in relative sensitivity found at increasing temperatures is attributed to the mobility deterioration of electrons in the 2DEG channel due to increased phonon scattering [6].”
[1] V. Marsic, S. Faramehr, J. Fleming, R. Bhagat, and P. Igic, “GaN Transistors’ Radiated Switching Noise Source Evidenced by Hall Sensor Experiments Toward Integration,” IEEE Access, vol. 12, pp. 13783–13794, 2024, doi: 10.1109/ACCESS.2024.3357239.
[2] J. Fleming, T. Amietszajew, and A. Roberts, “In-situ electronics and communications for intelligent energy storage,” HardwareX, vol. 11, p. e00294, 2022, doi: 10.1016/j.ohx.2022.e00294.
[3] T. Amietszajew, E. McTurk, J. Fleming, and R. Bhagat, “Understanding the limits of rapid charging using instrumented commercial 18650 high-energy Li-ion cells,” Electrochim. Acta, vol. 263, pp. 346–352, 2018, doi: 10.1016/j.electacta.2018.01.076.
[4] H. Wang et al., “Theoretical Investigation of performance enhancement in GeSn/SiGeSn Type-II staggered heterojunction tunneling FET,” IEEE Trans. Electron Devices, vol. 63, no. 1, pp. 303–310, 2016, doi: 10.1109/TED.2015.2503385.
[5] M. Liu et al., “Design of GeSn-Based Heterojunction-Enhanced N-Channel Tunneling FET with Improved Subthreshold Swing and ON-State Current,” IEEE Trans. Electron Devices, vol. 62, no. 4, pp. 1262–1268, 2015, doi: 10.1109/TED.2015.2403571.
[6] E. Pichonat et al., “Temperature analysis of AlGaN/GaN high-electron-mobility transistors using micro-Raman scattering spectroscopy and transient interferometric mapping,” Proc. 1st Eur. Microw. Integr. Circuits Conf. EuMIC 2006, pp. 54–57, 2006, doi: 10.1109/EMICC.2006.282748.
[7] R. K. U. S. P. and P. S. Mallick, “Effect of dislocation scattering on electron mobility in GaN,” Nat. Sci., vol. 03, no. 09, pp. 812–815, 2011, doi: 10.4236/ns.2011.39106.
[8] F. Schubert, S. Wirth, F. Zimmermann, J. Heitmann, T. Mikolajick, and S. Schmult, “Growth condition dependence of unintentional oxygen incorporation in epitaxial GaN,” Sci. Technol. Adv. Mater., vol. 17, no. 1, pp. 239–243, 2016, doi: 10.1080/14686996.2016.1178565.
[9] M. N. Gurusinghe, S. K. Davidsson, and T. G. Andersson, “Two-dimensional electron mobility limitation mechanisms in Al x Ga 1-x N/GaN heterostructures,” Phys. Rev. B, vol. 72, no. 4, Jul. 2005, doi: 10.1103/PHYSREVB.72.045316.
[10] M. S. Pramanick and A. Ghosal, “Effects of scattering on transport properties in GaN,” 2017 Devices Integr. Circuit, pp. 647–651, Oct. 2017, doi: 10.1109/DEVIC.2017.8074030.
[11] A. Asgari, S. Babanejad, and L. Faraone, “Electron mobility, Hall scattering factor, and sheet conductivity in AlGaN/AlN/GaN heterostructures,” J. Appl. Phys., vol. 110, no. 11, 2011, doi: 10.1063/1.3665124.
[Kindly note that the reference sequence number is different than the manuscript document]
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsAccepted the changes
Comments on the Quality of English LanguageAcceptable quality
