- Email: [email protected]
Superlattice structure modeling and simulation of High Electron Mobility Transistor for improved performance
Superlattices and Microstructures 64 (2013) 388–398
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Superlattice structure modeling and simulation of High Electron Mobility Transistor for improved performance Ravindiran Munusami ⇑, Bhaskar Rao Yakkala, Shankar Prabhakar Department of Electronics, Communication, Saveetha School of Engineering, Saveetha University, Thandalam, Chennai, India
a r t i c l e
i n f o
Article history: Received 8 July 2013 Accepted 7 October 2013 Available online 16 October 2013 Keywords: Magnetic tunnel junction Tunneling Spintronic Superlattice
a b s t r a c t Magnetic tunnel junction were made by inserting the magnetic materials between the source, channel and the drain of the High Electron Mobility Transistor (HEMT) to enhance the performance. Material studio software package was used to design the superlattice layers. Different cases were analyzed to optimize the performance of the device by placing the magnetic material at different positions of the device. Simulation results based on conductivity reveals that the device has a very good electron transport due to the magnetic materials and will amplify very low frequency signals. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction HEMT’s are becoming increasing popular due to the high frequency operation of the device, which can be used for various applications. HEMT’s were normally formed due to the superlattice structure due to the layers of two or more materials with different band gaps. AlGaN/GaN is a well known superlattice structure which is normally used to make HEMT’s. Tremendous research has been done using AlGaN/GaN to improve the device performance and to discover the applications. Reported by Mr. Ramachandran [1] in his doctoral thesis the superlattice structure InAlSb/InAs/AlGaSb HEMT has improved Dc and RF performance. Improvement of the device performance on various aspects has been done for the HEMT’s. Depletion to Enhancement mode conversion is made possible using the engineered capacitive layer on AlGaN/GaN HEMT device, where the threshold voltage is varied between 0.1 V and 0.14 V for the Enhancement mode and the Depletion mode [2]. Barrier thickness of
⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (R. Munusami). 0749-6036/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2013.10.009
R. Munusami et al. / Superlattices and Microstructures 64 (2013) 388–398
389
the AlGaN heterostructures will have an effect on the mechanical and electrical property of the HEMT devices such as robustness and very good high frequency power performance [3]. HEMT made of InGaN channel region has an improved electron mobility of 1070–1290 cm2/V s which improves the device performance considerably [4]. HEMT’s made of AlGaN/GaN over silicon substrate demonstrate high power operation and breakdown voltage which is most suited for high frequency operations [5]. Similarly when the AlGaN/GaN HEMT structure was epitaxially grown on diamond the device shows a maximum drain current of 800 mA/mm with little self-heating effect [6]. HEMT’s made of oxidized GaAs gate by liquid phase oxidation results in reduced leakage current, a higher breakdown voltage, and an improved subthreshold swing in an enhancement-mode (E-mode) pseudomorphic highelectron- mobility transistor (PHEMT) [7]. HEMT’s performance will have an adverse effect due to the dielectric layer used in the device. Dielectric based on the Al2O3, HfO2, and Ga2O3 + HfO2 dielectrics have been evaluated and the analysis have shown an great potential for high frequency application of HEMT [8]. Performance of the HEMT can be further improved by using better Ohmic contacts by metallization process for AlGaN/GaN HEMT device with silicon carbide substrate [9]. The above discussion provides the vital information on the performance optimization of HEMT’s by various approaches, which makes the devices more suitable for diverse applications. Some of the applications of HEMT are discussed in detail in the following discussion. AlGaN/GaN HEMTs are used to detect the HIV-1 RTs in a low detection limit which can be used in drug development for pharmaceutical industries [10]. Physisorbed gold nano particle on the HEMT will improve the electron mobility of the device which can be used for biosensing applications [11]. Biofunctionalized gate based HEMT’s made of AlGaN/GaN are used for label free electrical detection of deoxyribonucleic acid (DNA) hybridization. The hybridization was detected when there is a current drop in the gate well if a complementary DNA is introduced [12]. HEMT’s are used as ultraviolet photo detectors when transparent gate is used with AlGaN/GaN on silicon substrate. HEMT based photo detectors based on the transparent gate have a very good response of 2.0 105 A/W at 360 nm [13]. HEMT’s can be used as Microelectro Mechanical Systems (MEMS) when the channel region is interfaced with the micro cantilever structure which can control the current flow through it [14]. HEMT made of GaN/AlGaN can be used a pressure sensor when placed next to the drum skin, which has a considerable change in the drain source voltage [15]. The above discussion based on the HEMT’s applications has given an overview of the HEMT and its various applications. A similar HEMT device structure made of Si/ MnInSb/Si has been adopted and discussed in this paper on the following chapters. MnInSb, which is considered to be a ferromagnetic semiconductor composite, has been placed on the source and drain sides of the devices for different cases and its performance has been analyzed. 2. Methodology The proposed work includes modeling and simulation of HEMT with three different architectures. The novel modified architecture investigated in the present work relates to its effect in improving the performance of the HEMT. Three different architectures represented as Case I, Case II and Case III and were analyzed with respect to the placement of ferromagnetic layer in the device. 2.1. Case I Performance of the HEMT is analyzed by introducing the ferromagnetic layer before the channel formed by the semiconductor and ferromagnetic layer as shown in Fig. 1. In this case the electron passing through the ferromagnetic layer experiences a spin while tunneling through the channel and second tunnel junction and reaches the drain. Since the electron passes through the ferromagnetic layer on the source there will be an improvement in the mobility of the electron from source to drain. Change in the energy from the tunnel junction 1 is expressed in Eq. (1) [2]. When the spintronic layer is added next to the source the expression would be modified depending on the spintronic layers property. Change in energy of the electron in the above case can be analyzed by combining the magnetic moment equation with the tunnel junction equation. By combining Eqs. (3) and (4) the change in energy for the modified architecture of Case I is given as follows,
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Fig. 1. Illustration of NFPN.
o e n e mb DF1 ¼ þ 2C 2 þ C g V g C g þ ne CR 2 2
ð1Þ
Above equation gives the electron spin at the tunnel junction 1.
Electron movementðCase IÞ ¼ DF 1 les
ð2Þ
where
DF1 ¼
o e n e mb þ ½2C 2 þ C g V g C g þ ne les CR 2 2
les ¼ cðe=2mÞS
ð3Þ ð4Þ
2.2. Case II The ferromagnetic layer is introduced before the drain as shown in Fig. 2. Due to this electron experiences a spin while passing through the ferromagnetic layer and tunnels through the second tunnel junction to reach the drain. Electron mobility improves as similar to that of Case I due to the presence of the ferromagnetic layer on the drain side. Change in the energy on the second tunnel junction is expressed in Eq. (5) [2]. Electron tunneling is due to the total capacitance, tunneling rate and resistance of the tunnel layer. Eq. (6) gives the expression for the tunnel rate with the parameters [2].
DF2 ¼
o e n e mb þ ½2C 1 þ C g þ V g C g ne CR 2 2
CðDFÞ ¼
DF
ð5Þ
ð6Þ
e2 RC½expfdfk g 1
By combining the Eqs. (4) and (5) the change in energy for the modified architecture with ferromagnetic on drain side is represented as follows,
Fig. 2. Illustration of NPFN.
R. Munusami et al. / Superlattices and Microstructures 64 (2013) 388–398
DF2 ¼
o e n e mb þ ½2C 1 þ C g þ V g C g ne les CR 2 2
391
ð7Þ
In case II the spintronic material is placed next to the second tunnel junction near the drain region. The above equation explains the spin effect of the device for the case II architecture. 2.3. Case III Ferromagnetic layer is placed on both sides of the channel as shown in Fig. 3. Since the electron experiences spin effect on both the ferromagnetic layers on the source and drain the mobility of the electron is improved. The improved electron mobility with two ferromagnetic layers is expressed by the following equation:
Electron movementðCast IIIÞ ¼ DF1 les þ DF2 les
ð8Þ
Analysis of the above three cases reveals that the mobility of the electron improves due to the spin orientation by spintronic layers. When the spintronic layer is placed on both sides of the channel the net flow of electrons crossing the junction will be the product of electron tunneling and magnetic moment. Net electron tunneling with the spintronic layer placed next to the two tunnel junctions will be the cumulative product of the tunneling of junction 1 (near source) and junction 2 (near drain) with the dual magnetic moment. Since the magnetic moment doubles due to the spintronic layer on both the sides of the channel it is assumed that the movement of the electrons from the source to the drain will be doubled. 3. Results and discussion Material modeling using Material studio software package for HEMT was done for all the three cases. The atomic layers of Silicon and the spintronic materials were placed one over the other using Material Studio software package as follows.
NPN (standard Architecture) NFPN (Case I) NPFN (Case II) NFPFN (Case III) N–N type Silicon P–P type Silicon F–Ferromagnetic Layer (MnInSb)
Material Studio software package was used to build the atomic layer modeling of the device and CASTEP simulator was used to analyze the conductivity and band gap.
Fig. 3. Illustration of NFPFN.
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Fig. 4. NPN layer.
Fig. 5. NPN layer conductivity.
3.1. NPN modeling (Existing Standard Structure) Fig. 4 shows the configuration of the NPN layer modeled with material studio software package. Modeling was done for the layers NPN which is above the substrate. Fig. 5 shows the conductivity of the NPN layer which is almost 6.5 mV. Band structure of the NPN layer is shown in Fig. 6, where the energy levels ranging from 5.65 eV to 6.15 eV. 3.2. NFPN modeling (Case I) NFPN modeling is done by introducing a spintronic layer in between the N type and P type semiconductor as shown in Fig. 7. Manganese Indium Antimonide (MnInSb) is used as ferromagnetic (Spintronic) layer. Conductivity and the band structure of the NFPN device is different from that of the NPN layer due to the ferromagnetic layer sandwiched between the N and P type semiconductor.
R. Munusami et al. / Superlattices and Microstructures 64 (2013) 388–398
Fig. 6. NPN layer band structure.
Fig. 7. NFPN layer.
Fig. 8. NFPN layer conductivity.
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Fig. 9. NFPN layer band structure.
Conductivity and the Band gap of the NFPN layer is different from the NPN layer due to the presence of the spintronic layer between the source and the channel. Fig. 8 gives the conductivity of the NFPN device which varies from the NPN structure due to the modified architecture. Conductivity of the NFPN device is 18 mV which three times higher than the normal NPN architecture. This increased conductivity is due to the spintronic layer sandwiched in-between the N and P type semiconductor. With reference to Fig. 9 the energy levels the NFPN material is with range 5.64–5.98 eV which is slightly less than the NPN modeled device. Energy required for the free electrons to move from the conduction band of N type semiconductor material to the valance band of the Spintronic material will be less due to the reduced band gap. Spin of the electron helps the electrons to pass through the junctions faster. 3.3. NPFN modeling (Case II) In Fig. 10 shown the configuration of the NPFN model in which spintronic layer is sandwiched between the P type semiconductor and the N type semiconductor the spintronic layer is placed on the other side of the device as compared to the NFPN structure. Conductivity of the device is 10.5 V
Fig. 10. NPFN layer.
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395
Fig. 11. NPFN layer conductivity.
which is less when compared to the NFPN. This reduced conductivity is due to the tunneling of the electrons from the N type semiconductor through the P type semiconductor and the spintronic material. Fig. 11 gives the conductivity performance of the NPFN layer device. Curves of the NPFN show a region where the current increases to a peak and then gets reduced to a valley point. This region in the conductivity curve of NPFN region is called as negative resistance region. This is similar to the VI characteristics curves of Uni Junction Transistor and Tunnel diode. This negative resistance region in the devices is due to the tunneling phenomena of electrons to cross over the barrier. With reference to Fig. 12 the band gap of the NPFN model is 7.792–7.825 eV which is more when compared to the NFPN model device. The increase in Energy level would be due to the structure of the band when different materials like a P and N type semiconductor and spintronic materials placed one over the other. Conductivity of the NPFN Device is reduced for a small region called as Negative resistance region due to the tunneling of the electrons through the heterojunctions with the misaligned energy band states.
Fig. 12. NPFN layer band structure.
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Fig. 13. NFPFN layer.
3.4. NFPFN modeling (Case III) Fig. 13 shows the NFPFN device structure which is modeled by placing the spintronic material on both sides of the P type semiconductor. Due to the spin alignment of electrons in a spintronic layer the conductivity of the device is increased by ten times when compared to the NPN device structure. Fig 14 shows the conductivity of the NFPFN device which is 64.5 V, which is ten times more than the conductivity of the NPN device of 6.5 V. According to Fig. 15 the energy levels of the NFPFN structure are in the range of is 4.465–4.71 eV which are very less when compared to the other device structures. Reduced energy level in the device is due to the spintronic layer sandwiched on both sides of the device. The channel improves the tunneling of the electrons to cross through the junction. Comparison of all three cases is done in Table 1. All three cases and the normal device structure are compared for the band structure and the conductivity of the different modeled device. There is an improved conductivity in the NFPFN device structure which may be due to the spintronic layer on both
Fig. 14. NFPFN layer conductivity.
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Fig. 15. NFPFN layer band structure.
Table 1 Comparison of three cases. Device modeling
Band gap (eV)
Conductivity (V)
NPN NFPN NPFN NFPFN
5.65–6.15 5.64–5.98 7.792–7.825 4.465–4.71
6.5 18 10.5 64.5
sides of the device. Conductivity of the device is less for the NPFN device structure due to the tunneling of electron to cross from the P type semiconductor through the spintronic layer and again through a N type semiconductor. Band gap for all the three cases and the normal NPN device structure is also analyzed and the reduced energy levels are indicated for with the NFPFN device structure. Due to the reduced energy level the conductivity of the NFPFN device is better and ten times that of the normal NPN device structure.
4. Conclusion Performance optimization of HEMT with novel device architecture has been analyzed in this paper. Mathematical equations defining the tunneling and magnetic moment of electrons are used to explain the operation of the novel device architecture theoretically. To support the theoretical analysis, simulation was done using material studio. Structure of the device is built in layers and the spintronic layer is sandwiched between the Semiconductor and Channel with three different approaches. For each of the case conductivity and the band gap is analyzed to identify the optimized device performance. According to the simulation results NFPFN architecture has a better performance when compared to the other two novel architectures. Conductivity of the NFPFN architecture is ten times higher compared to that of existing NPN architectures. This increased conductivity may support the device to detect very low frequency signals which can be used as sensors in medical applications. Acknowledgement The authors thank the Management of Saveetha University for the continuous motivation to carry on this research work and for extending all necessary support. The motivation and the support received from colleagues at SSE are incredible which provides an excellent platform for free exchange of interdisciplinary ideas on advanced research topics. Special thanks to Dr. Narayanan of Mechanical department, School of Engineering, Saveetha University for his valuable suggestion in reviewing the manuscript.
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References [1] Vishwanath Ramachandran, Single-event mechanisms in InAlSb/InAs/AlGaSb High Electron Mobility Transistors, Dissertation Submitted to the Faculty of the Graduate School of Vanderbilt University in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Electrical Engineering, May 2013, Nashville, Tennessee. [2] Yuechan Kong, Jianjun Zhou, Cen Kong, Xun Dong, Youtao Zhang, Haiyan Lu, Tangsheng Chen, Monolithic integrated enhancement/depletion-mode AlGaN/GaN high electron mobility transistors with cap layer engineering, Applied Physics Letters 102 (2013) 043505. [3] P. Waltereit, W. Bronner, M. Musser, F. van Raay, M. Dammann, M. C€asar, S. M€uller, L. Kirste, K. K€ohler, R. Quay, M. Mikulla, O. Ambacher, Influence of AlGaN barrier thickness on electrical and device properties in Al0.14Ga0.86N/GaN high electron mobility transistor structures, Journal of Applied Physics 112 (2012) 053718. [4] O. Laboutin, Y. Cao, W. Johnson, R. Wang, G. Li, D. Jena, H. Xing, InGaN channel high electron mobility transistor structures grown by metal organic chemical vapor deposition, Applied Physics Letters 100 (2012) 121909. [5] Chia-Hua Chang, Heng-Tung Hsu⁄, Lu-Che Huang, Che-Yang Chiang⁄, Edward Yi Chang, Fabrication of AlGaN/GaN High Electron Mobility Transistors (HEMTs) on silicon substrate with slant field plates using deep-UV lithography featuring 5 W/mm power density at X-band, in: Proceedings of APMC 2012, Kaohsiung, Taiwan, December 4–7, 2012. [6] Kazuyuki Hirama, Yoshitaka Taniyasu, Makoto Kasu, Epitaxial growth of AlGaN/GaN high-electron mobility transistor structure on diamond (1 1 1) surface, Japanese Journal of Applied Physics 51 (2012) 090114. [7] Kuan-Wei Lee1,⁄, Hsien-Cheng Lin2, Yeong-Her Wang2,⁄⁄, Enhancement-mode pseudomorphic high-electron-mobility transistor with a nanoscale oxidized GaAs gate, 978-1-4673-1724-5/12/$31.00 Ó2013 IEEE. [8] Omair I. Saadat, Student Member, IEEE, Jinwook W. Chung, Edwin L. Piner, Member, IEEE, Tomás Palacios, Member, IEEE, Gate-First lGaN/GaN HEMT Technology for High-Frequency Applications, IEEE Electron Device Letters, vol. 30, no. 12, December 2009. [9] Cong Wang, Sung-Jin Cho, Nam-Young Kim, Optimization of ohmic contact metallization process for AlGaN/GaN high electron mobility transistor, Transactions on Electrical And Electronic Materials 14 (1) (2013) 32–35 (February 25). [10] Yen-Wen Kang,1 Geng-Yen Lee, 2 Jen-Inn Chyi, 2 Chen-Pin Hsu, 1 You-Ren Hsu, 1 Chia-Hsien Hsu, 3 Yu-Fen Huang, 4 YuhChang Sun, 4 Chih-Chen Chen, 1 Sheng Chun Hung, 5 Fan Ren, 6 J. Andrew Yeh, 1 Yu-Lin Wang1, Human immunodeficiency virus drug development assisted with AlGaN/GaN high electron mobility transistors and binding-site models, Applied Physics Letters 102, 2013, 173704. [11] M.S. Makowski, S. Kim, M. Gaillard, D. Janes, M.J. Manfra, I. Bryan, Z. Sitar, C. Arellano, J. Xie, R. Collazo, A. Ivanisevic, Physisorption of functionalized gold nanoparticles on AlGaN/GaN high electron mobility transistors for sensing applications, Applied Physics Letters 102 (2013) 074102. [12] Resham Thapa, Siddharth Alur, Kyusang Kim, Fei Tong, Yogesh Sharma, Moonil Kim, Claude Ahyi, Jing Dai, Jong Wook Hong, Michael Bozack, John Williams, Ahjeong Son, Amir Dabiran, Minseo Park, Biofunctionalized AlGaN/GaN high electron mobility transistor for DNA hybridization detection, Applied Physics Letters 100 (2012) 232109. [13] Tomotaka Narita, Akio Wakejima, Takashi Egawa, Ultraviolet photodetectors using transparent gate AlGaN/GaN high electron mobility transistor on silicon substrate, Japanese Journal of Applied Physics 52 (2013) 01AG06. [14] Irina Khmyrova⁄,1, Ryosuke Yamase1, Norikazu Watanabe1, Takao Maeda1, Elena Shestakova2, Evgeny Polushkin3, Anatoly Kovalchuk3, Sergei Shapoval, Analysis of resonant MEMS based on high-electron mobility transistor-like structure, Phys. Status Solidi C, vol. 9, no. 2, 2012, 399–402. doi: 10.1002/pssc.201100224. [15] E.D. Le Boulbara,⁄, M.J. Edwardsa, b, S. Vittozc, G. Vankod, K. Brinkfeldte, L. Ruferc, P. Johandere, T. Lalinsky´ d, C.R. Bowena, D.W.E. Allsoppa, Effect of bias conditions on pressure sensors based on AlGaN/GaN High Electron Mobility Transistor, Sensors and Actuators A, vol. 194, 2013, pp. 247–251.
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Superlattice structure modeling and simulation of High Electron Mobility Transistor for improved performance Ravindiran Munusami ⇑, Bhaskar Rao Yakkala, Shankar Prabhakar Department of Electronics, Communication, Saveetha School of Engineering, Saveetha University, Thandalam, Chennai, India
a r t i c l e
i n f o
Article history: Received 8 July 2013 Accepted 7 October 2013 Available online 16 October 2013 Keywords: Magnetic tunnel junction Tunneling Spintronic Superlattice
a b s t r a c t Magnetic tunnel junction were made by inserting the magnetic materials between the source, channel and the drain of the High Electron Mobility Transistor (HEMT) to enhance the performance. Material studio software package was used to design the superlattice layers. Different cases were analyzed to optimize the performance of the device by placing the magnetic material at different positions of the device. Simulation results based on conductivity reveals that the device has a very good electron transport due to the magnetic materials and will amplify very low frequency signals. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction HEMT’s are becoming increasing popular due to the high frequency operation of the device, which can be used for various applications. HEMT’s were normally formed due to the superlattice structure due to the layers of two or more materials with different band gaps. AlGaN/GaN is a well known superlattice structure which is normally used to make HEMT’s. Tremendous research has been done using AlGaN/GaN to improve the device performance and to discover the applications. Reported by Mr. Ramachandran [1] in his doctoral thesis the superlattice structure InAlSb/InAs/AlGaSb HEMT has improved Dc and RF performance. Improvement of the device performance on various aspects has been done for the HEMT’s. Depletion to Enhancement mode conversion is made possible using the engineered capacitive layer on AlGaN/GaN HEMT device, where the threshold voltage is varied between 0.1 V and 0.14 V for the Enhancement mode and the Depletion mode [2]. Barrier thickness of
⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (R. Munusami). 0749-6036/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2013.10.009
R. Munusami et al. / Superlattices and Microstructures 64 (2013) 388–398
389
the AlGaN heterostructures will have an effect on the mechanical and electrical property of the HEMT devices such as robustness and very good high frequency power performance [3]. HEMT made of InGaN channel region has an improved electron mobility of 1070–1290 cm2/V s which improves the device performance considerably [4]. HEMT’s made of AlGaN/GaN over silicon substrate demonstrate high power operation and breakdown voltage which is most suited for high frequency operations [5]. Similarly when the AlGaN/GaN HEMT structure was epitaxially grown on diamond the device shows a maximum drain current of 800 mA/mm with little self-heating effect [6]. HEMT’s made of oxidized GaAs gate by liquid phase oxidation results in reduced leakage current, a higher breakdown voltage, and an improved subthreshold swing in an enhancement-mode (E-mode) pseudomorphic highelectron- mobility transistor (PHEMT) [7]. HEMT’s performance will have an adverse effect due to the dielectric layer used in the device. Dielectric based on the Al2O3, HfO2, and Ga2O3 + HfO2 dielectrics have been evaluated and the analysis have shown an great potential for high frequency application of HEMT [8]. Performance of the HEMT can be further improved by using better Ohmic contacts by metallization process for AlGaN/GaN HEMT device with silicon carbide substrate [9]. The above discussion provides the vital information on the performance optimization of HEMT’s by various approaches, which makes the devices more suitable for diverse applications. Some of the applications of HEMT are discussed in detail in the following discussion. AlGaN/GaN HEMTs are used to detect the HIV-1 RTs in a low detection limit which can be used in drug development for pharmaceutical industries [10]. Physisorbed gold nano particle on the HEMT will improve the electron mobility of the device which can be used for biosensing applications [11]. Biofunctionalized gate based HEMT’s made of AlGaN/GaN are used for label free electrical detection of deoxyribonucleic acid (DNA) hybridization. The hybridization was detected when there is a current drop in the gate well if a complementary DNA is introduced [12]. HEMT’s are used as ultraviolet photo detectors when transparent gate is used with AlGaN/GaN on silicon substrate. HEMT based photo detectors based on the transparent gate have a very good response of 2.0 105 A/W at 360 nm [13]. HEMT’s can be used as Microelectro Mechanical Systems (MEMS) when the channel region is interfaced with the micro cantilever structure which can control the current flow through it [14]. HEMT made of GaN/AlGaN can be used a pressure sensor when placed next to the drum skin, which has a considerable change in the drain source voltage [15]. The above discussion based on the HEMT’s applications has given an overview of the HEMT and its various applications. A similar HEMT device structure made of Si/ MnInSb/Si has been adopted and discussed in this paper on the following chapters. MnInSb, which is considered to be a ferromagnetic semiconductor composite, has been placed on the source and drain sides of the devices for different cases and its performance has been analyzed. 2. Methodology The proposed work includes modeling and simulation of HEMT with three different architectures. The novel modified architecture investigated in the present work relates to its effect in improving the performance of the HEMT. Three different architectures represented as Case I, Case II and Case III and were analyzed with respect to the placement of ferromagnetic layer in the device. 2.1. Case I Performance of the HEMT is analyzed by introducing the ferromagnetic layer before the channel formed by the semiconductor and ferromagnetic layer as shown in Fig. 1. In this case the electron passing through the ferromagnetic layer experiences a spin while tunneling through the channel and second tunnel junction and reaches the drain. Since the electron passes through the ferromagnetic layer on the source there will be an improvement in the mobility of the electron from source to drain. Change in the energy from the tunnel junction 1 is expressed in Eq. (1) [2]. When the spintronic layer is added next to the source the expression would be modified depending on the spintronic layers property. Change in energy of the electron in the above case can be analyzed by combining the magnetic moment equation with the tunnel junction equation. By combining Eqs. (3) and (4) the change in energy for the modified architecture of Case I is given as follows,
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Fig. 1. Illustration of NFPN.
o e n e mb DF1 ¼ þ 2C 2 þ C g V g C g þ ne CR 2 2
ð1Þ
Above equation gives the electron spin at the tunnel junction 1.
Electron movementðCase IÞ ¼ DF 1 les
ð2Þ
where
DF1 ¼
o e n e mb þ ½2C 2 þ C g V g C g þ ne les CR 2 2
les ¼ cðe=2mÞS
ð3Þ ð4Þ
2.2. Case II The ferromagnetic layer is introduced before the drain as shown in Fig. 2. Due to this electron experiences a spin while passing through the ferromagnetic layer and tunnels through the second tunnel junction to reach the drain. Electron mobility improves as similar to that of Case I due to the presence of the ferromagnetic layer on the drain side. Change in the energy on the second tunnel junction is expressed in Eq. (5) [2]. Electron tunneling is due to the total capacitance, tunneling rate and resistance of the tunnel layer. Eq. (6) gives the expression for the tunnel rate with the parameters [2].
DF2 ¼
o e n e mb þ ½2C 1 þ C g þ V g C g ne CR 2 2
CðDFÞ ¼
DF
ð5Þ
ð6Þ
e2 RC½expfdfk g 1
By combining the Eqs. (4) and (5) the change in energy for the modified architecture with ferromagnetic on drain side is represented as follows,
Fig. 2. Illustration of NPFN.
R. Munusami et al. / Superlattices and Microstructures 64 (2013) 388–398
DF2 ¼
o e n e mb þ ½2C 1 þ C g þ V g C g ne les CR 2 2
391
ð7Þ
In case II the spintronic material is placed next to the second tunnel junction near the drain region. The above equation explains the spin effect of the device for the case II architecture. 2.3. Case III Ferromagnetic layer is placed on both sides of the channel as shown in Fig. 3. Since the electron experiences spin effect on both the ferromagnetic layers on the source and drain the mobility of the electron is improved. The improved electron mobility with two ferromagnetic layers is expressed by the following equation:
Electron movementðCast IIIÞ ¼ DF1 les þ DF2 les
ð8Þ
Analysis of the above three cases reveals that the mobility of the electron improves due to the spin orientation by spintronic layers. When the spintronic layer is placed on both sides of the channel the net flow of electrons crossing the junction will be the product of electron tunneling and magnetic moment. Net electron tunneling with the spintronic layer placed next to the two tunnel junctions will be the cumulative product of the tunneling of junction 1 (near source) and junction 2 (near drain) with the dual magnetic moment. Since the magnetic moment doubles due to the spintronic layer on both the sides of the channel it is assumed that the movement of the electrons from the source to the drain will be doubled. 3. Results and discussion Material modeling using Material studio software package for HEMT was done for all the three cases. The atomic layers of Silicon and the spintronic materials were placed one over the other using Material Studio software package as follows.
NPN (standard Architecture) NFPN (Case I) NPFN (Case II) NFPFN (Case III) N–N type Silicon P–P type Silicon F–Ferromagnetic Layer (MnInSb)
Material Studio software package was used to build the atomic layer modeling of the device and CASTEP simulator was used to analyze the conductivity and band gap.
Fig. 3. Illustration of NFPFN.
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Fig. 4. NPN layer.
Fig. 5. NPN layer conductivity.
3.1. NPN modeling (Existing Standard Structure) Fig. 4 shows the configuration of the NPN layer modeled with material studio software package. Modeling was done for the layers NPN which is above the substrate. Fig. 5 shows the conductivity of the NPN layer which is almost 6.5 mV. Band structure of the NPN layer is shown in Fig. 6, where the energy levels ranging from 5.65 eV to 6.15 eV. 3.2. NFPN modeling (Case I) NFPN modeling is done by introducing a spintronic layer in between the N type and P type semiconductor as shown in Fig. 7. Manganese Indium Antimonide (MnInSb) is used as ferromagnetic (Spintronic) layer. Conductivity and the band structure of the NFPN device is different from that of the NPN layer due to the ferromagnetic layer sandwiched between the N and P type semiconductor.
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Fig. 6. NPN layer band structure.
Fig. 7. NFPN layer.
Fig. 8. NFPN layer conductivity.
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Fig. 9. NFPN layer band structure.
Conductivity and the Band gap of the NFPN layer is different from the NPN layer due to the presence of the spintronic layer between the source and the channel. Fig. 8 gives the conductivity of the NFPN device which varies from the NPN structure due to the modified architecture. Conductivity of the NFPN device is 18 mV which three times higher than the normal NPN architecture. This increased conductivity is due to the spintronic layer sandwiched in-between the N and P type semiconductor. With reference to Fig. 9 the energy levels the NFPN material is with range 5.64–5.98 eV which is slightly less than the NPN modeled device. Energy required for the free electrons to move from the conduction band of N type semiconductor material to the valance band of the Spintronic material will be less due to the reduced band gap. Spin of the electron helps the electrons to pass through the junctions faster. 3.3. NPFN modeling (Case II) In Fig. 10 shown the configuration of the NPFN model in which spintronic layer is sandwiched between the P type semiconductor and the N type semiconductor the spintronic layer is placed on the other side of the device as compared to the NFPN structure. Conductivity of the device is 10.5 V
Fig. 10. NPFN layer.
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Fig. 11. NPFN layer conductivity.
which is less when compared to the NFPN. This reduced conductivity is due to the tunneling of the electrons from the N type semiconductor through the P type semiconductor and the spintronic material. Fig. 11 gives the conductivity performance of the NPFN layer device. Curves of the NPFN show a region where the current increases to a peak and then gets reduced to a valley point. This region in the conductivity curve of NPFN region is called as negative resistance region. This is similar to the VI characteristics curves of Uni Junction Transistor and Tunnel diode. This negative resistance region in the devices is due to the tunneling phenomena of electrons to cross over the barrier. With reference to Fig. 12 the band gap of the NPFN model is 7.792–7.825 eV which is more when compared to the NFPN model device. The increase in Energy level would be due to the structure of the band when different materials like a P and N type semiconductor and spintronic materials placed one over the other. Conductivity of the NPFN Device is reduced for a small region called as Negative resistance region due to the tunneling of the electrons through the heterojunctions with the misaligned energy band states.
Fig. 12. NPFN layer band structure.
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Fig. 13. NFPFN layer.
3.4. NFPFN modeling (Case III) Fig. 13 shows the NFPFN device structure which is modeled by placing the spintronic material on both sides of the P type semiconductor. Due to the spin alignment of electrons in a spintronic layer the conductivity of the device is increased by ten times when compared to the NPN device structure. Fig 14 shows the conductivity of the NFPFN device which is 64.5 V, which is ten times more than the conductivity of the NPN device of 6.5 V. According to Fig. 15 the energy levels of the NFPFN structure are in the range of is 4.465–4.71 eV which are very less when compared to the other device structures. Reduced energy level in the device is due to the spintronic layer sandwiched on both sides of the device. The channel improves the tunneling of the electrons to cross through the junction. Comparison of all three cases is done in Table 1. All three cases and the normal device structure are compared for the band structure and the conductivity of the different modeled device. There is an improved conductivity in the NFPFN device structure which may be due to the spintronic layer on both
Fig. 14. NFPFN layer conductivity.
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Fig. 15. NFPFN layer band structure.
Table 1 Comparison of three cases. Device modeling
Band gap (eV)
Conductivity (V)
NPN NFPN NPFN NFPFN
5.65–6.15 5.64–5.98 7.792–7.825 4.465–4.71
6.5 18 10.5 64.5
sides of the device. Conductivity of the device is less for the NPFN device structure due to the tunneling of electron to cross from the P type semiconductor through the spintronic layer and again through a N type semiconductor. Band gap for all the three cases and the normal NPN device structure is also analyzed and the reduced energy levels are indicated for with the NFPFN device structure. Due to the reduced energy level the conductivity of the NFPFN device is better and ten times that of the normal NPN device structure.
4. Conclusion Performance optimization of HEMT with novel device architecture has been analyzed in this paper. Mathematical equations defining the tunneling and magnetic moment of electrons are used to explain the operation of the novel device architecture theoretically. To support the theoretical analysis, simulation was done using material studio. Structure of the device is built in layers and the spintronic layer is sandwiched between the Semiconductor and Channel with three different approaches. For each of the case conductivity and the band gap is analyzed to identify the optimized device performance. According to the simulation results NFPFN architecture has a better performance when compared to the other two novel architectures. Conductivity of the NFPFN architecture is ten times higher compared to that of existing NPN architectures. This increased conductivity may support the device to detect very low frequency signals which can be used as sensors in medical applications. Acknowledgement The authors thank the Management of Saveetha University for the continuous motivation to carry on this research work and for extending all necessary support. The motivation and the support received from colleagues at SSE are incredible which provides an excellent platform for free exchange of interdisciplinary ideas on advanced research topics. Special thanks to Dr. Narayanan of Mechanical department, School of Engineering, Saveetha University for his valuable suggestion in reviewing the manuscript.
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