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A field effect transistor using highly nitrogen-doped CVD diamond for power device applications
Applied Surface Science 216 (2003) 483–489
A field effect transistor using highly nitrogen-doped CVD diamond for power device applications Yoko Yokoyamaa, Xueqing Lib, Kuang Shengb, Andrei Mihailac, Tanija Traikovicc, Florin Udreac, G.A.J. Amaratungac, Ken Okanoa,* b
a Department of Physics, International Christian University, 3-10-2 Osawa, Mitaka, Tokyo, Japan Department of Electrical and Computer Engineering, The State University of New Jersey, Rutgers, NJ, USA c Department of Engineering, University of Cambridge, Cambridge, UK
Abstract A new idea of power device, which contains highly nitrogen-doped CVD diamond and Schottky contact, is proposed to actualise a power device with diamond. Two-dimensional simulation is conducted using ISE TCAD device simulator. While comparably high current is obtained in a transient simulation as expected, this current does not contribute to the drain–source current because of the symmetry of the device. Using an asymmetric structure or bias conditions, the device has high potential as an electric device for extremely high power, high frequency and high temperature. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Highly nitrogen-doped CVD diamond; Schottky barrier; Power device; Carrier density
1. Introduction Although silicon is overwhelmingly used in high voltage power devices, these devices with silicon are reaching fundamental limits because their breakdown field is not high enough. In recent years, wide bandgap semiconductors have attracted great attention due to their specific electronic properties, which make them suitable for devices with high power, high temperature and high frequency [1]. Diamond is one of the most attractive materials for such devices because it exhibits large breakdown electric field, high thermal conductivity, small relative permittivity and high carrier mobility. High power devices consisting of diamond are expected to exhibit *
Corresponding author. Tel.: þ81-422-33-3254; fax: þ81-422-33-3254. E-mail address: [email protected] (K. Okano).
greater breakdown voltage and faster switching speed than those consisting of other materials [2]. Diamond, however, has severe problems when it is used as an electronic device. Although type Ib diamond contains certain amount of nitrogen (N), the donor level is too deep for electrons to be excited even if very high voltage is applied. Thus, there are few reports about electronic devices using diamond with some exceptions, such as FETs using the surface conductive layer reported by Itoh and Kawarada [3]. According to the recent publication of some of the authors, high concentrations of nitrogen can now be incorporated in diamond, using urea as a dopant. This technique makes it possible to increase impurity concentration of CVD diamond up to 1021 cm3. In our previous paper [4], more than 1020 cm3 of N atoms have been confirmed in doped CVD diamond films by RRS and XPS technique. These films were also identified as diamond with high crystalline perfection by
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00402-1
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simulation program. Characteristics of the device are computed and the effect of the geometry and applied voltage will be also discussed.
2. Idea and simulation
Fig. 1. Structure of proposed device.
RHEED, AES and Raman spectroscopy. From theoretical studies, significant numbers of N atoms are expected to be contained substitutionally in diamond mainly due to the small ion radius of nitrogen [5]. It is true that the number of electrons in the conduction band is fairly small at room temperature since the donor level (1.7 eV) is deep. However, at the metal–diamond interface of substitutional N-doped diamond as shown in Fig. 1, electrons are excited from the donor levels to the conduction band while the depletion region is creating or expanding. Therefore, at least some electrons are transiently available in the conduction band even at room temperature with a suitable bias. We tried to apply these electrons for the device operation. In this report, a new structure of a JFET-like power device with the usage of highly nitrogen-doped CVD diamond and Schottky contact will be discussed. Twodimensional device simulation of a diamond power device is investigated by using ISE TCAD device
Fig. 2 shows our new idea of the device structure. The device consists of single crystal diamond (type Ib, (1 0 0)) with layers of highly nitrogen-doped CVD diamond and Schottky contact (gate) as shown in the figure. Two ohmic contacts are placed on the top (drain) and bottom (source) of the diamond crystal. Although it is known that the ohmic contact for wide band-gap semiconductor is generally difficult to be formated, there are many reports about the formation of ohmic contact for n-type diamond [7]. For example, Teraji et al. reported that Ga implanted contact formed on n-type diamond surfaces exhibits ohmic property [8]. Even though high voltage is applied between two ohmic contacts, the resistivity of diamond is so high that current does not flow within its bulk. When reverse bias is applied to the Schottky contact, the depletion region will expand [6]. Within the depleted region, which is created in the highly nitrogen-doped CVD diamond layer, the donor level lies above the Fermi level as illustrated in Fig. 1. Thus, some electrons are excited from the donor levels to the conduction band. When greater reverse bias is applied to the Schottky contact, the depletion region further expands. This expansion leads to the excitation of more electrons.
Fig. 2. Energy-band diagram of the metal–diamond (N-doped) interface.
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485
Fig. 3. The mechanism of device operation (a) when gate is reverse biased and (b) gate is forward biased.
We may expect that the following mechanisms give rise to the expansion of the depleted region. Some of the excited electrons would move towards the center of the bulk since the potential is lower at the center (Fig. 3(a)). Those electrons would move to the drain contact. Hopefully, the same amount of electrons would be injected at the source in order to keep the electrical neutrality. This means that the current between the drain and the source is conserved. Therefore, when the gate is reverse biased, the drain–source current is switched on. On the contrary, when the Schottky contact is forward biased, the depletion region shrinks (Fig. 3(b)) and the density of the carriers at the center area decreases. Therefore, almost no current flows between the drain and the source contacts, which means the drain–source current is switched off. In the case of forward bias, leakage current is extremely low due to the high resistivity of diamond. This is the ideal characteristic for the switching devices. On the basis of these facts, the drain–source current can be controlled by applied gate bias. There are two main differences between the device we proposed and the existing FET. 1. Although diamond is known as an insulator, there are a number of reports that the diamond contained significant impurities has properties as a semiconductor. For example, according to the
previous reference by Geis et al., the depletion region is generated near the interface of Schottky contact and nitrogen-doped diamond [6]. In this study, we tried to apply both characteristics of diamond, namely, the high resistivity and the energy-band bending in the depletion region, to the device operation. 2. While the depletion layer expands, electrons at the metal–semiconductor interface move away in order to meet the Fermi level. Since diamond has few electrons in the conduction band at the room temperature, it is proposed by Geis et al. that the electrons need to be excited from donors to the conduction band and those electrons move away due to the electric field [6]. We tried to apply those electrons to the device operation under the high electric field towards the bulk center. This is the first trial as far as we know. Device simulation is conducted using ISE TCAD device simulator to confirm the device operation. ISE TCAD device simulator solves the following three equations for charge transport in semiconductor devices [9]; One is the Poisson equation. rE rC ¼ qðp n þ ND þ NA Þ where E is the electrical permittivity; q the elementary electronic charge; n and p the electron density and the hole density, respectively; and NDþ and NA are the
486
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number of ionized donors and acceptors, respectively. The others are the electron continuity equation and the hole continuity equation. @n r ~ Jn ¼ qR þ q ; @t
@p r ~ Jp ¼ qR þ q @t
where R is the net electron–hole recombination rate; and ~ Jn and ~ Jp are the electron current density and the hole current density, respectively. However, for simplicity, we solve them for a twodimensional geometry. Namely, the cross-sectional surface of the device is a square of 2 mm 2 mm, made of single crystal diamond, which contains 1016 cm3 of nitrogen. In two-dimensional simulation, the width of the device is always considered as 1 nm. When any effective result of the proposed device is obtained in two-dimensional simulation, we will apply the device to three-dimensional simulation. The right and left layers are made of highly nitrogen-doped diamond, which contains 1021 cm3 of nitrogen. There are four contacts on this device. The top is the drain, the bottom is the source, and the left side and the right side are the gates. The drain and the source are ohmic contacts and the gates are Schottky contacts. The work function of the gate is chosen as 4.9 eV. We also take into account the temperature dependence of carrier density in incomplete ionization model,
in contrast to the case of other materials in which the donor level is shallow. We also choose the values of parameters for diamond, which were obtained by Pan et al. [10]. To simplify the simulation, most of the parameters, such as the electron mobility, the hole mobility and the band-gap, are taken as constants. Temperature is fixed at 300 K in the simulation when we deal with Fermi-Dirac distribution. The dopant is nitrogen, whose activation energy is 1.7 eV and the electron affinity of diamond is chosen as 0 eV. The drain is grounded and the source is biased up to þ50 V. Then, reverse bias is applied to the gate contacts. The gate voltage is varied from 0 to 10 V. For each value of the gate bias, the drain current–drain voltage characteristics are computed. To clarify the mechanism of the device operation, the transient simulation is also conducted, with the same geometry and bias condition as in the static simulation by taking the ramped time as a parameter.
3. Results and discussions The obtained potential profiles indicate that the potential around the drain contact is higher than any other points within the bulk as illustrated in Fig. 4. It means that once the electrons are excited
Fig. 4. Potential profiles inside the device for following bias conditions: (a) VS ¼ 0, VD ¼ 10, and VG ¼ 0; (b) VS ¼ 0, VD ¼ 10, and VG ¼ 1.
Y. Yokoyama et al. / Applied Surface Science 216 (2003) 483–489
487
Fig. 5. Drain current–drain voltage characteristics in a static simulation where gate voltage is taken as a parameter.
within the bulk or injected from outside, they would travel towards the drain contact, which is same as we expected. From the potential profiles, it is also confirmed that the depletion layer expands more as the reverse bias at the gate contact increases. Fig. 5 shows the drain current–drain voltage characteristics for various values of the gate voltage in the static simulation. The drain current decreases when the gate is negatively biased, which is contrary to our expectation. It can also be seen that the drain current is saturated when the drain voltage increases. These characteristics, namely, control of the drain–source current by the gate bias and the existence of Pinch-off mode, are similar to those of JFET/MESFET. This current level is only of the order of 1016 A mm1 or below, because of the high resistivity. However, if the density of impurities is much higher than 1016 cm3, the current level increases since the number of ionized donors increases. Either applying high voltage or using the different geometry might also increase the device operation. If the current level increases, this device will work statically like JFET made of silicon. In the transient simulation, the electron density distribution is calculated. The reversed bias is varied from 0 to 5 V during 2 1013 s and the other conditions are the same as those of the static simulation. The series of maps in Fig. 6 are of the electron distribution in the order of time passing from the left to right. The numbers on the figures are corresponding to the numbers on the above graph. Note that the distribution is normalized and lighter region indicates low electron density while darker region indicates comparably high electron density. This result indeed
Fig. 6. Electron density distribution in transient simulation.
implies that the depletion region further expands as the reverse bias is increasing. Fig. 7 shows the amount of the current, which pass through each contact during the transient simulation. From the top to below, the plots indicate the amount of the current at the source contact, at the drain contact, and at the gate contact, respectively. Although the current level of drain–source is of the order of 1016 A mm1 in the static simulation, relatively high current passes through each contact. It is confirmed that the current, which passes through each contact, is proportional to dV/dT. This current seems to be generated by the electrons, which are excited while the depletion region is expanding. When the depletion
Fig. 7. Current–time characteristic in transient simulation.
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tance between the drain and the source. Therefore, the electrons are excited near the gate and move towards the drain contact. If the electrons were injected from the source, the current would flow from the source to the drain. Namely, using an asymmetric structure or asymmetric bias conditions might improve the device operation.
4. Conclusions Fig. 8. Possible schematic interpretation of the results.
region expands, some electrons are able to be excited from the donor level to the conduction band, since the donor level lies above the Fermi level within a depletion region. It is also confirmed that the gate–drain current and the gate–source current are of the same magnitude in opposite directions. The sum amount of the current passing through the drain and the source is the same as the amount of the current passing through the gate contact. It was assumed in Section 1 that most electrons move towards the drain contact. However, as interpreted in Fig. 8, only half of the excited electrons move towards the drain contact and the other half of excited electrons move towards the source contact. The reason of this contradiction is that the potential near the gate is lower than both near the source and the drain. Another reason is that this device has a symmetric structure. Hence, some modifications, which break symmetry of the device and enable the excited electrons to enhance the drain–source current, are required in order that the device may operate. For this purpose, either changing the bias conditions or changing its geometry can be considered. One example is to keep the source voltage same as the gate voltage. If the source voltage and the gate voltage are same, the potential only around the drain is higher. Then, the excited electrons might move towards the drain contact. Another example is to change the geometry into a rectangle. With this shape of the device, the distance between the drain and the source is shorter than the distance between the gate and the source/drain. Then, most electrons might reach the drain contact because of the electric field between the drain and the source rather than because of the dis-
A new idea of a power device for high power, for high frequency and for high temperature with the usage of highly nitrogen-doped CVD diamond and band bending, which is caused by the Schottky barrier, was proposed in this report. Although the drain– source current decreases when the gate is reverse biased in the static simulation, while the depletion region further expands, relatively high current is obtained in the transient simulation. However, the drain–source current cannot be enhanced because drain current and source current are cancelled out. Using asymmetric bias conditions and/or asymmetric geometry, the device operation may improve. If those problems are solved, this device with the usage of diamond will be an ideal electric device for extremely high power, high frequency and high temperature. Preliminary calculation shows that the drain–source current can be enhanced by the applied reverse bias to the gate contact with the asymmetric structure and bias conditions. Details will be reported elsewhere.
Acknowledgements The authors would like to express our gratitude to Prof. W.I. Milne (University of Cambridge), Prof. J.H. Zhao (Rutgers, The State University of New Jersey), Dr. T. Yamada (Tohoku University) and Mr. K. Umezawa (ISE Japan Ltd.). Y. Yokoyama appreciate Prof. K. Kitahara (International Christian University (ICU)) for his critical reading of the manuscript and for his encouragement. The authors also thank Mr. Y. Suzuki, H. Yamaguchi, Ms. R. Horiuchi (ICU) for the fruitful discussions. This work is supported by Grantin-Aid for Scientific Research (A) #13305006 of Ministry of Education, Science, Sports and Culture,
Y. Yokoyama et al. / Applied Surface Science 216 (2003) 483–489
Japan and the Research Grants-in-Aid Fund of the School Juridical Person International Christian University, 2001. References [1] G. Amaratunga, F. Udrea, Selected publications of High Voltage Microelectronics Group, Department of Engineering, University of Cambridge, 2001. [2] L.S. Pan, D.R. Kania, Diamond: Electronic Properties and Applications, Kluwer Academic Publishers, Dordrecht, 1995. [3] H. Itoh, K. Kawarada, Jpn. J. Appl. Phys. 1 34 (1995) 4677.
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[4] K. Okano, S. Koizumi, S.R.P. Silva, G.A.J. Amaratunga, Nature 381 (1996) 140. [5] S.A. Kajihara, A. Antonelli, J. Bernholc, Phys. Rev. Lett. 66 (1991) 2010. [6] M.W. Geis, J.C. Twichell, T.M. Lyszczarz, J. Vac. Sci. Tech. B14 (1996) 2060. [7] T. Tachibana, B.E. Williams, J.T. Glass, Phys. Rev. B45 (1992) 11975. [8] T. Teraji, S. Koizumi, H. Kanda, Appl. Phys. Lett. 76 (2000) 1303. [9] Integrated Systems Engineering AG, ISE TCAD Manuals 8.0, ISE Integrated Systems Engineering AG, 2002. [10] L.S. Pan, S. Han, D.R. Kania, K. Okano, Diamond Films and Technol. 2 (2–3) (1992).
A field effect transistor using highly nitrogen-doped CVD diamond for power device applications Yoko Yokoyamaa, Xueqing Lib, Kuang Shengb, Andrei Mihailac, Tanija Traikovicc, Florin Udreac, G.A.J. Amaratungac, Ken Okanoa,* b
a Department of Physics, International Christian University, 3-10-2 Osawa, Mitaka, Tokyo, Japan Department of Electrical and Computer Engineering, The State University of New Jersey, Rutgers, NJ, USA c Department of Engineering, University of Cambridge, Cambridge, UK
Abstract A new idea of power device, which contains highly nitrogen-doped CVD diamond and Schottky contact, is proposed to actualise a power device with diamond. Two-dimensional simulation is conducted using ISE TCAD device simulator. While comparably high current is obtained in a transient simulation as expected, this current does not contribute to the drain–source current because of the symmetry of the device. Using an asymmetric structure or bias conditions, the device has high potential as an electric device for extremely high power, high frequency and high temperature. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Highly nitrogen-doped CVD diamond; Schottky barrier; Power device; Carrier density
1. Introduction Although silicon is overwhelmingly used in high voltage power devices, these devices with silicon are reaching fundamental limits because their breakdown field is not high enough. In recent years, wide bandgap semiconductors have attracted great attention due to their specific electronic properties, which make them suitable for devices with high power, high temperature and high frequency [1]. Diamond is one of the most attractive materials for such devices because it exhibits large breakdown electric field, high thermal conductivity, small relative permittivity and high carrier mobility. High power devices consisting of diamond are expected to exhibit *
Corresponding author. Tel.: þ81-422-33-3254; fax: þ81-422-33-3254. E-mail address: [email protected] (K. Okano).
greater breakdown voltage and faster switching speed than those consisting of other materials [2]. Diamond, however, has severe problems when it is used as an electronic device. Although type Ib diamond contains certain amount of nitrogen (N), the donor level is too deep for electrons to be excited even if very high voltage is applied. Thus, there are few reports about electronic devices using diamond with some exceptions, such as FETs using the surface conductive layer reported by Itoh and Kawarada [3]. According to the recent publication of some of the authors, high concentrations of nitrogen can now be incorporated in diamond, using urea as a dopant. This technique makes it possible to increase impurity concentration of CVD diamond up to 1021 cm3. In our previous paper [4], more than 1020 cm3 of N atoms have been confirmed in doped CVD diamond films by RRS and XPS technique. These films were also identified as diamond with high crystalline perfection by
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00402-1
484
Y. Yokoyama et al. / Applied Surface Science 216 (2003) 483–489
simulation program. Characteristics of the device are computed and the effect of the geometry and applied voltage will be also discussed.
2. Idea and simulation
Fig. 1. Structure of proposed device.
RHEED, AES and Raman spectroscopy. From theoretical studies, significant numbers of N atoms are expected to be contained substitutionally in diamond mainly due to the small ion radius of nitrogen [5]. It is true that the number of electrons in the conduction band is fairly small at room temperature since the donor level (1.7 eV) is deep. However, at the metal–diamond interface of substitutional N-doped diamond as shown in Fig. 1, electrons are excited from the donor levels to the conduction band while the depletion region is creating or expanding. Therefore, at least some electrons are transiently available in the conduction band even at room temperature with a suitable bias. We tried to apply these electrons for the device operation. In this report, a new structure of a JFET-like power device with the usage of highly nitrogen-doped CVD diamond and Schottky contact will be discussed. Twodimensional device simulation of a diamond power device is investigated by using ISE TCAD device
Fig. 2 shows our new idea of the device structure. The device consists of single crystal diamond (type Ib, (1 0 0)) with layers of highly nitrogen-doped CVD diamond and Schottky contact (gate) as shown in the figure. Two ohmic contacts are placed on the top (drain) and bottom (source) of the diamond crystal. Although it is known that the ohmic contact for wide band-gap semiconductor is generally difficult to be formated, there are many reports about the formation of ohmic contact for n-type diamond [7]. For example, Teraji et al. reported that Ga implanted contact formed on n-type diamond surfaces exhibits ohmic property [8]. Even though high voltage is applied between two ohmic contacts, the resistivity of diamond is so high that current does not flow within its bulk. When reverse bias is applied to the Schottky contact, the depletion region will expand [6]. Within the depleted region, which is created in the highly nitrogen-doped CVD diamond layer, the donor level lies above the Fermi level as illustrated in Fig. 1. Thus, some electrons are excited from the donor levels to the conduction band. When greater reverse bias is applied to the Schottky contact, the depletion region further expands. This expansion leads to the excitation of more electrons.
Fig. 2. Energy-band diagram of the metal–diamond (N-doped) interface.
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485
Fig. 3. The mechanism of device operation (a) when gate is reverse biased and (b) gate is forward biased.
We may expect that the following mechanisms give rise to the expansion of the depleted region. Some of the excited electrons would move towards the center of the bulk since the potential is lower at the center (Fig. 3(a)). Those electrons would move to the drain contact. Hopefully, the same amount of electrons would be injected at the source in order to keep the electrical neutrality. This means that the current between the drain and the source is conserved. Therefore, when the gate is reverse biased, the drain–source current is switched on. On the contrary, when the Schottky contact is forward biased, the depletion region shrinks (Fig. 3(b)) and the density of the carriers at the center area decreases. Therefore, almost no current flows between the drain and the source contacts, which means the drain–source current is switched off. In the case of forward bias, leakage current is extremely low due to the high resistivity of diamond. This is the ideal characteristic for the switching devices. On the basis of these facts, the drain–source current can be controlled by applied gate bias. There are two main differences between the device we proposed and the existing FET. 1. Although diamond is known as an insulator, there are a number of reports that the diamond contained significant impurities has properties as a semiconductor. For example, according to the
previous reference by Geis et al., the depletion region is generated near the interface of Schottky contact and nitrogen-doped diamond [6]. In this study, we tried to apply both characteristics of diamond, namely, the high resistivity and the energy-band bending in the depletion region, to the device operation. 2. While the depletion layer expands, electrons at the metal–semiconductor interface move away in order to meet the Fermi level. Since diamond has few electrons in the conduction band at the room temperature, it is proposed by Geis et al. that the electrons need to be excited from donors to the conduction band and those electrons move away due to the electric field [6]. We tried to apply those electrons to the device operation under the high electric field towards the bulk center. This is the first trial as far as we know. Device simulation is conducted using ISE TCAD device simulator to confirm the device operation. ISE TCAD device simulator solves the following three equations for charge transport in semiconductor devices [9]; One is the Poisson equation. rE rC ¼ qðp n þ ND þ NA Þ where E is the electrical permittivity; q the elementary electronic charge; n and p the electron density and the hole density, respectively; and NDþ and NA are the
486
Y. Yokoyama et al. / Applied Surface Science 216 (2003) 483–489
number of ionized donors and acceptors, respectively. The others are the electron continuity equation and the hole continuity equation. @n r ~ Jn ¼ qR þ q ; @t
@p r ~ Jp ¼ qR þ q @t
where R is the net electron–hole recombination rate; and ~ Jn and ~ Jp are the electron current density and the hole current density, respectively. However, for simplicity, we solve them for a twodimensional geometry. Namely, the cross-sectional surface of the device is a square of 2 mm 2 mm, made of single crystal diamond, which contains 1016 cm3 of nitrogen. In two-dimensional simulation, the width of the device is always considered as 1 nm. When any effective result of the proposed device is obtained in two-dimensional simulation, we will apply the device to three-dimensional simulation. The right and left layers are made of highly nitrogen-doped diamond, which contains 1021 cm3 of nitrogen. There are four contacts on this device. The top is the drain, the bottom is the source, and the left side and the right side are the gates. The drain and the source are ohmic contacts and the gates are Schottky contacts. The work function of the gate is chosen as 4.9 eV. We also take into account the temperature dependence of carrier density in incomplete ionization model,
in contrast to the case of other materials in which the donor level is shallow. We also choose the values of parameters for diamond, which were obtained by Pan et al. [10]. To simplify the simulation, most of the parameters, such as the electron mobility, the hole mobility and the band-gap, are taken as constants. Temperature is fixed at 300 K in the simulation when we deal with Fermi-Dirac distribution. The dopant is nitrogen, whose activation energy is 1.7 eV and the electron affinity of diamond is chosen as 0 eV. The drain is grounded and the source is biased up to þ50 V. Then, reverse bias is applied to the gate contacts. The gate voltage is varied from 0 to 10 V. For each value of the gate bias, the drain current–drain voltage characteristics are computed. To clarify the mechanism of the device operation, the transient simulation is also conducted, with the same geometry and bias condition as in the static simulation by taking the ramped time as a parameter.
3. Results and discussions The obtained potential profiles indicate that the potential around the drain contact is higher than any other points within the bulk as illustrated in Fig. 4. It means that once the electrons are excited
Fig. 4. Potential profiles inside the device for following bias conditions: (a) VS ¼ 0, VD ¼ 10, and VG ¼ 0; (b) VS ¼ 0, VD ¼ 10, and VG ¼ 1.
Y. Yokoyama et al. / Applied Surface Science 216 (2003) 483–489
487
Fig. 5. Drain current–drain voltage characteristics in a static simulation where gate voltage is taken as a parameter.
within the bulk or injected from outside, they would travel towards the drain contact, which is same as we expected. From the potential profiles, it is also confirmed that the depletion layer expands more as the reverse bias at the gate contact increases. Fig. 5 shows the drain current–drain voltage characteristics for various values of the gate voltage in the static simulation. The drain current decreases when the gate is negatively biased, which is contrary to our expectation. It can also be seen that the drain current is saturated when the drain voltage increases. These characteristics, namely, control of the drain–source current by the gate bias and the existence of Pinch-off mode, are similar to those of JFET/MESFET. This current level is only of the order of 1016 A mm1 or below, because of the high resistivity. However, if the density of impurities is much higher than 1016 cm3, the current level increases since the number of ionized donors increases. Either applying high voltage or using the different geometry might also increase the device operation. If the current level increases, this device will work statically like JFET made of silicon. In the transient simulation, the electron density distribution is calculated. The reversed bias is varied from 0 to 5 V during 2 1013 s and the other conditions are the same as those of the static simulation. The series of maps in Fig. 6 are of the electron distribution in the order of time passing from the left to right. The numbers on the figures are corresponding to the numbers on the above graph. Note that the distribution is normalized and lighter region indicates low electron density while darker region indicates comparably high electron density. This result indeed
Fig. 6. Electron density distribution in transient simulation.
implies that the depletion region further expands as the reverse bias is increasing. Fig. 7 shows the amount of the current, which pass through each contact during the transient simulation. From the top to below, the plots indicate the amount of the current at the source contact, at the drain contact, and at the gate contact, respectively. Although the current level of drain–source is of the order of 1016 A mm1 in the static simulation, relatively high current passes through each contact. It is confirmed that the current, which passes through each contact, is proportional to dV/dT. This current seems to be generated by the electrons, which are excited while the depletion region is expanding. When the depletion
Fig. 7. Current–time characteristic in transient simulation.
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tance between the drain and the source. Therefore, the electrons are excited near the gate and move towards the drain contact. If the electrons were injected from the source, the current would flow from the source to the drain. Namely, using an asymmetric structure or asymmetric bias conditions might improve the device operation.
4. Conclusions Fig. 8. Possible schematic interpretation of the results.
region expands, some electrons are able to be excited from the donor level to the conduction band, since the donor level lies above the Fermi level within a depletion region. It is also confirmed that the gate–drain current and the gate–source current are of the same magnitude in opposite directions. The sum amount of the current passing through the drain and the source is the same as the amount of the current passing through the gate contact. It was assumed in Section 1 that most electrons move towards the drain contact. However, as interpreted in Fig. 8, only half of the excited electrons move towards the drain contact and the other half of excited electrons move towards the source contact. The reason of this contradiction is that the potential near the gate is lower than both near the source and the drain. Another reason is that this device has a symmetric structure. Hence, some modifications, which break symmetry of the device and enable the excited electrons to enhance the drain–source current, are required in order that the device may operate. For this purpose, either changing the bias conditions or changing its geometry can be considered. One example is to keep the source voltage same as the gate voltage. If the source voltage and the gate voltage are same, the potential only around the drain is higher. Then, the excited electrons might move towards the drain contact. Another example is to change the geometry into a rectangle. With this shape of the device, the distance between the drain and the source is shorter than the distance between the gate and the source/drain. Then, most electrons might reach the drain contact because of the electric field between the drain and the source rather than because of the dis-
A new idea of a power device for high power, for high frequency and for high temperature with the usage of highly nitrogen-doped CVD diamond and band bending, which is caused by the Schottky barrier, was proposed in this report. Although the drain– source current decreases when the gate is reverse biased in the static simulation, while the depletion region further expands, relatively high current is obtained in the transient simulation. However, the drain–source current cannot be enhanced because drain current and source current are cancelled out. Using asymmetric bias conditions and/or asymmetric geometry, the device operation may improve. If those problems are solved, this device with the usage of diamond will be an ideal electric device for extremely high power, high frequency and high temperature. Preliminary calculation shows that the drain–source current can be enhanced by the applied reverse bias to the gate contact with the asymmetric structure and bias conditions. Details will be reported elsewhere.
Acknowledgements The authors would like to express our gratitude to Prof. W.I. Milne (University of Cambridge), Prof. J.H. Zhao (Rutgers, The State University of New Jersey), Dr. T. Yamada (Tohoku University) and Mr. K. Umezawa (ISE Japan Ltd.). Y. Yokoyama appreciate Prof. K. Kitahara (International Christian University (ICU)) for his critical reading of the manuscript and for his encouragement. The authors also thank Mr. Y. Suzuki, H. Yamaguchi, Ms. R. Horiuchi (ICU) for the fruitful discussions. This work is supported by Grantin-Aid for Scientific Research (A) #13305006 of Ministry of Education, Science, Sports and Culture,
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