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GaAs heterostructure-emitter bipolar transistor grown by metalorganic chemical vapor deposition
Thin Solid Films 515 (2007) 3978 – 3981 www.elsevier.com/locate/tsf
δ-doped InGaP/GaAs heterostructure-emitter bipolar transistor grown by metalorganic chemical vapor deposition Y.S. Lin a,⁎, D.H. Huang b , Y.W. Chen c , J.C. Huang b , W.C. Hsu b a
Department of Materials Science and Engineering, National Dong Hwa University, 1, Sec. 2, Da Hsueh Rd. Shou-Feng, Hualien, Taiwan, R.O.C. b Institute of Microelectronics, Department of Electrical Engineering, National Cheng-Kung University, Tainan, Taiwan, R.O.C. c Chi Mei Optoelectronics, Tainan, Taiwan, R.O.C. Received 9 December 2005; received in revised form 4 August 2006; accepted 14 September 2006 Available online 23 October 2006
Abstract This article assesses the performance of a δ-doped In0.5Ga0.5P/GaAs heterostructure-emitter bipolar transistor (δ-HEBT) grown by lowpressure metalorganic chemical vapor deposition. Moreover, an HEBT without δ-doping design in the emitter and with spacer layers of various thicknesses is presented for comparison. The common-emitter current gain and offset voltage are 50 and 130 mV, respectively, for the HEBT without δ-doping layer. The offset voltage of the δ-doped HEBT is as low as 70 mV and its common-emitter current gain is increased to 55, because of the use of the optimum spacer thickness of 100 Å and the design of the δ-doped emitter layer. © 2006 Elsevier B.V. All rights reserved. Keywords: InGaP; HEBT; MOCVD; δ-doping; Offset voltage
1. Introduction Advances in epitaxy and fabrication approaches have led to the production of various compound semiconductor devices, such as real space transfer devices [1–3], heterostructure fieldeffect transistors (HFETs) [4–6] and heterojunction bipolar transistors (HBTs) [7–9]. Among these heterostructure devices, InGaP/GaAs HBTs have attracted considerable interest because of their potential microwave circuit applications based on their high speed and high current-handling capabilities [8]. The other strengths of InGaP/GaAs HBTs over AlGaAs/GaAs HBTs are that the former have a large valence-band discontinuity (ΔEV) at the emitter–base (E–B) heterojunction and extremely highly selective etching between InGaP and GaAs. Although reported values of conduction-band discontinuity (ΔEC) for the In0.5Ga0.5P/GaAs heterojunction vary between 0.03 and 0.39 eV, ΔEC ∼ 0.2 eV (and hence ΔEV ∼ 0.3 eV) is typical [10–15]. Thus, a potential spike at the E–B junction of the InGaP/GaAs single heterojunction bipolar transistor is expected, producing a collector-emitter offset voltage. Accordingly, heterostructure-emitter bipolar transistors (HEBTs) have ⁎ Corresponding author. Tel.: +886 3 8634218; fax: +886 3 8634200. E-mail address: [email protected] (Y.S. Lin). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.09.022
been reported to overcome the above shortcomings [16–19]. The heterostructure-emitter structure allows effective electron injection through the E–B homojunction and blocks the backinjection of holes from the base to the emitter. Moreover, the presence of the effective E–B homojunction is expected to eliminate the potential spike. Thus, the undesired offset voltage can be reduced and a high current gain maintained. Moreover, δ-doping technique has been recently applied to semiconductor quantum structures to improve the performance of devices [20– 23]. When the impurities are ionized, the charged dopants create a V-shaped potential well at the δ-doped layer. This study reports an improved δ-doped emitter In0.5Ga0.5P/GaAs HEBT with 100Å-thick undoped GaAs spacer layers on both sides of the base (represented by δ-HEBT). The improved In0.5Ga0.5P/ GaAs δ-HEBT has a common-emitter current gain of 55 and an offset voltage of as low as 70 mV. Moreover, an HEBT without δ-doped layers and with 300Å-thick undoped GaAs spacer layers (represented by HEBT) is also fabricated and compared. 2. MOCVD growth and device fabrication This developed δ-HEBT was grown by metalorganic chemical vapor deposition (MOCVD) on a GaAs substrate. The epitaxial layers comprised a 0.3 μm n+ -GaAs (n = 5 × 10 18 cm − 3 )
Y.S. Lin et al. / Thin Solid Films 515 (2007) 3978–3981
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Table 1 Device layer structure of HEBT Layer Emitter cap Emitter confinement Emitter Spacer Base Spacer Collector Subcollector
Material GaAs In0.5Ga0.5P GaAs GaAs GaAs GaAs GaAs GaAs
Thickness (Å) Type 3000 1000 300 300 1000 300 5000 3000
+
n n n Undoped p+ Undoped n− n+
Doping (cm− 3) 5 × 1018 7 × 1017 5 × 1017 – 5 × 1018 – 5 × 1016 5 × 1018
subcollector layer, followed by a 0.5 μm n − -GaAs (n = 5 × 1016 cm− 3) collector layer, a 100 Å undoped GaAs spacer layer, a 0.1 μm p+-GaAs (p = 5 × 1018 cm− 3) base layer, a 100 Å undoped GaAs spacer layer, a 100 Å n-GaAs (n = 5 × 1016 cm− 3) layer, a δ-doped layer (δ1), a 200 Å n-GaAs (n = 5 × 1016 cm− 3) layer, a δ-doped layer (δ2), a 100 Å n-GaAs (n = 5 × 1016 cm− 3) layer, a 0.1 μm n-In0.5Ga0.5P (n = 7 × 1017 cm− 3 ) emitter confinement layer and, finally, a 0.3 μm n + -GaAs (n = 5 × 1018 cm− 3) cap layer. δ1 = δ2 = 3 × 1012 cm− 2. Tables 1 and 2 describe the layer structures of HEBT and δ-HEBT, respectively. The temperature and pressure for growing the two proposed devices were 690 °C and 150 Torr, respectively. Trimethylgallium (TMG), trimethylindium (TMI), arsine (AsH3) and phosphine (PH3) were used as the growth precursors. Device fabrication was performed using photolithography and wet etching. The InGaP confinement layers were etched with 5:3:100 NH4OH:H2O2:H2O. GaAs was etched with solutions of 4:1 H3PO4:HCl. AuGe/Ag and AuZn were used to construct the n- and p-type ohmic contacts, respectively. Current–voltage (I–V) measurements were made on both the devices using an HP-4156 semiconductor parameter analyzer. 3. Results and discussion Fig. 1(A) plots the common-emitter I–V characteristics of HEBT at 300 K. The current gain of the HEBT is 50. The offset voltage of HEBT is 130 mV. Fig. 1(B) plots the commonemitter I–V characteristics of δ-HEBT at 300 K. δ-HEBT exhibits a current gain of the 55 and an offset voltage of as low as 70 mV. These values of δ-HEBT are better than those of the
Fig. 1. (A) Common-emitter I–V characteristics of HEBT. (B) Common-emitter I–V characteristics of δ-HEBT.
proposed HEBT, because the optimization of the spacer thickness, 100 Å, and the δ-doped layers in the δ-HEBT. Fig. 2 is an enlargement of Fig. 1(B) near the origin. A low offset voltage of 70 mV is clearly observed. This is smaller than those of the previously developed AlGaAs/GaAs and InGaP/GaAs HEBTs [24] and those of the InGaP/GaAs HBTs with nongraded and
Table 2 Device layer structure of δ-HEBT Layer Emitter cap Emitter confinement Emitter Emitter Emitter Emitter Emitter Spacer Base Spacer Collector Subcollector
Material GaAs In0.5Ga0.5P GaAs GaAs GaAs GaAs GaAs GaAs GaAs GaAs GaAs GaAs
Thickness (Å) 3000 1000 100 – 200 – 100 100 1000 100 5000 3000
Type +
n n n δ(n+) n− δ(n+) n− Undoped p+ Undoped n− n+
Doping 5 × 1018 7 × 1017 5 × 1016 3 × 1012 5 × 1016 3 × 1012 5 × 1016 – 5 × 1018 – 5 × 1016 5 × 1018
cm–3 cm–3 cm–3 cm–2 cm–3 cm–2 cm–3 cm–3 cm–3 cm–3
Fig. 2. Expanded view at the beginning of Fig. 1(B), showing the collectoremitter offset voltage in common-emitter I–V characteristics of δ-HEBT.
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Fig. 3. Common-emitter I–V characteristics of δ-HEBT operated in the low collector current region.
graded base doping [7]. Fig. 3 plots the common-emitter I–V characteristics of δ-HEBT at low collector currents. They indicate that δ-HEBT has a useful gain, even in the μA region. Hence, the base bulk current dominates the base current. Fig. 4 shows the semilog-forward bias characteristics for the E–B junction of δ-HEBT. The current varies as exp (qV / nkT) for several decades, where q is the electron charge, V is the applied voltage, k is the Boltzmann constant, T is the absolute temperature, and n is an ideality factor. Fig. 5 plots the I–V characteristics of the base–collector (B–C) junction of δ-HEBT. Notably, the breakdown voltage exceeds 16 V. Fig. 6 plots the semilog-forward bias characteristics of the B–C junction in δHEBT. The value of the ideality factor n is calculated from the slope of the semilog-forward bias curve. In the lower current regime, the ideality factor is around 1.52, indicating that the 1 kT diffusion current and the 2 kT recombination current
Fig. 4. Semilogarithmic plot of the forward I–V characteristics of the E–B junction in δ-HEBT.
Fig. 5. I–V characteristics of B–C junction in δ-HEBT.
dominate the transport of carriers. Furthermore, the composition of the collector and the base currents are analyzed from the Gummel plot. The transferred collector current and base current are functions of the base-emitter voltage when the basecollector bias of δ-HEBT is maintained at 0 V. The ideality factor of the collector current is 1.03, indicating that the recombination current in δ-HEBT is greatly reduced. The junction ideality factor can be determined from the properties of the device as follows [12,25]. g¼1þ
e1 Nd e2 Na
ð1Þ
where ε1 and ε2 are the dielectric constants of n-InGaP and pGaAs, respectively, and Nd and Na are the respective doping densities. Eq. (1) shows that, at an estimated doping ratio of ∼1:10, η is 1.09. The calculated value is consistent with the experimental value.
Fig. 6. Semilogarithmic plot of the forward I–V characteristics of the B–C junction in δ-HEBT.
Y.S. Lin et al. / Thin Solid Films 515 (2007) 3978–3981
4. Conclusion This paper develops an improved InGaP/GaAs HEBT with δ-doping layers grown by a MOCVD system. The InGaP/GaAs HEBT without the δ-doping layers exhibits a current gain of 50 and an offset voltage of 130 mV. Optimizing the thickness of the undoped-GaAs spacer layer adjacent to the base can effectively reduce the recombination current at the p–n interface. δ-HEBT has the advantages of a low offset voltage (70 mV), an improved current gain of 55 and ease of fabrication (by the highselectivity etching process). Consequently, the improved device has considerable potential for use in practical circuit. Acknowledgment The author would like to thank the National Science Council of the Republic of China for financially supporting this research under Contract No. NSC 95-2221-E-259-037. References [1] J.S. Su, W.C. Hsu, Y.S. Lin, W. Lin, C.L. Wu, M.S. Tsai, Y.H. Wu, IEEE Electron Device Lett. 17 (1996) 43. [2] J.S. Su, W.C. Hsu, W. Lin, Y.S. Lin, J. Appl. Phys. 82 (1997) 4076. [3] J.S. Su, W.C. Hsu, Y.S. Lin, W. Lin, Appl. Phys. Lett. 70 (1997) 1002. [4] Y.S. Lin, Y.L. Hsieh, J. Electrochem. Soc. 152 (2005) G778. [5] Y.S. Lin, W.C. Hsu, C.H. Wu, W. Lin, R.T. Hsu, Appl. Phys. Lett. 75 (1999) 1616. [6] Y.S. Lin, D.H. Huang, W.C. Hsu, T.B. Wang, K.H. Su, J.C. Huang, C.H. Ho, Semicond. Sci. Technol. 21 (2006) 540.
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[7] Y.W. Chen, W.C. Hsu, R.T. Hsu, Y.H. Wu, Y.J. Chen, Y.S. Lin, J. Vac. Sci. Technol., B 21 (2003) 2555. [8] Y.S. Lin, W.C. Hsu, F.C. Jong, Y.Z. Chiou, Y.J. Chen, J.J. Tang, Jpn. J. Appl. Phys. 43 (2004) 3285. [9] Y.S. Lin, D.H. Huang, W.C. Hsu, K.H. Su, T.B. Wang, Semicond. Sci. Technol. 21 (2006) 303. [10] K. Kodama, M. Masataka, K. Kitahara, M. Takikawa, M. Ozaki, Jpn. J. Appl. Phys. 25 (1986) L127. [11] M.A. Rao, E.J. Caine, H. Kroemer, S.I. Long, D.I. Babic, J. Appl. Phys. 61 (1987) 643. [12] T. Kobayashi, K. Taira, F. Nakamura, H. Kawai, J. Appl. Phys. 65 (1989) 4898. [13] D. Biswas, N. Debbar, P. Bhattacharya, M. Razeghi, M. Defour, F. Omnes, Appl. Phys. Lett. 56 (1990) 833. [14] J. Chen, J.R. Sites, I.L. Spain, M.J. Hafich, G.Y. Robinson, Appl. Phys. Lett. 58 (1991) 744. [15] M.A. Haase, M.J. Hafich, G.Y. Robinson, Appl. Phys. Lett. 58 (1991) 616. [16] L.F. Luo, H.L. Evans, E.S. Yang, IEEE Trans. Electron Devices 3 (1989) 1844. [17] X. Wu, Y.Q. Wang, L.F. Luo, E.S. Yang, IEEE Electron Device Lett. 11 (1990) 264. [18] W.C. Liu, W.S. Lour, IEEE Electron Device Lett. 12 (1991) 474. [19] Y.F. Yang, C.C. Hsu, E.S. Yang, IEEE Trans. Electron Devices 41 (1994) 643. [20] Y.S. Lin, J.H. Huang, J. Electrochem. Soc. 152 (2005) G627. [21] E. Tokumitsu, A.G. Dentai, C.H. Joyner, S. Chandrasekhar, Appl. Phys. Lett. 57 (1990) 2841. [22] K.W. Goossen, J.E. Cunningham, T.Y. Kuo, W.Y. Jan, C.G. Fonstad, Appl. Phys. Lett. 59 (1991) 682. [23] Y.S. Lin, Y.L. Hsieh, J. Electrochem. Soc. 153 (2006) G498. [24] W.C. Liu, J.H. Tsai, S.Y Cheng, W.L. Chang, H.J. Pan, Y.H. Shie, Thin Solid Films 324 (1998) 219. [25] T.W. Lee, P.A. Houston, R. Kumar, X.F. Yang, G. Hill, M. Hopkinson, P.A. Claxton, Appl. Phys. Lett. 60 (1992) 474.
δ-doped InGaP/GaAs heterostructure-emitter bipolar transistor grown by metalorganic chemical vapor deposition Y.S. Lin a,⁎, D.H. Huang b , Y.W. Chen c , J.C. Huang b , W.C. Hsu b a
Department of Materials Science and Engineering, National Dong Hwa University, 1, Sec. 2, Da Hsueh Rd. Shou-Feng, Hualien, Taiwan, R.O.C. b Institute of Microelectronics, Department of Electrical Engineering, National Cheng-Kung University, Tainan, Taiwan, R.O.C. c Chi Mei Optoelectronics, Tainan, Taiwan, R.O.C. Received 9 December 2005; received in revised form 4 August 2006; accepted 14 September 2006 Available online 23 October 2006
Abstract This article assesses the performance of a δ-doped In0.5Ga0.5P/GaAs heterostructure-emitter bipolar transistor (δ-HEBT) grown by lowpressure metalorganic chemical vapor deposition. Moreover, an HEBT without δ-doping design in the emitter and with spacer layers of various thicknesses is presented for comparison. The common-emitter current gain and offset voltage are 50 and 130 mV, respectively, for the HEBT without δ-doping layer. The offset voltage of the δ-doped HEBT is as low as 70 mV and its common-emitter current gain is increased to 55, because of the use of the optimum spacer thickness of 100 Å and the design of the δ-doped emitter layer. © 2006 Elsevier B.V. All rights reserved. Keywords: InGaP; HEBT; MOCVD; δ-doping; Offset voltage
1. Introduction Advances in epitaxy and fabrication approaches have led to the production of various compound semiconductor devices, such as real space transfer devices [1–3], heterostructure fieldeffect transistors (HFETs) [4–6] and heterojunction bipolar transistors (HBTs) [7–9]. Among these heterostructure devices, InGaP/GaAs HBTs have attracted considerable interest because of their potential microwave circuit applications based on their high speed and high current-handling capabilities [8]. The other strengths of InGaP/GaAs HBTs over AlGaAs/GaAs HBTs are that the former have a large valence-band discontinuity (ΔEV) at the emitter–base (E–B) heterojunction and extremely highly selective etching between InGaP and GaAs. Although reported values of conduction-band discontinuity (ΔEC) for the In0.5Ga0.5P/GaAs heterojunction vary between 0.03 and 0.39 eV, ΔEC ∼ 0.2 eV (and hence ΔEV ∼ 0.3 eV) is typical [10–15]. Thus, a potential spike at the E–B junction of the InGaP/GaAs single heterojunction bipolar transistor is expected, producing a collector-emitter offset voltage. Accordingly, heterostructure-emitter bipolar transistors (HEBTs) have ⁎ Corresponding author. Tel.: +886 3 8634218; fax: +886 3 8634200. E-mail address: [email protected] (Y.S. Lin). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.09.022
been reported to overcome the above shortcomings [16–19]. The heterostructure-emitter structure allows effective electron injection through the E–B homojunction and blocks the backinjection of holes from the base to the emitter. Moreover, the presence of the effective E–B homojunction is expected to eliminate the potential spike. Thus, the undesired offset voltage can be reduced and a high current gain maintained. Moreover, δ-doping technique has been recently applied to semiconductor quantum structures to improve the performance of devices [20– 23]. When the impurities are ionized, the charged dopants create a V-shaped potential well at the δ-doped layer. This study reports an improved δ-doped emitter In0.5Ga0.5P/GaAs HEBT with 100Å-thick undoped GaAs spacer layers on both sides of the base (represented by δ-HEBT). The improved In0.5Ga0.5P/ GaAs δ-HEBT has a common-emitter current gain of 55 and an offset voltage of as low as 70 mV. Moreover, an HEBT without δ-doped layers and with 300Å-thick undoped GaAs spacer layers (represented by HEBT) is also fabricated and compared. 2. MOCVD growth and device fabrication This developed δ-HEBT was grown by metalorganic chemical vapor deposition (MOCVD) on a GaAs substrate. The epitaxial layers comprised a 0.3 μm n+ -GaAs (n = 5 × 10 18 cm − 3 )
Y.S. Lin et al. / Thin Solid Films 515 (2007) 3978–3981
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Table 1 Device layer structure of HEBT Layer Emitter cap Emitter confinement Emitter Spacer Base Spacer Collector Subcollector
Material GaAs In0.5Ga0.5P GaAs GaAs GaAs GaAs GaAs GaAs
Thickness (Å) Type 3000 1000 300 300 1000 300 5000 3000
+
n n n Undoped p+ Undoped n− n+
Doping (cm− 3) 5 × 1018 7 × 1017 5 × 1017 – 5 × 1018 – 5 × 1016 5 × 1018
subcollector layer, followed by a 0.5 μm n − -GaAs (n = 5 × 1016 cm− 3) collector layer, a 100 Å undoped GaAs spacer layer, a 0.1 μm p+-GaAs (p = 5 × 1018 cm− 3) base layer, a 100 Å undoped GaAs spacer layer, a 100 Å n-GaAs (n = 5 × 1016 cm− 3) layer, a δ-doped layer (δ1), a 200 Å n-GaAs (n = 5 × 1016 cm− 3) layer, a δ-doped layer (δ2), a 100 Å n-GaAs (n = 5 × 1016 cm− 3) layer, a 0.1 μm n-In0.5Ga0.5P (n = 7 × 1017 cm− 3 ) emitter confinement layer and, finally, a 0.3 μm n + -GaAs (n = 5 × 1018 cm− 3) cap layer. δ1 = δ2 = 3 × 1012 cm− 2. Tables 1 and 2 describe the layer structures of HEBT and δ-HEBT, respectively. The temperature and pressure for growing the two proposed devices were 690 °C and 150 Torr, respectively. Trimethylgallium (TMG), trimethylindium (TMI), arsine (AsH3) and phosphine (PH3) were used as the growth precursors. Device fabrication was performed using photolithography and wet etching. The InGaP confinement layers were etched with 5:3:100 NH4OH:H2O2:H2O. GaAs was etched with solutions of 4:1 H3PO4:HCl. AuGe/Ag and AuZn were used to construct the n- and p-type ohmic contacts, respectively. Current–voltage (I–V) measurements were made on both the devices using an HP-4156 semiconductor parameter analyzer. 3. Results and discussion Fig. 1(A) plots the common-emitter I–V characteristics of HEBT at 300 K. The current gain of the HEBT is 50. The offset voltage of HEBT is 130 mV. Fig. 1(B) plots the commonemitter I–V characteristics of δ-HEBT at 300 K. δ-HEBT exhibits a current gain of the 55 and an offset voltage of as low as 70 mV. These values of δ-HEBT are better than those of the
Fig. 1. (A) Common-emitter I–V characteristics of HEBT. (B) Common-emitter I–V characteristics of δ-HEBT.
proposed HEBT, because the optimization of the spacer thickness, 100 Å, and the δ-doped layers in the δ-HEBT. Fig. 2 is an enlargement of Fig. 1(B) near the origin. A low offset voltage of 70 mV is clearly observed. This is smaller than those of the previously developed AlGaAs/GaAs and InGaP/GaAs HEBTs [24] and those of the InGaP/GaAs HBTs with nongraded and
Table 2 Device layer structure of δ-HEBT Layer Emitter cap Emitter confinement Emitter Emitter Emitter Emitter Emitter Spacer Base Spacer Collector Subcollector
Material GaAs In0.5Ga0.5P GaAs GaAs GaAs GaAs GaAs GaAs GaAs GaAs GaAs GaAs
Thickness (Å) 3000 1000 100 – 200 – 100 100 1000 100 5000 3000
Type +
n n n δ(n+) n− δ(n+) n− Undoped p+ Undoped n− n+
Doping 5 × 1018 7 × 1017 5 × 1016 3 × 1012 5 × 1016 3 × 1012 5 × 1016 – 5 × 1018 – 5 × 1016 5 × 1018
cm–3 cm–3 cm–3 cm–2 cm–3 cm–2 cm–3 cm–3 cm–3 cm–3
Fig. 2. Expanded view at the beginning of Fig. 1(B), showing the collectoremitter offset voltage in common-emitter I–V characteristics of δ-HEBT.
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Fig. 3. Common-emitter I–V characteristics of δ-HEBT operated in the low collector current region.
graded base doping [7]. Fig. 3 plots the common-emitter I–V characteristics of δ-HEBT at low collector currents. They indicate that δ-HEBT has a useful gain, even in the μA region. Hence, the base bulk current dominates the base current. Fig. 4 shows the semilog-forward bias characteristics for the E–B junction of δ-HEBT. The current varies as exp (qV / nkT) for several decades, where q is the electron charge, V is the applied voltage, k is the Boltzmann constant, T is the absolute temperature, and n is an ideality factor. Fig. 5 plots the I–V characteristics of the base–collector (B–C) junction of δ-HEBT. Notably, the breakdown voltage exceeds 16 V. Fig. 6 plots the semilog-forward bias characteristics of the B–C junction in δHEBT. The value of the ideality factor n is calculated from the slope of the semilog-forward bias curve. In the lower current regime, the ideality factor is around 1.52, indicating that the 1 kT diffusion current and the 2 kT recombination current
Fig. 4. Semilogarithmic plot of the forward I–V characteristics of the E–B junction in δ-HEBT.
Fig. 5. I–V characteristics of B–C junction in δ-HEBT.
dominate the transport of carriers. Furthermore, the composition of the collector and the base currents are analyzed from the Gummel plot. The transferred collector current and base current are functions of the base-emitter voltage when the basecollector bias of δ-HEBT is maintained at 0 V. The ideality factor of the collector current is 1.03, indicating that the recombination current in δ-HEBT is greatly reduced. The junction ideality factor can be determined from the properties of the device as follows [12,25]. g¼1þ
e1 Nd e2 Na
ð1Þ
where ε1 and ε2 are the dielectric constants of n-InGaP and pGaAs, respectively, and Nd and Na are the respective doping densities. Eq. (1) shows that, at an estimated doping ratio of ∼1:10, η is 1.09. The calculated value is consistent with the experimental value.
Fig. 6. Semilogarithmic plot of the forward I–V characteristics of the B–C junction in δ-HEBT.
Y.S. Lin et al. / Thin Solid Films 515 (2007) 3978–3981
4. Conclusion This paper develops an improved InGaP/GaAs HEBT with δ-doping layers grown by a MOCVD system. The InGaP/GaAs HEBT without the δ-doping layers exhibits a current gain of 50 and an offset voltage of 130 mV. Optimizing the thickness of the undoped-GaAs spacer layer adjacent to the base can effectively reduce the recombination current at the p–n interface. δ-HEBT has the advantages of a low offset voltage (70 mV), an improved current gain of 55 and ease of fabrication (by the highselectivity etching process). Consequently, the improved device has considerable potential for use in practical circuit. Acknowledgment The author would like to thank the National Science Council of the Republic of China for financially supporting this research under Contract No. NSC 95-2221-E-259-037. References [1] J.S. Su, W.C. Hsu, Y.S. Lin, W. Lin, C.L. Wu, M.S. Tsai, Y.H. Wu, IEEE Electron Device Lett. 17 (1996) 43. [2] J.S. Su, W.C. Hsu, W. Lin, Y.S. Lin, J. Appl. Phys. 82 (1997) 4076. [3] J.S. Su, W.C. Hsu, Y.S. Lin, W. Lin, Appl. Phys. Lett. 70 (1997) 1002. [4] Y.S. Lin, Y.L. Hsieh, J. Electrochem. Soc. 152 (2005) G778. [5] Y.S. Lin, W.C. Hsu, C.H. Wu, W. Lin, R.T. Hsu, Appl. Phys. Lett. 75 (1999) 1616. [6] Y.S. Lin, D.H. Huang, W.C. Hsu, T.B. Wang, K.H. Su, J.C. Huang, C.H. Ho, Semicond. Sci. Technol. 21 (2006) 540.
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[7] Y.W. Chen, W.C. Hsu, R.T. Hsu, Y.H. Wu, Y.J. Chen, Y.S. Lin, J. Vac. Sci. Technol., B 21 (2003) 2555. [8] Y.S. Lin, W.C. Hsu, F.C. Jong, Y.Z. Chiou, Y.J. Chen, J.J. Tang, Jpn. J. Appl. Phys. 43 (2004) 3285. [9] Y.S. Lin, D.H. Huang, W.C. Hsu, K.H. Su, T.B. Wang, Semicond. Sci. Technol. 21 (2006) 303. [10] K. Kodama, M. Masataka, K. Kitahara, M. Takikawa, M. Ozaki, Jpn. J. Appl. Phys. 25 (1986) L127. [11] M.A. Rao, E.J. Caine, H. Kroemer, S.I. Long, D.I. Babic, J. Appl. Phys. 61 (1987) 643. [12] T. Kobayashi, K. Taira, F. Nakamura, H. Kawai, J. Appl. Phys. 65 (1989) 4898. [13] D. Biswas, N. Debbar, P. Bhattacharya, M. Razeghi, M. Defour, F. Omnes, Appl. Phys. Lett. 56 (1990) 833. [14] J. Chen, J.R. Sites, I.L. Spain, M.J. Hafich, G.Y. Robinson, Appl. Phys. Lett. 58 (1991) 744. [15] M.A. Haase, M.J. Hafich, G.Y. Robinson, Appl. Phys. Lett. 58 (1991) 616. [16] L.F. Luo, H.L. Evans, E.S. Yang, IEEE Trans. Electron Devices 3 (1989) 1844. [17] X. Wu, Y.Q. Wang, L.F. Luo, E.S. Yang, IEEE Electron Device Lett. 11 (1990) 264. [18] W.C. Liu, W.S. Lour, IEEE Electron Device Lett. 12 (1991) 474. [19] Y.F. Yang, C.C. Hsu, E.S. Yang, IEEE Trans. Electron Devices 41 (1994) 643. [20] Y.S. Lin, J.H. Huang, J. Electrochem. Soc. 152 (2005) G627. [21] E. Tokumitsu, A.G. Dentai, C.H. Joyner, S. Chandrasekhar, Appl. Phys. Lett. 57 (1990) 2841. [22] K.W. Goossen, J.E. Cunningham, T.Y. Kuo, W.Y. Jan, C.G. Fonstad, Appl. Phys. Lett. 59 (1991) 682. [23] Y.S. Lin, Y.L. Hsieh, J. Electrochem. Soc. 153 (2006) G498. [24] W.C. Liu, J.H. Tsai, S.Y Cheng, W.L. Chang, H.J. Pan, Y.H. Shie, Thin Solid Films 324 (1998) 219. [25] T.W. Lee, P.A. Houston, R. Kumar, X.F. Yang, G. Hill, M. Hopkinson, P.A. Claxton, Appl. Phys. Lett. 60 (1992) 474.