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GaAs double delta-doped heterojunction bipolar transistor (D3HBT)
Thin Solid Films 345 (1999) 270±272
A new InGaP/GaAs double delta-doped heterojunction bipolar transistor (D 3HBT) Shiou-Ying Cheng, Wei-Chou Wang, Wen-Lung Chang, Jing-Yuh Chen, His-Jen Pan, Wen-Chau Liu* Department of Electrical Engineering, National Cheng-Kung University, 1 University Road, Tainan, Taiwan Received 10 November 1997; accepted 11 September 1998
Abstract A new InGaP/GaAs double delta-doped heterojunction bipolar transistor (D 3HBT) has been fabricated successfully and demonstrated. Due to the employment of delta-doped sheets, the potential spikes at emitter±base (E±B) and base±collector (B±C) heterojunctions are suppressed considerably. Therefore, good transistor performance including higher current gain and lower knee voltage are obtained. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Heterojunction bipolar transistors; Gallium arsenide
1. Introduction Recently, the lattice-matched In0.5Ga0.5P/GaAs heterojunction has become an attractive material system for the fabrication of high-performance heterojunction bipolar transistors (HBTs), ®eld effect transistors (FETs), a negativedifferential-resistance (NDR) device etc.[1±3]. The InGaP/ GaAs material system, when compared with the AlGaAs/ GaAs heterojunction, exhibits advantages including (i) higher ratio of valence band discontinuity to conduction band discontinuity (DEV/DEC), (ii) low DX center density, and (iii) higher chemical etching selectivity between InGaP and GaAs [4]. InGaP/GaAs HBTs have demonstrated superior properties. One of the advantages is the existence of current gain higher than unity even at very low collector current regimes [5]. On the other hand, the wide-gap InGaP was also used to increase the collector breakdown voltage in double-HBTs (DHBTs) [1,6,7]. However, the ®nite DEC at the collector±base heterointerface should cause a smaller current gain and an undesired large collector current saturation voltage, i.e., the knee voltage, in the current±voltage (I±V) characteristics [1,6,7]. In order to achieve high current gain performance, for conventional
* Corresponding author. Tel.: 1 88-662-345-482; fax: 1 88-662-345482. E-mail address: [email protected] (W.C. Liu)
InGaP/GaAs DHBTs, it is necessary to reduce the electron blocking effect which results mainly from conduction band discontinuity at the base±collector (B±C) heterojunction. Previously, this undesired potential spike has been removed by using the insertion of a d -doped sheet between the emitter±base (E±B) heterojunction. In this paper, we will demonstrate a new double delta-doped HBT (D 3HBT) based on the InGaP/GaAs material system. Due to the existence of delta-doped sheets located at E±B and B±C heterojunctions, the potential spikes at E±B and B±C heterointerfaces are suppressed signi®cantly. Experimental results show that this D 3HBT device exhibits the advantages of smaller offset voltage, smaller saturation voltage and high current-gain, simultaneously. These superior characteristics of the studied device provide good potential for high-speed applications. 2. Device fabrication The studied device structure was grown by metal organic chemical vapor deposition (MOCVD) on a (100)-oriented n 1-GaAs substrate as shown in Fig. 1a Trimethylindium (TMI), trimethylgallium (TEG), phosphine (PH3), and arsine (AsH3) were used as the In, Ga, P, and As sources, respectively. The dopants used for the n and p layers were silane (SiH4) and dimethylzinc (DMZ), respectively. The delta-doped sheet was performed by switching off TEG
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(98)0142 2-9
S.Y. Cheng et al. / Thin Solid Films 345 (1999) 270±272
271
Fig. 2. The output I±V characteristics of the studied D 3HBT.
3. Experimental results and discussion
Fig. 1. (a) Schematic cross-section and (b) microscopic photograph of the studied D 3HBT.
while keeping AsH3 and PH3 ¯owing in the growth chamÊ n 1-GaAs ber. The epitaxial layers included a 5000 A 1 18 23 Ê n-In0.5Ga0.5P cm ) buffer, a 2000 A (n 3 £ 10 (n 1 £ 1017 cm 23) collector, a d n1 2 £ 1012 cm 22 Ê undoped GaAs spacer, a 1000 A Ê delta-doped sheet, a 50 A Ê undoped GaAs p 1-GaAs (p1 1 £ 1019 cm 23) base, a 50 A spacer, a d n1 2 £ 1012 cm 22 delta-doped sheet, a Ê n-In0.5Ga0.5P (n 5 £ 1017 cm 23) emitter and a 1000 A Ê n 1-GaAs (n1 3 £ 1018 cm 23) cap layer. The 3000 A growth temperature and pressure were set at 725 and 150 Torr, respectively. The growth rates of InGaP and Ê /min and 325 A Ê /min, respectively. After GaAs were 375 A epitaxial growth, the conventional photolithography, vacuum evaporation and chemical wet selective etching process were used to fabricate the mesa-type devices as shown in Fig. 1b. The InGaP and GaAs layers were etched by the solution of 1HCl:1H2O and 3NH4OH:1H2O2:50H2O, respectively. The n- and p-type ohmic contact metals were Au±Ge±Ni and Au±Zn, respectively. The emitter area is 60 £ 60 mm 2. The DC current±voltage (I±V) characteristics of the studied device were measured by an HP-4156A precision semiconductor parameter analyzer. The schematic cross-section and the microscopic photograph of the studied devices are depicted in Fig. 1.
The typical output I±V characteristics of the studied D 3HBT are shown in Fig. 2. A common-emitter current gain of 210 at collector current I C 65 mA and an extremely small offset voltage VCE,OFFSET smaller than 70 mV are obtained. This high current-gain performance shows the potential spikes located both at E±B and B±C heterojunctions are indeed suppressed or deleted. The calculated energy band diagrams by solving Poisson's equation are illustrated in Fig. 3. Due to the employment of deltadoped sheets, obviously, the potential spikes are so small that they can be neglected both at E±B and B±C heterojunctions as indicated by the solid line in Fig. 3. This causes a
Fig. 3. The calculated energy band diagrams of the studied device at equilibrium (solid line) and under the applied collector±emitter voltage V CE 2:4 V (dashed lines).
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S.Y. Cheng et al. / Thin Solid Films 345 (1999) 270±272
higher emitter injection ef®ciency and current gain as shown in Fig. 2. Particularly, the suppressed and negligible potential spike at the B±C heterojunction cannot provide an effective barrier for electron transport under normal transistor operation as revealed by the dashed lines in Fig. 3. Therefore, a small collector±emitter voltage VCE (knee voltage) to initiate the saturation of collector current can be obtained. Experimentally, a knee voltage of 1.7 (1.3) V at collector saturation current I CS 70 35 mA is acquired. This value is superior to the conventional DHBTs without delta-doped sheet located at the B±C heterojunction [6,7]. Another feature of the studied D 3HBT device is the exhibition of current gain even at very low current regime, similar to the previously reported single delta-doped HBT [8]. A current gain of 4 is found at collector current I C 3:5 mA. This can improve and spread the device application potentiality to different current levels. The B±C junction breakdown voltage over 7 V is Ê n-InGaP obtained. This value is signi®cant for a 2000 A 17 23 (n 1 £ 10 cm ) collector structure. In addition, the maximum collector current ICmax over 220 mA is achieved. Therefore, the studied D 3HBT is also suitable for highspeed applications. 4. Conclusion In summary, we have demonstrated a new InGaP/GaAs D 3HBT. Due to the use of delta-doped sheets, the potential
spikes at E±B and B±C heterojunctions are suppressed substantially. Thus a higher emitter injection ef®ciency (current gain) and a lower knee voltage are obtained. From experimental results, it is shown that the studied device is a good candidate for high-speed applications.
Acknowledgements Part of this work was supported by the National Science Council of the Republic of China under Contact No.: NSC87-2215-E006-020
References [1] F. Ren, R.C. Abernathy, S.J. Perton, P.W. Wisk, R. Esagui, Electron. Lett. 28 (12) (1992) 1150. [2] L.W. Laih, S.Y. Cheng, W.C. Wang, P.H. Lin, J.Y. Chen, W.C. Liu, W. Lin, Electron. Lett. 33 (15) (1997) 98. [3] W.C. Liu, J.H. Tsai, W.S. Lour, S.Y. Cheng, K.B. Thei, C.Z. Wu, IEEE Trans. Electron Devices 44 (4) (1997) 520. [4] S.S. Lu, C.C. Huang, IEEE Electron Devices Lett. 13 (4) (1992) 214. [5] W. Liu, S.K. Fan, IEEE Electron Devices Lett. 13 (10) (1992) 510. [6] J.J. Song, C. Caneau, W.P. Hong, K.B. Chough, Electron. Lett. 33 (15) (1997) 98. [7] A.W. Hanson, S.A. Stockman, G.E. Stillman, IEEE Electron Devices Lett. 14 (1) (1993) 25. [8] H.R. Chen, C.H. Huang, C.Y. Chang, C.Y. Lee, K.L. Tsai, J.S. Tsang, IEEE Electron Devices Lett. 15 (8) (1994) 286.
A new InGaP/GaAs double delta-doped heterojunction bipolar transistor (D 3HBT) Shiou-Ying Cheng, Wei-Chou Wang, Wen-Lung Chang, Jing-Yuh Chen, His-Jen Pan, Wen-Chau Liu* Department of Electrical Engineering, National Cheng-Kung University, 1 University Road, Tainan, Taiwan Received 10 November 1997; accepted 11 September 1998
Abstract A new InGaP/GaAs double delta-doped heterojunction bipolar transistor (D 3HBT) has been fabricated successfully and demonstrated. Due to the employment of delta-doped sheets, the potential spikes at emitter±base (E±B) and base±collector (B±C) heterojunctions are suppressed considerably. Therefore, good transistor performance including higher current gain and lower knee voltage are obtained. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Heterojunction bipolar transistors; Gallium arsenide
1. Introduction Recently, the lattice-matched In0.5Ga0.5P/GaAs heterojunction has become an attractive material system for the fabrication of high-performance heterojunction bipolar transistors (HBTs), ®eld effect transistors (FETs), a negativedifferential-resistance (NDR) device etc.[1±3]. The InGaP/ GaAs material system, when compared with the AlGaAs/ GaAs heterojunction, exhibits advantages including (i) higher ratio of valence band discontinuity to conduction band discontinuity (DEV/DEC), (ii) low DX center density, and (iii) higher chemical etching selectivity between InGaP and GaAs [4]. InGaP/GaAs HBTs have demonstrated superior properties. One of the advantages is the existence of current gain higher than unity even at very low collector current regimes [5]. On the other hand, the wide-gap InGaP was also used to increase the collector breakdown voltage in double-HBTs (DHBTs) [1,6,7]. However, the ®nite DEC at the collector±base heterointerface should cause a smaller current gain and an undesired large collector current saturation voltage, i.e., the knee voltage, in the current±voltage (I±V) characteristics [1,6,7]. In order to achieve high current gain performance, for conventional
* Corresponding author. Tel.: 1 88-662-345-482; fax: 1 88-662-345482. E-mail address: [email protected] (W.C. Liu)
InGaP/GaAs DHBTs, it is necessary to reduce the electron blocking effect which results mainly from conduction band discontinuity at the base±collector (B±C) heterojunction. Previously, this undesired potential spike has been removed by using the insertion of a d -doped sheet between the emitter±base (E±B) heterojunction. In this paper, we will demonstrate a new double delta-doped HBT (D 3HBT) based on the InGaP/GaAs material system. Due to the existence of delta-doped sheets located at E±B and B±C heterojunctions, the potential spikes at E±B and B±C heterointerfaces are suppressed signi®cantly. Experimental results show that this D 3HBT device exhibits the advantages of smaller offset voltage, smaller saturation voltage and high current-gain, simultaneously. These superior characteristics of the studied device provide good potential for high-speed applications. 2. Device fabrication The studied device structure was grown by metal organic chemical vapor deposition (MOCVD) on a (100)-oriented n 1-GaAs substrate as shown in Fig. 1a Trimethylindium (TMI), trimethylgallium (TEG), phosphine (PH3), and arsine (AsH3) were used as the In, Ga, P, and As sources, respectively. The dopants used for the n and p layers were silane (SiH4) and dimethylzinc (DMZ), respectively. The delta-doped sheet was performed by switching off TEG
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(98)0142 2-9
S.Y. Cheng et al. / Thin Solid Films 345 (1999) 270±272
271
Fig. 2. The output I±V characteristics of the studied D 3HBT.
3. Experimental results and discussion
Fig. 1. (a) Schematic cross-section and (b) microscopic photograph of the studied D 3HBT.
while keeping AsH3 and PH3 ¯owing in the growth chamÊ n 1-GaAs ber. The epitaxial layers included a 5000 A 1 18 23 Ê n-In0.5Ga0.5P cm ) buffer, a 2000 A (n 3 £ 10 (n 1 £ 1017 cm 23) collector, a d n1 2 £ 1012 cm 22 Ê undoped GaAs spacer, a 1000 A Ê delta-doped sheet, a 50 A Ê undoped GaAs p 1-GaAs (p1 1 £ 1019 cm 23) base, a 50 A spacer, a d n1 2 £ 1012 cm 22 delta-doped sheet, a Ê n-In0.5Ga0.5P (n 5 £ 1017 cm 23) emitter and a 1000 A Ê n 1-GaAs (n1 3 £ 1018 cm 23) cap layer. The 3000 A growth temperature and pressure were set at 725 and 150 Torr, respectively. The growth rates of InGaP and Ê /min and 325 A Ê /min, respectively. After GaAs were 375 A epitaxial growth, the conventional photolithography, vacuum evaporation and chemical wet selective etching process were used to fabricate the mesa-type devices as shown in Fig. 1b. The InGaP and GaAs layers were etched by the solution of 1HCl:1H2O and 3NH4OH:1H2O2:50H2O, respectively. The n- and p-type ohmic contact metals were Au±Ge±Ni and Au±Zn, respectively. The emitter area is 60 £ 60 mm 2. The DC current±voltage (I±V) characteristics of the studied device were measured by an HP-4156A precision semiconductor parameter analyzer. The schematic cross-section and the microscopic photograph of the studied devices are depicted in Fig. 1.
The typical output I±V characteristics of the studied D 3HBT are shown in Fig. 2. A common-emitter current gain of 210 at collector current I C 65 mA and an extremely small offset voltage VCE,OFFSET smaller than 70 mV are obtained. This high current-gain performance shows the potential spikes located both at E±B and B±C heterojunctions are indeed suppressed or deleted. The calculated energy band diagrams by solving Poisson's equation are illustrated in Fig. 3. Due to the employment of deltadoped sheets, obviously, the potential spikes are so small that they can be neglected both at E±B and B±C heterojunctions as indicated by the solid line in Fig. 3. This causes a
Fig. 3. The calculated energy band diagrams of the studied device at equilibrium (solid line) and under the applied collector±emitter voltage V CE 2:4 V (dashed lines).
272
S.Y. Cheng et al. / Thin Solid Films 345 (1999) 270±272
higher emitter injection ef®ciency and current gain as shown in Fig. 2. Particularly, the suppressed and negligible potential spike at the B±C heterojunction cannot provide an effective barrier for electron transport under normal transistor operation as revealed by the dashed lines in Fig. 3. Therefore, a small collector±emitter voltage VCE (knee voltage) to initiate the saturation of collector current can be obtained. Experimentally, a knee voltage of 1.7 (1.3) V at collector saturation current I CS 70 35 mA is acquired. This value is superior to the conventional DHBTs without delta-doped sheet located at the B±C heterojunction [6,7]. Another feature of the studied D 3HBT device is the exhibition of current gain even at very low current regime, similar to the previously reported single delta-doped HBT [8]. A current gain of 4 is found at collector current I C 3:5 mA. This can improve and spread the device application potentiality to different current levels. The B±C junction breakdown voltage over 7 V is Ê n-InGaP obtained. This value is signi®cant for a 2000 A 17 23 (n 1 £ 10 cm ) collector structure. In addition, the maximum collector current ICmax over 220 mA is achieved. Therefore, the studied D 3HBT is also suitable for highspeed applications. 4. Conclusion In summary, we have demonstrated a new InGaP/GaAs D 3HBT. Due to the use of delta-doped sheets, the potential
spikes at E±B and B±C heterojunctions are suppressed substantially. Thus a higher emitter injection ef®ciency (current gain) and a lower knee voltage are obtained. From experimental results, it is shown that the studied device is a good candidate for high-speed applications.
Acknowledgements Part of this work was supported by the National Science Council of the Republic of China under Contact No.: NSC87-2215-E006-020
References [1] F. Ren, R.C. Abernathy, S.J. Perton, P.W. Wisk, R. Esagui, Electron. Lett. 28 (12) (1992) 1150. [2] L.W. Laih, S.Y. Cheng, W.C. Wang, P.H. Lin, J.Y. Chen, W.C. Liu, W. Lin, Electron. Lett. 33 (15) (1997) 98. [3] W.C. Liu, J.H. Tsai, W.S. Lour, S.Y. Cheng, K.B. Thei, C.Z. Wu, IEEE Trans. Electron Devices 44 (4) (1997) 520. [4] S.S. Lu, C.C. Huang, IEEE Electron Devices Lett. 13 (4) (1992) 214. [5] W. Liu, S.K. Fan, IEEE Electron Devices Lett. 13 (10) (1992) 510. [6] J.J. Song, C. Caneau, W.P. Hong, K.B. Chough, Electron. Lett. 33 (15) (1997) 98. [7] A.W. Hanson, S.A. Stockman, G.E. Stillman, IEEE Electron Devices Lett. 14 (1) (1993) 25. [8] H.R. Chen, C.H. Huang, C.Y. Chang, C.Y. Lee, K.L. Tsai, J.S. Tsang, IEEE Electron Devices Lett. 15 (8) (1994) 286.