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Investigation of GaAs-based heterostructure–emitter bipolar transistors (HEBTs)
Thin Solid Films 324 Ž1998. 219–224
Investigation of GaAs-based heterostructure–emitter bipolar transistors žHEBTs/ Wen-Chau Liu a
a,)
, Jung-Hui Tsai b, Shiou-Ying Cheng a , Wen-Lung Chang a , Hsi-Jen Pan a , Yung-Hsin Shie a
Department of Electrical Engineering, National Cheng-Kung UniÕersity, 1 UniÕersity Road, 70101 Tainan, Taiwan b Macronix International, No. 3, Creation Road III, Science-Based Industrial Park, Hsinchu, Taiwan Received 15 September 1997; accepted 18 December 1997
Abstract In this paper, we will investigate three GaAs-based heterostructure-emitter bipolar transistors ŽHEBTs.. These HEBTs have different heterostructure-confinement material systems, e.g., Al 0.5 Ga 0.5 AsrGaAs, In 0.49 Ga 0.51 PrGaAs, and Al 0.45 Ga 0.55 AsrIn 0.2 Ga 0.8 AsrGaAs. For the studied devices, an n-GaAs emitter layer inserted between the confinement and base layer is expected to eliminate the potential spike at emitter–base ŽE–B. junction. Therefore, the low collector–emitter offset voltage Ž DVCE . is obtained. For the AlGaAsrGaAs HEBT, experimental results show that a current gain of 180 and a low offset voltage of 80 mV are acquired. In addition, for the ˚ . which causes a large InGaPrGaAs HEBT, the current gain is only 60 attributed to the use of larger emitter layer thickness Ž700 A recombination current in neutral-emitter regime even when a large valence band discontinuity to conduction band discontinuity ratio Ž D EvrD Ec . is presented. On the other hand, for the AlGaAsrInGaAsrGaAs HEBT, the D Ev value can be enhanced due to the insertion of InGaAs quantum well ŽQW. between the n-GaAs emitter and the pq-GaAs base layer. Thus, the confinement effect of minority carriers is enhanced and a current gain of 280 is obtained, simultaneously. Consequently, our studied devices will provide a good promise for the transistor design and circuit applications. q 1998 Elsevier Science S.A. All rights reserved. Keywords: Electronic devices; Gallium arsenide; Heterostructures; Semiconductors
1. Introduction There has been a lot of activity surrounding heterojunction bipolar transistors ŽHBTs. because they offer many considerable interests in microwave and digital circuit applications due to their high speed and high current handling capabilities w1–3x. One of the main advantages of HBTs lies in the ability to suppress the hole current injecting from base into emitter regime attributed to the confinement effect of valence band discontinuity Ž D Ev . at heterojunction. Thus, the high emitter injection efficiency and current gain are obtained. In addition, the higher base concentration could be used in order to reduce the base resistance and achieve superior high-frequency performance w4x. However, some drawbacks including the difficulty in precise alignment of the compositional junction to doping junction and the large collector–emitter offset volt) Corresponding author. Tel.: q886-6-2744237; fax: q886-6-2345482; e-mail: [email protected].
0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 8 . 0 0 3 7 2 - 1
age Ž DVCE . are observed w5,6x. The large value of DVCE can increase the undesirable power consumption in the digital circuit applications. Over the past years, a significantly improved structure, i.e., the heterostructure–emitter bipolar transistor ŽHEBT. w7–11x, has been developed to overcome the disadvantages of conventional HBTs. For the HEBT structure, a widegap confinement layer is physically separated from p–n junction to prevent holes injecting toward the emitter regime. In addition, an n-type emitter layer controls the electrons injection. Due to the presence of the effective emitter–base ŽE–B. homojunction, the potential spike is expected to be eliminated. However, the insertion of n-type emitter layer causes a large recombination current in neutral-emitter region and reduces the current gain especially at large VBE regime. So, the optimum emitter layer thickness should be carefully designed to eliminate the potential spike at emitter–base ŽE–B. junction and decrease the magnitude of DVCE w11x. Recently, the InGaPrGaAs HBTs have attracted much attention due to the substantial benefits, e.g., the large valence band
220
W.-C. Liu et al.r Thin Solid Films 324 (1998) 219–224
discontinuity to conduction band discontinuity ratio Ž D E V rD Ec ., high etch selectivity, and low density of DX centers. Yet, the offset voltage of the previously reported InGaPrGaAs HBTs is still large. On the other hand, a pseudomorphic InGaAsrGaAs base structure has been successfully employed in HBTs to achieve a high emitter injection efficiency w12x. However, the base transport factor is decreased resulting from the degraded material quality of pseudomorphic InGaAs compared to GaAs layer. Thus, a low current gain only of 40 is obtained. In this paper, we will demonstrate and study three heterostructure–emitter heterojunction bipolar transistors ŽHEBTs. with different heterostructure-confinement material systems, i.e., Al 0.5 Ga 0.5 AsrGaAs, In 0.49 Ga 0.51 PrGaAs, and Al 0.45 Ga 0.55 AsrIn 0.2 Ga 0.8 AsrGaAs. For these structures, the low-offset voltage behaviors are obtained. The different transistor performances and confinement effects will be analyzed and compared.
2. Experiments The studied structures were grown on Ž100.-oriented nq-GaAs substrates. The studied Al 0.5 Ga 0.5 AsrGaAs HEBT structure included a 0.2 m m nqs 1 = 10 18 cmy3 GaAs buffer, a 0.5 m m nys 5 = 10 16 cmy3 GaAs collec˚ tor, a 0.1 m m pqs 5 = 10 18 cmy3 GaAs base, a 500 A
n s 5 = 10 17 cmy3 GaAs emitter, a 0.1 m m n s 5 = 10 17 cmy3 Al 0.5 Ga 0.5 As confinement layer, and a nq-GaAs cap lay er. T h e In 0 .4 9 G a 0 .5 1 P r G aA s H E B T an d Al 0.45 Ga 0.55 AsrIn 0.2 Ga 0.8 AsrGaAs HEBT had similar structures to the Al 0.5 Ga 0.5 AsrGaAs HEBT. However, a 0.1 m m n-In 0.49 Ga 0.51 P was employed to replace the nAl 0.5 Ga 0.5 As confinement layer in the InGaPrGaAs HEBT. In addition, as similar to AlGaAsrGaAs HEBT, a ˚ pq-In 0.2 Ga 0.8 As QW was inserted at base regime 100 A and an n-Al 0.45 Ga 0.55 As confinement layer was used for the AlGaAsrInGaAsrGaAs HEBT. The compositions of the studied Al 0.5 Ga 0.5 As, In 0.49 Ga 0.51 P, Al 0.45 Ga 0.55 As, and In 0.2 Ga 0.8 As epitaxial layers were obtained by the previous characterization from double crystal X-ray diffraction ŽDCD., photoreflectance ŽPR., and photoluminescence ŽPL. measurements. The doping concentrations of each layer were determined from Hall-effect measurement. The layer structures of the studied HEBTs are listed in Table 1. After the epitaxial growth, the studied devices were fabricated with a mesa-type structure using conventional photolithography and chemical wet etching techniques. The GaAs, AlGaAs and InGaP layers were etched by the solutions of 3 NH 4 OH:1 H 2 O 2 :50 H 2 0, 1 HF:1 H 2 0 and 1 HCl:1 H 2 0, respectively. AuGeNi and AuZn metals were used for n- and p-type layer ohmic contacts by vacuum evaporation, respectively. The effective areas of emitter–base and base–collector junctions were 5.1 = 10y5 cm2 and 5.5 = 10y4 cm2 , respectively. All of the
Table 1 The layer structure of the studied AlGaAsrGaAs HEBT, InGaPrGaAs HEBT and AlGaAsrInGaAsrGaAs HEBT AlGaAsrGaAs HEBT
InGaPrGaAs HEBT
AlGaAsrInGaAsrGaAs HEBT
CAP layer 0.2 m m nq-GaAs Ž1 = 10 18 cmy3 .
CAP layer 0.2 m m nq-GaAs Ž1 = 10 18 cmy3 .
CAP layer 0.2 m m nq-GaAs Ž1 = 10 18 cmy3 .
Cofinement 0.1 m m n-Al 0.5 Ga 0.5 As Ž5 = 10 17 cmy3 .
Confinement 0.1 m m n-In 0.49 Ga 0.51 P Ž5 = 10 17 cmy3 .
Confinement 0.1 m m n-Al 0.45 Ga 0.55 As Ž5 = 10 17 cmy3 .
Emitter
Emitter
Emitter
˚ n-GaAs 500 A Ž5 = 10 17 cmy3 .
˚ n-GaAs 700 A Ž5 = 10 17 cmy3 .
˚ n-GaAs 500 A Ž5 = 10 17 cmy3 . Quantum well
˚ pq-In 0.2 Ga 0.8 As 100 A Ž5 = 10 18 cmy3 . Base 0.1 m m pq-GaAs Ž5 = 10 18 cmy3 .
Base 0.1 m m pq-GaAs Ž5 = 10 18 cmy3 .
Base 0.1 m m pq-GaAs Ž5 = 10 18 cmy3 .
Collector 0.5 m m ny-GaAs Ž5 = 10 16 cmy3 .
Collector 0.5 m m ny-GaAs Ž5 = 10 16 cmy3 .
Collector 0.5 m m ny-GaAs Ž5 = 10 16 cmy3 .
Buffer 0.2 m m nq-GaAs Ž1 = 10 18 cmy3 .
Buffer 0.2 m m nq-GaAs Ž1 = 10 18 cmy3 . nq-GaAs substrate
Buffer 0.2 m m nq-GaAs Ž1 = 10 18 cmy3 .
W.-C. Liu et al.r Thin Solid Films 324 (1998) 219–224
221
comparable to the hole diffusion length, the recombination in the emitter region cannot be neglected and the emitter injection efficiency will be degraded seriously. On the other hand, if the emitter layer thickness is too small, most of the emitter depletion region extends to the wide-gap confinement layer and yield a potential spike at heterointerface and the increase of DVCE as similar to the conventional HBTs w7x. Thus, a minimum emitter layer thickness must be found to eliminate the undesired potential spike. By solving the Poisson’s equation, the minimum emitter layer thickness required to eliminate the potential spike can be obtained as w11x ds Fig. 1. The experimental common-emitter current–voltage Ž I – V . characteristics of the studied AlGaAsrGaAs HEBT.
current–voltage Ž I–V . characteristics shown in this paper were measured by a Tektronix 577 curve tracer.
3. Experimental results and discussion For the HEBTs structures, the confinement layer is used as a confinement barrier for holes only and is separated physically from the effective E–B homojunction. For the HEBTs, a critical parameter is the n-GaAs emitter layer thickness. If the emitter layer thickness is too large, i.e.,
(
´ D Ec 2
q ND
ž
NA NA q ND
/
Ž 1.
where D Ec , ND , and NA are the conduction band discontinuity between confinement and emitter layer, emitter, and base layer concentration, respectively. From theoretical ˚ are calculations, the d values of 260, 230, and 300 A obtained for the studied AlGaAsrGaAs, InGaPrGaAs, and AlGaAsrInGaAsrGaAs HEBTs, respectively. Thus, the employed emitter thickness is enough to eliminate the potential spike at base-emitter ŽB–E. junction. The common-emitter I–V characteristics of the studied AlGaAsrGaAs HEBT is shown in Fig. 1. The maximum common-emitter current gain of 180 and a low offset voltage of 80 mV are obtained for the studied AlGaAsrGaAs HEBT. The corresponding energy band
Fig. 2. The corresponding energy band diagrams Ža. at equilibrium and Žb. under normal operation mode of the studied AlGaAsrGaAs HEBT.
W.-C. Liu et al.r Thin Solid Films 324 (1998) 219–224
222
diagrams of the studied AlGaAsrGaAs HEBT at equilibrium and under normal operation mode are depicted in Fig. 2. Apparently, the potential spike is eliminated by the insertion of n-GaAs emitter layer. Since the offset voltage DVCE can be expressed as DVCE s R E I B q
KT q
ln
AC
ž / AE
KT q q
ln
ž
JCS
a N J ES
/
Ž 2.
where R E is the emitter series resistance, A C and A E are the collector and emitter area. a N is the forward common base current gain. JCS and J ES are the reverse saturation current of collector and emitter junction, respectively. Because the effective E–B and B–C junctions are homojunction, the JCS and J ES is almost equal. Thus, the offset voltage could be reduced. For the studied InGaPrGaAs HEBT, the device fabrication is relatively simple due to the large etch selectivity between InGaP and GaAs materials. Furthermore, attributed to the large D E V rD Ec ratio, the confinement effect for holes is prominent. The energy band diagrams at equilibrium and under normal operation mode are illustrated in Fig. 3. Significantly, there is no potential spike existing at the E–B junction attributed to the small D Ec and the insertion of n-GaAs emitter. However, the emitter ˚ is so large that the recombination layer thickness of 700 A current in neutral-emitter region is increased. Thus, a low offset voltage can be achieved while the performance of
current gain is not so good. The typical common-emitter I–V characteristics of the studied InGaPrGaAs HEBT is shown in Fig. 4. The control base current I B is 0.1 mArstep. The current gain of 60 is obtained without using emitter-edge thinning structure. In order to improve the transistor performances, as similar to the AlGaAsrGaAs HEBT, a pq-InGaAs QW is added between n-GaAs emitter and pq-GaAs base layer of the studied AlGaAsrInGaAsrGaAs HEBT. The energy band diagrams at equilibrium and under normal operation mode are illustrated in Fig. 5. Compared with the AlGaAsrGaAs HEBT, owing to the presence of InGaAs QW, the effective valence band discontinuity at the E–B heterojunction ŽN-AlGaAsrn-GaAsrp q-InGaAsrp qGaAs. is approximately equivalent to the summation of the valence band discontinuity at the InGaAsrGaAs and AlGaAsrGaAs heterointerface. So, the effective valence band discontinuity at the E–B heterojunction of the AlGaAsrInGaAsrGaAs HEBT is higher than that of the AlGaAsrGaAs HEBT and InGaPrGaAs HEBT. Therefore, for the studied AlGaAsrInGaAsrGaAs HEBT, the injecting holes from base to emitter are reduced, and the emitter injection efficiency can be increased. This certainly yields the enhancement of collector current and current gain. Fig. 6 shows the output I–V characteristics of the studied AlGaAsrInGaAsrGaAs HEBT under normal operation mode. The common-emitter current gain of 280
Fig. 3. The corresponding energy band diagrams Ža. at equilibrium and Žb. under normal operation mode of the studied InGaPrGaAs HEBT.
W.-C. Liu et al.r Thin Solid Films 324 (1998) 219–224
223
Fig. 4. The common-emitter I – V characteristics of the studied InGaPrGaAs HEBT.
Fig. 6. The common-emitter I – V characteristics of the studied AlGaAsrInGaAsrGaAs HEBT.
with the DVCE of 100 mV is obtained. The current gain is superior to the AlGaAsrGaAs HEBT devices without the emitter edge-thinning structure. Hence, the higher emitter injection efficiency and a relatively small offset voltage is
resulted from the insertion of InGaAs QW in the base region. Yet, despite the improvement in current gain of the HEBT with the pq-InGaAs QW, one may expect a relative sacrifice in the high-frequency performance Že.q., the cut-
Fig. 5. The corresponding energy band diagrams Ža. at equilibrium and Žb. under normal operation mode of the studied AlGaAsrInGaAsrGaAs HEBT.
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W.-C. Liu et al.r Thin Solid Films 324 (1998) 219–224
off frequency f T and maximum oscillation frequency f max . of this device.
4. Conclusion The confinement effects of the HEBTs with different material systems, i.e., AlGaAsrGaAs, InGaPrGaAs, and AlGaAsrInGaAsrGaAs, have been studied. Due to the presence of effective emitter–base homojunction, the low offset voltages are obtained. For the studied devices, the employment of the higher bandgap confinement layer not only provides a better confinement effect for minority carriers Žholes., but also maintains a relatively higher injection level for majority carriers Želectrons.. On the other hand, the emitter layer thickness must be carefully designed in order to achieve a good performance of high current gain and low offset voltage. For the studied InGaPrGaAs HEBT, the n-GaAs emitter layer thickness might be designed as a much lower value to improve tr a n s is to r p e r f o r m a n c e . F o r th e s tu d ie d AlGaAsrInGaAsrGaAs HEBT, the satisfactory proper transistor performances are obtained. Further, more excellent characteristics may be expected when an emitter-edge thinning design is employed. Consequently, the studied devices provide good promises for transistor design and circuit applications.
Acknowledgements Part of this work was supported by the National Science Council of the Republic of China under Contract No. NSC 87-2215-E-006-020.
References w1x N. Hayama, A. Okamto, M. Madihan, K. Honjo, IEEE Electron Device Lett. 8 Ž1987. 246. w2x J. Hu, Q.M. Zhang, R.K. Surridge, J.M. Xu, D. Pavlidis, IEEE Electron Device Lett. 14 Ž1993. 563. w3x Y. Matsuoka, E. Sano, Solid-State Electron. 31 Ž1995. 1703. w4x Y. Beter, D. Ritter, IEEE Electron Device Lett. 16 Ž1995. 97. w5x T. Won, S. Iyer, S. Agarwala, H. Morkoc, IEEE Electron Device Lett. 10 Ž1989. 274. w6x C.C. Wu, S.L. Lu, IEEE Electron Device Lett. 13 Ž1992. 418. w7x W.C. Liu, W.S. Lour, J. Appl. Phys. 69 Ž1991. 1063. w8x W.C. Liu, W.S. Lour, Solid-State Electron. 34 Ž1991. 717. w9x W.C. Liu, D.F. Guo, W.S. Lour, IEEE Trans. Electron. Devices 39 Ž1992. 2740. w10x H.R. Chen, C.P. Lee, C.Y. Chang, J.S. Tsang, K.L. Tsai, J. Appl. Phys. 74 Ž1993. 1398. w11x H.R. Chen, C.Y. Chang, C.P. Lee, C.H. Huang, J.S. Tsang, K.L. Tsai, IEEE Electron Device Lett. 15 Ž1994. 336. w12x P.M. Enquist, L.R. Ramberg, F.E. Najjar, W.J. Schaff, L.F. Eastman, Appl. Phys. Lett. 49 Ž1986. 179.
Investigation of GaAs-based heterostructure–emitter bipolar transistors žHEBTs/ Wen-Chau Liu a
a,)
, Jung-Hui Tsai b, Shiou-Ying Cheng a , Wen-Lung Chang a , Hsi-Jen Pan a , Yung-Hsin Shie a
Department of Electrical Engineering, National Cheng-Kung UniÕersity, 1 UniÕersity Road, 70101 Tainan, Taiwan b Macronix International, No. 3, Creation Road III, Science-Based Industrial Park, Hsinchu, Taiwan Received 15 September 1997; accepted 18 December 1997
Abstract In this paper, we will investigate three GaAs-based heterostructure-emitter bipolar transistors ŽHEBTs.. These HEBTs have different heterostructure-confinement material systems, e.g., Al 0.5 Ga 0.5 AsrGaAs, In 0.49 Ga 0.51 PrGaAs, and Al 0.45 Ga 0.55 AsrIn 0.2 Ga 0.8 AsrGaAs. For the studied devices, an n-GaAs emitter layer inserted between the confinement and base layer is expected to eliminate the potential spike at emitter–base ŽE–B. junction. Therefore, the low collector–emitter offset voltage Ž DVCE . is obtained. For the AlGaAsrGaAs HEBT, experimental results show that a current gain of 180 and a low offset voltage of 80 mV are acquired. In addition, for the ˚ . which causes a large InGaPrGaAs HEBT, the current gain is only 60 attributed to the use of larger emitter layer thickness Ž700 A recombination current in neutral-emitter regime even when a large valence band discontinuity to conduction band discontinuity ratio Ž D EvrD Ec . is presented. On the other hand, for the AlGaAsrInGaAsrGaAs HEBT, the D Ev value can be enhanced due to the insertion of InGaAs quantum well ŽQW. between the n-GaAs emitter and the pq-GaAs base layer. Thus, the confinement effect of minority carriers is enhanced and a current gain of 280 is obtained, simultaneously. Consequently, our studied devices will provide a good promise for the transistor design and circuit applications. q 1998 Elsevier Science S.A. All rights reserved. Keywords: Electronic devices; Gallium arsenide; Heterostructures; Semiconductors
1. Introduction There has been a lot of activity surrounding heterojunction bipolar transistors ŽHBTs. because they offer many considerable interests in microwave and digital circuit applications due to their high speed and high current handling capabilities w1–3x. One of the main advantages of HBTs lies in the ability to suppress the hole current injecting from base into emitter regime attributed to the confinement effect of valence band discontinuity Ž D Ev . at heterojunction. Thus, the high emitter injection efficiency and current gain are obtained. In addition, the higher base concentration could be used in order to reduce the base resistance and achieve superior high-frequency performance w4x. However, some drawbacks including the difficulty in precise alignment of the compositional junction to doping junction and the large collector–emitter offset volt) Corresponding author. Tel.: q886-6-2744237; fax: q886-6-2345482; e-mail: [email protected].
0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 8 . 0 0 3 7 2 - 1
age Ž DVCE . are observed w5,6x. The large value of DVCE can increase the undesirable power consumption in the digital circuit applications. Over the past years, a significantly improved structure, i.e., the heterostructure–emitter bipolar transistor ŽHEBT. w7–11x, has been developed to overcome the disadvantages of conventional HBTs. For the HEBT structure, a widegap confinement layer is physically separated from p–n junction to prevent holes injecting toward the emitter regime. In addition, an n-type emitter layer controls the electrons injection. Due to the presence of the effective emitter–base ŽE–B. homojunction, the potential spike is expected to be eliminated. However, the insertion of n-type emitter layer causes a large recombination current in neutral-emitter region and reduces the current gain especially at large VBE regime. So, the optimum emitter layer thickness should be carefully designed to eliminate the potential spike at emitter–base ŽE–B. junction and decrease the magnitude of DVCE w11x. Recently, the InGaPrGaAs HBTs have attracted much attention due to the substantial benefits, e.g., the large valence band
220
W.-C. Liu et al.r Thin Solid Films 324 (1998) 219–224
discontinuity to conduction band discontinuity ratio Ž D E V rD Ec ., high etch selectivity, and low density of DX centers. Yet, the offset voltage of the previously reported InGaPrGaAs HBTs is still large. On the other hand, a pseudomorphic InGaAsrGaAs base structure has been successfully employed in HBTs to achieve a high emitter injection efficiency w12x. However, the base transport factor is decreased resulting from the degraded material quality of pseudomorphic InGaAs compared to GaAs layer. Thus, a low current gain only of 40 is obtained. In this paper, we will demonstrate and study three heterostructure–emitter heterojunction bipolar transistors ŽHEBTs. with different heterostructure-confinement material systems, i.e., Al 0.5 Ga 0.5 AsrGaAs, In 0.49 Ga 0.51 PrGaAs, and Al 0.45 Ga 0.55 AsrIn 0.2 Ga 0.8 AsrGaAs. For these structures, the low-offset voltage behaviors are obtained. The different transistor performances and confinement effects will be analyzed and compared.
2. Experiments The studied structures were grown on Ž100.-oriented nq-GaAs substrates. The studied Al 0.5 Ga 0.5 AsrGaAs HEBT structure included a 0.2 m m nqs 1 = 10 18 cmy3 GaAs buffer, a 0.5 m m nys 5 = 10 16 cmy3 GaAs collec˚ tor, a 0.1 m m pqs 5 = 10 18 cmy3 GaAs base, a 500 A
n s 5 = 10 17 cmy3 GaAs emitter, a 0.1 m m n s 5 = 10 17 cmy3 Al 0.5 Ga 0.5 As confinement layer, and a nq-GaAs cap lay er. T h e In 0 .4 9 G a 0 .5 1 P r G aA s H E B T an d Al 0.45 Ga 0.55 AsrIn 0.2 Ga 0.8 AsrGaAs HEBT had similar structures to the Al 0.5 Ga 0.5 AsrGaAs HEBT. However, a 0.1 m m n-In 0.49 Ga 0.51 P was employed to replace the nAl 0.5 Ga 0.5 As confinement layer in the InGaPrGaAs HEBT. In addition, as similar to AlGaAsrGaAs HEBT, a ˚ pq-In 0.2 Ga 0.8 As QW was inserted at base regime 100 A and an n-Al 0.45 Ga 0.55 As confinement layer was used for the AlGaAsrInGaAsrGaAs HEBT. The compositions of the studied Al 0.5 Ga 0.5 As, In 0.49 Ga 0.51 P, Al 0.45 Ga 0.55 As, and In 0.2 Ga 0.8 As epitaxial layers were obtained by the previous characterization from double crystal X-ray diffraction ŽDCD., photoreflectance ŽPR., and photoluminescence ŽPL. measurements. The doping concentrations of each layer were determined from Hall-effect measurement. The layer structures of the studied HEBTs are listed in Table 1. After the epitaxial growth, the studied devices were fabricated with a mesa-type structure using conventional photolithography and chemical wet etching techniques. The GaAs, AlGaAs and InGaP layers were etched by the solutions of 3 NH 4 OH:1 H 2 O 2 :50 H 2 0, 1 HF:1 H 2 0 and 1 HCl:1 H 2 0, respectively. AuGeNi and AuZn metals were used for n- and p-type layer ohmic contacts by vacuum evaporation, respectively. The effective areas of emitter–base and base–collector junctions were 5.1 = 10y5 cm2 and 5.5 = 10y4 cm2 , respectively. All of the
Table 1 The layer structure of the studied AlGaAsrGaAs HEBT, InGaPrGaAs HEBT and AlGaAsrInGaAsrGaAs HEBT AlGaAsrGaAs HEBT
InGaPrGaAs HEBT
AlGaAsrInGaAsrGaAs HEBT
CAP layer 0.2 m m nq-GaAs Ž1 = 10 18 cmy3 .
CAP layer 0.2 m m nq-GaAs Ž1 = 10 18 cmy3 .
CAP layer 0.2 m m nq-GaAs Ž1 = 10 18 cmy3 .
Cofinement 0.1 m m n-Al 0.5 Ga 0.5 As Ž5 = 10 17 cmy3 .
Confinement 0.1 m m n-In 0.49 Ga 0.51 P Ž5 = 10 17 cmy3 .
Confinement 0.1 m m n-Al 0.45 Ga 0.55 As Ž5 = 10 17 cmy3 .
Emitter
Emitter
Emitter
˚ n-GaAs 500 A Ž5 = 10 17 cmy3 .
˚ n-GaAs 700 A Ž5 = 10 17 cmy3 .
˚ n-GaAs 500 A Ž5 = 10 17 cmy3 . Quantum well
˚ pq-In 0.2 Ga 0.8 As 100 A Ž5 = 10 18 cmy3 . Base 0.1 m m pq-GaAs Ž5 = 10 18 cmy3 .
Base 0.1 m m pq-GaAs Ž5 = 10 18 cmy3 .
Base 0.1 m m pq-GaAs Ž5 = 10 18 cmy3 .
Collector 0.5 m m ny-GaAs Ž5 = 10 16 cmy3 .
Collector 0.5 m m ny-GaAs Ž5 = 10 16 cmy3 .
Collector 0.5 m m ny-GaAs Ž5 = 10 16 cmy3 .
Buffer 0.2 m m nq-GaAs Ž1 = 10 18 cmy3 .
Buffer 0.2 m m nq-GaAs Ž1 = 10 18 cmy3 . nq-GaAs substrate
Buffer 0.2 m m nq-GaAs Ž1 = 10 18 cmy3 .
W.-C. Liu et al.r Thin Solid Films 324 (1998) 219–224
221
comparable to the hole diffusion length, the recombination in the emitter region cannot be neglected and the emitter injection efficiency will be degraded seriously. On the other hand, if the emitter layer thickness is too small, most of the emitter depletion region extends to the wide-gap confinement layer and yield a potential spike at heterointerface and the increase of DVCE as similar to the conventional HBTs w7x. Thus, a minimum emitter layer thickness must be found to eliminate the undesired potential spike. By solving the Poisson’s equation, the minimum emitter layer thickness required to eliminate the potential spike can be obtained as w11x ds Fig. 1. The experimental common-emitter current–voltage Ž I – V . characteristics of the studied AlGaAsrGaAs HEBT.
current–voltage Ž I–V . characteristics shown in this paper were measured by a Tektronix 577 curve tracer.
3. Experimental results and discussion For the HEBTs structures, the confinement layer is used as a confinement barrier for holes only and is separated physically from the effective E–B homojunction. For the HEBTs, a critical parameter is the n-GaAs emitter layer thickness. If the emitter layer thickness is too large, i.e.,
(
´ D Ec 2
q ND
ž
NA NA q ND
/
Ž 1.
where D Ec , ND , and NA are the conduction band discontinuity between confinement and emitter layer, emitter, and base layer concentration, respectively. From theoretical ˚ are calculations, the d values of 260, 230, and 300 A obtained for the studied AlGaAsrGaAs, InGaPrGaAs, and AlGaAsrInGaAsrGaAs HEBTs, respectively. Thus, the employed emitter thickness is enough to eliminate the potential spike at base-emitter ŽB–E. junction. The common-emitter I–V characteristics of the studied AlGaAsrGaAs HEBT is shown in Fig. 1. The maximum common-emitter current gain of 180 and a low offset voltage of 80 mV are obtained for the studied AlGaAsrGaAs HEBT. The corresponding energy band
Fig. 2. The corresponding energy band diagrams Ža. at equilibrium and Žb. under normal operation mode of the studied AlGaAsrGaAs HEBT.
W.-C. Liu et al.r Thin Solid Films 324 (1998) 219–224
222
diagrams of the studied AlGaAsrGaAs HEBT at equilibrium and under normal operation mode are depicted in Fig. 2. Apparently, the potential spike is eliminated by the insertion of n-GaAs emitter layer. Since the offset voltage DVCE can be expressed as DVCE s R E I B q
KT q
ln
AC
ž / AE
KT q q
ln
ž
JCS
a N J ES
/
Ž 2.
where R E is the emitter series resistance, A C and A E are the collector and emitter area. a N is the forward common base current gain. JCS and J ES are the reverse saturation current of collector and emitter junction, respectively. Because the effective E–B and B–C junctions are homojunction, the JCS and J ES is almost equal. Thus, the offset voltage could be reduced. For the studied InGaPrGaAs HEBT, the device fabrication is relatively simple due to the large etch selectivity between InGaP and GaAs materials. Furthermore, attributed to the large D E V rD Ec ratio, the confinement effect for holes is prominent. The energy band diagrams at equilibrium and under normal operation mode are illustrated in Fig. 3. Significantly, there is no potential spike existing at the E–B junction attributed to the small D Ec and the insertion of n-GaAs emitter. However, the emitter ˚ is so large that the recombination layer thickness of 700 A current in neutral-emitter region is increased. Thus, a low offset voltage can be achieved while the performance of
current gain is not so good. The typical common-emitter I–V characteristics of the studied InGaPrGaAs HEBT is shown in Fig. 4. The control base current I B is 0.1 mArstep. The current gain of 60 is obtained without using emitter-edge thinning structure. In order to improve the transistor performances, as similar to the AlGaAsrGaAs HEBT, a pq-InGaAs QW is added between n-GaAs emitter and pq-GaAs base layer of the studied AlGaAsrInGaAsrGaAs HEBT. The energy band diagrams at equilibrium and under normal operation mode are illustrated in Fig. 5. Compared with the AlGaAsrGaAs HEBT, owing to the presence of InGaAs QW, the effective valence band discontinuity at the E–B heterojunction ŽN-AlGaAsrn-GaAsrp q-InGaAsrp qGaAs. is approximately equivalent to the summation of the valence band discontinuity at the InGaAsrGaAs and AlGaAsrGaAs heterointerface. So, the effective valence band discontinuity at the E–B heterojunction of the AlGaAsrInGaAsrGaAs HEBT is higher than that of the AlGaAsrGaAs HEBT and InGaPrGaAs HEBT. Therefore, for the studied AlGaAsrInGaAsrGaAs HEBT, the injecting holes from base to emitter are reduced, and the emitter injection efficiency can be increased. This certainly yields the enhancement of collector current and current gain. Fig. 6 shows the output I–V characteristics of the studied AlGaAsrInGaAsrGaAs HEBT under normal operation mode. The common-emitter current gain of 280
Fig. 3. The corresponding energy band diagrams Ža. at equilibrium and Žb. under normal operation mode of the studied InGaPrGaAs HEBT.
W.-C. Liu et al.r Thin Solid Films 324 (1998) 219–224
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Fig. 4. The common-emitter I – V characteristics of the studied InGaPrGaAs HEBT.
Fig. 6. The common-emitter I – V characteristics of the studied AlGaAsrInGaAsrGaAs HEBT.
with the DVCE of 100 mV is obtained. The current gain is superior to the AlGaAsrGaAs HEBT devices without the emitter edge-thinning structure. Hence, the higher emitter injection efficiency and a relatively small offset voltage is
resulted from the insertion of InGaAs QW in the base region. Yet, despite the improvement in current gain of the HEBT with the pq-InGaAs QW, one may expect a relative sacrifice in the high-frequency performance Že.q., the cut-
Fig. 5. The corresponding energy band diagrams Ža. at equilibrium and Žb. under normal operation mode of the studied AlGaAsrInGaAsrGaAs HEBT.
224
W.-C. Liu et al.r Thin Solid Films 324 (1998) 219–224
off frequency f T and maximum oscillation frequency f max . of this device.
4. Conclusion The confinement effects of the HEBTs with different material systems, i.e., AlGaAsrGaAs, InGaPrGaAs, and AlGaAsrInGaAsrGaAs, have been studied. Due to the presence of effective emitter–base homojunction, the low offset voltages are obtained. For the studied devices, the employment of the higher bandgap confinement layer not only provides a better confinement effect for minority carriers Žholes., but also maintains a relatively higher injection level for majority carriers Želectrons.. On the other hand, the emitter layer thickness must be carefully designed in order to achieve a good performance of high current gain and low offset voltage. For the studied InGaPrGaAs HEBT, the n-GaAs emitter layer thickness might be designed as a much lower value to improve tr a n s is to r p e r f o r m a n c e . F o r th e s tu d ie d AlGaAsrInGaAsrGaAs HEBT, the satisfactory proper transistor performances are obtained. Further, more excellent characteristics may be expected when an emitter-edge thinning design is employed. Consequently, the studied devices provide good promises for transistor design and circuit applications.
Acknowledgements Part of this work was supported by the National Science Council of the Republic of China under Contract No. NSC 87-2215-E-006-020.
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