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GaAs NpN double heterojunction bipolar transistor with low turn-on voltage
Solid-State Electronics 46 (2002) 1±5
GaAs/Ga0:89In0:11N0:02As0:98/GaAs NpN double heterojunction bipolar transistor with low turn-on voltage R.J. Welty a,*, H.P. Xin a, K. Mochizuki b, C.W. Tu a, P.M. Asbeck a a
Department of Electrical and Computer Engineering, University of California at San Diego, La Jolla, CA 92093-0407, USA b Central Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185-8601, Japan Accepted 1 October 2001
Abstract GaAs based heterojunction bipolar transistors have a relatively large turn-on voltage of approximately 1.4 V that can only be decreased by reducing the bandgap energy of the base material. For a variety of applications, particularly operation with low power supply voltage and reduced power dissipation, it would be desirable to have a smaller value of turn-on voltage. We report the DC performance of NpN double heterojunction bipolar transistors fabricated on a GaAs substrate with a Ga0:89 In0:11 N0:02 As0:98 -base that has a bandgap energy (Eg ) of 0.98 eV. These devices have a turn-on voltage VBE that is 0.4 V lower than that of their GaAs-base counterparts. To overcome the large conduction band discontinuity between GaAs and Ga0:89 In0:11 N0:02 As0:98 both chirped superlattice and delta doping were employed. Peak incremental current gain of hfe 8:5 was obtained. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Double heterojunction bipolar transistor; GaInNAs
1. Introduction GaAs-based heterojunction bipolar transistors (HBTs) have achieved widespread use in high performance microwave and digital applications. A limitation of bipolar transistors is their non-scalable turn-on voltage. The turn-on voltage for MOSFETs (threshold voltage, VT ) is determined by process parameters, such as the channel doping and oxide thickness, which can readily be varied and also by the dielectric constant of the insulating layer. For bipolar technologies, the turnon voltage is predominately ®xed by the bandgap energy of the base material and also by the type of base±emitter (BE) heterojunction: abrupt or graded. For GaAs (Eg 1:424 eV) based HBTs this translates into a large BE turn-on voltage VBE of approximately 1.4 V (at high current density). As the power supply voltage continues
*
Corresponding author. Fax: +1-858-534-0556. E-mail address: [email protected] (R.J. Welty).
to shrink, it is becoming increasingly important to develop techniques to reduce the turn-on voltage. The turn-on voltage for InP based HBTs is inherently lower than for GaAs-based HBTs. Ga0:47 In0:53 As used as the base material, lattice matched to the InP substrate, has a bandgap energy of 0.75 eV [1]. GaAs0:5 Sb0:5 (Eg 0:72 eV) as the base material grown on InP also provides a corresponding reduction in turn-on voltage [2]. InGaAs as the base material in a GaAs technology has had limited success for reducing the turn-on voltage because the materials are not lattice matched. The thickness and In composition of the InGaAs layer must be kept within the critical thickness such that the strain due to lattice mismatch can be absorbed elastically. If the InGaAs is thicker than the critical thickness, mis®t dislocations will occur, which will cause excessive recombination. Ga0:92 In0:08 As-based HBTs grown on GaAs have been successfully fabricated [3]. This In composition translates into 0:1 V reduction in turn-on voltage, relative to a GaAs-base. This small reduction is a practical limit for GaInAs-based HBTs with conventional base widths before reaching the critical layer
0038-1101/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 1 0 1 ( 0 1 ) 0 0 3 1 5 - X
2
R.J. Welty et al. / Solid-State Electronics 46 (2002) 1±5
thickness. To further reduce the turn-on voltage for GaAs-based HBTs, alternative base materials must be investigated. Recently, GaInNAs has received wide attention. The incorporation of small amounts of nitrogen into GaInAs reduces the net strain of the material grown on GaAs, which is the limiting factor for HBTs with GaInAsbases, and also reduces the bandgap energy. These material characteristics have motivated considerable research activity in GaInNAs, particularly for longwavelength semiconductor lasers [4]. GaInNAs can be lattice matched to GaAs with roughly three times the amount of In as N. The use of GaInNAs in the base region of an HBT has recently been investigated [5±7]. Previous work has demonstrated a 0.2 V reduction in turn-on voltage VBE in GaInNAs-based HBTs using an abrupt GaInP emitter grown by metal±organic vapor phase epitaxy [5]. In this letter, we report a double heterojunction bipolar transistor (DHBT) with a Ga0:89 In0:11 N0:02 As0:98 -base together with GaAs emitter and collector regions that reduces the turn-on voltage VBE by 0.4 V relative to an HBT with a GaAs-base. This is the largest turn-on voltage reduction for a GaAs HBT technology reported to date. 2. Device design The collector current of an HBT with ideal grading between BE and the base±collector (BC) junctions, is base transport limited [1], and the collector current density can be calculated by Eqs. (1) and (2), [8] q exp qVBE =kT JC R p dx=Dn n2i x n2i NC NV exp
Eg kT
1 2
where VBE , p, Dn and ni are the BE voltage, base free carrier concentration, minority carrier diusion constant
and intrinsic carrier concentration, respectively, and the integral is across the quasi-neutral base. The eect of decreasing the bandgap energy Eg of the base is taken into account by the intrinsic carrier concentration ni , where NC and NV are the density of states of the conduction and valence band, respectively. From Eqs. (1) and (2), it is evident that the bandgap energy of the base region of an HBT with smooth transitions between junctions will de®ne the turn-on voltage of the device. In order to obtain the smallest turn-on voltage for an HBT, grading should be used in the BE junction to obtain a base transport limited device. Without such grading, BE barrier control of the current will also play a role, and increase the turn-on voltage. This is especially relevant for the Ga0:89 In0:11 N0:02 As0:98 -base (Eg 0:98 eV) HBT because of the large bandgap energy discontinuity in the conduction band, relative to the GaAs emitter. The bandgap energy of Ga0:89 In0:11 -N0:02 As0:98 was measured by photoluminescence. The band lineup for GaAs/Ga0:89 In0:11 N0:02 As0:98 is estimated by the reported change in the conduction band discontinuity (DEC ) from the incorporation of indium [9] and nitrogen [10], as DEC 0:28 eV and the valence band discontinuity as DEV 0:16 eV. The BC junction should also be graded to remove the barrier to electrons. The GaAs/Ga0:89 In0:11 N0:02 As0:98 /GaAs NpN HBT epilayer structure is shown in Table 1. The HBT was grown on semi-insulating GaAs and has a GaAs emitter and collector. On either side of the Ga0:89 In0:11 N0:02 As0:98 -base is a spacer layer to allow beryllium outdiusion. Such out-diusion is a concern during the rapid thermal anneal (RTA) which is necessary to partially recover the hydrogen-passivation of the base material as described below [11]. The grading between the BE and BC was done with a short period chirped su to allow electrons to readily tunnel perlattice of 11 A through the graded layer. Delta doping was also employed on either side of the chirped superlattice to counter the electric ®eld that is generated by the compositional grade. In order for the delta doping to be
Table 1 GaAs/GaInNAs/GaAs DHBT epitaxial device layers Layer
Material
Thickness (A)
Doping (cm 3 )
Cap Emitter Delta doping Graded Spacer Base Spacer Graded ``Delta doping'' Collector Sub-collector S.I. GaAs Substrate
GaAs GaAs GaAs GaAs)Ga0:89 In0:11 N0:02 As0:98 Ga0:89 In0:11 N0:02 As0:98 Ga0:89 In0:11 N0:02 As0:98 Ga0:89 In0:11 N0:02 As0:98 Ga0:89 In0:11 N0:02 As0:98 )GaAs GaAs GaAs GaAs
2000 2000 5 300 50 400 50 300 50 4000 7000
n 5 1018 n 5 1017 n 3 1019 n 3 1017 undoped p 8 1018 undoped n 3 1016 n 1:5 1018 n 3 1016 n 5 1018
R.J. Welty et al. / Solid-State Electronics 46 (2002) 1±5
3
eective it must be depleted. The delta doping on the emitter side of the grading was done by the conventional procedure of momentarily opening and closing the high ¯ux silicon shutter, this layer can be estimated to be thick. On the collector side, the ``delta dopabout 5 A This is allowable because the ing'' was done over 50 A. lowly doped region has more bandbending therefore the barrier is already reduced at equilibrium.
3. Device growth and fabrication The device epitaxial layers were grown by gas-source molecular beam epitaxy (GSMBE) and the nitrogen was generated from a radio-frequency plasma source. The n and the p type dopant were silicon and beryllium, respectively. As found by Hall measurements on a calibration sample, the hole mobility is 50 cm2 V 1 s 1 and free carrier concentration is 8 1018 cm 3 . The composition of indium and nitrogen was determined by Xray rocking curves: In 11% and N 2% and yields a bandgap energy of 0.98 eV. This composition yields a (compressive) strained material with a lattice mismatch of 0.4%. After crystal growth, the sample went through an RTA cycle at 700 °C in forming gas for 1 min. The samples were proximity annealed with a GaAs wafer on top of the device wafer, face to face, to prevent arsenic loss. This anneal step is critical to reduce the hydrogen-passivation in the Ga0:89 In0:11 N0:02 As0:98 layer which reduces the free carrier concentration. The hydrogen is incorporated into the material via the thermally cracked arsine, which is the arsenic source. The transistors were fabricated using a conventional mesa process. The emitter and collector contacts were formed by AuGe/Ni/Au electron beam evaporation and lifto. The base and sub-collector layers were reached by wet chemical etching using H3 PO4 H2 O2 H2 O. The base contact was formed by Pt/Ti/Pt/Au electron beam evaporation and lifto. The sample was furnace annealed at 365 °C in forming gas for 1 min.
4. Results and discussion All measurements were done on a device with a 120 120 lm2 emitter, using an HP 4155. The common emitter current±voltage (I±V) curves are shown in Fig. 1. The I±V curves show a positive output conductance that increases with increasing collector current. Frequently for DHBTs, this behavior is due to a barrier at the BC junction; by applying a larger reverse bias on the BC diode the barrier is reduced and more electrons can surmount the barrier, contributing to collector current. However, this mechanism was ruled out for the present device, by testing the transistor in inverse active mode. It
Fig. 1. Common emitter I±V curves with base current IB 2, 4, 6, 8 mA.
was veri®ed that the device has an n 1 ideality factor under these circumstances, indicating that the current is base-transport limited. The positive output conductance shown in Fig. 1 can be attributed to base width modulation. This was con®rmed by the close agreement between the observed changes in collector current and the changes expected on the basis of base sheet resistance measurements. The base sheet resistance qS was determined by using transmission line method (TLM) patterns on base and sub-collector layers of a non-isolated sample. Values of qS were measured for a set of reverse bias voltages VCB applied between the base and collector. For a change in VCB between 1 and 4 V, the collector current IC (measured for a constant base bias of IB 10 mA) changed by a factor of 1.3. The corresponding change of qS as VCB varied between 1 and 4 V is 1.24, and is in close agreement with the current change. A large oset voltage VCE;sat 0:5 V (for IC 0) is observed for this device with an emitter area of 120 120 lm2 and BC to BE junction area ratio of 2.4. The large oset voltage is attributed to the high base resistance in the extrinsic device and large contact resistance. The base sheet resistance (qS ) is 7 103 X= and contact resistance (RC ) is 3 10 3 X cm2 , determined by TLM measurements. Fig. 2 shows a measured common emitter Gummel plot taken at 294 K. The collector ideality factor is 1.05, indicating a nearly ideal collector current. The base current ideality factor has two distinct values: VBE < 0:3, n 1:95 and VBE > 0:3, n 1:59. The large base current ideality factors indicate that the base current is limited by both BE space charge and quasi-neutral base recombination. Fig. 3 shows a comparison of collector current IC vs. VBE for the Ga0:89 In0:11 N0:02 As0:98 and a conventional GaAs-base HBT. The GaAs-base HBT has an AlGaAs emitter with graded composition between the base and emitter. A 0.4 V shift in collector current is evident, in excellent agreement with theoretical
4
R.J. Welty et al. / Solid-State Electronics 46 (2002) 1±5
In order to improve the I±V characteristics of the GaInNAs-base HBT, the conductivity of the base needs to be increased. It is anticipated that higher current gain can be achieved by reducing recombination in the base and BE space charge region and also by compositional grading in the base. 5. Conclusion
Fig. 2. Common emitter Gummel plot, VBC 0 V.
We have demonstrated experimental results of a GaAs/Ga0:89 In0:11 N0:02 As0:98 /GaAs NpN DHBT with a 0.4 V reduction in turn-on voltage VBE relative to a conventional GaAs-base HBT. This is the largest reported reduction in turn-on voltage for a GaAs HBT technology. Delta doping and chirped superlattice regions between the BE and BC junctions were successfully designed to overcome the large conduction band discontinuity between GaAs and Ga0:89 In0:11 N0:02 As0:98 . The transistors in this work were grown by GSMBE and at present have lower base conductivity than their GaAs-based counterpart, due to hydrogen-passivation of acceptors during growth. Further anneal optimization or use of alternative growth techniques may mitigate this problem. These transistors should be advantageous for operation at low power supply voltages, which can be a limiting constraint for GaAs-based HBTs in scaled power supply applications.
Acknowledgements
Fig. 3. Collector current for Ga0:89 In0:11 N0:02 As0:98 and GaAsbase HBT.
The authors gratefully acknowledge Prof. S.S. Lau and D.J. Qiao for their assistance with RTA. This work was supported by Rockwell Science Center and UC MICRO programs and by OSD and ARO under the MURI ``Low Power Low Noise Electronics for Wireless Communications''. References
Fig. 4. Incremental current gain.
expectations. The incremental current gain reaches a peak value of hfe 8:5, as shown in Fig. 4.
[1] Asbeck PM. In: Sze SM, editor. High Speed Semiconductor Devices. New York: Wiley; 1990. [2] Matine N, Dvorak MW, Bolognesi CR, Xu X, Hu J, Watkins SP, et al. Electron Lett 1998;34(17):1700±2. [3] Ito H, Ishibashi T. Jpn J Appl Phys 1986;25(5):421±4. [4] Kondow M, Kitatani T, Nakatsuka S, Larson MC, Nakahara K, Yazawa Y, et al. IEEE J Sel Top Quantum Electron 1997;3(3):719±30. [5] Li NY, Chang PC, Baca AG, Xie XM, Sharps PR, Hou HQ. Electron Lett 2000;36(1):81±3. [6] Welty RJ, Xin HP, Mochizuki K, Tu CW, Asbeck M. In: 58th Device Research Conference Digest, Denver CO, 2000. p. 145±6. [7] Welser RE, DeLuca PM, Pan N. IEEE Electron Device Lett 2000;21(12):554±6.
R.J. Welty et al. / Solid-State Electronics 46 (2002) 1±5 [8] Kroemer H. Solid-State Electron 1985;28(11):1101±3. [9] Niki S, Lin CL, Chang WSC, Wieder HH. Appl Phys Lett 1989;55(13):1339±41.
5
[10] Xin HP, Tu CW. Appl Phys Lett 1998;72(19):2442±4. [11] Xin HP, Tu CW, Geva M. Appl Phys Lett 1999;75(10): 1416±8.
GaAs/Ga0:89In0:11N0:02As0:98/GaAs NpN double heterojunction bipolar transistor with low turn-on voltage R.J. Welty a,*, H.P. Xin a, K. Mochizuki b, C.W. Tu a, P.M. Asbeck a a
Department of Electrical and Computer Engineering, University of California at San Diego, La Jolla, CA 92093-0407, USA b Central Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185-8601, Japan Accepted 1 October 2001
Abstract GaAs based heterojunction bipolar transistors have a relatively large turn-on voltage of approximately 1.4 V that can only be decreased by reducing the bandgap energy of the base material. For a variety of applications, particularly operation with low power supply voltage and reduced power dissipation, it would be desirable to have a smaller value of turn-on voltage. We report the DC performance of NpN double heterojunction bipolar transistors fabricated on a GaAs substrate with a Ga0:89 In0:11 N0:02 As0:98 -base that has a bandgap energy (Eg ) of 0.98 eV. These devices have a turn-on voltage VBE that is 0.4 V lower than that of their GaAs-base counterparts. To overcome the large conduction band discontinuity between GaAs and Ga0:89 In0:11 N0:02 As0:98 both chirped superlattice and delta doping were employed. Peak incremental current gain of hfe 8:5 was obtained. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Double heterojunction bipolar transistor; GaInNAs
1. Introduction GaAs-based heterojunction bipolar transistors (HBTs) have achieved widespread use in high performance microwave and digital applications. A limitation of bipolar transistors is their non-scalable turn-on voltage. The turn-on voltage for MOSFETs (threshold voltage, VT ) is determined by process parameters, such as the channel doping and oxide thickness, which can readily be varied and also by the dielectric constant of the insulating layer. For bipolar technologies, the turnon voltage is predominately ®xed by the bandgap energy of the base material and also by the type of base±emitter (BE) heterojunction: abrupt or graded. For GaAs (Eg 1:424 eV) based HBTs this translates into a large BE turn-on voltage VBE of approximately 1.4 V (at high current density). As the power supply voltage continues
*
Corresponding author. Fax: +1-858-534-0556. E-mail address: [email protected] (R.J. Welty).
to shrink, it is becoming increasingly important to develop techniques to reduce the turn-on voltage. The turn-on voltage for InP based HBTs is inherently lower than for GaAs-based HBTs. Ga0:47 In0:53 As used as the base material, lattice matched to the InP substrate, has a bandgap energy of 0.75 eV [1]. GaAs0:5 Sb0:5 (Eg 0:72 eV) as the base material grown on InP also provides a corresponding reduction in turn-on voltage [2]. InGaAs as the base material in a GaAs technology has had limited success for reducing the turn-on voltage because the materials are not lattice matched. The thickness and In composition of the InGaAs layer must be kept within the critical thickness such that the strain due to lattice mismatch can be absorbed elastically. If the InGaAs is thicker than the critical thickness, mis®t dislocations will occur, which will cause excessive recombination. Ga0:92 In0:08 As-based HBTs grown on GaAs have been successfully fabricated [3]. This In composition translates into 0:1 V reduction in turn-on voltage, relative to a GaAs-base. This small reduction is a practical limit for GaInAs-based HBTs with conventional base widths before reaching the critical layer
0038-1101/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 1 0 1 ( 0 1 ) 0 0 3 1 5 - X
2
R.J. Welty et al. / Solid-State Electronics 46 (2002) 1±5
thickness. To further reduce the turn-on voltage for GaAs-based HBTs, alternative base materials must be investigated. Recently, GaInNAs has received wide attention. The incorporation of small amounts of nitrogen into GaInAs reduces the net strain of the material grown on GaAs, which is the limiting factor for HBTs with GaInAsbases, and also reduces the bandgap energy. These material characteristics have motivated considerable research activity in GaInNAs, particularly for longwavelength semiconductor lasers [4]. GaInNAs can be lattice matched to GaAs with roughly three times the amount of In as N. The use of GaInNAs in the base region of an HBT has recently been investigated [5±7]. Previous work has demonstrated a 0.2 V reduction in turn-on voltage VBE in GaInNAs-based HBTs using an abrupt GaInP emitter grown by metal±organic vapor phase epitaxy [5]. In this letter, we report a double heterojunction bipolar transistor (DHBT) with a Ga0:89 In0:11 N0:02 As0:98 -base together with GaAs emitter and collector regions that reduces the turn-on voltage VBE by 0.4 V relative to an HBT with a GaAs-base. This is the largest turn-on voltage reduction for a GaAs HBT technology reported to date. 2. Device design The collector current of an HBT with ideal grading between BE and the base±collector (BC) junctions, is base transport limited [1], and the collector current density can be calculated by Eqs. (1) and (2), [8] q exp qVBE =kT JC R p dx=Dn n2i x n2i NC NV exp
Eg kT
1 2
where VBE , p, Dn and ni are the BE voltage, base free carrier concentration, minority carrier diusion constant
and intrinsic carrier concentration, respectively, and the integral is across the quasi-neutral base. The eect of decreasing the bandgap energy Eg of the base is taken into account by the intrinsic carrier concentration ni , where NC and NV are the density of states of the conduction and valence band, respectively. From Eqs. (1) and (2), it is evident that the bandgap energy of the base region of an HBT with smooth transitions between junctions will de®ne the turn-on voltage of the device. In order to obtain the smallest turn-on voltage for an HBT, grading should be used in the BE junction to obtain a base transport limited device. Without such grading, BE barrier control of the current will also play a role, and increase the turn-on voltage. This is especially relevant for the Ga0:89 In0:11 N0:02 As0:98 -base (Eg 0:98 eV) HBT because of the large bandgap energy discontinuity in the conduction band, relative to the GaAs emitter. The bandgap energy of Ga0:89 In0:11 -N0:02 As0:98 was measured by photoluminescence. The band lineup for GaAs/Ga0:89 In0:11 N0:02 As0:98 is estimated by the reported change in the conduction band discontinuity (DEC ) from the incorporation of indium [9] and nitrogen [10], as DEC 0:28 eV and the valence band discontinuity as DEV 0:16 eV. The BC junction should also be graded to remove the barrier to electrons. The GaAs/Ga0:89 In0:11 N0:02 As0:98 /GaAs NpN HBT epilayer structure is shown in Table 1. The HBT was grown on semi-insulating GaAs and has a GaAs emitter and collector. On either side of the Ga0:89 In0:11 N0:02 As0:98 -base is a spacer layer to allow beryllium outdiusion. Such out-diusion is a concern during the rapid thermal anneal (RTA) which is necessary to partially recover the hydrogen-passivation of the base material as described below [11]. The grading between the BE and BC was done with a short period chirped su to allow electrons to readily tunnel perlattice of 11 A through the graded layer. Delta doping was also employed on either side of the chirped superlattice to counter the electric ®eld that is generated by the compositional grade. In order for the delta doping to be
Table 1 GaAs/GaInNAs/GaAs DHBT epitaxial device layers Layer
Material
Thickness (A)
Doping (cm 3 )
Cap Emitter Delta doping Graded Spacer Base Spacer Graded ``Delta doping'' Collector Sub-collector S.I. GaAs Substrate
GaAs GaAs GaAs GaAs)Ga0:89 In0:11 N0:02 As0:98 Ga0:89 In0:11 N0:02 As0:98 Ga0:89 In0:11 N0:02 As0:98 Ga0:89 In0:11 N0:02 As0:98 Ga0:89 In0:11 N0:02 As0:98 )GaAs GaAs GaAs GaAs
2000 2000 5 300 50 400 50 300 50 4000 7000
n 5 1018 n 5 1017 n 3 1019 n 3 1017 undoped p 8 1018 undoped n 3 1016 n 1:5 1018 n 3 1016 n 5 1018
R.J. Welty et al. / Solid-State Electronics 46 (2002) 1±5
3
eective it must be depleted. The delta doping on the emitter side of the grading was done by the conventional procedure of momentarily opening and closing the high ¯ux silicon shutter, this layer can be estimated to be thick. On the collector side, the ``delta dopabout 5 A This is allowable because the ing'' was done over 50 A. lowly doped region has more bandbending therefore the barrier is already reduced at equilibrium.
3. Device growth and fabrication The device epitaxial layers were grown by gas-source molecular beam epitaxy (GSMBE) and the nitrogen was generated from a radio-frequency plasma source. The n and the p type dopant were silicon and beryllium, respectively. As found by Hall measurements on a calibration sample, the hole mobility is 50 cm2 V 1 s 1 and free carrier concentration is 8 1018 cm 3 . The composition of indium and nitrogen was determined by Xray rocking curves: In 11% and N 2% and yields a bandgap energy of 0.98 eV. This composition yields a (compressive) strained material with a lattice mismatch of 0.4%. After crystal growth, the sample went through an RTA cycle at 700 °C in forming gas for 1 min. The samples were proximity annealed with a GaAs wafer on top of the device wafer, face to face, to prevent arsenic loss. This anneal step is critical to reduce the hydrogen-passivation in the Ga0:89 In0:11 N0:02 As0:98 layer which reduces the free carrier concentration. The hydrogen is incorporated into the material via the thermally cracked arsine, which is the arsenic source. The transistors were fabricated using a conventional mesa process. The emitter and collector contacts were formed by AuGe/Ni/Au electron beam evaporation and lifto. The base and sub-collector layers were reached by wet chemical etching using H3 PO4 H2 O2 H2 O. The base contact was formed by Pt/Ti/Pt/Au electron beam evaporation and lifto. The sample was furnace annealed at 365 °C in forming gas for 1 min.
4. Results and discussion All measurements were done on a device with a 120 120 lm2 emitter, using an HP 4155. The common emitter current±voltage (I±V) curves are shown in Fig. 1. The I±V curves show a positive output conductance that increases with increasing collector current. Frequently for DHBTs, this behavior is due to a barrier at the BC junction; by applying a larger reverse bias on the BC diode the barrier is reduced and more electrons can surmount the barrier, contributing to collector current. However, this mechanism was ruled out for the present device, by testing the transistor in inverse active mode. It
Fig. 1. Common emitter I±V curves with base current IB 2, 4, 6, 8 mA.
was veri®ed that the device has an n 1 ideality factor under these circumstances, indicating that the current is base-transport limited. The positive output conductance shown in Fig. 1 can be attributed to base width modulation. This was con®rmed by the close agreement between the observed changes in collector current and the changes expected on the basis of base sheet resistance measurements. The base sheet resistance qS was determined by using transmission line method (TLM) patterns on base and sub-collector layers of a non-isolated sample. Values of qS were measured for a set of reverse bias voltages VCB applied between the base and collector. For a change in VCB between 1 and 4 V, the collector current IC (measured for a constant base bias of IB 10 mA) changed by a factor of 1.3. The corresponding change of qS as VCB varied between 1 and 4 V is 1.24, and is in close agreement with the current change. A large oset voltage VCE;sat 0:5 V (for IC 0) is observed for this device with an emitter area of 120 120 lm2 and BC to BE junction area ratio of 2.4. The large oset voltage is attributed to the high base resistance in the extrinsic device and large contact resistance. The base sheet resistance (qS ) is 7 103 X= and contact resistance (RC ) is 3 10 3 X cm2 , determined by TLM measurements. Fig. 2 shows a measured common emitter Gummel plot taken at 294 K. The collector ideality factor is 1.05, indicating a nearly ideal collector current. The base current ideality factor has two distinct values: VBE < 0:3, n 1:95 and VBE > 0:3, n 1:59. The large base current ideality factors indicate that the base current is limited by both BE space charge and quasi-neutral base recombination. Fig. 3 shows a comparison of collector current IC vs. VBE for the Ga0:89 In0:11 N0:02 As0:98 and a conventional GaAs-base HBT. The GaAs-base HBT has an AlGaAs emitter with graded composition between the base and emitter. A 0.4 V shift in collector current is evident, in excellent agreement with theoretical
4
R.J. Welty et al. / Solid-State Electronics 46 (2002) 1±5
In order to improve the I±V characteristics of the GaInNAs-base HBT, the conductivity of the base needs to be increased. It is anticipated that higher current gain can be achieved by reducing recombination in the base and BE space charge region and also by compositional grading in the base. 5. Conclusion
Fig. 2. Common emitter Gummel plot, VBC 0 V.
We have demonstrated experimental results of a GaAs/Ga0:89 In0:11 N0:02 As0:98 /GaAs NpN DHBT with a 0.4 V reduction in turn-on voltage VBE relative to a conventional GaAs-base HBT. This is the largest reported reduction in turn-on voltage for a GaAs HBT technology. Delta doping and chirped superlattice regions between the BE and BC junctions were successfully designed to overcome the large conduction band discontinuity between GaAs and Ga0:89 In0:11 N0:02 As0:98 . The transistors in this work were grown by GSMBE and at present have lower base conductivity than their GaAs-based counterpart, due to hydrogen-passivation of acceptors during growth. Further anneal optimization or use of alternative growth techniques may mitigate this problem. These transistors should be advantageous for operation at low power supply voltages, which can be a limiting constraint for GaAs-based HBTs in scaled power supply applications.
Acknowledgements
Fig. 3. Collector current for Ga0:89 In0:11 N0:02 As0:98 and GaAsbase HBT.
The authors gratefully acknowledge Prof. S.S. Lau and D.J. Qiao for their assistance with RTA. This work was supported by Rockwell Science Center and UC MICRO programs and by OSD and ARO under the MURI ``Low Power Low Noise Electronics for Wireless Communications''. References
Fig. 4. Incremental current gain.
expectations. The incremental current gain reaches a peak value of hfe 8:5, as shown in Fig. 4.
[1] Asbeck PM. In: Sze SM, editor. High Speed Semiconductor Devices. New York: Wiley; 1990. [2] Matine N, Dvorak MW, Bolognesi CR, Xu X, Hu J, Watkins SP, et al. Electron Lett 1998;34(17):1700±2. [3] Ito H, Ishibashi T. Jpn J Appl Phys 1986;25(5):421±4. [4] Kondow M, Kitatani T, Nakatsuka S, Larson MC, Nakahara K, Yazawa Y, et al. IEEE J Sel Top Quantum Electron 1997;3(3):719±30. [5] Li NY, Chang PC, Baca AG, Xie XM, Sharps PR, Hou HQ. Electron Lett 2000;36(1):81±3. [6] Welty RJ, Xin HP, Mochizuki K, Tu CW, Asbeck M. In: 58th Device Research Conference Digest, Denver CO, 2000. p. 145±6. [7] Welser RE, DeLuca PM, Pan N. IEEE Electron Device Lett 2000;21(12):554±6.
R.J. Welty et al. / Solid-State Electronics 46 (2002) 1±5 [8] Kroemer H. Solid-State Electron 1985;28(11):1101±3. [9] Niki S, Lin CL, Chang WSC, Wieder HH. Appl Phys Lett 1989;55(13):1339±41.
5
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