P. C. Chang, A. G. Baca. N; Y. Li, P. R. Sharps, H. Q. Hou. J. R. Laroche, F. Ren. Abstract

Transcription

1 #.,.. AGaAs/nGaAsN/G~ PnP Double Heterojunction Bipolar Transistor P. C. Chang, A. G. Baca SandiaNationalLaboratories,Albuquerque,New Mexico. N; Y. Li, P. R. Sharps, H. Q. Hou EmcorePhotovoltaics,EmcoreCorporation,Albuquerque,New Mexico. J. R. Laroche, F. Ren Departmentof ChemicalEngineering,Universityof Florida, Gainesville,Florida. Abstract We have demonstrated a functional PnP double heterojunction bipolar transistor (DH13T) using AlGaAs, ngaasn, and GaAs. The band aliegunent between ngaasn and GaAs has a large AEc and negligible AEv, this unique chiractenstic is very suitable for PnP DHBT applications. The metalorganic vapor phase epitaxy (MOCVD) grown ~o.3@o.7as~0.03@0.97as0.99n0.01/gasr@ DHBT is lattice matched to GaAs and has a peak current gain of 25. Because of the smaller bandgap (E~= 1.20 ev) of no.03ga0.g7aso.99n0.01 used for the base layer, this device has a low VONof 0.79 V, which is 0.25 V lower than in a -.<,. -. comparable Pnp AGaAs/GaAs HBT. And because GaAs is used for the collector, its BVCEOis 12 V, consistent with BVCEOof AGa.As/GaAsHBTs... 1

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3 _..-.. ~ DSCLAMER Portions of this document may be illegible in electronic image products. mages are produced from the best available original document.

4 ngaasn has received a lot of attention lately. By incorporating a small amount of nitrogen (N) into ngaas results in a reduction of its lattice constant, thus reducing the strain of 1S2 Jn addition, due to a large bandgap (&) bowing, the EG ngaas layer grown on GaAs. decreases asnisaddedl 2, a desirable characteristic for GaAs based device structures that require material with a smaller EGthan the 1.42 ev of GaAs. Recent advances in the ngaasn materials ystem has led to much progress on the application of this material system for a variety ofdevices An heterojunction bipolar transistor (HBT) for low-power electronic application is a device that could take advantage of ngaasn. To reduce the power dissipation of HBT circuits, a lower device turn on voltage (VON)would be very beneficial. The HBT with a small. EGin the base has lower VON,a desirable characteristic for low-power electronics. One approach uses strained ngaas on GaAs, however, the range of n composition for growing strained ngaas on GaAs without formation of misfit dislocations is very limited. n addition, due to the compressive strain, the &of ngaas increases, reducing the benefits of having ngaas in the base layer6. That is why most of the earlier works on low-power HBT have focused on the np/ngaas material system. However, the np technology is expensive, and its application has.. been lhnited7 8. The ngaasn material system should be an excellent zdtemative for low-power HBTs. t is compatible with the existing GaAs foundry technology; thus, it is more likely to be used in inexpensive low-power electronics. The band alignment of the ngaasn material system is illustrated in Figure 1? As N is incorporated into GaAs, a tensile strain develops. Adding N to GaAs lowers both the conduction band (Ec) and the valence band (EV). On the other hand, a compressive strain builds up as iridium (n).is added to GaAs, the Ec is lowered, and the Ev is raised. By incorporating proper amount of n and N into GaAs simultaneously, ngaasn that is lattice matched to GaAs can be 2

5 obtained. The Ec of the resulting lhgaasn would be significantly lower because of the aggregated lowering effect from the incorporation of N and n. The effects on Ev from the incorporation of N and n are compensated, and the EVis relatively unchanged compared to the Ev level of GaAs. The resulting band alignment, as shown in Figure 2, is favorable for PnP DHE3Tapplications. Using AlGaAs for emitter and GaAs for the collector, a large conduction band discontinuity (AEc) on the emitter side suppresses the electrons from being injected into the emitter from the base, while a small valence band discontinuity (AEv) facilitates hole transport from the emitter to the base. On the collector side, the AEv is negligible, thus GaAs cw be used for the collector layer and the device does not have to resort to complicated grading or doping schemes to eliminate the non-ideal effects suffered by most DHBTs. n this letter, we report the operation of an AGaAs/nGaAsN/GaAs PnP DHBT. The structure of the PnP Alo.3G~.7As/no.03Gx.g7Aso.ggNo.odGaAs DHBT investigated in. this work is shown in Table. The base layer is made of &.03G~.g7Aso.wNo.ol,which is lattice matched to GaAs and its E~is approximately 1.2 ev. The resulting band stmcture should resemble the diagram shown in Figure 2. For the Alo.3G~.7As/no.03G~.g7Aso.YNo.olemitter-base junction, the AEv at the base emitter junction would be around 0.15 ev, while the AEc would be more than 0.5 ev. At the base-collector junction, the AEv between no.03g~.waso.wno.oland GaAs is negligible. As discussed earlier, this is very suitable for PnP DH13Tapplications. Thus, GaAs can be used instead of ngaasn in the collector without typical penalties suffered by DHBTs, at the same time, taking advantage of the larger EGof GaAs, which allowsfor higher breakdown voltages, especially when compared to others low-power HBTs based on np/ngaas material system. n addition, the hole mobility (pp)of the best ngaasn reported to date is about half of the pp typically observed in GaAs, therefore using GaAs as the collector material would 3

6 , affect the rf performance of this device positively. The DHBT structure understudy was grown by metalorganic-chemical vapor deposition (MOCVD) using an Emcore D180 system. The material compositions were calibrated using photoluminescence measurement (PL) and X-Ray diffraction (XDR). The doping levels were confirmed with polaron and Hall measurement. The surface morphology of the sample is uniform and smooth. The DHBT device was fabricated using a triple mesa process. Wet etching was used to expose the base and the subcollector surface, as well as for achieving device isolation. All three etches were done using the 1 HsPOA: 4 H202: 45 H20 solution. Pt/Ti/Pt/Au forms non-alloyed ohmic contacts for the emitter and the collector. n order to avoid spiking through the base layer, Pd/Ge/Au annealed at 175 C for 1 hour was used for the base contact metal. 0 For this work, the device was not passivated. The emitter area of the final device is about 500 ymz. The fabricated device was tested using a HP-4145 Semiconductor Parameter Analyzer. The Gummel plot and the measured common-emitter current-voltage (V) plot are shown in Figure 3 and Figure 4, respectively. As shown in Figure 3, the A.lo.3G~07As/no.03G~.g7Aso.wNo.ol/GaAs DHBT has a current gain (~) of 25, which is sufficient >: gain to be useful for many circuit applications. More importantly, the VONof the Alo.3G~.7As/no.03G~.g7Aso.wNo.ol/Ga4s DHBT, as defined by the base-emitter junction bias (V~~)at which the collector current (L-) exceeds 1.0 pa, is only 0.79 V, which is significantly lower than the 1.03 V measured in a Alo.3G~.7As/GaAs HBT with similar structure, confirming that HBTs with ngaasn base layer can be used as an alternate approach for reducing power dissipation in a low-power circuit. Also, because GaAs is used for the collector layer, the emitter collector breakdown voltage (BVCEO)is about 12 V, comparable to the BVCEOobsened in a similar AGaAs/GaAs HBT.

7 Other important parameters considered are the offset voltage (Voff,ei)and the saturation voltage (VSaJ. As can be observed from Figure 4, the VoffS~t our device is about 220 mv, which is slightly higher than what is measured in an AGaAs/GaAs HBT. The V~t is also slightly higher than expected, ranging from 0.55 V to 0.85 V for ~ ranging from about 1.4 ma to ma. This discrepancy probably arises because the material quzdity of the no.03g~.g7aso.ggno.ol still does not match that of the GaAs. The base sheet resistance (RB)of the no.03g~.g7aso.ggno.ol base layer is about 3 K2/Square, this is because the electron mobility (U.) in the base layer is significantly lower than the p. in a comparable GaAs material. At about 350 cm2v s-1, it is much lower than the p. typically observed in GBAs,which is around 2000 cm2v- s-. Therefore, despite a base doping concentration (NDB)of 1.2x10 8 cm 3, the RBis still high. The high value of RB leads to the high VOff,.tand high V=, as observed in Figure 4. n addition, the ~ for a typical Alo.3Gm.TAs/GaAsPnp HBT is greater than 100. Considering the presence of a larger AEc at the Ao.sGao.TAs/no.osGao.gTAso.99N0.ol base-emitter junction, and the fact increase exponentially with increasing ~, ~ should ideally be greater t&m 25. A possible cause maybe the presence of recombination centers in the.. no.03g~.g7aso.ggno.ol base, thus resulting in high levels of recombination current and lower ~. One indication that the material could be improved is the high ideality factor of the base current (nm). A high nm indicates high level of recombination current, thus reducing the value of ~. However, as shown in Figure 3,,the nmis very large (around 4), indicating that there is more than just intrinsic base recombination effects. Since the device tested has not been passivated, a possible source of recombination current is the surface recombination. The we of surface states present on no.03g~.g7aso.ggno.ol is still unknown. More study would be need to understand it 5.

8 . better, and a proper passivation method for ngaasn would need to be determined to improve the performance of this device. n conclusion, the quality of the ngaasn material has now been improved to the point that an operational Alo.3G~.7As/no.03Ga0.g7As0.ggNo.ol/GaAs PnP DHBT is demonstrated. The near ideal band alignment between igaasn and GaAs results in near ideal V characteristics without resorting to grading or delta doping schemes needed in typical DHBTs. The AGaAs/nGaAsN/GaAs DHBT has a peak j3of 25. Since the collector is made of GaAs, the BVCEOof 12 V is comparable to the BVCEOobserved in a AGaAs/GaAs HBTs with similar collector thickness and doping. The narrower EGof no.03g~.g7aso.wno.ol has led to the low VON of 0.79 V, an important parameter for HBT application in circuits that require reduced power dissipation. However, due to the limitation of the ngaasn material available today, the RBis still high, causing the VOff,.tand the V=t to be high. Further improvements on the ngaasn material would benefit the performance of AGaAs/nGaAsN/GaAs DHBTs for low-power applications. And, the existence of surface states on ngaasn also needs to be characterized, so that a proper passivation scheme can be determined to improve the performance of the AGaAs/nGaAsN/GaAs DHBT. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94& ,. - ~y,..,:

9 Reference S. Sakai, Y. Ueta, and Y. Terauchi, Jpn. J. Appl. Phys., 32,4413 (1993). 2J. A. Van Vechten, Phys. Rev., 182,891 (1969). 3 S. Sate, and S. Satoh, Electron. L.ett., 35,1251 (1999). 4N. Y. Li, C. P. Hains, K. Yang, J. Lu, J. Cheng, and P. W. Li, Appl. Phys. Lett., 75, 1051 (1999). 5 S. R. Kurtz, A. A. Allen-mm,E. D. Jones, J. M. Gee, J. J. EMMs, Appl.phys.~tt., 74, 729. (1999). GH. P. Xin, C. W. Tu, and M. Geva, Appl. Phys. Lett., 75, 1416 (1999). 7P. C. Chang, A. G. Baca, J. F. Kem, M. J. Hailch, C. L H. Ashby, V. M. Hietala, Proceeding of State of the Art Programs on Compound Semiconductors XXX, to be published in Oct H. to, S. Yamahata, N. Shigekawa, and K. Kurishima, Jpn J. Appl. Phys., 35,648 (1997). 9 M. Kondow, K. Uomi, A. Niwa, T. Kitatani, S. Watahiki, and y. Yazawa, Jpn. J. Appl. Phys., 35, 1273 (1996). 0L. C. Wang, P. H. Hao, and B. J. Wu, Appl. Phys. Lett., 67,509 (1995).

10 The structure of the PnP A.10.sGao.TAslnO.03Ga0.97ASo.9gNo.o/G* D~T investigated in this work. The device was grown on S.. GaAs substrate by MOCVD using the Emcore D180 system. Material Thickness [~] Doping [cm-3] Contact Cap Layer p GaAs E+19 Emitter Layer p Alo.3Gao7As E+18 Spacer Layer u- Alo.3Gao7As 50 undoped Base Layer n no.03ga0.97f4so.ggno.ol E+18 Collector Layer p- GaAs E+16 Subcollector Layer p GaAs 5000 ~.ooe+19 Substrate S.. Gak 8 - % r,., u,,,, x >= ~.-...,;,.:-.r:,.@al,.._:- : ~.;,

11 Figure 1: The effect on the conduction band and valence band edges by incorporating N and n into GaAs? Figure 2: The band alignment of the Ao.3G4.7As/no.03G~.g7Aso.wNo.ol/GaAs DHBT. At the base-emitter junction, has a large AEc and a small AEv with respect to A0.3G~.7As. Compared to GaAs at the base collector junction, ngaasn has negligible AEv. This band alignment is very suitable for PnP DHBT applications... Figure 3: The Gurnmel plot of the Alo.3G~.7As/no.03Ga0.g7Aso.wNo.o/GaAs PnP DHBT. The ~ of this device is 25. The Vo~ is only 0.79 V, significantly lower than the V of a comparable AGaAs/GaAs HBT. But the nm is poor, indicating high level of recombination cument. Figure 4: The common-emitter V plot of the Alo.3G~.7As/no.03G~.g7Aso.wNo.ol/GaAs PnP DHBT. The five curves corresponds to 1~of 0.2,0.4,0.6,0.8, and 1.0 ma. The VOff,.lis about 220 mv, and the V=t varies from about 0.55 V to 0.85 V. 9 ~....,--.---:,.,Vm:,...

12 Ec +N Ga.As + n Compressive Figure 1. -=-m-.7 -t-..,,.; -,.V7,m ,..,-, ~.:/,

13 . Ec h) 03% 97f4so.99No.ol.. Emitter Al.. 3Gao7As Base Collector GaAs Figure 2

14 . # 1.E c + A 1.E-02 T 20 ~ 1.E-03 B 15 k [All. E P 1.E E-06 o 1 2 VB [V] 3 0 Figure 3

15 ,. k -1.2E-02-1.OE-02-8.OE-03 [A]-6.OE-03-4.OE-03-2.OE-03 O.OE+OO o -1 1~=- 1.0 rna ~ -2-3 Figure 4