GaAs HBTs

June 2001

Materials Letters 49 Ž2001. 219–223 www.elsevier.comrlocatermatlet

Effects of emitter structure variation on the RF characteristics of AlGaAsrGaAs HBTs Il-Ho Kim Department of Materials Science and Engineering, Chungju National UniÕersity, 123 Komdan-ri, Iryu-myon, Chungju, Chungbuk, 380-702, South Korea Received 22 June 2000; received in revised form 3 November 2000; accepted 6 November 2000

Abstract Emitter structure effects on the microwave characteristics of AlGaAsrGaAs HBTs have been investigated. Cutoff frequency and maximum oscillation frequency were changed with emitter dimension, and this was attributed to the variation of resistances and junction capacitances with emitter structure. Emitter perimeter and junction area also affected the high frequency performance of HBTs. To enhance the RF characteristics of HBTs, it is necessary to minimize the intrinsic emitter–collector transit time as well as the parasitic components. q 2001 Elsevier Science B.V. All rights reserved. PACS: 85.30.De; 85.30Pq Keywords: Gallium arsenide; Heterojunction bipolar transistor; Emitter; High frequency; Microwave; RF

1. Introduction AlGaAsrGaAs heterojunction bipolar transistors ŽHBTs. are recognized as promising electronic devices for high frequency and high performance circuit applications, such as monolithic microwave integrated circuit ŽMMIC. and optoelectronic integrated circuit ŽOEIC. w1–4x. The advantages of HBTs come from the wide band gap emitter that results in high emitter injection efficiency and high DC current gain because the hole or electron injection across the heterojunction interface from the base to emitter is reduced. It is also possible to reduce the parasitic components of the device, such as the base resistance and emitter–base capacitance, without any sacrifice of current gain for enhancing the high frequency

characteristics of HBTs. Therefore, HBTs show excellent DC and RF performance. Such a high potential for superior microwave performance of HBTs has been limited by the fabrication technology that results in large extrinsic parasitics, such as extrinsic resistances, contact resistances and junction capacitances. However, these drawbacks could be overcome by advanced technologies. The extrinsic resistances may be greatly reduced by the self-aligned technology w5,6x. Extensive researches of various material systems have been performed to make good ohmic contact w7,8x. The extrinsic junction capacitances can be reduced by proton or boron ion implantation that includes damage to compensate the donors in the collector region w9x.

00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 3 7 3 - 6

I.-H. Kim r Materials Letters 49 (2001) 219–223

220

Table 1 Epitaxial layer structure of AlGaAsrGaAs HBT Epitaxial

Material

Al or In Thickness Dopant ˚. ŽA Žcmy3 .

nq-InGaAs nq-InGaAs nq-GaAs n-AlGaAs n-AlGaAs pq-GaAs n-GaAs nq-GaAs i-AlGaAs i-GaAs s.i.-GaAs

0.5 0.5™ 0 0 0 ™ 0.3 0.3 0 0 0 0.3 0 0

layer Cap III Cap II Cap I Emitter II Emitter I Base Collector Subcollector Buffer II Buffer I Substrate

reported that f max can be improved by reduction of collector–base junction capacitance if an emitter width decreases at constant emitter–base spacing. Nagata et al. w15x have also reported that f T shows a more enhancement than f max if an emitter length increases at constant emitter width, i.e. the ratio of emitter area to perimeter increases. However, systematic and quantitative studies have been not made on the device structure effects in the high frequency performance of the HBTs. In this study, the dependence of the cutoff frequency and maximum oscillation frequency on the emitter structure of AlGaAsrGaAs HBTs is investigated by varying the emitter width and length, the emitter area to emitter perimeter ratio and the emitter area to collector area ratio.

1=10 19 Si 1=10 19 Si 3.7=10 18 Si 5=10 17 Si 2=10 17 Si 3=10 19 C 2=10 16 Si 4=10 18 Si undoped undoped undoped

400 400 1000 500 1500 700 4000 5000 3000 3000 6=10 6

The total emitter–collector transit time is a dominant factor in limiting the high speed performance when the extrinsic parasitic RC time constant is minimized by the improved fabrication technologies mentioned above. Two figures of merit to design the optimal structure and to analyze the microwave performance of the HBTs are the cutoff frequency Ž f T . and the maximum oscillation frequency Ž f max ., which are related to intrinsic and extrinsic resistances, junction capacitances, and emitter–collector transit time w10,11x. For good design, the HBT structures are optimized to obtain the highest possible f T and f max while retaining a higher fmax than f T w12x. To enhance the high frequency characteristics of the HBTs, optimization of an epitaxial structure has been intended by variation of thickness and doping level in epitaxial layers. Extensive researches have been also made to minimize the emitter–base spacing, parasitic components, and surface recombination area. Kim et al. w13x and Gao et al. w14x have

2. Fabrication of AlGaAsr r GaAs HBTs AlGaAsrGaAs heterojunction epitaxial layers for the fabrication of HBTs were grown by metal organic chemical vapor deposition ŽMOCVD. on 3-in. semi-insulating GaAs wafer. They consisted of an n-AlGaAs emitter doped with Si at 2 = 10 17 cmy3 , a p-GaAs base doped with C at 3 = 10 19 cmy3 , and an n-GaAs collector doped with Si at 2 = 10 16 cmy3 . As an emitter capping layer, n-InGaAs doped with Si at 1 = 10 19 cmy3 was also grown. The detailed layer structure of the HBT is shown in Table 1. H 3 PO4rH 2 O 2rH 2 O s 4:1:90 and NH 4 OHrH 2 O 2r H 2 O s 20:7:973 solutions were used for emitter mesa etching, and H 3 PO4rH 2 O 2rH 2 O s 4:1:50 solution was utilized for base mesa and device isolation ˚ .rNi Ž500 A˚ .rAu etching. In this study, Au Ž900 A ˚ ˚ ˚ Ž600 A.rGe Ž300 A.rPd Ž400 A. system was used as emitter ohmic contact Žactually contacted to emit-

Table 2 Dimensions of AlGaAsrGaAs HBTs Device name

A

B

C

D

E

F

G

H

I

J

K

L

Emitter width We Žmm. Emitter length Le Žmm. Emitter junction area A e Žmm2 . Emitter perimeter Pe Žmm. Collector junction area A c Žmm2 .

1.9 10.4 19.8 24.6 120.0

1.6 10.4 16.6 24.0 101.6

1.4 10.4 14.6 23.6 89.0

2.4 10.4 25.0 25.6 169.5

1.9 10.4 19.8 24.6 108.3

1.6 10.4 16.6 24.0 93.2

1.4 10.4 14.6 23.6 89.7

1.6 12.9 20.6 29.0 118.2

1.6 7.4 11.8 18.0 84.8

1.6 5.4 8.6 14.0 73.6

1.6 10.4 16.6 24.0 83.4

1.6 10.4 16.6 24.0 73.0

I.-H. Kim r Materials Letters 49 (2001) 219–223

˚ .rPt Ž300 ter capping layer of n-InGaAs.. Au Ž800 A ˚ .rTi Ž300 A˚ .rPt Ž50 A˚ . and Au Ž600 A˚ .rTi Ž200 A ˚ .rAu Ž600 A˚ .rGe Ž300 A˚ .rNi Ž100 A˚ . were A deposited as base and collector ohmic contacts, which are widely used as contact systems on p- and n-GaAs, respectively. Dielectric SiN layer was employed by plasma enhanced chemical vapor deposition ŽPECVD. for the device protection and electrical insulation, and it was etched by magnetically enhanced reactive ion etching ŽMERIE. with C 2 F6 plasma for the formation of via-contacts. Table 2 shows the dimensions of AlGaAsrGaAs HBTs fabricated in this work. 3. RF characteristics of AlGaAsr r GaAs HBTs Cutoff frequency Ž f T . and maximum oscillation frequency Ž f max . were measured by using Cascade

221

Microtech probe station and HP8510B network analyzer. f T is the frequency at which the common emitter current gain drops to unity w10x, and expressed by Eq. Ž1.

fT s

1 2ptec 1

1

s 2p re C be q t b q tc q Ž re q R e q R c . C bc

Ž 1.

where tec is the emitter–collector transit time, re is the intrinsic emitter resistance, C be is the emitter– base junction capacitance, t b is the base transit time, tc is the collector charging time, R e is the emitter resistance, R c is the collector resistance and C bc is the base–collector junction capacitance. f max is the frequency at which maximum available power gain

Fig. 1. Variation of cutoff and maximum oscillation frequencies with collector current Ž Vce s 1.5 V.: Ža., Žb. We variation, Žc. Le variation and Žd. A erA c variation.

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becomes unity w11x, and approximately given by Eq. Ž2. f max s

(

fT 8p R b C bc

Ž 2.

where R b is the base resistance. To increase f T and f max , it is necessary to minimize the total transit time as well as the parasitic components. Emitter structure Ždimension. change leads to the variation of emitter–base junction capacitance as well as emitter resistance. Base and collector resistances and their junction capacitances are also dependent on emitter structure variation in view of device integration. Fig. 1 shows the variation of f T and f max with collector current Ž Ic . at emitter–collector voltage Ž Vce . of 1.5 V. It was found that RF characteristics of the HBTs are strongly dependent on the emitter structure. At constant collector current of 4 mA where all the f T and f max do not reach to maximum value in this experiment, variations of f T and f max with emitter width ŽWe . are shown in Fig. 2. f T and f max abruptly decrease with increasing We above 1.9 mm because of the increase of the intrinsic base resistance and base–collector capacitance. In the range below 1.9 mm, f T and f max show the decreasing behavior. The reason for this is that the increase of emitter resistance with decreasing We dominates in this range. The alignment error Žresolution. of

Fig. 2. Variation of cutoff and maximum oscillation frequencies with emitter width Ž Vce s1.5 V, Ic s 4 mA, Le s10.4 mm..

Fig. 3. Variation of cutoff and maximum oscillation frequencies with emitter length Ž Vce s1.5 V, Ic s 4 mA, We s1.6 mm..

i-line stepper in the fabrication process could be another reason. Kim et al. w13x and Gao et al. w14x have reported that f max increased with decreasing We because the collector–base junction capacitance was reduced. In this study, the similar behavior is shown up to decreasing We s 1.9 mm; however, if We is narrower than 1.9 mm, both f T and f max rather decrease because the increase in emitter resistance is dominant. Fig. 3 indicates the variation of f T and f max with emitter length Ž Le .. Both f T and f max increase to maximum with increasing Le and drop with further

Fig. 4. Variation of cutoff and maximum oscillation frequencies with the ratio of emitter–base junction area to collector–base junction area Ž Vce s1.5 V, Ic s 4 mA..

I.-H. Kim r Materials Letters 49 (2001) 219–223

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width, length, perimeter and junction area ratio. However, their dependencies are somewhat different. Therefore, the tradeoff between f T and f max is very important, and they should be optimized to obtain the best design for specific circuit applications.

Acknowledgements This work was supported by Grant No. 2000-130100-002-1 from the Basic Research Program of the Korea Science and Engineering Foundation. Fig. 5. Variation of cutoff and maximum oscillation frequencies with the ratio of emitter area to emitter perimeter Ž Vce s1.5 V, Ic s 4 mA..

increasing Le . Nagata et al. w15x have reported that f T and f max increased with increasing Le . However, in this study, f T shows a peak at Le of 10.4 mm and f max also shows a peak at Le of 7.4 mm. The reason for this is that reduction of emitter resistance and increase of junction capacitance are correlated and competitive to each other. This behavior is also found in Fig. 4, which shows the variation of f T and f max with the ratio of emitter–base junction area Ž A e . to collector–base junction area Ž A c .. Fig. 5 shows the variation of RF characteristics with the ratio of emitter area to emitter perimeter Ž Pe .. When the emitter area is constant, f T increases with increasing emitter perimeter, i.e. with increasing the A erPe ratio. However, f max decreases beyond the ratio of 0.65. This means that parasitic effects such as junction capacitances due to an increase of emitter area are dominant in f max .

4. Conclusions Emitter structure dependence of the high frequency performance of AlGaAsrGaAs HBTs has been examined. Cutoff frequency and maximum oscillation frequency are strongly dependent on emitter

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