007) 3978–3981www.elsevier.com/locate/tsf

Thin Solid Films 515 (2

δ-doped InGaP/GaAs heterostructure-emitter bipolar transistor grown bymetalorganic chemical vapor deposition

Y.S. Lin a,⁎, D.H. Huang b, Y.W. Chen c, J.C. Huang b, W.C. Hsu b

a Department of Materials Science and Engineering, National Dong Hwa University, 1, Sec. 2, Da Hsueh Rd. Shou-Feng, Hualien, Taiwan, R.O.C.b Institute of Microelectronics, Department of Electrical Engineering, National Cheng-Kung University, Tainan, Taiwan, R.O.C.

c Chi Mei Optoelectronics, Tainan, Taiwan, R.O.C.

Received 9 December 2005; received in revised form 4 August 2006; accepted 14 September 2006Available online 23 October 2006

Abstract

This article assesses the performance of a δ-doped In0.5Ga0.5P/GaAs heterostructure-emitter bipolar transistor (δ-HEBT) grown by low-pressure metalorganic chemical vapor deposition. Moreover, an HEBT without δ-doping design in the emitter and with spacer layers of variousthicknesses is presented for comparison. The common-emitter current gain and offset voltage are 50 and 130 mV, respectively, for the HEBTwithout δ-doping layer. The offset voltage of the δ-doped HEBT is as low as 70 mV and its common-emitter current gain is increased to 55,because of the use of the optimum spacer thickness of 100 Å and the design of the δ-doped emitter layer.© 2006 Elsevier B.V. All rights reserved.

Keywords: InGaP; HEBT; MOCVD; δ-doping; Offset voltage

1. Introduction

Advances in epitaxy and fabrication approaches have led tothe production of various compound semiconductor devices,such as real space transfer devices [1–3], heterostructure field-effect transistors (HFETs) [4–6] and heterojunction bipolartransistors (HBTs) [7–9]. Among these heterostructure devices,InGaP/GaAs HBTs have attracted considerable interest becauseof their potential microwave circuit applications based on theirhigh speed and high current-handling capabilities [8]. The otherstrengths of InGaP/GaAs HBTs over AlGaAs/GaAs HBTs arethat the former have a large valence-band discontinuity (ΔEV)at the emitter–base (E–B) heterojunction and extremely highlyselective etching between InGaP and GaAs. Although reportedvalues of conduction-band discontinuity (ΔEC) for theIn0.5Ga0.5P/GaAs heterojunction vary between 0.03 and0.39 eV, ΔEC∼0.2 eV (and hence ΔEV∼0.3 eV) is typical[10–15]. Thus, a potential spike at the E–B junction of theInGaP/GaAs single heterojunction bipolar transistor isexpected, producing a collector-emitter offset voltage. Accord-ingly, heterostructure-emitter bipolar transistors (HEBTs) have

⁎ Corresponding author. Tel.: +886 3 8634218; fax: +886 3 8634200.E-mail address: yslindh@mail.ndhu.edu.tw (Y.S. Lin).

0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2006.09.022

been reported to overcome the above shortcomings [16–19].The heterostructure-emitter structure allows effective electroninjection through the E–B homojunction and blocks the back-injection of holes from the base to the emitter. Moreover, thepresence of the effective E–B homojunction is expected toeliminate the potential spike. Thus, the undesired offset voltagecan be reduced and a high current gain maintained. Moreover,δ-doping technique has been recently applied to semiconductorquantum structures to improve the performance of devices [20–23]. When the impurities are ionized, the charged dopants createa V-shaped potential well at the δ-doped layer. This studyreports an improved δ-doped emitter In0.5Ga0.5P/GaAs HEBTwith 100Å-thick undoped GaAs spacer layers on both sides ofthe base (represented by δ-HEBT). The improved In0.5Ga0.5P/GaAs δ-HEBT has a common-emitter current gain of 55 and anoffset voltage of as low as 70 mV. Moreover, an HEBTwithoutδ-doped layers and with 300Å-thick undoped GaAs spacerlayers (represented by HEBT) is also fabricated and compared.

2. MOCVD growth and device fabrication

This developed δ-HEBTwas grown bymetalorganic chemicalvapor deposition (MOCVD) on a GaAs substrate. The epitaxiallayers comprised a 0.3 μm n+-GaAs (n=5×1018 cm−3)

Table 1Device layer structure of HEBT

Layer Material Thickness (Å) Type Doping (cm−3)

Emitter cap GaAs 3000 n+ 5×1018

Emitter confinement In0.5Ga0.5P 1000 n 7×1017

Emitter GaAs 300 n 5×1017

Spacer GaAs 300 Undoped –Base GaAs 1000 p+ 5×1018

Spacer GaAs 300 Undoped –Collector GaAs 5000 n− 5×1016

Subcollector GaAs 3000 n+ 5×1018

Fig. 1. (A) Common-emitter I–V characteristics of HEBT. (B) Common-emitterI–V characteristics of δ-HEBT.

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subcollector layer, followed by a 0.5 μm n−-GaAs(n=5×1016 cm−3) collector layer, a 100 Å undoped GaAs spacerlayer, a 0.1 μm p+-GaAs (p=5×1018 cm−3) base layer, a 100 Åundoped GaAs spacer layer, a 100 Å n-GaAs (n=5×1016 cm−3)layer, a δ-doped layer (δ1), a 200 Å n-GaAs (n=5×1016 cm−3)layer, a δ-doped layer (δ2), a 100 Å n-GaAs (n=5×1016 cm−3)layer, a 0.1 μm n-In0.5Ga0.5P (n=7×1017 cm−3) emitterconfinement layer and, finally, a 0.3 μm n+-GaAs(n=5×1018 cm−3) cap layer. δ1=δ2=3×1012 cm−2. Tables 1and 2 describe the layer structures of HEBT and δ-HEBT,respectively. The temperature and pressure for growing the twoproposed devices were 690 °C and 150 Torr, respectively.Trimethylgallium (TMG), trimethylindium (TMI), arsine (AsH3)and phosphine (PH3) were used as the growth precursors.

Device fabrication was performed using photolithographyand wet etching. The InGaP confinement layers were etchedwith 5:3:100 NH4OH:H2O2:H2O. GaAs was etched withsolutions of 4:1 H3PO4:HCl. AuGe/Ag and AuZn were usedto construct the n- and p-type ohmic contacts, respectively.Current–voltage (I–V) measurements were made on both thedevices using an HP-4156 semiconductor parameter analyzer.

3. Results and discussion

Fig. 1(A) plots the common-emitter I–V characteristics ofHEBT at 300 K. The current gain of the HEBT is 50. The offsetvoltage of HEBT is 130 mV. Fig. 1(B) plots the common-emitter I–V characteristics of δ-HEBT at 300 K. δ-HEBT ex-hibits a current gain of the 55 and an offset voltage of as low as70 mV. These values of δ-HEBT are better than those of the

Table 2Device layer structure of δ-HEBT

Layer Material Thickness (Å) Type Doping

Emitter cap GaAs 3000 n+ 5×1018 cm–3

Emitter confinement In0.5Ga0.5P 1000 n 7×1017 cm–3

Emitter GaAs 100 n 5×1016 cm–3

Emitter GaAs – δ(n+) 3×1012 cm–2

Emitter GaAs 200 n− 5×1016 cm–3

Emitter GaAs – δ(n+) 3×1012 cm–2

Emitter GaAs 100 n− 5×1016 cm–3

Spacer GaAs 100 Undoped –Base GaAs 1000 p+ 5×1018 cm–3

Spacer GaAs 100 Undoped –Collector GaAs 5000 n− 5×1016 cm–3

Subcollector GaAs 3000 n+ 5×1018 cm–3

proposed HEBT, because the optimization of the spacer thick-ness, 100 Å, and the δ-doped layers in the δ-HEBT. Fig. 2 is anenlargement of Fig. 1(B) near the origin. A low offset voltage of70 mV is clearly observed. This is smaller than those of thepreviously developed AlGaAs/GaAs and InGaP/GaAs HEBTs[24] and those of the InGaP/GaAs HBTs with nongraded and

Fig. 2. Expanded view at the beginning of Fig. 1(B), showing the collector-emitter offset voltage in common-emitter I–V characteristics of δ-HEBT.

Fig. 3. Common-emitter I–V characteristics of δ-HEBT operated in the lowcollector current region.

Fig. 5. I–V characteristics of B–C junction in δ-HEBT.

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graded base doping [7]. Fig. 3 plots the common-emitter I–Vcharacteristics of δ-HEBT at low collector currents. Theyindicate that δ-HEBT has a useful gain, even in the μA region.Hence, the base bulk current dominates the base current.

Fig. 4 shows the semilog-forward bias characteristics for theE–B junction of δ-HEBT. The current varies as exp (qV /nkT)for several decades, where q is the electron charge, V is theapplied voltage, k is the Boltzmann constant, T is the absolutetemperature, and n is an ideality factor. Fig. 5 plots the I–Vcharacteristics of the base–collector (B–C) junction of δ-HEBT.Notably, the breakdown voltage exceeds 16 V. Fig. 6 plots thesemilog-forward bias characteristics of the B–C junction in δ-HEBT. The value of the ideality factor n is calculated from theslope of the semilog-forward bias curve. In the lower currentregime, the ideality factor is around 1.52, indicating that the1 kT diffusion current and the 2 kT recombination current

Fig. 4. Semilogarithmic plot of the forward I–V characteristics of the E–Bjunction in δ-HEBT.

dominate the transport of carriers. Furthermore, the compositionof the collector and the base currents are analyzed from theGummel plot. The transferred collector current and base currentare functions of the base-emitter voltage when the base-collector bias of δ-HEBT is maintained at 0 V. The idealityfactor of the collector current is 1.03, indicating that therecombination current in δ-HEBT is greatly reduced. Thejunction ideality factor can be determined from the properties ofthe device as follows [12,25].

g ¼ 1þ e1Nd

e2Nað1Þ

where ε1 and ε2 are the dielectric constants of n-InGaP and p-GaAs, respectively, and Nd and Na are the respective dopingdensities. Eq. (1) shows that, at an estimated doping ratio of∼1:10, η is 1.09. The calculated value is consistent with theexperimental value.

Fig. 6. Semilogarithmic plot of the forward I–V characteristics of the B–Cjunction in δ-HEBT.

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4. Conclusion

This paper develops an improved InGaP/GaAs HEBT withδ-doping layers grown by a MOCVD system. The InGaP/GaAsHEBTwithout the δ-doping layers exhibits a current gain of 50and an offset voltage of 130 mV. Optimizing the thickness of theundoped-GaAs spacer layer adjacent to the base can effectivelyreduce the recombination current at the p–n interface. δ-HEBThas the advantages of a low offset voltage (70 mV), an im-proved current gain of 55 and ease of fabrication (by the high-selectivity etching process). Consequently, the improved devicehas considerable potential for use in practical circuit.

Acknowledgment

The author would like to thank the National Science Councilof the Republic of China for financially supporting this researchunder Contract No. NSC 95-2221-E-259-037.

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