IEEE ELECTRON DEVICE LETTERS, VOL. 17, NO. 3, MARCH 1996 133

AlInAs/GaInAs/InP Double Heterojunction Bipolar Transistor with a Novel Base-Collector

Design for Power Applications Chanh Nguyen, Member, ZEEE, Takyiu Liu, Mary Chen, Hsiang-Chih Sun, and David Rensch

Abstract- We report the performance of an A1InAslGaInAsl InP DHBT with a new collector design. The base-collector junc- tion was formed with an all arsenide chirped superlattice with linear variation in the average composition. A doping dipole was inserted at the ends of the superlattice to cancel the quasielec- tric field. The conduction band offset between AlInAs and InP enabled hot electrons to be launched into the InP collector. The new design resulted in an excellent combination of speed and breakdown voltage with superior microwave power performance at X-band. Output power of 2 W (5.6 W/mm) with 70% power- added-efficiency at 9 GHz was achieved.

I. INTRODUCTION

HE speed advantages of InP/GaInAs and AlInAdGaInAs T compared to AlGaAdGaAs HBT’s are well known [ 11-[3]. However, InP-based SHBT’s with a GaInAs collector are not suitable for microwave power applications which require a high output power density. The high rates of impact ionization in GaInAs severely limit the range of collector voltage for safe operation. Therefore the majority of microwave power HBT’s today are AlGaAdGaAs and GaInP/GaAs.

Previous work in our laboratory indicated that A1I- nAs/GaInAs/InP DHBT’s, in which the collector is made of InP, are very promising for high-performance power amplifiers due to the low impact ionization rate, high peak and saturated velocities, and high thermal conductivity of InP [4]. The conduction band edge discontinuity at the base-collector heterointerface, however, often results in a current-blocking potential which increases the saturation voltage and causes gain compression to occur at a lower current density [ 5 ] . This is a major problem for all DHBT’s and have to be eliminated in order to exploit the advantages of the wide-gap collector. Secondly, since the thickness of the collector of a X-band power HBT is on the order of one micrometer to withstand collector voltages larger than 20 V, the speed of the device is generally limited by the transit time of electrons through the collector, and the high-frequency performance of the device can be noticeably enhanced by reducing the collector transit time.

In this letter, we report the performance of a novel transistor design for of AlInAs/GaInAs/InP DHBT’s in which the base-

Manuscript received September 21, 1995; revised November 7, 1995. The authors are with Hughes Research Laboratories, Malibu, CA 90265

Publisher Item Identifier S 0741-3106(96)01972-6. USA.

collector junction is band-gap engineered to (i) completely eliminate the current blocking barrier and (ii) allow hot elec- trons to be injected into the collector to enhance the speed of the device. The combination of high breakdown voltage and high speed has resulted in a record power performance for HBT’s at 9 GHz.

We have proposed a practical base-collector design for an npn DHBT which involves a linear bandgap variation (which requires an essentially linear variation in composition) and two delta doping layers of the same concentration at the ends of the grade, acceptor at the base end and donor at the collector end, to form a dipole [6]. The quasielectric field created by the linear bandgap variation is canceled by the electrostatic field arising from the ionized impurities of the doping sheets. As a result, the entire bandgap difference is transferred into the valence band, and the conduction band becomes barrier-free similar to that of a SHBT as long as the delta doping sheets are depleted. Therefore, the need for a spacer layer, which would increase the transit time, is also eliminated. Clearly, the donor delta doping sheet can also be replaced by a thin, heavily doped layer such that the integrated impurity concentration per unit area is maintained, and the heavily doped base autornatically provides the acceptor sheet for the charge dipole. Thus, the linear-delta design offers much tolerance in the growth of the DHBT structure. Recently, Liu and Pan [7] also proposed and analyzed a similar base-collector structure for GaAdAlGaAs DHBT.

Extending the concept of a linear-delta design, we can use a composite collector involving two wide-bandgap semicon- ductors to launch hot electrons into the collector as shown in Fig. 1. For example, in the present work, the base collector junction consists of a linear-delta region for GaInAs and AlInAs (both lattice matched to InP) to remove the AE, between the GaInAs base and the AlInAs launcher, followed by a 1.0 pm thick InP layer. Hence, the energetic distribution of electrons entering the InP collector is shifted upward by the AE, between AlInAs and InP (250 meV). By using a quaternary A10 48-,GaZIno.52As alloy, we can also vary the kinetic energy of hot electrons since AE, can be adjusted continuously from 0 (x = 0.24) to 250 meV (x = 0). The design therefore offers much flexibility in the optimization of DHBT structures for different applications.

The device structure consisted of an n+ 700 nm GaInAs subcollector, a composite collector involving a 1.0 pm thick InP layer doped at n = 1.5 x 10l6 cm3 and an AlInAdGaInAs

0741-3106/96$05.00 0 1996 IEEE

NGUYEN et al.: DOUBLE HETEROJUNCTION BIPOLAR TRANSISTOR 135

Large-signal performance of multifinger cells was measured on unthinned wafers at 9 GHz with an active load-pull system. The base-collector breakdown voltage was 38 V and the emitter collector breakdown voltage was approximately 32 V with the base terminal open. Fig. 2 shows the Gummel plot of a typical single-finger device. The ideality factors of the base and collector currents from the Gummel plot are 1.1 and 1.0 respectively. The DC current gain was approximately 20 for a large range of collector current densities from below 1 A/cm2 to above 6 x lo4 A/cm2, at which point the gain began to compress due to the onset of the Kirk effect. As the ionized donors in the base-collector junction became neutralized, base-pushout caused the negative charge required to form a dipole with the donor sheet in the collector to become naturalized. Consequently, a current blocking barrier was formed, exacerbating the Kirk effect, and essentially prevented further increase in collector current. Below this current limit, however, the device behaved as if it were a SHBT due to the cancellation of the quasielectric field and doping dipole. Clearly, we could increase the limit of collector current by raising the collector doping level, at the price of reducing the breakdown voltage, similarly to the case of SHBT’s.

In contrast to SHBT’s, the CSL DHBT had a very small offset voltage. Since the LIE, between the base and collector was shifted to the valence band by the linear-delta design, hole injection from the base into the collector at low collector voltage in common-emitter configuration was suppressed. As a result, the device had an offset voltage of less than 50 mV as shown in Fig. 3, where the turn-on characteristics of the CSL DHBT and an AlInAdGaInAs SHBT are compared.

Microwave S-parameter measurements were performed on wafers from 1 to 30 GHz using a Wiltron 360 Network Analyzer. For single-finger devices (2 x 30 pm2) we obtained a peak f~ of 71 GHz at VCE = 1.5 V and J c = 5.2 x lo4 A/cm2. We believe this is the highest f~ achieved for a 1.0 pm thick collector and a breakdown voltage exceeding 35 V. The peak fmax of the device was 80 GHz, which, given the high f T , can be substantially increased by reducing the base-collector capacitance.

Multiple-finger power cells were fabricated on the same wafer to characterize the large-signal performance of devices with the linear-delta design. The power cell consisted of 12 (2 x 30 pm2) emitter fingers. The high breakdown voltage allowed us to bias the power cell at 14 V collector-emitter voltage for high output power. The high breakdown voltage is also particularly desirable for satellite and airborne applica- tions where the voltage on the system bus is of the order of

50 V or even higher. Fig. 4 shows the output power and power added efficiency of the power cell operating in class B and measured on-wafer at 9 GHz using an active load-pull system. The solid lines were obtained with a load tuned for high output power. The output power was 2.5 W, corresponding to a power density of 6.9 W/mm, at 61% power-added-efficiency (PAE). The associated gain was 7.6 dB. When the load was tuned for high PAE at 2 W output power (5.6 W/mm), the PAE is 70% with an associated gain of 10 dB as shown in Fig. 4. This excellent power performance of the power cell, which already surpassed the recently reported performance of GaAs-based HBT power cells [8]-[ 111, clearly demonstrated the advantage of the linear-delta InP-DHBT for power applications.

ACKNOWLEDGMENT

The authors would like to thank P. Greiling for his support and encouragement, P. MacDonald, R. Virk, and M. Hafizi for technical assistance, B. Hughes, and A. Cognata (HP Santa Rosa) for loadpull measurements.

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