Shoji Yamahata

Ultrahigh-speed InP/InGaAs DHPTs for OEMMICs

This paper presents an ultrahigh-speed InP/InGaAs double-heterostructure phototransistor (DHPT) with a record optical gain cutoff frequency of 82 GHz. This excellent performance originates from the double-heterostructures compatibility with high-performance double-heterostructure bipolar transistor (DHBT) and a new self-aligned process. To demonstrate the excellent performance of the DHPT, two kinds of optoelectronic MMICs (OEMMICs) were designed and fabricated. One is a 40-GHz-band DHPT/DHBT photoreceiver that shows the DHPTs ability to be simultaneously integrated with a high-performance DHBT. The 40 GHz operation frequency is also the highest reported for monolithically integrated HPT/HBT photoreceivers. The other is a direct optical injection-locked oscillator that can extract an electrical clock signal from optical data streams. The OEMMICs are promising for compact and low-power-consumption optical receivers on an InP platform for millimeter-wave photonics and ultrahigh-speed optical communication systems.

Effects of a Compositionally-Graded InxGa1-xAs Base in Abrupt-Emitter InP/InGaAs Heterojunction Bipolar Transistors

A compositionally-graded In x Ga 1−x As base is experimentally shown to improve electron transport properties in abrupt-emitter InP/InGaAs heterojunction bipolar transistors (HBTs). The built-in field in the base of 6 kV/cm enables a more than 50% improvement in current gains, compared to a uniform-base structure. The peak current-gain cutoff frequency f T for the graded-base HBT is 143 GHz versus 121 GHz for the uniform-base HBT. It is also shown that the graded-base structure is effective in suppressing the space charge in the vicinity of the base-collector junction. The built-in field in the base accelerates low-speed energy-relaxed electrons and thereby increases the velocity of electrons injected into the collector. The minimized base widening effect, combined with low base resistance, yields a maximum oscillation frequency f max over 200 GHz even at a collector bias voltage V CE as low as 1 V

Ultrahigh-Speed InP/InGaAs Double-Heterostructure Bipolar Transistors and Analyses of Their Operation

Novel hexagonal emitters are proposed for heterostructure bipolar transistors (HBTs) with a base-metal-overlaid emitter-base self-alignment structure to reduce parasitic effects. Two different layer structures for InP/InGaAs double-heterostructure bipolar transistors (DHBTs) that can more fully utilize the inherent potential of the materials are used to enhance unity current gain cutoff frequency, f T, and maximum oscillation frequency, fmax . On a wafer with a 180-nm-thick collector, a transistor with a 20-µ m2 hexagonal emitter electrode shows an f T of 230 GHz and an fmax of 147 GHz, while with a 4-µ m2 hexagonal emitter electrode the corresponding values are 225 GHz and 241 GHz. fmax of 300 GHz is achieved for a transistor with a 4-µ m2 emitter electrode and a 330-nm-thick collector. Transistor operation is analyzed using a simple but appropriate small-signal equivalent circuit model of a transistor that includes internal and external base/collector capacitances and yields good estimates of the measured scattering (s-) parameters. Even in these InP-based (D)HBTs, the internal collector capacitance increases with collector current density due to the Kirk effect which degrades performance. In thin-collector (D)HBTs, the increase in the internal collector capacitance is suppressed, which increases the collector current density at which the transistor can operate normally, and f T is increased by both transit time reduction and high-collector-current operation.

High-Speed and High-Reliability InP-Based HBTs With a Novel Emitter

This paper describes InP HBTs with a novel emitter simply consisting of a degenerately doped n+-InGaAs layer and an undoped InP thin layer. An n+-InP layer is not necessary because the quasi-Femi level in the n+-InGaAs layer is high enough to exceed the conduction band discontinuity between the n+ -InGaAs layer and the undoped InP layer. In the proposed structure, a thin ( ~ 10 nm) ledge structure can easily be fabricated by etching the n+-InGaAs layer. The fabricated HBTs with a 15-nm-thick ledge structure provide a high collector current density of over 6 mA/¿m2 . There is almost no degradation of current gain, although the emitter width is reduced to as small as 0.5 ¿m. The HBTs also exhibit an ft of 324 GHz at a collector current density of 5.5 mA/¿m2, which is comparable with that of HBTs with a conventional emitter consisting of an n+ -InGaAs layer, an n+-InP layer, and an n-InP layer. From the results of accelerated life tests, the activation energy of the degradation in HBTs is estimated to be around 1.8 eV, and the extrapolated mean time to failure is estimated to be over 108 h at a junction temperature of 125°C.

Performance and Stability of MOVPE-Grown Carbon-Doped InP/InGaAs HBT's Dehydrogenated by an Anneal after Emitter Mesa Formation

A high-temperature anneal of 500°C for 5 min after emitter mesa formation is effective in completely reversing the hydrogen passivation of carbon acceptors in InP/InGaAs heterostructure bipolar transistors. Fabricated devices show a base hole concentration as high as 5 ×1019 cm-3 and a maximum oscillation frequency above 200 GHz. However, this technique simultaneously causes damage to the emitter mesa surface and degrades current gain. In order to avoid such undesirable effects, one has to carefully optimize the anneal conditions and/or select the optimum crystallographic orientation of the emitter mesa so as to increase the thermal stability of InP sidewalls. Preliminary bias-temperature stress tests were also performed to examine the stability of base-emitter junctions. The results show that the stability strongly depends on the emitter mesa orientation. Promising results are obtained from devices whose emitter orientation is parallel to the Primary Flat of (100)-oriented InP substrates.

Initial degradation of base-emitter junction in carbon-doped InP/InGaAs HBTs under bias and temperature stress

Bias-temperature stress tests were performed to examine the stability of base-emitter junction characteristics of carbon-doped InP/InGaAs heterojunction biopolar transistors (HBTs). Two different kinds of degradation modes were observed from the Gummel I-V characteristics. One is characterized by the gradual increase in a nonideal base current. The generation of the nonideal current strongly depends on the crystallographic orientation of the emitter mesa. The other degradation mode was observed when a large current (200 kA/cm/sup 2/) was injected under a high ambient temperature (180/spl deg/C). This degradation is characterized by an initial decrease in turn-on voltage and significant drop in current gain.

Growth and Characterization of High-Speed Carbon-Doped-Base InP/InGaAs Heterojunction Bipolar Transistors by Metalorganic Chemical Vapor Deposition

The dependences of the hole concentration and hydrogenation ratio in C-doped InGaAs on growth parameters such as growth temperature, V/III ratio, and CBr4 flow rate are clarified. The hydrogenation ratio is shown to increase with increasing carbon concentration in InGaAs. Several annealing procedures in different ambients and with different cap layers are examined to establish a procedure to effectively re-activate hydrogenated carbons in InGaAs base of heterojunction bipolar transistor (HBT) structure. It is revealed that the re-hydrogenation of C acceptors at the growth of emitter layer determines the final hole concentration in the base layer. With an annealing in H2 with a growth interruption at the emitter/base interface, an as-grown hole concentration of 1.1×1019/cm3 is obtained in the device structure. An HBT with a 1.6×9.6-µ m2 emitter area exhibits f T and fmax values of 185 and 105 GHz.

High/f/sub max/ collector-up AlGaAs/GaAs heterojunction bipolar transistors with a heavily carbon-doped base fabricated using oxygen-ion implantation

AlGaAs/GaAs collector-up heterojunction bipolar transistors (HBTs) with a heavily carbon-doped base layer were fabricated using oxygen-ion implantation and zinc diffusion. The high resistivity of the oxygen-ion-implanted AlGaAs layer in the external emitter region effectively suppressed electron injection from the emitter, allowing collector current densities to reach values above 10/sup 5/ A/cm/sup 2/. For a transistor with a 2- mu m*10- mu m collector, f/sub T/ was 70 GHz and f/sub max/ was as high as 128 GHz. It was demonstrated by on-wafer measurements that the first power performance of collector-up HBTs resulted in a maximum power-added efficiency of as high as 63.4% at 3 GHz.<<ETX>>

InP/InGaAs double-heterojunction bipolar transistors for high-speed optical receivers

We fabricated monolithically integrated pin/HBT photoreceivers using FPIGA (full-potential InGaAs) DHBTs with various collector thicknesses. An HBT figure-of-merit was deduced from the relationship between measured bandwidths of the preamplifiers and the f/sub T/s and f/sub max/s of the DHBTs. A phenomenological device model of the DHBTs is proposed to find the optimum collector thickness that gives the highest bandwidth of the photoreceivers. Finally, we discuss the feasibility of monolithically integrating a pin-PD, preamplifier, buffer amplifier, and D-type flip-flop with an operating speed of 40 Gbit/s.

Enhancement of f/sub max/ in InP/InGaAs HBTs by selective MOCVD growth of heavily-doped extrinsic base regions

InP/InGaAs heterojunction bipolar transistors (HBTs) with selectively grown heavily-doped extrinsic base layers have been fabricated. A new selective metalorganic chemical vapor deposition (MOCVD) method using a very high-speed rotating susceptor, which can attain high selectivity even at low growth temperature, is employed for the extrinsic-base regrowth. The maximum f/sub max/ of the HBT with the selectively grown extrinsic-base layer is 141 GHz, which is more than 50% larger than that of a HBT without the selective growth. The base resistances are estimated by a small-signal equivalent-circuit analysis and transmission line model measurements, and we find that the resistance is reduced to be about a half by the selective regrowth. This significant reduction is achieved by the decrease of base contact resistance as well as the low regrowth-interface resistance. We also discuss Zn redistribution during the extrinsic base regrowth.