Kenji Kurishima

Fabrication and characterization of high-performance InP/InGaAs double-heterojunction bipolar transistors

This paper is on high-performance InP/InGaAs double-heterojunction bipolar transistors (DHBTs) utilizing compositionally step-graded InGaAsP layers between the InGaAs base and InP collector to suppress the current blocking effect. These DHBTs exhibit current gains of 200 and excellent breakdown behavior. Moreover, the DHBTs permit collector current density levels J/sub C/ up to 3/spl times/10/sup 5/ A/cm/sup 2/ at V/sub CE/=1.5 V. A current gain cutoff frequency of 155 GHz and a maximum oscillation frequency of 90 GHz have been successfully obtained at J/sub C/=1.6/spl times/10/sup 5/ A/cm/sup 2/. We have also investigated electron transport properties in the InP collector using a set of DHBTs with different injection energies into the InP collector. By increasing the injection energies, electron velocity is found to decrease from 3.5/spl times/10/sup 7/ cm/s to 1.6/spl times/10/sup 7/ cm/s, due to increased population of upper valleys. This result clearly demonstrates the significant role of nonequilibrium /spl Gamma/-valley transport in determining the high-speed performance of InP/InGaAs DHBTs. >

Over 300 GHz f/sub T/ and f/sub max/ InP/InGaAs double heterojunction bipolar transistors with a thin pseudomorphic base

Describes 150-nm-thick collector InP-based double heterojunction bipolar transistors with two types of thin pseudomorphic bases for achieving high f/sub T/ and f/sub max/. The collector current blocking is suppressed by the compositionally step-graded collector structure even at J/sub C/ of over 1000 kA/cm/sup 2/ with practical breakdown characteristics. An HBT with a 20-nm-thick base achieves a record f/sub T/ of 351 GHz at high J/sub C/ of 667 kA/cm/sup 2/, and a 30-nm-base HBT achieves a high value of 329 GHz for both f/sub T/ and f/sub max/. An equivalent circuit analysis suggests that the extremely small carrier-transit-delay contributes to the ultrahigh f/sub T/.

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.

60-GHz bidirectional radio-on-fiber links based on InP-InGaAs HPT optoelectronic mixers

We demonstrate 60-GHz band bidirectional radio-on-fiber (RoF) links based on InP-InGaAs heterojunction phototransistor (HPT) optoelectronic mixers. They employ remote up/down conversion scheme with optical local oscillator signals distributed from the central office and intermediate frequency (IF) fiber transmission for both up- and down-links. Since frequency up/down conversions and photodetection are carried out by a single HPT optoelectronic mixer, base station architecture is greatly simplified. In order to validate its feasibility, both up- and down-link RoF transmissions of 16 quadrature amplitude modulator data are successfully demonstrated at 60-GHz band using 1.25-GHz IF for down-link and 2.0-GHz IF for up-link.

Improvement of current gain of C-doped GaAsSb-base heterojunction bipolar transistors by using an InAlP emitter

Large conduction band edge discontinuity (ΔEc) at the emitter/base interface in InP∕GaAs0.51Sb0.49∕InP heterojunction bipolar transistor (HBT) is one of the possible reasons that the recombination process in the emitter/base depletion region dominates the characteristics of this HBT. We fabricate an InAlP emitter∕GaAs0.51Sb0.49base∕InP collector HBT for reducing the ΔEc at the emitter/base interface. It is demonstrated that a HBT with an InAlP emitter shows a relatively lower recombination current than one with an InP emitter, resulting in the higher current gain. It is also found that the decrease of recombination current depends on the Al content of InAlP emitter. Additionally, the ideality factor of the emitter-base current is smallest at the Al content of 0.15 in the InAlP emitter. These results indicate that using an InAlP emitter is effective for improving the current gain of GaAsSb-base HBTs.

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.

A 24-Gsps 3-bit Nyquist ADC using InP HBTs for electronic dispersion compensation

A 3-bit flash analog-to-digital converter (ADC) for electronic dispersion compensation (EDC) was developed using InP HBTs. Nyquist operation was developed using InP HBT. Nyquist operation was confirmed up to 24 Gsps, which enables oversampling acquisition for 10 Gbits/s nonreturn-to-zero (NRZ) signals. The ADC can also be operated at up to 37 Gsps for low input frequencies. To reduce aperture jitter and achieve a wideband of over 7 GHz, an analog input signal for all latched comparators are provided as travelling waves through coplanar transmission lines.

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.