Yoshino K. Fukai

Highly Reliable Submicron InP-Based HBTs with over 300-GHz ft

We have developed 0.5-μm-emitter InP-based HBTs with high reliability. The HBTs incorporate a passivation ledge structure and tungsten-based emitter metal. A fabricated HBT exhibits high collector current density and a current gain of 58 at a collector current density of 4mA/μm2. The results of dc measurements indicate that the ledge layer sufficiently suppresses the recombination current at the emitter-base periphery. The HBT also exhibits an ft of 321GHz and an fmax of 301GHz at a collector current density of 4mA/μm2. The ft does not degrade even though the emitter size is reduced to as small as 0.5μm×2μm. The results of an accelerated life test show that the degradation of dc current gain is due to thermal degradation of the interfacial quality of semiconductors at the passivation ledge. The activation energy is expected to be around 1.5eV, and the extrapolated mean time to failure is expected to be over 108 hours at a junction temperature of 125°C. These results indicate that this InP HBT technology is promising for making over-100-Gbit/s ICs with high reliability.

Electron Velocity Overshoot Effect in Collector Depletion Layers of InP/InGaAs Heterojunction Bipolar Transistors

InP/InGaAs heterojunction bipolar transistors with a 550-A-thick base and 3000-A-thick collector layers are fabricated to study the electron velocity overshoot effect in collector depletion layers. The intrinsic delay time evaluated at room temperature measurement remains almost constant at 0.7-0.8 ps in the investigated base/collector bias range of 0 to 1.1 V at a collector current density of 1×105 A/cm2. According to Monte Carlo simulation results, such bias-independent behavior is associated with the sufficiently large overshoot effect caused at high collector biases, where the overshoot velocity is enhanced by the high electric field.

Measuring the junction temperature of AlGaAs/GaAs heterojunction bipolar transistors using electroluminescence

Rises in the junction temperatures of heterojunction bipolar transistors (HBTs) due to self‐heating effects during transistor operation are measured using the electroluminescence of the band‐to‐band recombination. This method is useful for the direct junction temperature monitoring of small geometry devices. Junction temperatures measured in AlGaAs/GaAs HBTs with five 2×20 μm2 emitter fingers are raised 115 °C when the product of the collector current and the emitter‐collector voltage is 0.25 W. The thermal resistance is determined to be 260 °C/W at 300 K.

Emitter layer design for high-speed InP HBTs with high reliability

We investigated the influence of emitter doping level on the performance for high-speed InP HBTs with high reliability. The HBTs show high current gain and excellent reliability characteristics under stress current density of 5 mA/mum2.

Highly reliable InP-based HBTs with a ledge structure operating at current density over 2 mA//spl mu/m/sup 2/

We have developed highly reliable InP-based HBTs with a ledge structure. The lifetime of discrete transistors, which is independent of stress current density over 2mA//spl mu/m/sup 2/, is predicted to be 1/spl times/10/sup 7/ h at 125/spl deg/C.

Reliability study on InP/InGaAs emitter-base junction for high-speed and low-power InP HBT

The reliability of sub-micrometers InP-based heterostructure bipolar transistors (HBTs), which are being applied in over-100-Gbit/s ICs, was examined at high current injection conditions. These HBTs had a ledge structure and an emitter electrode consisting with a refractory metal of W, which suppressed surface degradation and metal diffusion, respectively. We conducted bias-temperature (BT) stress tests in several stress conditions of current densities, Jc, up to 10 mA/μιη2 in order to investigate the stability of InP/InGaAs emitter-base (E-B) junction. At 10 mA/μιη2 operation with the junction temperature of 210 °C, dc current gain, ß, was stable for 1000 h. The activation energy for the reduction of β, however, decreased to 1.1 eV, which is suggesting the degradation of the emitter-base (E-B) junction. For the reliability of sub-micrometer, high-speed and low-power InP HBTs at high current densities, stability around the E-B junction has become more dominant.

Hot-Carrier-Related Increase in Drain Resistance and Its Suppression by Reducing Contaminants in InP-Based HEMTs

We examined the issue of reliability of InP-based high-electron mobility transistors (HEMTs), focusing on the increase of drain resistance Rd. In investigations of the mechanism of Rd increase, we took note of contaminant incorporation and of the relations between the device lifetime and the strength of the channel electric field. In the fabrication process, reducing contaminants, especially fluorine, significantly suppressed the increase of source and drain resistances. Cross-sectional views of the gate of improved devices, which had a long lifetime, confirmed an almost contaminant-free surface around the gate. In acceleration tests, the most negative impact on drain resistance stability among several bias conditions was found when the current density was high and the channel electric field was large at the same time. The dependence of drain-gate electric field strength E showed that the device lifetimes of HEMTs determined from Rd increase obeyed exp(1/E), which means that impact ionization was the main cause of degradation. We elucidate that the interactions of hot carriers with contaminants around the gate are the main causes of the Rd increase in HEMTs. Suppression of device degradation was achieved by optimizing the fabrication process around the gate. In this way, device lifetime was remarkably enhanced.

InP HEMT IC technology for 40 Gbit/s and beyond

InP-based HEMT integrated circuit (IC) technology has contributed much to the research and development of highspeed optical communications systems and microwave/millimeter-wave wireless systems. This paper describes recent progress in our InP HEMT IC technology for 40 Gbit/s optical communications systems and discusses future prospects for 100 Gbit/s and beyond.

Electroluminescence measurement of ballistic electron distribution in AlGaAs/GaAs heterojunction bipolar transistor structures with base width modulation

Abstract Electroluminescence from the base region of Npn-AlGaAs/GaAs heterojunction bipolar transistors (HBTs) are analyzed to investigate the behavior of electrically injected ballistic electrons in hetero-structure. The low temperature luminescence reveals ballistic electron spectra, subsequent optical-phonon emissions and quasi-equilibrium hot electron spectra, which are the first observations in HBT structures. Modulation of the neutral base width by varying the base-collector voltage enables the evaluation of the spatial electron distribution. The ballistic mean free path is determined to be 130 ± 20 nm at 18 K for an acceptor concentration of 2.5 × 1017 cm-3.

Electroluminescence of Ballistic and Phonon Emitting Electrons in the p-Type Base of AlGaAs/GaAs HBT Structures

We investigate the transport behavior of high-energy ballistic electrons in the p-type region using Npn-Al x Ga 1−x As/GaAs heterojunction bipolar transistors (HBT) with an abrupt emitter-base heterojunction. The observed electroluminescence spectra from the base region show several high-energy peaks that are associated with ballistic electrons and with LO-phonon emissions. By varying the emitter-collector voltage, the neutral base width can be modulated, leading to a change in spatial electron distributions and thus determining the mean free paths of ballistic electrons