We report here on the electrical and structural characteristics of InGaP/GaInAsN DHBTs with up to a 50 mV reduction in turn-on voltage relative to standard InGaP/GaAs HBTs. High p-type doping levels ( 3 1019 cm-3) and dc current gain (βmax up to 100) are achieved in GaInAsN base layer structures ranging in base sheet resistance between 250 and 750 Ω/. The separate effects of a base-emitter conduction band spike and base layer energy-gap on turn-on voltage are ascertained by comparing the collector current characteristics of several different GaAs-based bipolar transistors. Photoluminescence measurements are made on the InGaP/GaInAsN DHBTs to confirm the base layer energy gap, and double crystal x-ray diffraction spectrums are used to assess strain levels in the GaInAsN base layer.

The long-term reliability of heterojunction bipolar transistor (HBT) continues to be a subject of great interest due to the increased acceptance of this device in a wide range of applications. The most demanding requirements for long-term reliability include high performance microwave instrumentation, X-band radar, and lightwave communication (OC-192). A significant leap in the long-term reliability performance was observed in HBT as the AlGaAs emitter material was replaced with lattice matched InGaP. A dramatic improvement in the long-term reliability was also observed in AlGaAs emitter HBT's as the turn on voltage (Vbe) was lowered. The typical failure mechanism in HBT devices at high current density and high temperature long-term reliability testing was a dramatic increase in the base current at low current densities. One of the limiting factors in obtaining MTTF in InGaP HBT was the long time required to promote failures in the HBT device. Furthermore, a large sample size is necessary to extract a reliable MTTF. Significant increases in the current density as high as 180 kA/cm2 during reliability testing was used to promote failures in order to obtain an MTTF within a reasonable amount of time. The MTTF at a junction temperature of 334C and at a current density of 180 kA/cm2 was 1159 hours. The extrapolated MTTF at a junction temperature of 150C exceeded 106 hours for all of the tested devices. An attempt to predict the MTTF of AlGaAs and InGaP HBT using a simple model based upon the fitting of the initial Gummel plots of large area devices was made. The model was based upon the estimation of the trap defect density at the base/emitter junction, the hole injection component of the base current, and the turn-on Vbe. Degradation of the HBT was assumed to occur at the base/emitter junction and this corresponded to an increase in the trap density at this heterojunction. A factor of 5 improvement in the MTTF of the reliability of AlGaAs HBT with a lower turn on voltage was estimated based upon the above model, which confirmed the experimental results. These results suggested that the emitter material is primarily responsible in determining the long-term reliability characteristics of HBT. The combination of a high effective hole barrier and a low turn-on Vbe are highly desirable for long-term reliability characteristics.

Photoelectric techniques, such as photoluminescence are commonly used to evaluate and qualify heterostructure materials. These studies provide invaluable information on the energy configuration of these devices. In this paper, we extend photoelectric techniques to the evaluation of fully fabricated HBTs. We describe photoconduction measurements performed on the base/collector junctions in GaAs based HBTs. The devices studied contained a window in the emitter metal and monochromatic, chopped light was focused through a microscope into the window. The measurements are performed on wafer at room temperature. The spectral characteristic of the photocurrent provides information on the material and allows the determination of the source of the measured photocurrent. The dependence of the photocurrent on the junction bias allows the profiling of the junction. Three different collector structures were investigated, containing GaAs, AlGaAs, and InGaP. The effects of electron and hole barriers are evaluated. The information obtained allows for the design of improved HBTs.

Heterojunction bipolar transistors (HBTs) are key devices for a variety of applications including L-band power amplifiers, high speed A/D converters, broadband amplifiers, laser drivers, and low phase noise oscillators. AlGaAs emitter HBTs have demonstrated sufficient reliability for L-band mobile phone applications. For applications which require extended reliability performance at high junction temperatures (>250) and large current densities (>50 kA/cm2), InGaP emitter HBTs are the preferred devices. The excellent reliability of InGaP/GaAs HBTs has been confirmed at various laboratories. At a moderate current density and junction temperature, Jc = 25 kA/cm2 and Tj = 264, no device failures were reported out to 10,000 hours in a sample of 10 devices. Reliability testing performed up to a junction temperature of 360 and at a higher current density (Jc = 60 kA/cm2) showed an extrapolated MTTF of 5 105 hours at Tj = 150. The activation energy for AlGaAs/GaAs HBTs was 0.57 eV, while the activation energy for InGaP/GaAs HBTs was 0.68 eV, which indicated a similar failure mechanism for both devices.