Using InGaP emitters in GaAs heterojunction bipolar transistors not only offers advantages in terms of cost and performance it makes for excellent reliability too. Barry Lin and Larry Wang summarize the findings of EiC's latest research studies.

In recent years InGaP has begun to replace AlGaAs as the material of choice for the emitter layer in GaAs HBTs, affording superior device performance and providing a highly suitable process for power and high-frequency ICs in wireless and optoelectronic applications. One of the main reasons stems from the reliability and processing issues that have plagued AlGaAs devices. This article looks at the evolution of InGaP HBTs and discusses new reliability data obtained for EiC s new InGaP HBT broadband MMIC amplifiers. The HBT advantage HBTs offer a number of advantages compared with existing technologies (see ). HBT MMIC power amplifiers offer a combination of high frequency of operation, high linearity, excellent power-added efficiency and a high breakdown voltage for power RF applications. The device also features a near-zero leakage current in the off-state, which has allowed handset manufacturers to simplify module design by eliminating the negative voltage supply. The new generation of InGaP HBT emitter structures has also given design engineers further advantages, such as the combination of a highly reproducible manufacturing process, tighter DC and RF parameter distributions, and smaller die. These factors have led to cheaper MMIC power amplifiers and modules. Early HBTs were based on the well-known AlGaAs structure. These devices employed MBE to achieve the beryllium doping required to form the p-doped base region. This technology met the enhanced performance and cost goals of the industry, but created two problems. The first is that the beryllium dopant, being a very small atom, is unstable and diffuses relatively rapidly. At elevated junction temperatures and higher operating current densities, this phenomenon is accelerated resulting in DC current gain (beta) degradation. The second issue is that the base access surface near the emitter-base junction of the AlGaAs HBT is relatively unstable. This requires a careful surface passivation technique (ledge passivation) to reduce surface recombination effects. This surface effect leads to further beta degradation due to the increase in base current. Solving the problems The former problem was overcome by substituting the base dopant, beryllium, with the larger and more stable carbon atom. The high effusion cell temperatures required meant a switch in growth technique to MOCVD. While MOCVD did help, a problem existed with beta degradation at the HBT base access region due to the surface effect at the AlGaAs HBT. In addition, a higher current density across the emitter junction further aggravated the current-gain phenomenon. Applications requiring continuous current flow or high-temperature environments including wireless and optical components and CATV infrastructure equipment are particularly vulnerable. Fortunately, replacing AlGaAs with InGaP to form the HBT emitter has solved these problems, and the InGaP emitter structure has been shown to offer a robust solution to the reliability issues of AlGaAs structures. Indeed, InGaP HBTs have been successfully used in the optical communications industry since the early 1990s. MBE-grown AlGaAs HBTs gave a good indication of potential reliability problems that exist as a result of a high junction temperature in certain applications. As a result, a decision to manufacture InGaP HBT material by MOCVD paying special attention to circuit design as it relates to the emitter finger junction temperature has paid dividends. Reliability data shows an example of long-term reliability data (mean-time-to-failure or MTTF) collected for InGaP HBTs. The data are obtained from devices with a measured ambient temperature of 250 C and a junction temperature of 315 C. The devices operated at 2.5 V and a current density of 50 kA/cm2. For testing, each chip was mounted on a ceramic package, and the current density and junction temperature conditions documented for accelerated life testing. Several different conditions were also chosen. These included higher (5 V) voltage operation at 25 kA/cm2, and 75 kA/cm2 at a higher junction temperature of 330 C. To date, those devices with bias at 25 kA/cm2 and 50 kA/cm2, with a junction temperature of 315 C, have run under continuous operation for more than 6500 hours and most of them have not failed yet. Noticeable degradation has only occurred in the devices driven at the highest current density of 75 kA/cm2 and with the higher junction temperature of 330 C. Using a 20% degradation of current gain as the maxim for failure, the MTTF at this level of stress is approximately 3200 hours. The projected MTTF, which is shown in , is expected to be greater than 7.4 million hours for a junction temperature equal to 125 C and current density of 75 kA/cm2. This assumes a conservative value of activation energy of 0.8 eV. Contributing further to the robust performance of InGaP HBTs is the relatively cool junction temperature of the transistors under normal bias conditions. EiC has employed a careful thermal budget design in the device circuits that results in a relatively low temperature for the emitter finger junction. To demonstrate this, the company s EC-1019 broadband amplifier is shown in . This component supplies 18.5 dB gain from DC to 3 GHz and an IP3 of +34 dBm. The junction temperature profile of the surface of this device is measured with an infrared scanning microscope, and the temperature gradients are shown to be evenly distributed across the surface. The data also confirm the thermal design target as lower than 125 C. These results are highly encouraging and show the advantages of the combination of InGaP HBT technology and a careful thermal budget in MMIC circuit design. EiC s proprietary InGaP HBT design provides excellent reliability and is inherently superior to AlGaAs HBTs. Although handset PA applications do not require such stringent operating requirements as MMIC devices used for the infrastructure market, the high reliability of InGaP HBTs assures a high-quality MMIC suited to high-volume production.