Paul Kurpas

Reliability issues of GaN based high voltage power devices

Abstract GaN based power devices for high efficiency switching applications in modern power electronics are rapidly moving into the focus of world wide research and development activities. Due to their unique material properties GaN power devices are distinguished by featuring high breakdown voltages, low on-state resistances and fast switching properties at the same time. Finally, these properties are the consequences of extremely high field and current densities that are possible per unit device volume or area. Therefore, in order to obtain very high performance, the material itself is stressed significantly during standard device operation and any imperfection may lead to wear out and reliability problems. Thus material quality, the specific epitaxial design as well as the device topology will directly influence device performance, reliability and mode of degradation. The paper will mainly discuss those degradation mechanisms that are especially due to the specific material combinations used in GaN based high voltage device technology such as epitaxial layer design, chip metallization, passivation schemes and general device topology and layout. It will then discuss technological ways towards engineering reliability into these devices. Generally, device designs are required that effectively minimize high field regions in the internal device or shift them towards less critical locations. Furthermore, an optimized thermal design in combination with suitable chip mounting technologies is required to enable maximum device performance.

A 2.4 GHz GaAs-HBT Class-E MMIC Amplifier with 65% PAE

A class-E amplifier in the 2 GHz band is presented. It is realized as a coplanar MMIC using a high-voltage GaAs-HBT process. At 37 dBm output power, a high PAE of 65% with 71% collector efficiency are achieved. The gain of the amplifier in the switch-mode region reaches 11 dB. These are very competitive values for PAE, collector efficiency, and output power and the highest ones using GaAs-HBT technology. The measured data is supported by in-depth circuit simulation results highlighting the special conditions and requirements of switch-mode operation.

A flexible GaN MMIC enabling digital power amplifiers for the future wireless infrastructure

This paper presents a GaN power-switch MMIC and demonstrates its potential and its versatility in realizing power amplifier (PA) modules for future LTE base station transmitters with an increased digital content. The MMIC provides a compact high-gain broadband voltage-mode PA. With a TTL-level input voltage swing of 0.4 Vpp it reaches a large-signal gain of up to 40 dB. The PA can be used as a building block for various class-S and related applications. As examples, a single-chip and an H-bridge PA module for the 800 MHz band are reported as well as a digital Doherty PA.

Critical issues of growth optimization for Ga0.5In0.5P/GaAs heterojunction bipolar transistors

This work reports on optimization of MOVPE growth procedures for high-quality, highly uniform (4 inch) and highly reliable Ga0.5In0.5P/GaAs-HBTs in a commercial multiwafer reactor. Appropriately to this application the main focus of research have been improvements in base-layer transport properties and their implications for HBT device characteristics and reliability. Different carbon-doping techniques are compared with regard to GaAs:C material quality and GaInP/GaAs-HBT device performance. Base-doping homogeneity and hydrogen incorporation were further aspects of optimization. The obtained differences in HBT device performance and reliability depending on base-growth conditions confirm the importance of optimization of this particular step in the growth process.

Degradation properties of MOVPE-grown GaInP/GaAs HBTs under combined temperature and current stressing

Abstract Reliability tests at different ambient temperatures and biasing conditions have been performed on GaInP/GaAs heterojunction bipolar transistors. The epitaxial layer structure of the transistors has been grown by MOVPE, device processing utilized a triple mesa approach to access the base and collector regions and to perform interdevice isolation. A GaInP ledge structure suppresses excessive surface recombination currents. Reliability tests demonstrated a characteristic degradation behaviour of the transistors. It consists of a burn-in phase at the beginning of lifetime, followed by a rather stable phase with only minor degradation and a rapid decay of performance at the end of lifetime. It could be shown that the course of degradation and burn-in with time strongly depends on the parameters of reliability testing. Furthermore there is an indication that reliability also depends on crystal growth technology of the GaAs substrates. A device lifetime (mean time to failure, MTTF) of up to typically 3×107 h could be determined for a collector current density of 1×10 5 A /cm2 and a junction temperature of 125°C.

Investigation of short-term current gain stability of GaInP/GaAs-HBTs grown by MOVPE

Device performance and operation stability of GaInP/GaAs-based Heterojunction Bipolar Transistors (HBTs) are important factors for an increasing market share among the competing RF technologies. MOVPE growth of GaInP/GaAs HBT device structures with a carbon-doped base layer has been proven to ensure excellent transistor performance based on optimized base material quality This work deals with changes in DC-current gain of MOVPE-grown GaInP/GaAs HBTs during the first hours of operation under high current density stress. The influence of base-dopant concentration and hydrogen passivation on initial gain reduction were in the focus of research.

Current gain collapse in HBTs analysed by transient interferometric mapping method

Thermal distribution during a current gain collapse event is investigated in multi-finger InGaP/GaAs HBTs using the transient interferometric mapping method. The onset of the collapse is observed at time of about 1ms in devices with a low emitter ballasting resistance RE, while for HBTs with a high RE, the current is distributed equally over the fingers. 3D thermal simulation supports the results and allows an estimation of temperature at which the collapse occurs.

Investigation of breakdown and DC behavior in HBTs with (Al,Ga)As collector layer

We report on the realization of an InGaP-GaAs-based double heterojunction bipolar transistor with high breakdown voltages of up to 85 V using an Al/sub 0.2/Ga/sub 0.8/As collector. These results were achieved with devices with a 2.8 /spl mu/m collector doped to 6/spl times/10/sup 15/ cm/sup -3/ (with an emitter area of 60/spl times/60 /spl mu/m/sup 2/). They agree well with calculated data from a semi-analytical breakdown model. A /spl beta//R/sub SBI/ (intrinsic base sheet resistance) ratio of more than 0.5 by introducing a 150-nm-thick graded Al-content region at the base-collector heterojunction was achieved. This layer is needed to efficiently suppress current blocking, which is otherwise caused by the conduction band offset from GaAs to Al/sub 0.2/Ga/sub 0.8/As. The thickness of this region was determined by two-dimensional numerical device simulations that are in good agreement with the measured device properties.