Monolithically Integrated GaInP/GaAs High-Voltage HBTs and Fast Power Schottky Diodes for Switch-Mode Amplifiers

Based on mature and high-yield high-voltage (HV) HBT technology monolithically integrated ultra-fast Schottky diodes are developed. The Schottky diodes take full advantage of the optimized HV-HBT layer structure allowing for diode ́s breakdown voltage of 80 V. Due to optimized thermal mounting using priopriatery flip-chip soldering high current switching capability up to 4 A at 60 V was demonstrated. On the other hand, short diode ́s recovery time of 9 12 ps was evaluated. Thus, the integrated HV-HBTs and Schottky diodes are well suited for high speed MMICs for power applications. Novel switched-mode power amplifier circuits were fabricated yielding a digital output power of 5.4 W with very high collector efficiency of 92% at data rates of 1.8 Gbps. INTRODUCTION GaInP/GaAs heterojunction bipolar transistors (HBTs) are available for microwave power applications not only in the low-voltage region as commonly used in mobile handsets but also at higher operation voltages up to 36 V delivering 10+ W of output power at 2 GHz [1]. Due to its high efficiency and high linearity [2] such high-voltage HBTs (HV-HBTs) are well suited for power amplifiers (PAs) as required e.g. for applications in cell-phone base stations. Recently, efficiency record of PAE ~ 58% for a single-stage wideband HV-HBT-based PA was reported [3]. In this paper, we report on successful monolithic integration of such 'high-voltage' HBT power cells with ultra-fast high-voltage blocking Schottky diodes as required for next generation switch-mode power amplifiers. The challenge in realizing such diodes is to combine 70 V blocking voltage with high forward current (2...3 A) and an ultra-low time constant. The basic idea was to use a Schottky diode formed by a metal contact on the collector part of the HBT layer structure for this purpose. The lowly n-doped (~5x10 cm) and 3 μm thick collector layer ensures the high breakdown voltage. Since the Schottky diode should be capable of handling high currents up to 3 A, proper heat sinking is important for this device, too. Hence, we adapted here the thermal concept of the HV-HBTs including FBH ́s proprietary flip-chip soldering technology [4]. EXPERIMENTAL The high-voltage HBT structures (HV-HBTs) are grown in-house on 100 mm GaAs substrates in a MOVPE reactor. The layer structures mainly consist of a 700 nm GaAs subcollector layer (n=5x10cm), an up to 3500 nm thick GaAs collector layer (lowly doped in a region of 4 6x10 cm), a 100 nm GaAs base layer (p=4x10 cm), a 40 nm Ga0.51In0.49P emitter layer (n=5x10 17 cm), and GaAs and InGaAs contact layers. Si and C are used for the n-type and p-type doping, respectively. Higher resistance layers are included in the HBT structure as emitter ballast in order to increase the electrical and thermal stability of the device. The HBT process technology is based on a two-mesa approach in order to access the base and the collector layers. Due to the very thick collector layer the mesa formation is based on a combined dryand wet-etching approach. For device isolation He-ion implantation of the highly doped sub-collector layer is used. For a full MMIC processing MIM-capacitors with SiNx dielectric layer and NiCr thin film resistors are included. Interconnections are made by Ti/Pt/Au metal and 3.5 μm thick electroplated Au air bridges. For power cells emitter thermal shunts are formed by a 20 μm thick electroplated Au layer. Since during HV-HBT front-end processing emitter and base layers are etched as required for the mesa-type HBT device the collector layer is directly accessible. Thus, Schottky diodes can be fabricated by evaporating the Schottky-metal on the thick collector layer as grown for the CS MANTECH Conference, May 18th-21st, 2009, Tampa, Florida, USA HBTs. The n-contact of the diode is made by the same metallization as used for the collector contact for the HBTs. Thus, easy integration of both device types can be performed. The devices are then thermally shunted by thick Au plating, which is used also for flip-chip mounting. Fig. 1 shows integrated HV-HBTs and Schottky diodes as a part of an amplifier circuit. WSiNx as the Schottky metal was chosen due to its sufficiently high barrier height and its thermal stability combined with its suitability for operation at high current densities. Semiconductor surface conditioning prior to Schottky metal deposition is of crucial importance for the diode properties. Therefore, careful process optimization has been performed including the comparison of wet and/or dryetching for surface preparation. The successful process optimization translates into good ideality factors as obtained for the Schottky diodes given in Table 1. Furthermore, high process stability and high yield is verified by the low standard deviation values obtained from mappings of 70 single devices on a 100mm-wafer. Fig. 1 Viewgraph of integrated HV-HBTs and Schottky diodes prepared for flip-chip mounting. SCHOTTKY DIODE CHARACTERIZATION Firstly, the power performance of the Schottky diodes was evaluated by DC-measurements. Fig. 2 shows the forward characteristics of the largest of the fabricated Schottky diodes underlining its current handling capability at high currents up to 2.7 A. Table 1 gives the forward currents at a forward voltage of 2 V for 3 types of Schottky diodes with different active areas. Already the smallest diode with 10 diode fingers in parallel handles currents higher than 1 A. Fig. 3 confirms that the Schottky diode fully utilizes the potential of the HBT ́s collector layer with breakdown voltages higher than 70 V. Such high breakdown voltages were obtained regardless of the size of the Schottky diode (Table 1). These values correspond to the base-collector breakdown voltage of the HV-HBTs [4]. Detailed evaluation of power switching capability of the Schottky diodes was performed on packaged single diodes at Technical University Berlin. The 20-finger Schottky diodes are able to switch currents as high as 4 A at operating voltage of 60 V. Furthermore, superior performance in terms of lower recovery current and shorter recovery time in comparison with a commercially available 100V-class Schottky diode (MBRS3100T3) was observed [5]. 0,0 0,5 1,0 1,5 2,0 2,5 3,0 1x10 1x10 1x10 1x10 1x10 1x10 1x10 1x10