RF power transistors: material and performance issues

Introduction While wireless communications standards may come and go with developments in the latest digital coding technology, or the liberation of new fragments of the electromagnetic spectrum, the common denominator among the various communications systems is the power amplifier. Over the last few decades, the transition from vacuum tubes and other forms of amplification to solid state devices has been almost complete, especially at power levels less than 1 kW. Nowadays, at the heart of the power amplifier, we find the power transistor. In the world of RF wireless communications, the base-stations and long range transmitters use silicon LDMOS (laterally-diffused MOS) high power transistors almost exclusively. In addition to modern cellular communication systems, LDMOS devices are also used in a wide range of applications requiring radio-frequency power amplification: HF, VHF, and UHF communications systems; pulsed radar; industrial, scientific and medical (ISM) applications; avionics; and most recently in WiMAX™ communication systems. The frequency range of these applications is from a few megahertz and up to 4 GHz. While LDMOS technology is pre-eminent in high-power RF and lower frequency microwave applications, a wide variety of compound (III–V) semiconductors are used as effective power amplification devices, especially for application frequencies above about 5 GHz, and also for lower power applications such as cellular handsets, Bluetooth™ and other wireless local area networks (WLANs), which generally demand output powers of about 1 watt or below.

This paper reviews the history, evolution, and applications of silicon microwave & RF power transistors and electric power devices. The current status of silicon microwave & RF power transistors (BJT, DMOSFET, SIT) and a new kind of electric power device - RSD in HSRI are mainly presented. Important techniques for manufacturing such power devices are introduced. Future trends of such power devices are given briefly.

RF transistors can be found in several present day electronics including mobile radio and telecommunication applications, TV & radio broadcast, avionics, radar and defense communications as well as industrial, scientific, medical (ISM) and broadcast applications. The increasing demands for data anywhere/anytime (mobile internet) requires increased deployment of wireless communication infrastructure. This has pushed upwards across the supply chain (suppliers and customers alike). At the core are RF power amplifiers (PA) with a very costly design, hence important to get it right as rework may not be possible. The assembly of RF power transistors in PA modules is the back-bone of these applications. Such modules are usually generating high power (> 100 watts) with different package outlines and lead configuration. The high power demands excellent thermal management, else product performance and lifetime (reliability) are compromised. As a consequence, the transistors are often packaged in "chunky" S OT outlines, with a cerami c/plastic body and a metal flange. Packaging such RF power modules (or PA) has historically been achieved with bolt down devices, where only the leads of the components were soldered (more akin to manual assembly operations). A thermal compound was used underneath the transistor. High volume assembly is a basic capability (with automation a necessity) to meet industry demand. There are a variety of issues to consider module assembly since there are strong interplay between design, materials and process in order to achieve the desired performance (electrical, thermal, mechanical, and reliability). Due to thermal management requirement, the transistors are normally soldered directly onto the pallet (laminate bonded onto a heat sink) with the leads and SMDs on to the laminate on the pallet. Base on the SOT outline (i.e. gull wing), a cavity or pedestal is required in the pallet with a cut-out in the laminate. Each PA module will be different in i.e. size, weight, materials, number of transistors, board construction with no two modules the same. With such spectra of possibilities in the module design and permutations on assembly processes, the paper outlines and addresses the key issues to consider and approach to achieve robust PA modules assembly. It includes but not limited to substrates, solders, heat sinks, plating, board design and reflow soldering (assembly process flow). Key to achieving the robust assembly process is an iterative approach with feedback at each step in the process including a modeled reflow profiling. In addition, a novel use of Rth (thermal impedance) in combination to acoustic microscopy / x-rays to assess the quality of the soldered interface is presented. The solder volume is key to accommodate manufacturing tolerances and solder joint reliability. Additionally, the issues presented constitute a "toll-box" for PA module assembly, including microwave pallet modules with matched input/output impedance.

RF power amplifier circuits other than Class A have a previously unreported operating characteristic which can cause emitter-base reverse breakdown once each RF cycle. The breakdown is not necessarily observable at the external terminals of the transistor; it can be deduced from the observed emitter-base terminal voltage and base current and the transistor internal lead inductances and junction capacitances. Certain symptoms of RF power transistor deterioration and failure reported by other workers appear to be consistent with the possible effects of such repetitive breakdowns. Common-base amplifiers should be more likely to suffer from this problem than common-emitter amplifiers. Transistor specialists disagree in their predictions of whether such operation would damage the RF power transistor. Controlled experiments could provide definitive information as to the effects on the transistor of such operation.

RF power amplifiers use a wide range of active devices, such as vacuum tubes and transistors (usually BJTs or MOSFETs). Vacuum tubes, still widely used in high-power RF amplifiers.

An enhanced electrical method is presented for measuring the average junction temperature of an RF bipolar transistor. A two-step procedure, previously developed for DC operation, is extended to include the junction temperature measurement for an RF power transistor in a tuned amplifier circuit. The measurement technique is convenient, since it can be used with normal, packaged devices, and does not require a complex heat flow model or ambient temperature measurements. >

RF power transistors employing aluminum metallization can degrade and fail when the aluminum carries high-current densities at elevated temperatures. Two distinct mechanisms promoted by electromigration lead to failure. These are 1) the growth of etch pits filled with aluminum which penetrate and short the emitter-base junctions and 2) a reconstruction of the metallization which increases the electrical resistance reducing the device efficiency.

This chapter discusses memory effects exhibited by transistors and how they affect their large-signal RF performance. Memory effects include self-heating, traps, and parasitic bipolar effects in SOI-MOSFETs. Distributed and transient thermal models will be discussed first. Large-signal measurement techniques for characterizing memory effects in transistors using pulsed biased and pulsed-RF large-signal measurements will be presented. Results from combined deep-level optical spectroscopy and large-signal measurement will then be introduced. Finally, the correlation between trapping and noise will be discussed. The chapter will conclude then with a discussion of the cyclostationary effect, according to which the average trapping population and device noise characteristics can be altered by the fast RF signals under large-signal RF operation.

X-parameters are supersets of S-parameters and applicable to both linear and nonlinear system modeling. In this paper, a packaged 6 W Gallium Nitride (GaN) RF power transistor is modeled using load-dependent X-parameters by simulations. During the device characterization the load impedance is tuned only up to the 2 nd -order harmonic. However, it proves that the model can still accurately approximate the behavior of the transistor under impedance tuning up to the 3 rd -order harmonic. The simulation results preliminarily validate the concept of utilizing the X-parameter based modeling technique to decrease the complexity of a harmonic load-pull measurement setup. A high-efficiency 2 GHz power amplifier is also designed for further validation of the concept.

Due to their large bandgap, high critical electric field, and availability of high-quality large-size melt-grown bulk substrates, III-oxides including Ga2O3, Al2O3, In2O3, and their alloys have been extensively investigated for a myriad of electronic and optoelectronic applications. Recently, β-Ga2O3 based power electronics, RF transistors, and ultraviolet (UV) photodetectors have been demonstrated with promising performance. However, p-type β-Ga2O3 is still elusive due to high dopant activation energy (>1 eV), large hole effective mass, and hole trapping. This significantly limits the design freedom for β-Ga2O3 devices. Other p-type semiconductors have been proposed to form heterostructures with β-Ga2O3 such as p-NiO, p-Cu2O, and p-type III-nitrides. As popular wide bandgap semiconductors, III-nitrides are promising candidates to form III-oxide/III-nitride heterostructures to enable advanced device structures and new functionalities. Furthermore, III-oxides and III-nitrides can be epitaxially grown on each other with small lattice mismatch (< 5% for GaN and β-Ga2O3) by the industrial standard epitaxial method MOCVD. For example, vertical GaN violet LEDs grown on n-type β-Ga2O3 substrates have been reported. This talk will present our recent work on III-oxide/III-nitride heterostructures in power electronics and optoelectronics.For power electronics, β-Ga2O3/GaN p-n heterojunctions will first be discussed. The heterojunction via mechanical exfoliation shows decent forward rectifying behaviors and thermal stability up to 200 °C but relatively low breakdown voltages (BV). To improve the breakdown capability, we carried out a comprehensive TCAD simulation study to design mesa edge termination for kV-class β-Ga2O3/GaN p-n heterojunctions. It was found that the electric field crowding effect is the main reason for the low BV. Several mesa edge termination structures were investigated such as deeply-etched mesa, step mesa, and p-GaN guard ring. Second, normally-off AlN/β-Ga2O3 field-effect transistors using polarization-induced doping will be discussed. A large two-dimensional electron gas is formed at the AlN/β-Ga2O3 interface due to polarization effects, and p-GaN gate is used to realize tunable positive threshold voltage. The device transfer and output characteristics with different device structures are also studied. For optoelectronics applications, self-powered spectrally distinctive Ga2O3/GaN heterojunction UV photodetectors grown by MOCVD will be discussed. Opposite current polarities are observed under different illumination wavelengths due to different carrier transports, which can be utilized to distinguish different spectra. These results indicate that (ultra)wide bandgap III-oxide/III-nitride heterostructures are a promising platform to enable new device structures and functionalities.

The results of studies of the temperature dependence of the voltage UKL of current lacing in power RF and microwave bipolar transistors (PBT) in the temperature range from - 60 to 90 ° C on the upgraded unit UITEP-M for measuring the thermoelectric parameters of high-power transistors are presented. The principle of operation of the installation is based on measuring the amplitude of the variable component of the voltage at the emitter junction of the PBT when a constant emitter current is passed and the sum of linearly increasing and small alternating voltage is applied to the collector; current lacing in the transistor structure manifests itself in a sharp increase in the steepness of the dependence. On a sample of PBT type KT903A, it was experimentally established that with an increase in the temperature of the housing, the current lacing voltage decreases noticeably and at a certain temperature, the TCR reaches a minimum value of UKLmin, and then increases again with an increase in the temperature of the housing. It is shown that the greater the thermal resistance of the transistor junction-housing in the diode connection, the weaker the temperature dependence of the lacing voltage. The obtained results are qualitatively explained on the basis of a one-dimensional model.

The developments in RF power transistor technology makes solid state transmitters economically attractive for UHF DTV terrestrial transmissions. These transmitters will probably use silicon bipolar, MOS or silicon carbide static induction transistors (SIT) in the RF power amplifiers. This paper examines these RF transistor technologies. In addition, this paper provides an overview of a typical DTV solid state transmitter. The authors have conducted DTV transmission tests and demonstrations using a silicon carbide, solid state transmitter. This paper provides a brief description of these tests.

Exploiting load testing and profiling for Performance Antipattern Detection

Dynamic load modulation is a technique for increasing RF power amplifier efficiency by manipulating a tuneable matching network (TMN) between the RF transistor and the antenna in harmony with the envelope of the transmitted signal. Generally, TMNs are based on varactor diodes with control voltage swings approaching 100 V, but future devices are likely to need far greater. The varactor driver amplifier proposed here uses a split frequency architecture to amplify a 3 MHz bandwidth 3GPP 4G long‐term evolution envelope signal. Under these conditions, it can produce a peak voltage of 166 V at a power consumption of 0.7 W.

Drawing is a key component of ‘traditional’ practice in classical animation that is sometimes seen as an outmoded form lacking relevance to a digital age. On the contrary, the use of digital tools and virtual materials can still be seen as problematic by some animators schooled in traditional methods. In contemporary art, however, there is an explosion of experimentation with different tools, processes and paradigms, and a resurgence of interest in drawing around issues of time, performance and materiality. This article considers the processes and theorization of drawn animation in light of these different practices and tools. Drawing is reflected on in terms of material basis and conceptualization. It is explored as a process that can represent the passage of time and the performance of a character, yet is in itself a performative activity with duration.