RF/Microwave Transistor Structure, Semiconductors, and Configuration Roundup

by Tim Galla, Pasternack

RF solid-state transistors are used in an increasingly wide range of applications, and as the power levels and bandwidth of these solid state devices improve, so does the expanse of applications RF transistors find themselves. Essentially, RF transistors are used in signal amplification, oscillator, switching, buffer, isolation, and other applications that output to several kilowatts of power and can range in frequencies to hundreds of gigahertz. Hence, such a wide range of operation and use cases isn’t covered by a single transistor type or technology, and is instead done with a diverse combination of transistors. 

This fact makes interpreting the nuances of a particular transistor somewhat complicated, as there are now so many variations of common transistor technologies, as well as proprietary and branded transistor technology. Moreover, there is a trend for RF/microwave suppliers to have diverse fabrication facilities that they work with, and hence there are an increasing number of tweaked and modified transistor fabrication processes unique to a specific company as well as shared processes. Therefore, understanding the basic types, semiconductors, and substrates can help navigate the muddy waters of modern RF transistors.

Overview of Transistor Structure

Though many of the fabrication methods and technology used with RF transistors are similar to analog and digital transistor and active device manufacture, there are many differences unique to RF transistors and custom fabrication technologies. For instance, many RF transistors are made using epitaxial materials that offer greater control of the fabrication material properties. Moreover, the structure of RF transistors can be modified to enable higher frequency operation, higher power operation, lower noise, or other desirable factors for a given RF application. In general, RF transistors feature small and very refined horizontal and vertical structures used for contacts, junctions, insulators, or for other features which are often repeated in complex patterns with tight pitches. 

There are also some semiconductor materials that are generally only used for high power, high-speed switching, or low noise, that have been adapted for RF applications. Moreover, RF transistors, due to the frequency or power dependent effects of conductors and insulators, are often placed in packages specifically designed for RF operation. For instance, there are certain types of ceramic packages specifically used for high power RF applications, just as there are also specialized packages for use with RF applications in the tens to over one hundred gigahertz. 

RF Transistor Structures

There are now an increasing assembly of transistor structures that are used in RF applications. Beyond the original BJT and MOSFET, there are now a couple of BJT variations and several FET variations that have been adapted to RF uses. Each of these transistor structures, or types, either provides some improvements over basic transistor structures, or involves a balance of cost, complexity, materials, and performance factors that are attractive for particular applications. For instance, many of the FET structures were originally developed for high-speed switching or power, and have been customized for use in specific RF transistor processes. 

In the case of BJTs, there are several technologies out there that still use a typical BJT structure, often assembled from multiple small BJTs. However, for more recent power applications, HBTs have become common. HBTs are a variant of the BJT structure that leverages heterostructure junctions, which are most often developed using compound semiconductor materials, such as AlGaAs/GaAs, InP, or SiGe. HBTs improve over traditional BJTs as the bandgap difference between the base and emitter results in greater emitter gain. Moreover, HBTs feature a lower base sheet resistance than typical BJTs, and the vertical current flow path along with the use of semi-insulating substrates leads to reduced parasitics. Hence HBTs exhibit higher operating frequency than typical BJTs.

For FETs, there are now several structure types being used with various semiconductors. In general, the variations in the gate and channel region structure along with the use of different substrate structures and materials are used to realize a specific FET structure. Many of the higher frequency FET structures are designed with short channels, which enables the device to behave more linearly by ensuring the device operates in a saturated mode, such as with LDMOS. There are also vertically diffused (VDMOS) devices, which are also used as RF power transistors, though they are typically used at lower frequencies (to ~100 MHz) than LDMOS transistors (to ~4 GHz).

Other common FET variations include MESFETs and HEMTs. MESFETs are structured similarly to MOSFETs, with the exception that a metal semiconductor is used for the gate material instead of an oxide, and MESFETs are often fabricated using III-V compound semiconductors. These semiconductors include SiC, GaAs, InP, and GaN, and as they don’t allow for a stable oxide to be formed for a gate dielectric, a Schottky metal semiconductor contact is used instead. This structure is capable of higher frequency operation compared to a MOSFET, as the MESFET avoids traps in the gate insulator and eliminates the parasitic capacitance formed between the channel and gate terminal that plagues the typical MOSFET structure.

Like with BJTs, there are FET structures that employ a heterostructure along the current flow region of the device. These HEMTs are designed to form a two-dimensional electron gas (2-DEG) region between the layered heterostructure. The undoped layers surrounding the 2-DEG experience reduced collision phenomenon and higher frequency response compared to traditional MESFETs, as this region isn’t susceptible to scattering by ionized donor atoms. HEMTs are also generally fabricated using III-V compound semiconductors, of which GaN HEMTs have become popular for high power and high frequency applications.

There are also enhanced versions of the HEMT structure that include a pseudomorphic HEMT (p-HEMT) that usesmaterial layers that exhibit different lattice constants. This lattice mismatch results in a larger bandgap discontinuity between the layers, and leads to higher charge capability in the 2-DEG and higher transconductance. Metamorphic HEMTs (M-HEMTs), include an additional relaxing layer between the substrate and channel, which are used for high frequency and low-noise applications, though they are not as well suited to high power applications as p-HEMTs. There are also enhancement-mode p-HEMTs (Ep-HEMTs), which have low enough drain currents that a negative voltage supply isn’t needed.

Other common FET types include JFETs and FinFETs, of which JFETs are only generally used for RF switching applications. Research has been conducted that discusses the use of FinFETs for RF applications in the case of mixed-signal circuits, and FinFETs are often compared with bulk MOSFET technology for their suitability for analog and RF circuits. This is part of a larger discussion surrounding RF CMOS and other Si CMOS technologies that may lead to lower cost and adequate performance RF and analog circuits that can be made with the same process technology as modern digital circuits.

Combinations of BJT and CMOS structure features and technology also exist, such as IGBTs and BiCMOS technology. With IGBTs, an insulated gate material is used which enables both high voltage and high current operation, as well as lower ON losses than high voltage MOSFETs and simpler gate drive circuitry than typical BJTs, though at the sacrifice of lower switching speeds. 

BiCMOS technology enables the fabrication of both BJTs and CMOS transistors in a single IC. This allows for circuits that take advantage of both transistor types, though this involves a trade-off of increased process complexity (additional mask layers) and reduced performance compared to pure processes. Currently, compound semiconductor BiCMOS technology, specifically SiGe HBTs in BiCMOS, is widely used for many modern applications. These applications include high-speed wireless links, optical data links, high-precision analog circuits, and automotive radar (24 GHz and 77 GHz). 

RF Transistor Semiconductors

The semiconductors used to develop a transistor are often the most critical aspect that determines the overall performance of the resulting transistor technology. This is why for certain applications, such as high power, high frequency, low noise, etc., there are popular semiconductors used for those applications. Often, there is a trade-off between attractive figures-of-merit for a semiconductor material and the transistor application. For instance, semiconductors with features ideal for power devices (GaN and SiC) may not be the best suited to low-noise applications. However, there are instances of GaN low-noise amplifiers (LNAs) that benefit from the high breakdown voltage of GaN devices for high survivability applications.

Table 1 includes a listing of several semiconductors, some commonly used in RF applications, and their figure-of-merit (FOM) performance in several key parameters used to gauge the suitability of a semiconductor for RF applications. These FOMs include dielectric constant, bandgap energy, electron mobility, and critical field. Other FOMs key to RF use cases are saturated electron drift velocity, thermal conductivity, coefficient of thermal expansion, and a synthesized measure sometimes used in the power device market, power device FOM (PD-FOM).

Other RF semiconductor considerations include cost, complexity, technology maturity, investment, material availability, and other features that result in the overall viability of using a particular semiconductor for a given application. Currently, GaAs and SiGe are mature technologies with a wide range of RF applications, while GaN and SiC devices are more recent to the market and are growing in adoption and even enabling new applications. 

RF Transistor Processes

Some RF semiconductors can be epitaxially deposited, in layers, on insulative substrate materials. This is most common with GaN and Si semiconductors, and enable the development of hetero-epitaxy of GaN and AlGaN using either molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD). The insulative substrate influences many key factors of the resulting device performance. This includes high frequency performance, power handling, thermal conductivity, ruggedness, mean-time-between-failure (MTBF), and other important factors that are used to gauge a semiconductor process. 

Importantly, the use of a thin layer of a semiconductor on an insulative substrate can be used to develop very high frequency operating transistors, which is the case for Silicon on Insulator (SoI) and Silicon on Sapphire (SoS) technologies. For instance, with GaN, there are several common insulative materials used in modern applications, namely GaN, Si, SiC, Sapphire, and Diamond. The most common of these are currently GaN on Si and GaN on SiC, which also illustrate the significance that the insulative substrate has on the performance of such devices.

For example, GaN on Si devices exhibit roughly one third the thermal conductivity of GaN on SiC. Moreover, GaN and SiC are closely lattice-matched, meaning that the lattice structures between the layers don’t require modification of the structure to allow for a region of band gap change. Hence, the result is lower defect density of the crystals, with improved reliability and reduced leakage. SiC can also be operated at higher temperatures than Si, and SiC’s superior strength along with a much closer coefficient of thermal expansion compared to GaN, means that at high temperatures and through temperature cycling, GaN on SiC devices are likely to be more robust than GaN on Si devices.

That being said, SiC is also generally much more expensive to grow than Si, as SiC grows at several hundredths the rate of Si and the cost of semiconductors is heavily dependent on the time-related production costs. GaN on Si wafers are also typically larger than GaN on SiC wafers, further reducing the cost of GaN on Si devices compared to GaN on SiC. Therefore, for high power and high reliability applications, GaN on SiC is preferred, and if cost is the key concern, GaN on Si is generally used.

As a side note, GaN on Diamond is potentially the best performing combination of epitaxial GaN. However, the prohibitive cost of Diamond limits its use to niche military, aerospace, and space applications. 

RF Transistor Processes

• GaN
  • GaN on GaN
  • GaN on Diamond
  • GaN on Si
  • GaN on SiC
  • GaN on Sapphire

• Si
  • Silicon on Insulator (SoI)
  • Silicon on Sapphire (SoS)

Common RF Transistor Configurations

As the specific nuances of each semiconductor fabrication process must be designed and determined according to a very complex approach involving thousands of variables, there are a limited number of common RF transistor configurations. Meaning, there are only specific transistor types employed with distinct semiconductors and technologies. Some of the factors that determine the exact combination are the maturity of a technology and the development of methods to overcome fabrication challenges, such as doping, structure tolerances, yield, and other factors. Figure 3 shows some of the common transistor configurations for the top RF semiconductors.

Hence, each RF transistor technology results in a transistor device with unique performance features that are based on the transistor structure, semiconductor, and fabrication process. Also, a specific transistor technology may be branded or otherwise trademarked/named to abstract a company’s solution from its specific background. Often, companies will also have competing technologies that appear to have a very similar makeup, but result in different end performance features. This could be a result of the companies deciding on different trade-offs, Intellectual Property (IP) considerations, or other nuances of their fabrication process.

Conclusion

The landscape of RF transistors continues to grow in complexity with ever greater diversity of transistor types, semiconductors, and technology. This is further compounded by proprietary and branded technologies with unique performance features. It now requires greater discernment and understanding of the subtleties of transistor technology and semiconductors to zero-in on an optimized solution for a given application. Furthermore, the diversity of RF transistor technologies is likely only to explode as wireless communications and sensing technologies become more mainstream and move from niche military and aerospace applications to penetrate the majority of electronics applications.

Glossary

• RF/Microwave Transistor Structures
  • Bipolar Junction Transistor (BJT)
  • Heterojunction Bipolar Transistor (HBT)
  • Insulated-gate Bipolar Transistor (IGBT)
  • Field Effect Transistor (FET)
  • High Electron Mobility Transistor (HEMT)
  • Pseudomorphic HEMT (pHEMT)
  • Enhancement Mode pHEMT (E-pHEMT)
  • Metamorphic HEMT (M-HEMT)
  • Junction Gate Field Effect Transistor (JFET)
  • Metal Oxide Semiconductor FET (MOSFET)
  • Metal-Semiconductor FET (MESFET)
  • Linear Diffusion MOSFET (LDMOS)
  • Vertical Double Diffused MOSFET (VDMOS)
  • Bipolar Complementary Metal Oxide Semiconductor (BiCMOS)
  • Fin Field Effect Transistor (FinFET)

• RF Transistor Semiconductors
  • Silicon (Si)
  • Silicon-on-Insulator (SoI)
  • Silicon-on-Sapphire (SoS)
  • Silicon Carbide (SiC)
  • Indium Phosphide (InP)
  • Indium Aluminum Arsenide (InAlAs)
  • Indium Gallium Arsenide (InGaAs)
  • Gallium Arsenide (GaAs)
  • Aluminum GaAs (AlGaAs)
  • Gallium Nitride (GaN)
  • Aluminum GaN (AlGaN)
  • Silicon Germanium (SiGe)
  • Diamond (C)
  • Graphene (C)
  • Indium Antimonide (InSb)
  • Indium Arsenide (InAs)
  • Gallium Antimonide
  • Gallium Phosphide (GaP)
  • Germanium (Ge)
  • Aluminum Antimonide (AlSb)
  • Cadmium Sulfide (CdS)
  • Cadmium Selenide (CdSe)
  • Zinc Oxide (ZnO)
  • Zinc Sulfide (ZnS)

References

Radio Frequency Transistors: Principles and Practical Applications, Norman Nye, Helge Granberg, 2nd Edition.

High Efficiency RF and Microwave Solid State Power Amplifiers, Paolo Colantonio, Franco Giannini, Ernesto Limiti.

Microwave Devices and Circuits, Third Edition, Samuel Y. Liao.

RF And Microwave Passive and Active Technologies, Mike Golio.

“Top 10” RF Technologies, Terry Edwards, Microwave Journal, July 2018.

(1388)