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Pasternack BlogThe growing number of RF, microwave, and millimeter-wave applications has led to a growth in the number of RF transistor types, semiconductors, and configurations being used today. As more devices use RF/wireless technology to communicate and sense, there will be an even greater diversity of RF transistor technologies. RF Amplifiers and RF Mixers are a key example of how RF transistors are used.
Some of the RF transistor types, configurations, and semiconductors have been in use for decades, and are often still used in certain niche applications, while other variations of RF transistor technology have recently reached the market and are enabling new applications. In essence, there are three categories of RF semiconductor technologies, small-signal, switching, and power transistors. There are also a wide variety of transistor types, each with their unique performance metrics. RF transistors are commonly made from HEMT, LDMOS, FET, and MOSFET type technologies. This is a generalization because these transistor types can present better high frequency characteristics, though there are many subtypes and variations with different fabrication processes.
RF Transistor Types
- Bipolar Junction Transistor (BJT)
- Heterojunction Bipolar Transistor (HBT)
- Insulated-gate Bipolar Transistor (IGBT)
- High Electron Mobility Transistor (HEMT)
- Pseudomorphic HEMT (pHEMT)
- Enhancement Mode pHEMT (E-pHEMT)
- Field Effect Transistor (FET)
- Junction Gate Field Effect Transistor (JFET)
- Metal Oxide Semiconductor FET (MOSFET)
- Metal-Semiconductor FET (MESFET)
- Linear Diffusion MOSFET (LDMOS)
- Vertical Double Diffused MOSFET (VDMOS)
- Si Bipolar Complementary Metal Oxide Semiconductor (Si BiCMOS)
Certain RF transistor types and semiconductor configurations are best suited to certain applications, which is why there is such a diversity of configurations. Small-signal technologies are typically used for low-noise signal processing and amplification, where power transistors are used for high-gain and high power applications. Switching transistors are used for their ability to rapidly be set to conduction-/off-states with minimal transient performance degrading factors. As most RF transistors are used to make amplifiers, there are a few main categories of amplifiers, Low-Noise Amplifiers (LNAs), Gain Block Amplifiers (GBAs), and High Power Amplifiers, and Variable Gain Amplifiers.
RF Transistor Semiconductors
- Silicon (Si)
- Silicon-on-insulator (SoI)
- Silicon Carbide (SiC)
- Indium Phosphide (InP), Indium Aluminum Arsenide (InAlAs), or Indium Gallium Arsenide (InGaAs)
- Gallium Arsenide (GaAs) or Aluminum GaAs (AlGaAs)
- Gallium Nitride (GaN) or Aluminum GaN (AlGaN)
- Silicon Germanium (SiGe)
- Diamond (C)
The final electrical performance characteristics of RF Amplifiers, Mixers, and other devices is ultimately dictated by the RF Transistor semiconductor type. New RF Transistor technologies and variations are being developed in order to provide enhancements over legacy technology, but these developments usually require many years and substantial development facilities and cost to realize a new RF Transistor fabrication process.
| Table: Common Semiconductor Properties | ||||||||
| Material | Bandgap
(eV) ~300K |
Saturated Electron Velocity (x10^7 cm/s) | Electron Mobility (cm2/Vs) | Critical Field Ec
(V/cm) |
Thermal Conductivity
(W/m·K) |
Coefficient of Thermal Expansion (ppm/K) | Power Device Figure of Merit
(u_n*E_c^3) |
Dielectric Constant
(epsilon_r) |
| InSb | 0.17, D | – | 77,000 | 1,000 | 18 | 5.37 | – | – |
| InAs | 0.354, D | – | 44,000 | 40,000 | 27 | 4.52 | – | – |
| GaSb | 0.726, D | – | 3,000 | 50,000 | 32 | 7.75 | – | – |
| InP | 1.344, D | 1 | 5,400 | 500,000 | 68 | 4.6 | – | 12.5 |
| GaAs | 1.424, D | 1 | 8500 | 400,000 | 55 | 5.73 | 20 | 12.8 |
| GaN
(AlGaN/GaN) |
3.44, D | 1.5 | 900-2000 | 3,000,000 | 110
(200 Film) |
5.4-7.2 | 3000 | 9 |
| Ge | 0.661, I | – | 3,900 | 100,000 | 58 | 5.9 | – | |
| Si | 1.12, I | 1 | 1,400 | 300,000 | 130 | 2.6 | 1 | 11.8 |
| GaP | 2.26, I | – | 250 | 1,000,000 | 110 | 4.65 | – | – |
| SiC (3C, b) | 2.36, I | ~2 | 300-900 | 1,300,000 | 700 | 2.77 | – | – |
| SiC (6H, a) | 2.86, I | ~2 | 330 – 400 | 2,400,000 | 700 | 5.12 | – | – |
| SiC (4H, a) | 3.25, I | ~2 | 700 | 3,180,000 | 700 | 5.12 | 675 | 10 |
| C (diamond) | 5.46-5.6, I | – | 2,200 | 6,000,000 | 1,300 | 0.8 | – | – |
In many cases RF Transistor technologies are combinations of two or more semiconductor technologies. This includes semiconductor-on-insulator technologies, such as Silicon-on-insulator (SOI) and compound semiconductor-on-insulator (CSOI), including GaN on SiC and GaN on Diamond. One of the reasons that this is done is due to the thermal properties of the insulating material, such as SiC and Diamond, which are far more thermally conductive or present better electrical characteristics than the semiconductor layered on these insulators. For example, both SiC and Diamond exhibit better thermal conductivity than GaN, which can allow for much higher power density transistors than can be achieved with typical GaN transistors.
Common RF Transistor Configurations
- GaN (AlGaN/GaN)
- GaN on GaN
- GaN on Diamond
- GaN on Si
- GaN on SiC
- GaN HEMT
- GaAs
- GaAs FET
- GaAs IGBT
- GaAs MESFET
- AlGaAs/GaAs HEMT
- GaAs pHEMT
- InP
- InP HEMT
- InP HBT
- SiC
- SiC MESFET
- SiC MOSFET
- SiGe
- SiGe HBT
- SiGe/SiC (SiGe:C)
- Si
- Si MOSFET
- Si LDMOS
- Si VDMOS
- Si BJT (Si-Bipolar)
- SoI
- Sapphire-on-Silicon (Ultra CMOS)
- Si BiCMOS
Learn more about Pasternack’s expansive line of RF, microwave, and millimeter-wave amplifiers here.
