RF Transistors Offering Remarkable Advances

RF transistors are designed to handle high-power radio frequency signals and are made of materials such as Si or Ge, including silicon bipolar junction transistors (BJT), laterally diffused metal oxide silicon (LDMOS), silicon metal oxide semiconductor field effect transistor (MOSFET), and gallium arsenide (GaAs) metal semiconductor field effect transistor (MESFET) technologies. There are several basic types of RF transistors, including:

• Bipolar RF transistors that consist of an n-type or p-type layer sandwiched between two layers of the opposite type.
• MOSFET RF transistors that have a channel made of either an n-type or p-type material.
• Heterojunction field effect transistors (HFETs) that require a negative power supply and are used mainly for driver or power amplification applications.
• Pseudomorphic high-electron-mobility transistors (PHEMTs) that are used mainly in wireless devices and satellite communication systems.

Some other types of RF transistors made from silicon carbide (SiC) and gallium nitride (GaN) semiconductors have recently entered the market for high-performance applications such as radar and military applications.

The SiC type provides transistors with peak output powers of 2 kW or more from a single device. However, GaN shows promise for the high-power markets at frequencies above 4 GHz, according to ABI Research.

Another study on GaN – “Gallium Nitride Markets: Commercial Markets Diver Power Electronics” by Strategy Analytics – says that military and high-power electronic (HPE) applications will be the catalysts for the development of a GaN device market through 2010. While commercial wireless infrastructure applications will drive demand in the future, the military and high-power microwave communications have future needs that are expected to tap the benefits offered by wide-bandgap materials such as GaN. Currently, there is a huge price gap between GaN devices and silicon power transistors in conventional amplifier circuits. Eventually, we can expect to see process improvements that will reduce the price gap. The GaN devices are expected to capture a portion of the RF power amplifier business for mobile wireless infrastructure over the next few years.

A recently introduced RF transistor from IBM Research (www.research.ibm.com) is a 100-GHz graphene device on two-inch wafers. The company says that the transistors, which operate at room temperature, are more than twice as fast as silicon transistors and are the fastest available except the fastest GaAs transistors. IBM says it expects to increase the speed of the graphene transistor to 1 THz. These graphene RF transistors were fabricated at the wafer scale using epitaxially grown graphene processing techniques that are compatible with those used to fabricate silicon transistors.

The world of RF transistors has improved dramatically since the inception of the devices, and by the looks of the cutting-edge designs, many new and exciting possibilities are just around the corner. For example, a very new and exciting frontier of RF transistors from MIT uses biomedical circuits – a cross between electronics and biology. They are cells that can be viewed as circuits that use molecules, ions, proteins, and DNA instead of electrons and transistors.

Research suggests that it should be possible to build electronic chips that mimic chemical reactions very efficiently and on a very fast timescale. Using this concept, MIT has designed a low-power radio chip that mimics the structure of the human cochlea to separate and process cell phone, Internet radio, and television signals. Known as an RF cochlea, it is an example of neuromorphic electronics and may lead to more discoveries of circuit designs that use ultralow power and are highly efficient. The future looks exciting, but the here and now is also pretty impressive.