Despite Continuing Rumors of This Segment’s Demise, This Legacy Market Continues to Resist GaN’s Intrusion

Vacuum Electron Devices (VEDs), also known as electron tubes, represent one of electronics’ early legacy markets. They have been around for well over 100 years and their basic operation is still the same.

One section of this market segment is for VEDs that operate in the microwave and millimeter wave areas (>1 GHz). Most of this class of tubes are characterized by operation in strong magnetic fields, and their power ranges can go from a few watts to tens of megawatts. ABI Research classifies these tubes into five categories: Travelling-Wave Tubes (TWT), klystrons, magnetrons, Crossed-Field Amplifiers (CFA), and gyrotrons.

All of these types of tubes are still with us today.

When Gallium Nitride (GaN) devices burst onto the scene in a practical sense a few years ago, it was thought that microwave and millimtere wave VEDs would rapidly see their demise. Intially, in some low power (<100 W) and low frequency areas (<4 GHz) this appeared to be true. Instrumentation amplifiers and other similar fixed applications indeed did get swallowed up by the newer technology.

The predicted onslaught, however, quickly lost steam and the microwave tube business continued to hold its own. Penetration rates for GaN replacements of TWTs have never met expectations and are not likely to do so in the near to mid future. All of the “low hanging fruit,” if one can use the phrase, has already been picked.

Extremely high-power VEDs (tens of kilowatts and up) are unlikely to ever be replaced by solid-state devices for size, space, cost, and efficiency reasons. Although inroads into the microwave and millimeter wave VED markets will obviously continue, the rate of replacement and breadth will be more measured.

The microwave and millimeter wave VED business is essentially flat with some incremental growth and very stable and is forecast to continue to be so over the next few years. While there are some mix changes that are likely to occur, they will be marginal in nature. Microwave Power Modules (MPMs), which use micro and mini helix TWTs, are no larger than a similar power level solid-state amplifier and performance has been improved to cover multi-octave and up frequency ranges.

Magnetron sales are actually growing very slightly, as it is difficult to generate high levels of power in such a small space. This is coupled with the fact that a magnetron is a power oscillator, so as a Radio Frequency (RF) source it is essentially self-contained.

Little comment is necessary with tubes in the megawatt range as there are no real economical solid-state solutions.

Having said all of this, look for the paradigm for GaN devices to penetrate the tube market to change. Instead of the expectation for GaN to replace microwave and millimeter wave tubes in existing applications, look for new designs to start off with GaN technology from the outset. C and X band combination radars and military airborne radars are good examples.

Although the idea of retrofitting older tube designs with solid-state is still in use, it has generally been discarded as not cost-effective and it makes more sense to just continue tube production until the system in question becomes obsolete.