Fujitsu has developed a GaN high-electron mobility transistor (HEMT) power amplifier for use in W-band (75-110GHz) transmissions, offering both high output power and high efficiency while improving transistor performance through the reduction of electrical current leakage and internal GaN-HEMT resistance.

In order to build a high capacity next-generation wireless communications network, attention has been focused on wireless communication technology using the high frequency W-band. The range of frequencies that can be used in the W-band is very broad, and because communication speed can be rapidly increased in this band, it is well-suited for this kind of high bandwidth wireless communication.

However, it will be necessary to expand the frequency bandwidth that can be amplified while simultaneously supporting modulation methods that can transmit more information within the same frequency bandwidth, and a strong requirement is to have less distortion when the signal is amplified. Another pursuit is keeping in check the energy consumption of communication systems that accompanies greater distances and capacities, and the improved energy efficiency in power amplifiers.

And so to increase the distance and capacity of wireless communications while decreasing energy consumption with indium-aluminium-gallium-nitride (InAlGaN) HEMTs, Fujitsu developed a technology that reduces internal resistance to one-tenth that of previous technology when current flows between the source or drain electrodes and the GaN-HEMT device.

20170727_EETI_Fujitsu-GaN-amplifier_01 (cr) Figure 1: Schematic cross-sectional view of GaN-HEMT device structure. (Source: Fujitsu)

Fujitsu said the technology utilises a manufacturing process that embeds GaN plugs directly below the source and drain electrodes, which generate electrons at high densities. The structure of the previous technology causes the electron supply layer to become a barrier, however, and internal resistance increases between the source electrode and the two dimensional electron gas. By applying this new technology, Fujitsu succeeded in running high currents through the transistor with significantly less resistance.

20170727_EETI_Fujitsu-GaN-amplifier_02 (cr) Figure 2: Comparison of transistor characteristics. (Source: Fujitsu)

Meanwhile, a current leakage occurs when the two dimensional electron gas, which moves at high speed on the boundary at the top of the channel layer, takes a detour below the gate when the transistor is in its off-state. This leakage causes deterioration in the operational performance of the power amplifier. Normally, it is possible to reduce current leakage by placing a barrier layer beneath the channel layer, but in that case the amount of two dimensional electron gas also decreases, and leads to a reduction of the drain current. Fujitsu's new technology maintains high drain currents by effectively distributing indium-gallium-nitride (InGaN) to create a barrier layer below the channel layer, which, in turn, reduces electron detours during operation, successfully providing significant reductions in current leakage.

Breaking world record

The previous world record for power amplifier output density in the W-band for transmitters was 3.6 watts per millimetre of gate width with technology developed by Fujitsu Laboratories. The new technology, however, has achieved 4.5 watts per millimetre of gate width—the world's highest output density in the W-band—and has confirmed a 26% reduction in energy consumption compared to previous technology through a reduction in current leakage.

20170727_EETI_Fujitsu-GaN-amplifier_03 (cr) Figure 3: Picture of the newly developed W-band GaN-HEMT power amplifier chip. (Source: Fujitsu)

Setting the power amplifier between wireless communication systems in two locations will achieve high-bandwidth communications at 10Gbit/s over a distance of 10km, Fujitsu said.

20170727_EETI_Fujitsu-GaN-amplifier_04 (cr) Figure 4: Comparison of GaN-HEMT power amplifier performance (Source: Fujitsu)

Fujitsu plans to commercialise the technology in high speed wireless communication systems by 2020, with an aim to employ it in such situations as a method of restoring communications when fibre optic cables have been severed by natural disasters or as a way of setting up temporary communications infrastructure when holding events.