Gallium Nitride based Power Conversion Systems

1. High Band Gap: Gallium Nitride is a tough semiconductor material with a wide band gap. The band gap of GaN is 3.2eV, almost three times as compared to Silicon, which has a bandgap of 1.1eV. This means GaN has good thermal stability even at high temperatures and higher breakdown voltages compared to Silicon. This allows GaN to have higher efficiency in the power conversion stages and is a better replacement for silicon in voltage converters, Schottky diodes, and power MOSFETs.

2. High Electron Mobility: The electron mobility for GaN is 2000 cm2/Vs., which is approximately 30% higher than silicon. This high mobility gives the Gallium Nitride an advantage in having higher switching frequencies. This also indicates that the power density of the GaN devices is improved, making the devices more efficient. 

3. High breakdown voltages: GaN breakdown voltage is about 3.3 MV/cm, whereas the breakdown voltage of silicon is 0.3 MV/cm, which is 10 times greater. This property of GaN allows its use in high voltage applications. 

4. Thermal Conductivity: Thermal conductivity of GaN is 1.3 W/cmK, while silicon has a thermal conductivity of just 1.5 W/cmK at 300K. This allows GaN devices to remain cool while operating. This provides an added advantage of compactness. Since the GaN devices are cooler, components can be placed closely on them. 

GaN based Power Transistors

The high electron mobility, gain at higher frequencies, and activation energy of GaN together results in excellent thermal properties and a significantly higher breakdown voltage. Further, GaN has fast-switching capabilities and are more compact. Due to these reasons, GaN has many vital applications in communication and power conversion systems.

Comparison of Always-ON, D-mode, and E-mode p-FETs:

GaN-based devices are naturally available in “always-on” mode or d-mode. However, the commercial demand is high for “always-off” or enhancement-mode (E-mode) p-FETs.

One of the concerns with an e-GaN device is the critical nature of its gate-drive design. The e-GaN device has low on-resistance and zero body diode reverse recovery. So, the gate-drive voltage is around its breakdown voltage, leaving the safety margin to only about 1V—such low safety margin results in device failure on the occurrence of a voltage spike or parasitic ringing. Another concern is lower gate threshold voltage which results in lower noise margins. Adding to this is the higher gate leakage current, which causes an increase in gate driver dissipation.

On the other side, the nature of the d-mode GaN device faces issues during power-up due to its always-on nature. It also needs a negative supply. The depletion-mode GaN HEMT can be connected in series with a low voltage silicon MOSFET in the cascade d-GaN structure to resolve this. The gate terminal is connected to the source terminal of the MOSFET, and the source terminal of HEMT is connected to the drain of MOSFET. This structure results in the complete cascaded d-GaN switch with the gate characteristics of a low-voltage silicon MOSFET. Therefore, existing commercial MOSFET gate drivers can quickly drive the cascaded d-GaN switch.

Further, to realize an E-mode p-FET, p-GaN is heavily doped and made sufficiently thick to avoid punch-through at high gate bias voltage. It is also essential to have a recess in the design; otherwise, it will never be depleted by the gate due to catastrophic breakdown due to the high electric field.

One exemplary GaN E-Mode FET from GaN Systems is GS-065-011-1-L. It is a 650V Enhancement Mode GaN on a Silicon Power Transistor.

GaN-based Type-C USB Charger

‘Battery is Low! Please connect to the charger’- we try to avoid this message when we are late for office meetings or when we are going out. Our life and work revolve around laptops and mobile devices, and we have seen increased usage in recent years. These devices are battery-operated, and people need to recharge them more than once daily. Additionally, the sizes of mobiles or laptops are increasing yearly because these devices need bigger batteries. This led to the invention of fast chargers. It is a fantastic invention that shoots up the battery percentage from 0 – 100% in approximately 25-35 mins. Without this, we would have to wait hours for our devices to be fully charged.

Fast charging generally relates to the higher watt of current input in the phone’s battery. As you would have seen, the Type-C USB cables with 15W, 25 W, or 65W are imprinted on them. But what goes inside of this? How are we able to achieve such high input power?

In the beginning, smartphones had around 5W charging speed. This was done to avoid overheating. However, with the emergence of newer technologies to ensure that the batteries don’t use too much power, i.e., to avoid overheating, internal circuitries were implemented to control the current flow. Now, these circuitries knew the amount of voltage acceptable to the battery and communicated that to the charging brick. The latest USB charging bricks are computers themselves, and they contain chips. When made of GaN, these chips provide greater efficiencies due to their electrical properties. GaN is better at conducting higher voltage over a more extended period than Silicon. Electric currents also travel faster through it, allowing for faster processing. This better conductivity leads to higher efficiency. That’s because it doesn’t need as much energy to get the same output as Silicon based transistors. It also allowed manufacturers to create chips in a denser, more compact form since less energy meant less heat. GaN chips also have higher voltage capacity and are more resistant to heat, perfect for power transfer applications.