Gallium nitride (GaN) transistor is high electron mobility (HEMT) semiconductor device that consists of three terminals – gate, source, and drain. High electron mobility means that the GaN transistor has higher electric-field strength compared to silicon-based transistors. The GaN transistor offers benefits such as low on-state resistance, lower conduction losses, lower switching losses, high-frequency switching, high efficiency, small form factor, higher power density, and can withstand high temperature.

GaN transistors are ideal for use in consumer power supplies, multi-level converters, solar inverters, industrial motor drives, UPS, high voltage battery chargers, telecom, SMPS, totem pole PFC, high-frequency LLC, and enterprise and networking power applications.

GaN is a compound formed by Gallium (atomic no 31) and Nitrogen (atomic no 7). Gallium does not exist in elemental form, hence it is derived as a by-product from the smelting of bauxite ore into aluminum and from the processing of sphalerite ore for zinc.

The atomic structure of GaN wurtzite crystal looks like this:

The resulting hexagonal crystal structure formed is very robust and stable with a melting point of 4532 ̊ F/1700 ̊C. This means that GaN can operate at a much higher temperature compared to its silicon counterparts. It also has a much larger bandgap as compared to silicon. Bandgap is the energy needed to free an electron from its orbit. At room temperature, GaN is insoluble in water, acid, and alkali. GaN also has a high electron mobility of 2000 cm²/Vs, which makes it suitable for high-frequency applications (in the range of 1 THz).

The figure of merit is a relative measure of the intrinsic, frequency-independent, power-handling capability of the basic material. The thermal conductivity of GaN is slightly lower than that of silicon but due to its higher efficiency, it can operate cooler than silicon-based devices at high voltages.

A table summarising the differences between GaN, and silicon is presented below:

GaN devices offer significant advantages over other semiconductor technologies in terms of reduced energy costs as GaN is more efficient than silicon hence, less energy is dissipated as heat, resulting in lesser cooling costs and smaller and fewer cooling systems like heat sinks, fans, etc. 

GaN also has a much higher power density which reduces the size of the system, and associated magnetics and parasitic concerns. It offers higher switching frequencies, which means less switching losses and reduces the inductive and capacitive components required for power circuits. This leads to a smaller system size and weight, less interference and noise generated, a smaller requirement for filters,  and more spatial freedom in the design process. 

While GaN devices offer many distinct advantages over silicon devices but there are some drawbacks too. GaN semiconductors are more expensive to manufacture than silicon ones. However, the auxiliary and overall system costs are greatly reduced and can compensate for high GaN costs. The overall savings could range typically from 10-20%. Due to its complex manufacturing and fabrication process, the dislocation density is quite high (108~1010/cm2), although strides are being made to bring the number down. It is impossible to crystallize GaN from melted material because the bulk GaN crystal development process is more challenging compared to Si. Bulk GaN commercialization is difficult to achieve not only by the high cost and small size of these substrates but also by the enormous initial investment required to set up GaN-specific fabs, which drives up the average selling price of the devices and further impedes the mass adoption of these devices by the industry.

Another major disadvantage is the difficulty in creating ohmic contact. GaN is a wide band gap semiconductor, with too large a polarity, it is difficult to obtain good metal-semiconductor ohmic contact through high doping, which is a difficult problem in GaN device manufacturing. Therefore, the performance of GaN devices is often related to the production result of ohmic contact.

Types of GaN Transistors

The GaN transistors are classified into two types - Enhancement mode GaN power transistor (e-GaN) and Depletion mode GaN power transistor (d-GaN).

• Enhancement mode GaN Transistor: In enhancement mode, the GaN transistor is equivalent to the “Normally Open” switch (normally OFF). This means that when there is no voltage applied across the gate terminal, the GaN transistor does not conduct. It is turned on by applying positive gate-source voltage.
• Depletion mode GaN Transistor: In this mode, the GaN transistor is equivalent to the “Normally closed” switch (normally ON). The mode indicates that the transistor is at an ON state at zero gate-source voltage. It is turned off by applying a negative voltage relative to the drain and source electrodes.

Key Specifications of GaN Transistors

• Configuration of GaN transistor: The GaN transistors are available with different configurations – single, dual, and half-bridge.
• Gate Threshold Voltage: It represents the minimum voltage that is applied between the gate and source terminal to make the GaN transistor turn ON. It is expressed in volts (V).
• Drain source voltage: It denotes the maximum voltage that can be applied across drain and source terminals, after which the GaN enters an off state. It is expressed in volts (V).
• Drain source resistance: It represents the drain to source on-state resistance and is usually measured in the milli-ohm range.
• Continuous drain current: It is the maximum continuous drain current that the GaN transistor can handle and is expressed in Ampere (A).
• Pulsed Drain Current: It is the maximum pulsed drain current that the GaN transistor can handle and is expressed in Ampere (A).
• Total charge: It represents the total charge that is accumulated at the gate terminal. The gate charge value is used to find how fast a GaN Transistor switches from ON to OFF state, and vice-versa.
• Turn-on Delay Time: It is the time required to charge the input capacitance of the GaN Transistor before the drain current conduction starts.
• Turn-off Delay Time: It is the time interval at which the voltage across the gate and source terminal drops below 90 % when the drain current falls below load current.
• Rise time: The time taken for the drain current to reach from 10% of its initial value to 90% of its final value.
• Fall time: The time taken for the drain current to reach from 90% of its maximum value to 10% of its initial value.

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