Gallium Nitride FETs are now providing designers and customers with a practical and viable alternative to Silicon MOSFETs in higher density applications. GaN devices operate faster with high speed switching in the Mega Hertz range. GaN devices are also smaller, allowing high power density systems. Additionally, GaN devices are more efficient with lower switching energy and reverse recovery losses.
GaN FET nomenclature is very similar to that of Si MOSFETs. Similar to Si MOSFETs, GaN FETs also have a Gate, Source and Drain terminal. However, GaN power switches do not have a body diode as Si MOSFETs do.
Currently, the major commercial GaN FETs are lateral high-electron-mobility transistors (HEMTs). These GaN switching devices come in two different types: enhancement mode (e-GaN) and depletion mode (d-GaN).
An enhancement mode transistor is normally off and is turned on by positive voltage applied at the gate.
A depletion mode transistor is normally on and requires the use of negative voltage applied at the gate to turn off.
To allow normally off operation of a depletion mode GaN HEMT, it can be packaged in cascode with a low voltage Silicon switch as shown in Figure 1-1. The GaN FET is turned on or off by switching the Si FET on or off in every cycle.
TI’s solution is to have the GaN FET driven directly as shown in Figure 1-1, where the Silicon switch is used as an enable switch at start-up, so the Si switch does not switch in every cycle, only the GaN switch does.
Figure 1-2 shows a basic example of a lateral structure of GaN FETs. In this structure, we have the silicon substrate, gallium nitride buffer, aluminum gallium nitride barrier, our three terminals: source, gate and drain, a layer of passivisation (protection dielectric) and a field plate extending from the source terminal.
GaN HEMTs require a passivisation layer to reduce the effects of surface electron traps between the gate and drain contacts. The benefits of the field plate are an increase of the breakdown voltage and a reduction of the surface electron traps.
The hetero junction of the AlGaN barrier and GaN buffer – which is a type of junction between two different semiconductors – forms a 2-dementional electron gas channel (2DEG). This channel has very high charge density and mobility. Current flows in the 2DEG channel versus in a Si MOSFET where the channel for current flow is the depletion region between the source and drain.
Previously, we mentioned that GaN FETs have no body diode. The body diode in Si MOSFETs forms from the pn junction between the source and drain. However, we do not have this junction in GaN FETs and that is why there is no body diode.
TI PROVIDES TECHNICAL AND RELIABILITY DATA (INCLUDING DATASHEETS), DESIGN RESOURCES (INCLUDING REFERENCE DESIGNS), APPLICATION OR OTHER DESIGN ADVICE, WEB TOOLS, SAFETY INFORMATION, AND OTHER RESOURCES “AS IS” AND WITH ALL FAULTS, AND DISCLAIMS ALL WARRANTIES, EXPRESS AND IMPLIED, INCLUDING WITHOUT LIMITATION ANY IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT OF THIRD PARTY INTELLECTUAL PROPERTY RIGHTS.
These resources are intended for skilled developers designing with TI products. You are solely responsible for (1) selecting the appropriate TI products for your application, (2) designing, validating and testing your application, and (3) ensuring your application meets applicable standards, and any other safety, security, or other requirements. These resources are subject to change without notice. TI grants you permission to use these resources only for development of an application that uses the TI products described in the resource. Other reproduction and display of these resources is prohibited. No license is granted to any other TI intellectual property right or to any third party intellectual property right. TI disclaims responsibility for, and you will fully indemnify TI and its representatives against, any claims, damages, costs, losses, and liabilities arising out of your use of these resources.
TI’s products are provided subject to TI’s Terms of Sale (www.ti.com/legal/termsofsale.html) or other applicable terms available either on ti.com or provided in conjunction with such TI products. TI’s provision of these resources does not expand or otherwise alter TI’s applicable warranties or warranty disclaimers for TI products.
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2022, Texas Instruments Incorporated