Electrostatic Influence of Diamond Heat Spreaders on GaN
Electrostatic Influence of Diamond Heat Spreaders on GaN FETs: Design Guidelines
Diamond is increasingly being integrated with GaN FETs as a device-level heat spreader because of its exceptional thermal conductivity. By efficiently removing heat from the active region, diamond can reduce channel temperature and improve the high-temperature performance of GaN power and RF devices. However, diamond integration introduces an important consideration beyond thermal management: the diamond/GaN interface can actively influence the electrostatics of the transistor.
Recent TCAD analysis shows that a diamond heat spreader can produce significant band bending at the diamond/GaN interface. In the modeled structure, a built-in electric field approaching 1 MV/cm was observed near the interface, accompanied by upward bending of the valence band and formation of an interfacial hole-accumulation region. This interfacial charge can create a parasitic conduction pathway, alter carrier distribution, and deplete carriers near the source and drain regions, potentially reducing carrier injection and limiting the achievable on-state current.
This effect demonstrates a fundamental tradeoff in GaN-on-diamond technology: diamond can provide a major thermal advantage while simultaneously modifying the electrical boundary conditions of the GaN device. Therefore, diamond should not be treated simply as a passive heat-spreading material. Its surface condition, interface chemistry, band alignment, dielectric environment, and electrical accessibility must all be considered during device design.
Design Guidelines
1. Engineer the diamond/GaN interface
The interface should be designed to minimize uncontrolled charge, surface states, and band bending while maintaining an efficient thermal path. An engineered ultrathin interlayer can be introduced between GaN and diamond to control both thermal boundary resistance (TBR) and electrostatic coupling. The study demonstrates the use of ultrathin interlayers as a means of independently controlling interfacial thermal resistance in TCAD.
The objective should not simply be to maximize electrical isolation. Instead, the interlayer should provide controlled electrostatics with minimum thermal penalty.
2. Avoid excessive dielectric thickness
A dielectric inserted between GaN and diamond does not automatically eliminate the electrostatic problem. Simulations show that the GaN/dielectric/diamond structure can develop capacitive coupling and induce charge accumulation at the dielectric/diamond interface, producing another parasitic lateral conduction channel.
Consequently, the interlayer thickness and dielectric properties should be optimized together with GaN polarization charge and doping.
3. Electrically isolate the diamond from the contacts
One of the most important findings is that electrical accessibility of the diamond interface strongly determines the observed leakage. Introducing dielectric isolation along the contact sidewalls reduced leakage to approximately the level of the fully passivated reference structure while retaining the diamond thermal pathway.
This suggests a practical architecture:
thermally coupled vertically, electrically isolated laterally.
The diamond should remain closely integrated with the device for heat spreading, while source/drain metallization and exposed diamond edges should be prevented from providing an electrical path to the interfacial charge region.
4. Optimize thermal and electrical properties simultaneously
Diamond thickness, interface TBR, interlayer thickness, dielectric properties, and contact geometry should be optimized as a coupled electrothermal system. Increasing diamond thickness can improve thermal performance, but the resulting electrical behavior must also be monitored.
Key design metrics should therefore include:
channel temperature and thermal resistance;
diamond/GaN TBR;
interfacial electric field;
band bending;
hole accumulation;
leakage current;
source/drain carrier injection;
on-state current.
