Normally-off GaN FET with Bottom Barrier Gate
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Solution Overview
Problem
High-power field-effect transistors like GaN-based HFETs are typically normally-on devices, which are not suitable for power electronics due to potential gate voltage source failures leading to high currents and damage, and existing solutions for achieving normally-off operation result in performance degradation such as excessive leakage currents and lower breakdown voltage.
Innovation Solution
A field-effect transistor design featuring a barrier above the channel and an additional barrier layer below it, where the bandgap of each barrier is larger than the channel's, inducing polarization charge for depletion and allowing voltage-controlled on-state operation, with the bottom barrier acting as a gate.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a GaN-based HFET is designed as a normally-on device, then high power and breakdown voltage are achieved, but the device is unsuitable for power electronics due to gate voltage source failures causing high currents and damage
Solution Approach 1:
The patent inverts the conventional approach by designing a normally-off device instead of a normally-on device. This is achieved by introducing a p-type barrier layer at the AlGaN/GaN interface, which creates a depletion region that blocks current flow in the absence of gate voltage. The device only conducts when a sufficient gate voltage is applied to overcome the polarization-induced depletion, thereby ensuring safety against gate voltage failures while maintaining high power capability.
Solution Approach 2:
The patent changes the electrical parameters of the device by introducing a p-type barrier layer with specific doping concentrations (1×10^16 to 1×10^18 atoms/cm³) and thickness (50 nm to 5 μm). This modification alters the device's default state from normally-on to normally-off by creating a controlled depletion region through polarization effects, while preserving the high breakdown voltage and power handling capabilities through careful parameter selection.
2Reliability
If etching is used to achieve normally-off condition, then normally-off operation is achieved, but performance degrades with excessive leakage currents and lower breakdown voltage
Solution Approach 1:
Instead of physically removing material through etching, the patent changes the electrical parameters by introducing a p-type barrier layer with controlled doping concentration (1×10^16 to 1×10^18 atoms/cm³) and thickness (50 nm to 5 μm). This approach achieves normally-off operation through polarization-induced depletion while maintaining high breakdown voltage by avoiding the physical damage and performance degradation associated with etching processes.
Solution Approach 2:
The patent employs a composite structure combining p-type AlGaN barrier layer with n-type GaN channel layer. This composite material approach creates a heterostructure that utilizes polarization effects at the interface to achieve normally-off operation without the performance penalties of conventional etching methods, preserving both high breakdown voltage and low leakage current characteristics.
3Reliability
If cascade connections with silicon devices are used, then normally-off operation is achieved, but parasitic parameters and series resistance increase
Solution Approach 1:
The patent merges the normally-off functionality directly into the GaN HFET structure by integrating a p-type barrier layer within the device architecture. This eliminates the need for external cascade connections with silicon devices, thereby reducing parasitic parameters and series resistance while achieving normally-off operation. The combined structure operates as a single unified device rather than a complex cascade connection.
Solution Approach 2:
The patent extracts the normally-off control mechanism from external silicon-based cascade circuits and integrates it directly into the GaN HFET structure through the p-type barrier layer. This extraction eliminates the need for separate control devices and their associated parasitic elements, simplifying the overall device structure while maintaining normally-off operation.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables high operating voltage in a normally-off state with reduced leakage currents and improved breakdown voltage, enhancing reliability for power electronics applications.
Implementation Method 1
A polarization charge induced at the interface between the first barrier and the channel depletes the channel
Implementation Method 2
A voltage can be applied to the bottom barrier to induce free carriers into the channel, thus turning the channel on
Data Source
AI summary
A normally-off transistor with a high operating voltage is provided. The transistor can include a barrier above the channel and an additional barrier layer located below the channel. A source electrode and a drain electrode are connected to the channel and a gate electrode is connected to the additional barrier layer located below the channel. The bandgap for each of the barrier layers can be larger than the bandgap for the channel. A polarization charge induced at the interface between the additional barrier layer below the channel and the channel depletes the channel. A voltage can be applied to the bottom barrier to induce free carriers into the channel and turn the channel on.