Normally-off HEMT with Insulated Gate Recess
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Solution Overview
Problem
Conventional normally-on high electron mobility transistors (HEMTs) require complex and expensive circuitry to turn off, and attempts to make them normally-off result in unreliable devices with non-uniform threshold voltages due to manufacturing errors in the gate recess process.
Innovation Solution
A normally-off heterojunction field effect transistor with a two-dimensional electron gas controlling layer of sufficient thickness (>5 nm) and a recessed second barrier layer, where the gate electrode is separated from the two-dimensional electron gas controlling layer by an insulating layer, allowing the device to operate without generating two-dimensional electron gases under the gate electrode.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a gate recess process is used to make HEMT normally-off, then the device can operate as a normally-off switch, but the threshold voltage changes substantially with manufacturing errors in the depth of the recess, resulting in non-uniform threshold voltage and low reliability
Solution Approach 1:
An insulating layer is introduced as an intermediary between the gate electrode and the two-dimensional electron gas controlling layer. This insulating layer mediates the interaction, allowing the gate to control the channel while preventing direct contact that would cause 2DEG formation under the gate. This resolves the contradiction by enabling normally-off operation without the reliability issues of direct gate-to-channel contact.
Solution Approach 2:
The patent creates a localized structure where the gate electrode is separated from the 2DEG controlling layer only in the region directly under the gate, while maintaining heterojunctions elsewhere. The insulating layer is applied locally at the gate interface, allowing different regions to have different properties: the gate region is insulated for normally-off operation, while other regions maintain high electron mobility through heterojunctions.
2Ease of operation
If a gate recess process is used to make HEMT normally-off, then the device can operate as a normally-off switch, but the etching tolerance between the electron supply layer and underlying layer is poor, leading to manufacturing complexity
Solution Approach 1:
The insulating layer serves as a mediator that simplifies the manufacturing process. Instead of requiring precise etching to create a recess through multiple layers with poor etching tolerance, the insulating layer is deposited conformally on the existing structure. This eliminates the need for complex selective etching processes and reduces sensitivity to manufacturing variations.
Solution Approach 2:
The insulating layer is formed preliminarily on the 2DEG controlling layer before the gate electrode is deposited. This preliminary action of insulating the surface simplifies subsequent gate fabrication, as the gate can be deposited directly on the insulating layer without requiring precise recess formation. The preliminary insulation step makes the overall process more robust and easier to manufacture.
3Ease of operation
If the two-dimensional electron gas controlling layer is made thick enough to prevent 2DEG generation under the gate, then normally-off operation is achieved, but the device structure becomes more complex
Solution Approach 1:
The insulating layer acts as an intermediary that eliminates the need for a thick 2DEG controlling layer. By placing the insulator between the gate and the 2DEG controlling layer, the patent achieves normally-off operation with a standard-thickness 2DEG controlling layer, thus preventing 2DEG formation under the gate without requiring excessive thickness that would complicate the structure.
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
The solution enables a field effect transistor with a uniform threshold voltage and high reliability, capable of operating as a normally-off switch with enhanced electron mobility and reduced complexity in circuitry.
Implementation Method 1
The heterojunction is configured with two types of nitride semiconductors having different bandgap energies from each other and is able to generate a two-dimensional electron gas layer (2DEG layer) near the junction plane
Data Source
AI summary
A field effect transistor device is provided by the invention. The field effect transistor device includes: a substrate; a buffer layer, a channel layer, and a first barrier layer sequentially disposed on the substrate; a two-dimensional electron gas controlling layer disposed on the first barrier layer; a second barrier layer disposed on the two-dimensional electron gas controlling layer, wherein the second barrier layer has a recess passing through the second barrier layer; and a gate electrode filled into the recess and separated from the second barrier layer and the two-dimensional electron gas controlling layer by an insulating layer.