HEMT Normally OFF Conversion via Segmented Gate and Ion Implantation
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Solution Overview
Problem
High Electron Mobility Transistors (HEMTs) are naturally 'normally ON' due to the two-dimensional electron gas (2DEG) generated at the interface of the channel and active layers, making them conductive without applied voltage, which is undesirable in power applications where a 'normally OFF' state is required to prevent current flow and reduce power consumption.
Innovation Solution
A gate structure is configured over the active layer to deplete the 2DEG under the gate, with a negatively charged region in the active layer further depleting the 2DEG, increasing the threshold voltage without reducing carrier density or increasing channel resistance, achieved by forming a p-doped and n-doped layer gate structure and implanting negative ions into the active layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a gate structure is added to deplete 2DEG and convert HEMT to normally OFF state, then the threshold voltage increases and the device can be turned off, but the device complexity increases due to additional layers and processing steps
Solution Approach 1:
The gate structure is segmented into multiple functional layers: a first doped semiconductor layer (p-type) and a second doped semiconductor layer (n-type) stacked vertically. This segmentation allows independent optimization of each layer's doping concentration and thickness to achieve the desired threshold voltage while maintaining manufacturing feasibility.
Solution Approach 2:
Different regions of the gate structure have different doping types and concentrations. The p-type layer provides hole carriers while the n-type layer provides electron carriers, creating localized charge distributions that collectively achieve the normally OFF state. This local quality variation enables precise control of the depletion region without requiring uniform complex structures throughout.
2Reliability
If ion implantation is used to create negatively charged region in active layer, then the 2DEG depletion is enhanced and threshold voltage increases, but the manufacturing precision requirements increase
Solution Approach 1:
The negatively charged region is created in the active layer through ion implantation before the gate structure is formed. This preliminary action establishes the charge distribution foundation that works synergistically with the subsequently formed doped gate layers, allowing the threshold voltage to be tuned through combined effects rather than requiring extreme precision in a single step.
Solution Approach 2:
The ion implantation process parameters (ion type, energy, dose) are optimized to create a specific negative charge distribution in the active layer. By controlling these parameters, the depletion effect is enhanced without requiring excessive manufacturing precision, as the effect can be tuned through parameter adjustment rather than perfect geometric control.
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
This configuration converts the HEMT to a 'normally OFF' state with a positive threshold voltage, suitable for power applications, while maintaining high carrier density and low channel resistance, thus addressing the design concerns of power consumption and start-up issues.
Implementation Method 1
A two-dimensional electron gas (2DEG) is generated in the channel layer, adjacent an interface with the active layer
Implementation Method 2
The gate structure is configured to deplete the 2DEG under the gate structure
Implementation Method 3
A negatively charged region is formed in a gate area of the active layer
Data Source
AI summary
A semiconductor device includes a substrate, a channel layer formed over the substrate, an active layer formed over the channel layer, and a gate structure formed over the active layer. The active layer is configured to cause a two dimensional electron gas (2DEG) to be formed in the channel layer along an interface between the channel layer and the active layer. The gate structure is configured to deplete the 2DEG under the gate structure. The active layer has a negatively charged region under the gate structure. The negatively charged region is configured to further deplete the 2DEG under the gate structure.