Multi-Gate HEMT Recess Depth Tuning for Linear Transconductance
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Solution Overview
Problem
Conventional recessed-gate aluminum gallium nitride (AlGaN)/GaN high electron mobility transistors (HEMTs) face challenges in improving transconductance linearity while maintaining a positive threshold voltage, often resulting in reduced carrier concentration and decreased range of linear transconductance.
Innovation Solution
The implementation of multi-gate HEMTs with tuned recess depth gates, where at least one gate is recessed deeper than others, allows for a positive shift in threshold voltage with minimal impact on transconductance linearity, achieved by adjusting the recess depths and distances between gates to optimize transconductance linearity and threshold voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the gate-to-channel distance is increased to improve transconductance linearity, then transconductance linearity is improved, but the threshold voltage shifts to a more negative voltage causing the HEMT to operate in depletion mode rather than enhancement mode
Solution Approach 1:
The patent divides the single gate structure into multiple gates (first gate and second gate) with different recess depths. The first gate has a greater recess depth to provide stronger control and positive threshold voltage shift, while the second gate has a smaller recess depth to maintain transconductance linearity. This segmentation allows independent optimization of each gate's function to resolve the contradiction between threshold voltage control and transconductance linearity.
2Ease of operation
If recess depth is increased to shift threshold voltage positively, then threshold voltage is shifted positively, but carrier concentration in the 2DEG layer is reduced impacting transconductance linearity
Solution Approach 1:
The patent applies different recess depths to different gates based on their specific functions. The first gate has a greater recess depth locally to achieve strong threshold voltage control, while the second gate has a smaller recess depth locally to preserve carrier concentration and transconductance linearity. This local differentiation of quality (recess depth) allows each gate to optimize its specific function without compromising the other.
3Ease of operation
If a single gate is recessed to achieve enhancement mode operation, then the HEMT operates in enhancement mode, but the range of operation in which transconductance remains approximately linear is reduced
Solution Approach 1:
The patent segments the gate control function into multiple gates with different recess depths. The first gate with greater recess ensures enhancement mode operation, while the second gate with smaller recess extends the linear operation range. This segmentation enables the system to achieve both enhancement mode operation and extended linear range simultaneously.
Solution Approach 2:
The patent creates a dynamic multi-gate system where the first gate and second gate work together with different recess depths to dynamically control the 2DEG channel. This dynamic control structure allows the HEMT to maintain enhancement mode operation while extending the voltage range over which transconductance remains linear, overcoming the limitations of single-gate recessed structures.
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 approach enhances transconductance linearity, maintaining peak transconductance over a broader range of input voltages compared to conventional single-gate HEMTs, while ensuring the HEMT operates in enhancement mode, thus improving device performance in RF applications.
Implementation Method 1
A barrier layer is a wide band-gap donor-supply layer disposed in contact above a narrow band-gap channel layer ('buffer layer'). A heterojunction is created at the intersection of the barrier layer and the buffer layer. The conduction channel in a HEMT can be gate controlled to form a two-dimensional electron gas (2DEG) layer, which is a very thin layer of highly mobile conducting electrons with very high concentration formed at the heterojunction.
Implementation Method 2
Recessing the gate 114 into the barrier layer 102 reduces the thickness in the Y-axis direction of the barrier layer 102 under the gate 114, causing the Fermi level at the AlGaN/GaN heterojunction 108 to fall below the minimum conduction band of the barrier layer 102. This depletes the 2DEG 106 of high mobility carriers creating an opening 118 in the 2DEG 106.
Data Source
AI summary
A multi-gate HEMT includes at least two gates, with at least one recessed the same depth or at a deeper depth in a barrier layer than at least one other gate. Recessing a gate decreases the thickness of the barrier layer beneath the gate, reducing a density of high mobility carriers in a two-dimensional electron gas layer (2DEG) conductive channel formed at the heterojunction of a barrier layer and a buffer layer below the recessed gate. The recessed gate can increase gate control of the 2DEG conductive channel. The multi-gate HEMT has at least one gate recessed the same depth or a deeper depth into the buffer layer than another gate, which forms at least two different turn-on voltages for different gates. This can achieve improvement of transconductance linearity and a positive shift of the threshold voltage.