HEMT Access Region Structure for Faster Switching and Less Trapping
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Solution Overview
Problem
Conventional High Electron Mobility Transistors (HEMTs) suffer from trapped charges in the barrier and channel layers, leading to signal distortion and reduced switching speed due to the inability to effectively manage charge injection and trapping at the gate contact.
Innovation Solution
A modified access region with a lower surface barrier height is introduced at the upper surface of the barrier layer, forming a charge emission path that allows charges to conduct to the drain contact, reducing the likelihood of charge trapping and improving switching characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional barrier layer with high bandgap is used in HEMT, then high breakdown field and high power capability are achieved, but charge trapping occurs leading to signal distortion and reduced switching speed
Solution Approach 1:
The barrier layer is segmented into two distinct regions: a first barrier layer region with higher bandgap for high breakdown field, and a second barrier layer region with lower bandgap for reduced charge trapping. This segmentation allows each region to fulfill its specific function independently, resolving the contradiction between reliability and switching speed.
Solution Approach 2:
Different regions of the barrier layer are assigned different material compositions and bandgap characteristics tailored to their specific functional requirements. The first region near the gate contact uses wider bandgap material for high field tolerance, while the second region uses narrower bandgap material for efficient charge emission, achieving local optimization of both reliability and speed.
2Speed
If the barrier layer bandgap is reduced to improve switching speed, then charge trapping is reduced, but breakdown field strength and power capability deteriorate
Solution Approach 1:
The barrier layer is divided into functional segments where the first region maintains high bandgap for breakdown strength while the second region uses lower bandgap for fast charge emission. This segmentation enables the device to achieve both high switching speed and high breakdown field strength simultaneously.
Solution Approach 2:
The barrier layer exhibits spatially varying material properties: the first region has optimized composition for high field tolerance near the gate, while the second region has optimized composition for rapid charge emission toward the drain. This local quality differentiation resolves the trade-off between speed and reliability.
3Power
If charges are allowed to accumulate in the barrier layer to maintain high carrier concentration, then transconductance is improved, but signal distortion increases due to trapped charges
Solution Approach 1:
The second barrier layer region provides a localized low-bandgap pathway that enables trapped charges to be emitted without affecting the overall high carrier concentration in the channel. This local quality variation allows charge management that preserves transconductance while eliminating signal distortion.
Solution Approach 2:
The second barrier layer region acts as an intermediary emission pathway that facilitates charge release to the drain contact. This intermediary region with lower bandgap serves as a dedicated channel for charge emission, separating the charge accumulation function (maintaining transconductance) from the charge emission function (reducing distortion).
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 modified access region enhances switching speed and reduces signal distortion by providing a path for charges to flow to the drain, thereby improving the transient behavior and Schottky barrier height of the transistor.
Implementation Method 1
A modified access region with a lower surface barrier height is introduced at the upper surface of the barrier layer, forming a charge emission path that allows charges to conduct to the drain contact
Implementation Method 2
A two-dimensional electron gas (2DEG) may be formed at the heterojunction of two semiconductor materials with different bandgap energies, where the smaller bandgap material has a higher electron affinity than the wider bandgap material
Implementation Method 3
The conductivity of the 2DEG channel can be modulated by applying a voltage to a gate contact 32 formed on the barrier layer 22
Data Source
AI summary
A transistor device includes a semiconductor epitaxial layer structure including a channel layer and a barrier layer on the channel layer, wherein the barrier layer has a higher bandgap than the channel layer. A modified access region is provided at an upper surface of the barrier layer opposite the channel layer. The modified access region includes a material having a lower surface barrier height than the barrier layer. A source contact and a drain contact are formed on the barrier layer, and a gate contact is formed between source contact and the drain contact.