GaN HEMT Back Surface Field Electrode for Current Collapse
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Solution Overview
Problem
High electron mobility transistors based on gallium nitride (GaN) face challenges with current collapse phenomena and increased on-resistance due to negative charge generation in trap regions, leading to reduced two-dimensional electron gas (2DEG) in the channel region, which affects their high-frequency and high-power performance.
Innovation Solution
The design includes a second field electrode disposed on the rear surface of the substrate, electrically connected to the source or gate electrode, which helps in preventing current collapse and increasing breakdown voltage by reducing the peak electric field around the drain-side edge of the gate electrode, and a method for fabricating this transistor involves forming a via hole through the substrate to expose the active layer and depositing a second field electrode made of conductive materials.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a gallium nitride-based high electron mobility transistor is manufactured to achieve high frequency and high power characteristics, then electronic mobility and saturated electron velocity are improved, but current collapse phenomenon occurs and on-resistance increases due to negative charge generation in trap regions
Solution Approach 1:
The patent introduces a back surface field electrode configured on the rear surface of the substrate, adding a spatial dimension (back surface) to the device structure. This back surface electrode creates an electric field that extends through the substrate thickness, counteracting the negative charge accumulation in trap regions and preventing current collapse while maintaining high electron mobility in the channel.
Solution Approach 2:
The back surface field electrode acts as an intermediary element that mediates between the positive and negative charges in the device. By applying a positive voltage to the back surface electrode, it attracts negative charges away from the trap regions in the active layer, thereby preventing current collapse and reducing on-resistance without affecting the high electron mobility characteristics.
2Speed
If a gallium nitride-based high electron mobility transistor is manufactured to achieve high frequency and high power characteristics, then electronic mobility and saturated electron velocity are improved, but on-resistance increases due to negative charge generation in trap regions
Solution Approach 1:
The back surface field electrode utilizes the thickness dimension of the substrate to create a vertical electric field that extends from the rear surface through the active layer. This additional dimensional approach allows the electrode to influence charge distribution in the trap regions without interfering with the horizontal electron transport in the channel, thereby maintaining high saturated electron velocity while reducing on-resistance.
Solution Approach 2:
The back surface field electrode serves as an intermediary that indirectly controls the charge distribution in the active layer. By applying voltage to the back surface electrode, it modulates the electric field in the trap regions, preventing negative charge accumulation that would otherwise increase on-resistance, while leaving the high velocity electron transport in the channel unaffected.
3Reliability
If a via hole is formed through the substrate to expose the active layer and a second field electrode is deposited, then leakage current is reduced and breakdown voltage is enhanced, but device complexity and fabrication difficulty increase
Solution Approach 1:
The fabrication process is segmented into distinct sequential steps: forming the via hole through the substrate, depositing the back surface field electrode material, and forming the gate electrode and insulating layer. This segmentation allows each step to be optimized independently and performed using standard semiconductor fabrication techniques, reducing overall complexity despite the additional process steps.
Solution Approach 2:
The via hole is formed preliminarily before depositing the back surface field electrode, and the insulating layer is formed preliminarily to define the gate electrode region. These preliminary actions prepare the substrate and structure in advance, allowing subsequent steps to proceed smoothly and reducing the need for complex rework or adjustments during fabrication.
Data Source
AI summary
A high electron mobility transistor includes a substrate including a first surface and a second surface facing each other and having a via hole passing through the first surface and the second surface, an active layer on the first surface, a cap layer on the active layer and including a gate recess region exposing a portion of the active layer, a source electrode and a drain electrode on one of the cap layer and the active layer, an insulating layer on the source electrode and the drain electrode and having on opening corresponding to the gate recess region to expose the gate recess region, a first field electrode on the insulating layer, a gate electrode electrically connected to the first field electrode on the insulating layer, and a second field electrode on the second surface and contacting the active layer through the via hole.