GaN FET Composition-Modulated Buffer for Short Channel Effects
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Solution Overview
Problem
GaN-type HJFETs face performance degradation due to short channel effects when gate length is shortened for millimeter or submillimeter wave bands, leading to electron injection from the channel layer to the buffer layer, which affects break-down voltage and pinch-off performance.
Innovation Solution
A multilayered epitaxial film structure with a composition-modulated buffer layer is used, where the Al composition is monotonically reduced from the substrate to the channel layer, generating negative polarized charges and creating a convex conduction band edge, thereby suppressing electron injection and enhancing carrier confinement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If gate length is shortened for millimeter or submillimeter wave bands, then high frequency performance is improved, but short channel effects cause electron injection from channel layer to buffer layer, degrading device performance
Solution Approach 1:
The buffer layer is designed with composition modulation, creating different Al composition regions (higher Al content near substrate, lower Al content near channel layer) to provide different local properties. This gradient structure generates negative polarized charges that form a convex conduction band edge, creating an energy barrier that prevents electron injection while maintaining high frequency performance
Solution Approach 2:
The buffer layer uses composite AlGaN material with varying composition ratios, combining regions of different Al content to achieve both carrier confinement and lattice mismatch management. The composite structure provides both the necessary electrical properties for high frequency operation and mechanical properties for device reliability
2Reliability
If Al composition in buffer layer is increased to suppress electron injection, then carrier confinement is improved, but lattice mismatch increases causing dislocation and reducing device reliability
Solution Approach 1:
The buffer layer employs composition modulation with different Al content in different regions. The Al composition is higher near the substrate to provide strong carrier confinement and lower near the channel layer to reduce lattice mismatch, achieving both goals simultaneously through spatially varying local properties
Solution Approach 2:
The Al composition ratio in the buffer layer is varied continuously or step-wise from the substrate interface to the channel layer interface, creating a gradient structure. This parameter change generates negative polarized charges that form a convex conduction band edge, providing carrier confinement without excessive lattice mismatch
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 effectively prevents device performance degradation, improves break-down voltage, and enhances high-frequency operation gains by reducing carrier injection and leak current, while maintaining excellent pinch-off characteristics.
Implementation Method 1
the spontaneous polarization and piezo polarization effects continuously changes in magnitude with composition change of the semiconductor material
Implementation Method 2
the spontaneous polarization and piezo polarization effects continuously changes in magnitude with composition change of the semiconductor material
Implementation Method 3
band gap difference ΔEc in a conduction band therebetween is use to achieve confinement of two-dimensional electron gas at a heterojunction interface between the GaN layer of the channel layer and AlGaN electron supply layer
Data Source
AI summary
In a group III nitride-type field effect transistor, the present invention reduces a leak current component by conduction of residual carriers in a buffer layer, and achieves improvement in a break-down voltage, and enhances a carrier confinement effect (carrier confinement) of a channel to improve pinch-off characteristics (to suppress a short channel effect). For example, when applying the present invention to a GaN-type field effect transistor, besides GaN of a channel layer, a composition-modulated (composition-gradient) AlGaN layer in which aluminum composition reduces toward a top gradually or stepwise is used as a buffer layer (hetero buffer). To gate length Lg of a FET to be prepared, a sum a of layer thicknesses of an electron supply layer and a channel layer is selected so as to fulfill Lg/a≥5, and in such a case, the layer thickness of the channel layer is selected in a range of not exceeding 5 times (about 500 Å) as long as a de Broglie wavelength of two-dimensional electron gas accumulated in the channel layer in room temperature.