Double Recess FET Stabilization via Depleted Ohmic Layer
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Solution Overview
Problem
Compound semiconductor FETs with a double recess structure are unstable due to high surface state densities at the outer recess section, which affect the depletion layer width and drain current, making them sensitive to surface conditions.
Innovation Solution
A field effect transistor with a double recess structure where the first and second ohmic contact layers are completely depleted, forming a high-concentration space charge within the ohmic contact layer to absorb changes in surface state density, thereby minimizing the influence on the channel region.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a double recess structure is used to improve breakdown voltage, then breakdown voltage is improved, but transistor characteristics become unstable due to surface state density variations
Solution Approach 1:
An undoped spacer layer is introduced as an intermediary between the channel layer and the doped ohmic contact layer. This spacer layer acts as a buffer that isolates the channel region from the harmful surface state effects at the outer recess section, while still allowing the double recess structure to maintain its breakdown voltage improvement function.
Solution Approach 2:
Different regions of the device are assigned different doping characteristics: the channel layer remains undoped or lightly doped for optimal transport, while the ohmic contact layer is heavily doped for low contact resistance. The spacer layer specifically positioned at the outer recess has undoped or lightly doped regions to control the depletion layer and suppress surface state influence locally.
2Manufacturing precision
If the surface state density at the outer recess section varies due to dust adhesion or process conditions, then the depletion layer width changes, but this causes drain current to fluctuate
Solution Approach 1:
The undoped spacer layer serves as a protective intermediary that decouples the channel layer from surface state variations at the outer recess. By positioning this spacer layer between the channel and the surface-prone ohmic contact layer, changes in surface state density are absorbed by the spacer's depletion layer rather than directly affecting the channel.
Solution Approach 2:
The device structure is designed with a spacer layer that anticipates and cushions against future surface state variations. This preemptive structural design ensures that even if surface conditions change during manufacturing or operation, the channel region is already protected from these variations.
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 stabilizes the channel region by suppressing changes in the depletion layer width, maintaining consistent drain current and transistor characteristics despite variations in surface state density.
Implementation Method 1
each of the first and second ohmic contact layers of the recess sections has a completely depleted thickness
Implementation Method 2
a high-concentration space charge is formed within the ohmic contact layer
Data Source
AI summary
A field effect transistor having a double recess structure, which minimizes an influence exerted on a channel region depending upon the surface state of an outer recess section. In the field effect transistor having such a double recess structure, an ohmic contact layer at the surface of the outer recess section is made to have a thickness so as to be in a to completely depleted state.