Nitride Semiconductor Transistor with Charge Storage Layer
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Solution Overview
Problem
Conventional nitride semiconductor FETs are normally-on devices, which are unsuitable for high-power switching due to safety concerns, and existing methods to make them normally-off, such as p-type nitride semiconductor layers or Metal-Insulator-Semiconductor (MIS) junctions, face limitations in threshold voltage and current handling.
Innovation Solution
A nitride semiconductor transistor device with a substrate, nitride semiconductor layers, an insulating film, a charge storage layer, and control electrodes, where the charge storage layer accumulates negative charge to control the threshold voltage and enhance current flow, reducing the impact of interface traps and etching depth variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a p-type nitride semiconductor layer is inserted beneath the gate to form a PN junction type gate electrode to achieve normally-off operation, then the FET can be made normally-off, but the positive threshold voltage is limited to less than 2V which is insufficient for power-supply apparatuses requiring more than 3V
Solution Approach 1:
A charge storage layer is introduced as an intermediary between the gate electrode and the AlGaN layer. This charge storage layer accumulates negative charges that shift the threshold voltage to positive values greater than 3V, enabling normally-off operation with sufficient threshold voltage for power-supply apparatuses without being limited by the PN junction on-voltage
Solution Approach 2:
The threshold voltage is controlled by adjusting the amount of negative charges stored in the charge storage layer. By changing the charge storage amount, the threshold voltage can be precisely tuned to achieve the required positive value greater than 3V while maintaining normally-off operation
2Reliability
If the AlGaN layer thickness is reduced to make the threshold voltage positive and achieve normally-off operation, then the FET becomes normally-off, but the etching depth must be precisely controlled to suppress threshold voltage variation
Solution Approach 1:
The charge storage layer serves as a mediator that decouples the threshold voltage control from the AlGaN layer thickness. Instead of relying on precise etching depth control, the threshold voltage is controlled by adjusting the charge storage amount in the charge storage layer, significantly reducing manufacturing precision requirements
Solution Approach 2:
The gate structure is segmented into multiple functional layers: the gate electrode, the charge storage layer for threshold voltage control, and the AlGaN layer for channel formation. This segmentation allows independent optimization of each layer's thickness and properties without affecting overall device performance
3Object-generated harmful factors
If an insulating film is inserted beneath the gate to form a MIS junction gate electrode to enable normally-off operation with large positive gate voltage, then the amplitude range of gate voltage can be wide, but the conduction electron density is reduced and on-current is limited
Solution Approach 1:
The gate structure uses a composite configuration combining the insulating film for voltage isolation and the charge storage layer for electron accumulation. The charge storage layer is made of materials with appropriate electron affinity to accumulate negative charges while maintaining good interface with the AlGaN layer, enabling both wide voltage range and high conduction electron density
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 solution enables a normally-off nitride semiconductor transistor with improved threshold voltage uniformity and enhanced switching properties, including lower on-resistance and higher on-current, making it suitable for high-power applications.
Implementation Method 1
a charge storage layer made of metal or a low resistivity layer having a portion thereof formed over the first insulating film formed over the second nitride semiconductor layer
Implementation Method 2
a first control electrode which is electrostatic-capacitively coupled with the charge storage layer via a first capacitor
Data Source
AI summary
A nitride semiconductor transistor device provides a normally-off nitride semiconductor transistor device which is excellent in switching properties with less dispersion of the properties. The nitride semiconductor transistor device has a buffer layer, a GaN layer, and an AlGaN layer in turn grown on a substrate. A first insulating film, a charge storage layer, a second insulating film, and a control electrode are in turn grown on the AlGaN layer. A source electrode and a drain electrode are formed to sandwich the charge storage layer over the AlGaN layer. A threshold voltage to shut off an electric current flowing between the source and drain electrodes through a conductive channel induced at an interface of the AlGaN layer and the GaN layer is made positive by adjusting the charge stored in the charge storage layer.