GaN FET with p-type Gate Region for Normally OFF Operation
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Solution Overview
Problem
GaN-based field-effect transistors (FETs) used as power transistors face challenges in being normally OFF type, which is essential for safety, while maintaining high current density and breakdown voltage, due to the difficulty in reducing polarization charge without compromising current density.
Innovation Solution
A field-effect transistor structure with a p-type nitride semiconductor layer and an undoped nitride semiconductor layer forming a pn junction at the gate region, allowing for a normally OFF state with high gate turn-on voltage and reduced gate leakage current, achieved by selectively etching the nitride semiconductor layers and introducing n-type impurities under source and drain electrodes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the Al composition ratio in the n-type AlGaN layer is reduced or the thickness of the n-type AlGaN layer is reduced to reduce polarization charge, then the transistor can be made normally OFF type, but the current density decreases
Solution Approach 1:
The invention changes the doping type parameter of the AlGaN layer from n-type to p-type. This fundamental parameter change allows the formation of a pn junction at the gate region, enabling normally OFF type operation through the depletion region formed by the pn junction while preserving high current density through the undoped GaN layer that maintains polarization charge for high electron mobility.
2Reliability
If a pn junction gate structure is used to achieve normally OFF type operation, then gate turn-on voltage increases and gate leakage current reduces, but device structure complexity increases
Solution Approach 1:
The invention applies local quality by introducing p-type doping only in the AlGaN layer at the gate region, while keeping the rest of the AlGaN layer undoped or with different doping characteristics. This localized modification creates the pn junction necessary for normally OFF operation and reduced gate leakage without requiring comprehensive structural changes throughout the entire device.
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 large current density and high breakdown voltage while ensuring the transistor is normally OFF, reducing series resistance and gate leakage current, thus enhancing safety and performance in power applications.
Implementation Method 1
electric charges are generated at a hetero interface due to spontaneous polarization and piezoelectric polarization
Implementation Method 2
electric charges are generated at a hetero interface due to spontaneous polarization and piezoelectric polarization
Implementation Method 3
a p-type nitride semiconductor layer and an undoped nitride semiconductor layer forming a pn junction at the gate region
Data Source
AI summary
An AlN buffer layer, an undoped GaN layer, an undoped AlGaN layer, a p-type GaN layer and a heavily doped p-type GaN layer are formed in this order. A gate electrode forms an Ohmic contact with the heavily doped p-type GaN layer. A source electrode and a drain electrode are provided on the undoped AlGaN layer. A pn junction is formed in a gate region by a two dimensional electron gas generated at an interface between the undoped AlGaN layer and the undoped GaN layer and the p-type GaN layer, so that a gate voltage can be increased.