FET-BJT Integrated Varactor for Wide Tuning Range
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Solution Overview
Problem
Existing semiconductor technologies lack a method to implement a field effect transistor (FET) as a varactor with a wide tuning range for use in tunable LC circuits and RF matching networks, where the FET can exhibit continuously variable capacitance.
Innovation Solution
A varactor is created by integrating a field effect transistor (FET) with a bipolar junction transistor (BJT), where the back gate of the FET shares an electrical connection with the base of the BJT, allowing a reverse voltage applied to the FET to create a continuously variable capacitance in its channel, enabling wide tuning range capabilities.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a FET is used as a varactor in tunable LC circuits, then the device can provide electronically tunable capacitance, but the tuning range is limited and the characteristics are not continuously variable
Solution Approach 1:
The patent merges a FET with a bipolar junction transistor (BJT) to create an integrated device that combines the capacitance modulation capability of the FET with the continuous current control of the BJT. The FET's gate controls the channel conductivity, while the BJT provides continuous base current control, enabling continuously variable capacitance over a wide tuning range without the limitations of conventional FET varactors.
Solution Approach 2:
The integrated FET-BJT device serves multiple functions simultaneously: the FET portion provides voltage-controlled capacitance modulation, while the BJT portion provides continuous current control and amplification. This multi-functionality allows the single device to achieve both wide tuning range and continuously variable capacitance characteristics that would otherwise require separate components.
2Adaptability or versatility
If a varactor diode is used to provide variable capacitance, then the capacitance can be tuned, but the tuning range is narrow and the device cannot provide wide frequency rejection
Solution Approach 1:
By integrating the FET with a BJT, the device combines the capacitance modulation capability of the FET with the frequency-selective properties of the BJT. The BJT's transistor action provides broad frequency rejection characteristics while the FET enables continuous capacitance tuning, achieving both wide tuning range and superior frequency rejection in a single device.
3Adaptability or versatility
If separate components are used to achieve wide tuning range and continuously variable capacitance, then the individual functions can be optimized, but the device complexity increases
Solution Approach 1:
The patent merges the FET and BJT into a single integrated device structure, reducing the number of discrete components while maintaining the ability to provide wide tuning range and continuously variable capacitance. The shared substrate and integrated architecture minimize inter-component parasitics and simplify the overall device complexity compared to using separate components.
Solution Approach 2:
The integrated FET-BJT device performs multiple functions within a single structure: voltage-controlled capacitance modulation, continuous current control, and frequency selection. This multi-functionality eliminates the need for separate components, reducing device complexity while achieving the desired tuning characteristics.
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 allows for improved RF isolation and tunable capacitance, enabling the FET to function as a continuous tunable LC resonator, suitable for applications such as RF pass gates and tunable output match elements with broad frequency rejection characteristics.
Implementation Method 1
a reverse voltage applied to the back gate of the FET creates a continuously variable capacitance in a channel of the FET
Implementation Method 2
The capacitance is variable and is a function of the voltage applied at its terminals
Data Source
AI summary
A device includes a field effect transistor (FET) integrated with at least a portion of a bipolar junction transistor (BJT), in which a back gate of the FET shares an electrical connection with a base of the BJT, and in which a reverse voltage can be applied to the back gate of the FET.