Three-Terminal Transcap Decouples AC and DC Signals
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Solution Overview
Problem
Existing semiconductor variable capacitors, such as varactors, face limitations in achieving high capacitance density, linearity, and low distortion, especially in integrated circuits, due to their two-terminal structure and sensitivity to process variations, which restricts their use in high-frequency applications.
Innovation Solution
A three-terminal MOS-based semiconductor variable capacitor (transcap) design that decouples AC and DC signals by modulating the capacitor area using a control terminal, reducing process sensitivity and enhancing linearity and capacitance density.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a two-terminal varactor structure is used to achieve variable capacitance, then the device can be manufactured with simple structure, but the capacitance value is distorted by AC voltage superimposed on DC control voltage and process variations affect performance
Solution Approach 1:
The patent divides the capacitor into two separate MOS capacitors (first and second MOS capacitors) with independent control terminals. This segmentation allows each capacitor to be controlled independently, enabling the AC and DC signals to be decoupled. The first MOS capacitor is controlled by a first control voltage while the second MOS capacitor is controlled by a second control voltage, preventing signal distortion while maintaining manufacturing simplicity.
2Quantity of substance
If reverse bias voltage is increased to obtain high capacitance value, then the capacitance increases, but the reverse-biased saturation current prevents achieving extremely high capacitance values
Solution Approach 1:
The patent changes the operating parameter from reverse bias voltage (used in varactors) to forward bias voltage control in MOS capacitors. By controlling the capacitance through gate voltage modulation of MOS capacitors rather than reverse bias modulation of pn-junction capacitors, the device can achieve extremely high capacitance values without being limited by reverse-biased saturation current.
3Adaptability or versatility
If varactor is used in input stages of high-quality RF receivers, then frequency tuning is achieved, but inter-modulation distortion increases due to capacitance sensitivity to signal voltage
Solution Approach 1:
The patent extracts the control function from the signal path by providing separate control terminals for each MOS capacitor. The control voltages are applied to the gates of the MOS capacitors independently from the signal terminals, removing the mechanism that causes inter-modulation distortion while preserving frequency tuning capability through electronic control.
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 transcap device achieves high capacitance density and linearity over a wide range of control voltages, minimizing distortion and process variations, making it suitable for integrated circuits and high-frequency applications.
Implementation Method 1
use the voltage of the control terminal to modulate the area of one of the capacitor plates of a MOS capacitance rather than the distance between the two equivalent electrodes of a pn-junction capacitor
Implementation Method 2
allow the control of the capacitance without overlapping the DC control voltage with the AC signal avoiding the distortion of the capacitance value during the circuit operation
Data Source
AI summary
A novel semiconductor variable capacitor is presented. The semiconductor structure is simple and is based on a semiconductor variable MOS capacitor structure suitable for integrated circuits, which has at least three terminals, one of which is used to modulate the equivalent capacitor area of the MOS structure by increasing or decreasing its DC voltage with respect to another terminal of the device, in order to change the capacitance over a wide ranges of values. Furthermore, the present invention decouples the AC signal and the DC control voltage minimizing the distortion and increasing the performance of the device, such as its control characteristic. The present invention is simple and only slightly dependent on the variations due to the fabrication process. It exhibits a high value of capacitance density and, if opportunely implemented, shows a quasi linear dependence of the capacitance value with respect to the voltage of its control terminal.