Level Shift Circuit Speed via Diode Merging
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Solution Overview
Problem
The operation speed of level shift circuits in active matrix liquid crystal display devices using polysilicon thin film transistors is limited due to high ON resistance, which increases the voltage rise time constant, restricting the speed of the level shifting operation.
Innovation Solution
Incorporating a diode element connected in parallel with the polysilicon thin film transistor, where the diode is formed of a p-type semiconductor region within the n-type semiconductor region, allows for faster charging of the input node by reducing the effective resistance and increasing the voltage rise speed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a polysilicon thin film transistor is used in the level shift circuit, then the circuit can be fabricated on the same substrate as the active elements using polysilicon thin film transistors, but the ON resistance of the transistor is high which increases the voltage rise time constant and limits the operation speed
Solution Approach 1:
The patent combines the polysilicon thin film transistor with a diode element in a merged structure where the diode's anode is connected to the transistor's source and cathode to the drain. This merging allows the diode to provide a low-resistance discharge path that compensates for the transistor's high ON resistance, thereby reducing the voltage rise time constant and improving operation speed while maintaining fabrication compatibility on the same substrate
Solution Approach 2:
The diode element acts as an intermediary component that mediates the voltage discharge process. When the transistor is off, the diode provides a controlled discharge path for the load capacitance, enabling faster voltage transitions without requiring changes to the polysilicon transistor itself, thus improving speed while preserving the ease of manufacture benefit
2Reliability
If the load resistance element has high resistance to enable stable operation, then the level shift circuit can operate stably, but the voltage rise time constant increases which limits the operation speed
Solution Approach 1:
The patent introduces dynamic behavior through the diode element that switches between forward and reverse bias states. During the discharge phase, the diode becomes forward-biased and provides a low-resistance path, dynamically adjusting the effective resistance to achieve both stable operation and fast switching, thereby resolving the contradiction between reliability and speed
3Ease of manufacture
If the ON resistance of the polysilicon thin film transistor is high, then the transistor can be fabricated with standard polysilicon processes, but the voltage rise time constant at the input node increases which restricts the level shifting speed
Solution Approach 1:
The patent segments the voltage discharge function into two parts: the polysilicon transistor handles the switching control function while the separately added diode element handles the rapid discharge function. This segmentation allows each component to optimize its specific function, with the diode providing fast discharge to reduce voltage rise time without compromising the process compatibility of the polysilicon transistor
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 enhances the speed of the level shifting operation in display devices with polysilicon thin film transistors by reducing the rise time constant of the input node voltage, thereby improving the overall performance of the level shift circuit.
Implementation Method 1
allowing the input signal to charge the input node via the diode when the voltage exceeds the diode's forward voltage, thereby reducing the rise time constant
Data Source
AI summary
An object of the present invention is to increase the speed of the level shifting operation in a display device having a level shift circuit formed of polysilicon thin film transistors. The present invention provides a display device having a level shift circuit wherein the above described level shift circuit has: a thin film transistor having a semiconductor layer formed of a polysilicon layer; a load resistance element connected between a second electrode of the above described thin film transistor and a reference power supply; and a waveform rectifying circuit connected to the second electrode of the above described thin film transistor, and a diode element of which the anode region is connected to the first electrode of the above described thin film transistor and of which the cathode region is connected to the second electrode of the above described thin film transistor.