Complementary Diodes Stabilize Threshold Voltage
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Solution Overview
Problem
In thin film transistor (TFT) processes, diode-connected transistors used to simulate diodes often suffer from significant threshold voltage variations due to process-induced shifts, making them unsuitable for reference voltage supply circuits in RFID and EAS applications.
Innovation Solution
The development of process variation-tolerant diodes and diode-connected TFTs, which utilize a complementary pair of NMOS and PMOS diodes in series, stabilizing the threshold voltage by leveraging the constant difference between their threshold voltages, and incorporating these in circuits like voltage clamp circuits and RFID tags.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If diode-connected transistors are used to simulate diodes in thin film processes, then the fabrication process becomes convenient and simple, but the threshold voltage varies significantly from run to run and lot to lot
Solution Approach 1:
The invention divides a single diode-connected transistor into two separate transistors (first transistor and second transistor) with different conductivity types. By segmenting the device and connecting them in series, the patent achieves process variation tolerance while maintaining fabrication simplicity. Each transistor can be manufactured using standard thin film processes, and their series combination compensates for individual variations.
Solution Approach 2:
The patent changes the electrical parameters of the system by using transistors with different conductivity types (n-type and p-type) having different threshold voltages. The series connection of these transistors creates a combined device where the total voltage drop is the sum of individual voltage drops, making the overall device less sensitive to process-induced threshold voltage shifts in any single transistor.
2Device complexity
If diode-connected TFTs are used in reference voltage supply circuits, then the circuit implementation is straightforward, but the forward voltage drop shows unacceptably large variation
Solution Approach 1:
The reference voltage supply circuit uses a segmented approach by employing two separate transistors in series instead of a single diode-connected transistor. This segmentation allows each transistor to contribute to the overall voltage drop, and the series combination provides stability against process variations, thereby improving reliability while keeping the circuit implementation relatively simple.
Solution Approach 2:
The invention creates a composite device structure by combining two transistors with different conductivity types in series. This composite structure leverages the complementary characteristics of n-type and p-type transistors to achieve a more stable forward voltage drop than either transistor could provide alone, enhancing the reliability of the reference voltage supply circuit.
3Device complexity
If a single diode-connected transistor is used, then the device structure is simple, but process-induced threshold voltage shifts cause significant performance variation
Solution Approach 1:
The patent segments the single diode-connected transistor into two separate transistors connected in series. While this increases the device structure slightly, it provides a significant advantage in threshold voltage control by making the overall device performance less sensitive to process-induced shifts in any individual transistor's threshold voltage.
Solution Approach 2:
The invention changes the device parameters by using transistors with different conductivity types and different threshold voltages. The series connection of these transistors creates a combined device with a total voltage drop that is more stable against process variations, thereby improving manufacturing precision without requiring complex fabrication processes.
Data Source
AI summary
Process variation-tolerant diodes and diode-connected thin film transistors (TFTs), printed or patterned structures (e.g., circuitry) containing such diodes and TFTs, methods of making the same, and applications of the same for identification tags and sensors are disclosed. A patterned structure comprising a complementary pair of diodes or diode-connected TFTs in series can stabilize the threshold voltage (Vt) of a diode manufactured using printing or laser writing techniques. The present invention advantageously utilizes the separation between the Vt of an NMOS TFT (Vtn) and the Vt of a PMOS TFT (Vtp) to establish and/or improve stability of a forward voltage drop across a printed or laser-written diode. Further applications of the present invention relate to reference voltage generators, voltage clamp circuits, methods of controlling voltages on related or differential signal transmission lines, and RFID and EAS tags and sensors.