Forem: INSU SHIN

What is the LED & FND?

INSU SHIN — Sat, 14 Feb 2026 06:53:57 +0000

Overview of LED and FND
Before presenting the experimental results, this section provides an overview of the operational principles of LEDs and FNDs.

In modern society, we interact closely with a myriad of electronic devices in our daily lives, such as televisions, smartphones, smartwatches, and tablet PCs. As the primary user interface for these devices, display technology that visually communicates information is an indispensable component. Even the most advanced, high-resolution displays of today are fundamentally rooted in the operating principles of basic optoelectronic components, such as Light-Emitting Diodes (LEDs) and Seven-Segment Displays. Therefore, this experiment aims to build a foundational understanding of modern display systems by practically implementing and analyzing the electrical characteristics and driving mechanisms of LEDs and Seven-Segment Displays, which form the cornerstone of display technology.

1. Operating Principle of LEDs
First, an LED (Light Emitting Diode) is a semiconductor device that emits light when an electric current flows through it. As a type of diode, it operates only when current flows in a specific direction (forward bias). When current flows, electrons recombine with holes to release energy, which is emitted in the form of light (photons).
Since LEDs share the same PN junction structure as standard diodes, the color of the emitted light depends on the energy band gap of the semiconductor material. A larger energy difference results in shorter wavelengths (e.g., blue), while a smaller energy difference results in longer wavelengths (e.g., red).

Forward Voltage vs. Cut-in Voltage
The forward voltage of an LED refers to the minimum voltage required for light emission, which differs slightly from the standard diode "cut-in" voltage. While it varies by device, the forward voltage typically ranges from 1.8V to 3.3V. In contrast, the cut-in voltage refers to the threshold where the diode begins to conduct significant current, leading to a rapid exponential increase in current flow.

Circuit Protection and Resistance
When designing circuits with LEDs, a current-limiting resistor is essential because LEDs are highly sensitive to current changes. If the current exceeds the rated maximum, the device can be easily damaged. Therefore, an appropriate resistor value must be calculated based on the supply voltage and the LED's forward voltage to ensure safe operation and accurate measurement.

Reverse Bias and Protection
Since LEDs are PN junction diodes, they are subject to breakdown phenomena. However, LEDs typically have a very low reverse breakdown voltage (often below 5V) and can be damaged before or immediately upon reaching this threshold. For this reason, extreme caution is required when applying reverse bias, or a standard rectifier diode should be connected in parallel for protection.

Why is a parallel connection necessary?
A series connection can control the direction of the current, but it cannot absorb the reverse voltage itself. Conversely, a parallel connection protects the LED by bypassing the current (providing an alternative path) when a reverse voltage or transient spike occurs.

2. Overview of FND (7-Segment Display)
The FND (Flexible Numeric Display), commonly referred to as a 7-Segment Display, is an electronic display component capable of representing decimal numerals (0 through 9) and specific alphabetic characters by illuminating combinations of seven distinct segments.

Structure and Configuration The device consists of the following key elements
Seven LED Segments : The bars that form the digits.
Decimal Point (DP) - A dot-shaped LED used to indicate a decimal point.
Depending on the internal wiring, 7-segment displays are classified into two types:
Common Anode - All anodes of the LEDs are connected together. A specific segment lights up when a LOW signal (logic 0) is applied to its cathode.
Common Cathode - All cathodes of the LEDs are connected together. A specific segment lights up when a HIGH signal (logic 1) is applied to its anode.

Driving Methods There are two primary methods for driving 7-segment displays
Direct Drive (Static Drive)
Each segment is connected to an individual I/O pin.
Pros/Cons - While the circuit design is simple, the required number of I/O pins increases significantly as the number of digits increases.

Multiplexing (Dynamic Drive)
The common terminals of multiple digits are selected sequentially. Although only one digit is lit at a time, high-speed scanning creates the illusion that all digits are lit simultaneously due to the persistence of vision.

Pros/Cons
This method conserves I/O pins. Although brightness may decrease due to the duty cycle, this can be compensated for by adjusting the resistance values and the scanning frequency.

Definition of I/O Pins I/O (Input/Output) pins are terminals designed to exchange data between digital circuits (such as microcontrollers) and external devices.

Input : Reads digital signals from external sensors, switches, or buttons.
Output : Sends processed signals from the microcontroller to external devices like LEDs.

Measurement Data
We constructed a circuit using Red and Yellow LEDs and measured the voltage drop across the LED and the current flowing through the circuit while increasing the power supply voltage from 0.5V to 6.0V.

Analysis and Observations
Based on the measured data, the following characteristics were observed

Light Emission Threshold
The LEDs remained off at low voltages. Light emission began only after the voltage exceeded a specific threshold (approximately 1.8V to 2.0V). The brightness increased as the voltage rose, reaching its maximum intensity at around 5V.

Voltage Clamping
As shown in the tables, when the input voltage was low (below 1.5V), the measured voltage across the LED was nearly identical to the input voltage, indicating an open circuit state. However, once the input exceeded 2.0V ~ 2.5V, the voltage across the LED did not increase significantly, remaining clamped between 1.9V and 2.3V, even as the input voltage continued to rise.

Current Surge
In contrast to the voltage, the current remained negligible until the voltage approached the LED's forward voltage (approx. 1.8V). Beyond 2.0V, the current increased rapidly, demonstrating the exponential nature of the diode's I-V characteristic.

Discussion
Current-Controlled Brightness
The experiment confirms that an LED is a current-controlled device. Although the voltage across the LED remained relatively constant after turning on, the brightness continued to increase. This indicates that the intensity of light emission is determined by the magnitude of the current flowing through the device, not the voltage across it.

Non-Ohmic Behavior
Unlike resistors that follow Ohm's Law linearly, the LED exhibited non-linear (Non-Ohmic) behavior. While the current increased drastically from roughly 1mA to 37mA, the voltage across the LED only changed slightly (approx. 0.2V ~ 0.3V). This implies that the dynamic resistance of the LED decreases significantly in the forward-biased region.

Safety Consideration (Over-Current)
It is important to note that at a 6V input, the current reached approximately 37mA. Since standard 5mm LEDs typically have a maximum rated current of 20mA, driving the LED at 37mA exceeds its safe operating range. Prolonged operation at this level can lead to overheating and permanent damage. Therefore, in practical circuit design, a current-limiting resistor with a higher resistance value must be selected to maintain the current within a safe range (typically 10mA ~ 20mA).

Conclusion
Through this experiment, we verified the operational characteristics of LEDs and 7-segment displays. We confirmed that LEDs require a minimum forward voltage to conduct and that their brightness is controlled by current. Additionally, the experimental data highlighted the importance of using appropriate resistors to protect the semiconductor elements from over-current damage due to their non-linear I-V characteristics.

LED Serial & parallel measurement & conclusion

In this experiment, the illumination of four light-emitting diodes (LEDs) was evaluated by configuring them in both parallel and series circuits with an applied source voltage of 3.5V. As observed in the visual data, the LEDs successfully illuminated in the parallel configuration but failed to emit light in the series configuration. This discrepancy can be attributed to Kirchhoff's circuit laws and the forward voltage (cut-in voltage) characteristics of the LEDs.
Based on previous experiments, the forward voltage required to activate a single LED is approximately 2V. In a series configuration, the applied voltage is divided across each component. Consequently, applying 3.5V across four LEDs results in an insufficient voltage drop per LED (less than 1V), which is well below the 2V threshold required for activation. Therefore, to successfully illuminate the four LEDs in a series circuit, a total supply voltage of approximately 7V to 8V is necessary. Conversely, in a parallel configuration, the full 3.5V source voltage is applied uniformly across each parallel branch. Because this applied voltage exceeds the 2V forward voltage requirement, all four LEDs illuminate successfully.

FND measurement & conclusion

In this experiment, numeric characters were implemented using a Seven-Segment Display. Among the two primary configurations—common anode and common cathode—the common anode type was utilized first. In a common anode display, all segment anodes are tied together to a positive voltage supply ($V_{CC}$), while the individual segments are activated by connecting their respective cathode terminals to ground.
Although only a single display unit was used, rendering specific numbers required a substantial number of jumper wires. While the theoretical principle of displaying alphanumeric characters is straight forward once the pin configuration is understood, the practical implementation proved to be complex the dense cluster of jumper wires significantly increased the probability of wiring errors. Through these consecutive LED and Seven-Segment Display experiments, the fundamental principles of basic display implementation were successfully comprehended.

Furthermore, the complexity and error-proneness experienced during manual wiring highlight the critical limitations of directly driving displays in practical circuits. Relying solely on jumper wires is inefficient and unscalable for multi-digit displays. To mitigate these issues in real-world applications, it is essential to utilize dedicated decoder/driver ICs (e.g., BCD-to-Seven-Segment decoders) or to implement multiplexing techniques. Ultimately, this foundational experiment not only demonstrates basic display operation but also provides practical insight into why digital logic controllers and driver circuits are indispensable in modern electronic display systems.

ABOUT DISPLAY , TFT and Wire Bonding

INSU SHIN — Sun, 08 Feb 2026 11:52:11 +0000

This study is based on research and experimental work conducted on display devices and thin-film transistors (TFTs). The manuscript is written from the perspective of hands-on experience, describing the characteristics of each device as well as the wire bonding process, focusing on the concepts and practical knowledge acquired through experimentation.

PROJECT DISPLAY & TFT

Glass Cleaning
OLED Manufacturing Process
OLED Measurement – (I-V-L Measurement system)
TFT Measurement - (Probe Station)
OLED & TFT Wire Bonding

** Glass Cleaning**
This process is designed to remove foreign substances and impurities from the glass substrate prior to the attachment of the PET film.

It can classify total 9 steps.

Ultrasonic Cleaning (Acetone) - Submerge the glass in an acetone solution. Place it in the ultrasonic cleaner and process at 40℃ for 10 minutes.
Rinsing - After 10 minutes, rinse the glass thoroughly with DI (Deionized) water.
Ultrasonic Cleaning (IPA) - Submerge the glass in an IPA (Isopropy Alcohol) solution. Place it in the ultrasonic cleaner and process at 40℃ for 10 minutes.
Rinsing - Rinse the glass again with DI water.
Ultrasonic Cleaning (DI Water) - Submerge the glass in DI water. Place it in the ultrasonic cleaner and process at 40℃ for 10 minutes.
Drying - After the final DI water rinse, hold the glass firmly with tweezers and remove all residual moisture and solution using N_2gas.
Dehydration Bake - Place aluminum foil on a hot plate and set the glass on top (face up). Heat at 270℃for 30 minutes to ensure complete dehydration.
NOA Coating (Planarization) - To ensure a smooth and level OLED surface, coat the glass with NOA (Norland Optical Adhesive). This step prevents potential contamination during the PET attachment process. The substrate is secured via vacuum. Ensure the center is properly fixed to prevent the substrate from detaching. Apply only enough NOA to cover the surface without overflowing. Start the device using the dial; the coating process is complete once the waveform is confirmed on the display.
UV Treatment - Place the glass on a temporary foil tray and transfer it to the UV chamber. Position the glass so its center aligns with the exposure mark. Operate the UV machine for 30 to 40 minutes to remove any remaining organic residues.

OLED Manufacturing Process
Substrate Preparation
Glass is used as the base substrate. Once the glass cleaning is complete, a PET film is attached to the deposition side of the glass substrate using adhesive tape.

Deposition Process(CVD)
OLED fabrication utilizes CVD (Chemical Vapor Deposition). CVD is a process where organic and inorganic materials are deposited onto OLED devices through chemical reactions. These reactions are triggered by heating materials in a high-temperature, negative-pressure (vacuum) environment.

Chamber Configuration and Material Handling
When facing the equipment, the left chamber is dedicated to organic material deposition, while the right chamber is used for metal deposition.
Transfer System - The interior of the transfer chamber, which houses the robotic arm, is filled with Nitrogen and maintained at positive pressure. The devices are moved between chambers exclusively by the robotic arm.

Patterning with Masks
Patterns are created by placing the substrate and overlaying it with a mask of the desired design. When organic and inorganic materials are deposited, they pass through the common openings of the masks, ensuring the materials are applied only to the specific intended areas.

System Control and Precision
Process parameters such as recipes, pressure, and temperature are configured and managed via a PC. This stage requires extreme precision, as even minute variations in these conditions can significantly impact the final quality and performance of the OLED device.

** TFT(Thin Film Transistor) Manufacturing Process **
A Thin Film Transistor (TFT) is a fundamental component of modern displays, acting as a microscopic switch that controls each individual pixel. By regulating the flow of electricity, TFTs determine when a pixel turns on or off and how much light it emits, which is essential for rendering high-resolution images.

Substrate Preparation and Dehydration
The process begins with thorough glass cleaning (using the method previously described) to remove all foreign particles. After cleaning, the glass is baked in an industrial oven to eliminate any residual microscopic moisture. This dehydration step is critical to minimizing impurities that could compromise the thin film's integrity.

Masking and Alignment
A patterned mask is placed over the cleaned device and secured firmly. Before loading it into the sputtering system, a transparent acrylic plate is used to fix the masked device in the optimal position for IGZO (Indium Gallium Zinc Oxide)deposition.
Purpose of Rigid Fixing - These multiple securing steps are implemented to prevent any displacement or vibration of the substrate during the process, ensuring maximum precision and accuracy.

Characteristics of IGZO Material
IGZO is a cutting-edge semiconductor material with a unique atomic arrangement that ensures stable performance. Its key advantages include:
High Electron Mobility - It offers approximately 30 times higher electron mobility than conventional amorphous silicon (a-Si).
Low Leakage Current - It significantly reduces power consumption, contributing to longer battery life in portable devices.

Sputtering Process (Physical Vapor Deposition - PVD)
The sputtering equipment utilizes the PVD (Physical Vapor Deposition)method. This is a physical deposition technique where high-energy ions collide with the target material, causing atoms to be ejected and subsequently form a thin film on the substrate.
Operational Note - Due to the nature of this physical mechanism, the substrate must be loaded into the sputtering chamber with the deposition side facing the source material (IGZO) to ensure effective film formation.

** * OLED I-V-L Characterization (Device Performance Measurement)**
The I-V-L system is used to measure and analyze the electro-optical characteristics of the fabricated OLED devices.

System Startup and Calibration
Power on the measurement equipment in the designated sequence.
Launch the control software on the PC and verify that all three main components are successfully connected (indicated by green status lights).

Sample Loading and Configuration
Identify whether the device is Top Emission or Bottom Emission.
Place the OLED device into the Sample Test Fixture (Dark Box)and toggle the corresponding switches to align with the device's pixel positions.
Adjust the camera focus to monitor the emission area clearly.

Functional Verification and Measurement (Sweep)
Functional Check - In the 'One Point' session, apply a voltage of approximately 3V to 4V (Cell On)to verify that the device emits light normally. Once confirmed, turn the cell off.
IVL Sweep Setup - Access the IVL Sweep mode to configure the measurement parameters:
Start/Stop Value - Set the initial and final voltage levels.
Step Value - Define the voltage increment for the sweep.
Execution - Start the measurement to capture the electrical and optical data.

Data Analysis and Visualization
Upon completion, the measured data is exported to an Excel file to generate the following characteristic graphs Current-Voltage (I-V), Luminance-Voltage (L-V), Luminance-Current Density (L-J), Electroluminescence (EL) Spectrum
In this session, measurements were conducted for Blue, QD (Quantum Dot) Blue, and Red OLED devices.

** Analysis of OLED Characterization Results**

Current-Voltage (I-V) Characteristics
Comparison of Blue and QD Blue - The current flow for both Blue and QD Blue devices is nearly identical. This confirms that the application of a QD (Quantum Dot) film is a passive process that does not alter the fundamental electrical properties of the underlying OLED device.
Blue/QD Blue vs. Red - A significant difference in current behavior was observed between the Blue and Red devices. While the current for Blue and QD Blue increased sharply starting from 4V, the Red device did not show a substantial increase within the measured range (0V to 5V).

Luminance - Current Density (L-J) and Efficiency
Efficiency Metric - The Luminance vs. Current Density graph serves as a key indicator of luminous efficiency, showing which device produces the most light for a given amount of electrical current.
Device Performance - The data reveals that the Red device is the most efficient, as it reaches the benchmark luminance of 30cd/m² at the lowest current density among the three.
Impact of QD Film - The QD Blue device exhibited the lowest luminance. This is attributed to the presence of the QD film on top of the Blue device, which causes some light loss or absorption, resulting in a dimmer appearance compared to the bare Blue device.

Spectral Analysis and Color Conversion
Wavelength Distribution - The spectral peaks follow the order of Blue < QD Blue < Red, with Blue having the shortest wavelength.
Color Shifting - It was observed that the Blue light shifted toward the red spectrum upon the application of the QD film.
Conclusion - This experiment successfully demonstrated that attaching a QD film to a standard Blue OLED device allows for color conversion (wavelength shifting)without modifying the device's basic electrical characteristics, such as the I-V profile.

** TFT Characterization using a Probe Station**
The Probe Station is utilized to measure the electrical characteristics (such as I–V curves)of the fabricated Thin Film Transistor (TFT) devices.

Device Loading and Stabilization
Place the TFT substrate onto the stage inside the probe station.
Activate the vacuum system to securely fix the device in place, ensuring stability during the measurement.

Microscopic Alignment and Probing
Using the integrated microscope, identify the precise locations of the Drain, Source, and Gate electrodes.
Carefully adjust the probe arms to ensure that each probe tip makes accurate electrical contact with its corresponding electrode.

Software Configuration and Parameter Setting
Once a stable contact is confirmed, configure the measurement software on the PC by assigning the Drain, Gate, and Source terminals.
Set the graph display to a Logarithmic (Log) Scale to clearly observe the subthreshold behavior and off-current.
Define the specific measurement conditions, including the Drain Voltage V_D and Gate Voltage VGs weep ranges.

Execution and Data Management
Initiate the measurement to capture the transfer and output characteristics of the TFT.
Upon completion, the data is automatically exported and saved as an Excel file, allowing for the analysis of current and voltage values under various conditions.

** Analysis of TFT Characterization Results**
Our team conducted 5 to 6 measurements for each of the two fabricated TFT devices. Based on the resulting graphs, Device 1 (Left)demonstrated superior performance compared to Device 2 (Right). The analysis is based on the following observations:
Consistency of Vth and Turn-on Voltage - The graphs indicate that Device 1 has a highly consistent Turn-on voltage and Threshold Voltage Vth across multiple sweeps.
Stability in the Saturation Region - At a fully turned-on state Vgs = 30V, the Drain Current ID in Device 1 remained stable and uniform. In contrast, Device 2 exhibited fluctuating and inconsistent current flow in the same region.

Conclusion - Device 2 showed higher deviation and lower reliability compared to Device 1. Therefore, Device 1 is identified as the better-performing TFT due to its superior electrical consistency and stability.

** Wire Bonding (TFT-OLED Interconnection)**
Wire Bonding is the critical final stage where the TFT and OLED components are electrically connected. This step is vital as the quality of the interconnection directly determines the final device's overall performance.
Experimental Approach (Innovative Solution)
For this experiment, our team utilized the internal metal wire from a bread twist tie. We carefully removed the outer plastic casing and adhesive to extract the conductive metal core.
Conductivity Verification
Using an ohmmeter, we measured the resistance of the extracted wire, which was approximately 1.2Ω ~ 1.3Ω. We confirmed that this resistance level is low enough for effective signal transmission.
Interconnection Method
We used Silver Paste to establish a secure electrical contact between the components. Specifically, we bonded the Source electrode of the TFT to the Anode of the OLED to complete the integrated circuit.

** Analysis of OLED-TFT Wire Bonding Characterization Results**

First Measurment(Failed) During the first attempt, the process required extreme caution as any leakage of silver paste into the channel could cause a short circuit. While we were able to measure the I-V curve of the TFT, the OLED device failed to emit light.

Failure Analysis
After the unsuccessful trial, our team discussed and identified the following potential causes for the failure

OLED Device Issues - Since current was flowing but the device remained dark, we suspect an issue with the OLED itself. It is possible that the required driving current was excessively high, suggesting a need for a more efficient OLED device.
Silver Paste Contamination (Short Circuit) - There is a possibility that the silver paste penetrated the channel, leading to leakage current where the current flows through unintended paths instead of the device.
Poor Probe Contact - The failure may have resulted from unstable electrical contact between the probes and the electrodes (Poor Contact), preventing proper signal delivery to the device.

Second Measurment (Failed) Recognizing the issues encountered in the previous attempt, our team conducted re-measurements on a different day. We meticulously applied conductive silver paste to ensure a precise connection solely between the Source and the Anode, avoiding any short circuits. Additionally, the probe contact was verified by the teaching assistant (TA) before proceeding.

However, despite these efforts, only the I-V characteristic curves of the TFT were obtained, and the OLED device failed to emit light.
Upon analyzing the measurement data, we suspected that the current flowing through the circuit was lower than the required driving current for the OLED.

Consequently, we replaced the device with an OLED having a lower driving current requirement. Nevertheless, light emission was still not observed.

Third Measurment (Succes) After seeking assistance from the TA, we inspected the device and observed that a specific section of the OLED was not turning on consistently. Hypothesizing that the operating voltage range was the issue, we shifted the Gate Voltage sweep range by +10V.

Instead of the original range of -30V to 30V, we adjusted the measurement range to -20V to 40V. Following this adjustment, all devices operated normally, and successful light emission was observed.

Discussion & Conclusion Through the success of the final experiment, we clearly identified the root causes of the previous OLED emission failures
Insufficient Current The driving current required for the initial OLED was too high for the supplied current.
Device Inconsistency Certain parts of the OLED device showed inconsistent activation.
Operating Voltage Range The OLED device required a Gate Voltage above 30V to operate normally (The initial 30V limit was insufficient).

Through the failure and subsequent success of the TFT & OLED Wire Bonding experiment, we gained a comprehensive understanding of the OLED's driving current, operating voltage ranges, and the critical importance of precise bonding. Furthermore, we concluded that observing the TFT I-V characteristic curve alone is insufficient to verify the wire bonding result; the operating Gate Voltage plays a decisive role in the actual activation of the OLED.