PD438C 4.8mm Semi-Lens Silicon PIN Photodiode Datasheet - 4.8mm Diameter - 32V Reverse Voltage - 150mW Power Dissipation - English Technical Document

1. Product Overview

The PD438C is a high-speed, highly sensitive silicon PIN photodiode housed in a cylindrical side-view plastic package. Its primary function is to convert incident light, particularly in the infrared spectrum, into an electrical current. A key feature of this component is that the epoxy package itself acts as an integrated infrared (IR) filter, which is spectrally matched to common IR emitters. This design simplifies system integration by reducing the need for external filtering. The device is characterized by fast response times, high photosensitivity, and a small junction capacitance, making it suitable for applications requiring quick and accurate light detection.

2. In-Depth Technical Parameter Analysis

2.1 Absolute Maximum Ratings

The device is designed to operate reliably within the following absolute limits, beyond which permanent damage may occur. The maximum reverse voltage (V_R) is 32V. The power dissipation (P_d) must not exceed 150 mW. The operational temperature range (T_opr) is from -40°C to +85°C, while the storage temperature (T_stg) extends from -40°C to +100°C. For assembly, the soldering temperature (T_sol) should be kept at 260°C for a duration not exceeding 5 seconds to prevent thermal damage to the package and semiconductor die.

2.2 Electro-Optical Characteristics

Under standard test conditions (Ta=25°C), the PD438C exhibits the following key performance parameters. Its spectral response bandwidth (λ_0.5) ranges from 400 nm to 1100 nm, with a peak sensitivity wavelength (λ_p) typically at 940 nm, aligning it perfectly with common infrared light sources. When illuminated with an irradiance of 5 mW/cm² at 940 nm, the typical open-circuit voltage (V_OC) is 0.35V. The short-circuit current (I_SC) is typically 18 µA under 1 mW/cm² at 940 nm. Under a reverse bias of 5V and the same irradiance, the reverse light current (I_L) is typically 18 µA (min. 10.2 µA). The dark current (I_d), which is the leakage current in the absence of light, is typically 5 nA (max. 30 nA) at a reverse voltage of 10V. The total terminal capacitance (C_t) is typically 25 pF at 3V reverse bias and 1 MHz. The rise and fall times (t_r/t_f) are both typically 50 ns when operating with a 10V reverse bias and a 1 kΩ load resistor.

3. Performance Curve Analysis

The datasheet provides several characteristic curves that are crucial for design engineers. The Spectral Sensitivity curve shows the relative responsivity of the photodiode across its operational wavelength range, confirming the peak at 940 nm. The Power Dissipation vs. Ambient Temperature graph illustrates the derating of the maximum allowable power as the operating temperature increases, which is essential for thermal management. The Dark Current vs. Ambient Temperature curve demonstrates how the leakage current increases with temperature, a critical factor for low-light or high-temperature applications. The Reverse Light Current vs. Irradiance (Ee) plot shows the linear relationship between incident light power and the generated photocurrent, confirming the device's predictable response. The Terminal Capacitance vs. Reverse Voltage curve indicates how the junction capacitance decreases with increasing reverse bias, which directly impacts the device's speed. Finally, the Response Time vs. Load Resistance graph shows how the rise/fall time is affected by the external load, guiding the selection of an appropriate load resistor for speed-critical circuits.

4. Mechanical and Package Information

4.1 Package Dimensions

The PD438C is packaged in a cylindrical side-view format with a nominal diameter of 4.8mm. The detailed mechanical drawing specifies all critical dimensions including lead spacing, package height, and lens geometry. The drawing notes that dimensional tolerances are typically ±0.25mm unless otherwise specified. The side-view configuration is particularly useful for applications where the light path is parallel to the mounting surface, such as in slot sensors or edge detection systems.

4.2 Polarity Identification

The device is a two-terminal component. The cathode is typically identified by a longer lead, a notch, or a flat spot on the package body. Correct polarity connection is essential when applying reverse bias for optimal performance in photoconductive mode.

5. Soldering and Assembly Guidelines

The component is rated for soldering at a peak temperature of 260°C. It is critical that the time above the liquidus temperature (typically around 217°C for lead-free solder) is limited to a maximum of 5 seconds to prevent excessive thermal stress on the epoxy package and the internal wire bonds. Standard reflow or wave soldering profiles for lead-free assemblies are generally applicable. Care should be taken to avoid mechanical stress on the leads during handling and placement.

6. Packaging and Ordering Information

The standard packing specification is 500 pieces per bag. Six bags are combined into one inner carton, and ten inner cartons constitute one master shipping carton, resulting in a total of 30,000 pieces per master carton. The product label includes fields for the customer's part number (CPN), the manufacturer's part number (P/N), packing quantity (QTY), and lot traceability information (LOT No.).

7. Application Suggestions

7.1 Typical Application Scenarios

The PD438C is well-suited for a variety of optoelectronic applications. Its high speed makes it ideal for high-speed photo detection in data communication links or pulse sensing. It is commonly used in consumer electronics such as cameras and camcorders (VCRs, video cameras) for autofocus systems, light metering, or tape-end detection. It serves as a reliable sensor in optoelectronic switches and interrupters for position sensing, object detection, and rotary encoder systems. The integrated IR filter makes it particularly effective in systems paired with 940 nm IR LEDs, filtering out unwanted visible light.

7.2 Design Considerations

When designing a circuit with the PD438C, several factors must be considered. For speed optimization, operate the photodiode with a sufficient reverse bias (e.g., 5V-10V) to minimize junction capacitance and use a low-value load resistor, as shown in the response time vs. load resistance curve, though this trades off with output voltage swing. A transimpedance amplifier (TIA) configuration is often preferred for converting the small photocurrent into a usable voltage while maintaining bandwidth. For noise-sensitive applications, the dark current specification and its temperature dependence are critical; cooling the device or using synchronous detection techniques may be necessary. The linearity of the photocurrent with irradiance simplifies optical power measurement designs. Ensure the optical aperture and alignment are correct for the side-view package orientation.

8. Technical Comparison and Differentiation

Compared to standard photodiodes without a lens or filter, the PD438C offers a distinct advantage due to its integrated semi-lens and IR-filtering epoxy. This eliminates the need for a separate optical filter, reducing component count, assembly complexity, and cost. The side-view package is a specific form factor that solves integration challenges in space-constrained designs where top-view sensors cannot be used. Its combination of relatively high speed (50 ns) and good sensitivity (18 µA at 1 mW/cm²) offers a balanced performance profile for many mid-range applications, positioning it between very high-speed, low-sensitivity devices and slower, high-sensitivity photodiodes.

9. Frequently Asked Questions (Based on Technical Parameters)

Q: What is the purpose of the "semi-lens"?
A: The semi-lens helps to focus incoming light onto the active area of the silicon chip, increasing the effective collection area and thus the responsivity (sensitivity) of the device compared to a flat window.

Q: Why is the peak sensitivity at 940 nm?
A: Silicon's inherent absorption properties peak in the near-infrared region. 940 nm is a very common wavelength for infrared emitters (LEDs), as it is invisible to the human eye and readily available. The epoxy is tuned to match this.

Q: Should I use this photodiode in photovoltaic (zero bias) or photoconductive (reverse bias) mode?
A: For highest speed and linearity, photoconductive mode (applying a reverse bias, e.g., 5V) is recommended. It reduces junction capacitance and widens the depletion region. Photovoltaic mode (zero bias) offers lower noise (no dark current) but is slower.

Q: How does temperature affect performance?
A: As shown in the curves, dark current increases significantly with temperature, which can be a source of noise. The photocurrent itself also has a slight temperature coefficient. For stable operation, temperature compensation or a controlled environment may be necessary in precision applications.

10. Practical Design and Usage Examples

Example 1: Infrared Proximity Sensor: An IR LED pulses at 940 nm. The reflected light is detected by the PD438C. The side-view package allows both emitter and detector to be placed on the same PCB, facing the same direction. The integrated IR filter in the PD438C helps reject ambient visible light, improving the signal-to-noise ratio of the reflected IR signal. A microcontroller measures the photodiode's current via a TIA to determine object presence or distance.

Example 2: Slotted Optical Switch: The PD438C is mounted on one side of a U-shaped bracket, facing an IR LED on the other side. An object passing through the slot interrupts the beam. The fast response time (50 ns) allows for detecting very high-speed events or encoding rapid motion.

11. Operating Principle Introduction

A PIN photodiode is a semiconductor device with a wide, lightly doped intrinsic (I) region sandwiched between a P-type and an N-type region. When photons with energy greater than the semiconductor's bandgap (for silicon, wavelengths shorter than ~1100 nm) strike the device, they generate electron-hole pairs in the intrinsic region. Under the influence of the built-in electric field (in photovoltaic mode) or an applied reverse bias (in photoconductive mode), these charge carriers are swept apart, creating a measurable photocurrent that is proportional to the incident light intensity. The wide intrinsic region allows for a larger depletion volume, which improves quantum efficiency (sensitivity) and reduces junction capacitance, enabling higher speed operation compared to a standard PN photodiode.

12. Industry Trends and Developments

The market for photodiodes like the PD438C continues to be driven by trends in automation, consumer electronics, and communication. There is a constant push for higher speed to support faster data transmission in optical links. Improved sensitivity (lower noise, higher responsivity) allows for operation with lower-power emitters or over longer distances. Miniaturization is another key trend, leading to photodiodes in smaller surface-mount packages. Furthermore, integration is advancing, with more devices incorporating the photodiode, amplifier, and sometimes even digital logic into a single package (e.g., photodiode arrays, integrated optical sensors). The PD438C, with its integrated optical filter, represents a step in this integration trend, simplifying the bill of materials for system designers.