Phototransistor: Spectral Range 400-1100 nm (Si)

Component Specifications

Definition

A phototransistor is a bipolar junction transistor (BJT) or field-effect transistor (FET) with a light-sensitive base or gate region. It operates by generating electron-hole pairs when exposed to light, which modulates the transistor's current flow. In optical couplers, it serves as the receiver component, detecting light from an LED or laser emitter to provide electrical isolation between circuits. Key characteristics include spectral response (typically 400-1100 nm for silicon), responsivity, response time, and dark current.

Working Principle

The phototransistor works on the photoelectric effect. When photons strike the base-collector junction, they generate electron-hole pairs, creating a base current proportional to light intensity. This current is amplified by the transistor's gain (β), producing a larger collector-emitter current. In optical couplers, light from an emitter (e.g., infrared LED) crosses an isolation barrier and activates the phototransistor, transmitting signals without electrical contact.

Materials

Typically made from silicon (Si) for visible to near-infrared detection, or germanium (Ge) for longer wavelengths. Package materials include epoxy resin (black for light blocking) or ceramic for high-temperature applications. Lead frames are often copper alloy with tin plating.

Technical Parameters

Parameter	Typical range	Notes & selection driver
Dark Current	10-100 nA
Responsivity	0.5-0.8 A/W at 850 nm
Response Time	2-10 μs
Spectral Range	400-1100 nm (Si)
Isolation Voltage	2500-5000 Vrms
Current Transfer Ratio (CTR)	20-600%
Collector Emitter Voltage (Vceo)	30-70 V

Ranges are indicative industry figures for RFQ preparation, not a supplier commitment. Confirm every value and standard with the legal manufacturer before ordering.

Standards

IEC 60747-5-5, JEDEC JESD22

Parent Products

This component is used in the following industrial products

Optical Coupler

An electronic component that transfers electrical signals between two isolated circuits using light waves.

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Engineering Analysis

Risks & Mitigation

Degradation of CTR over time due to LED aging
Temperature sensitivity affecting performance
Potential for saturation at high light levels
Electrostatic discharge (ESD) damage during handling

FMEA Triads

Trigger: LED emitter degradation in the optical coupler
Failure: Reduced CTR leading to signal loss or circuit malfunction
Mitigation: Use high-reliability LEDs, implement burn-in testing, and design circuits with CTR margin

Trigger: ESD exposure during installation
Failure: Permanent damage to the phototransistor junction, causing open or short circuits
Mitigation: Follow ESD protection protocols (e.g., grounded workstations, antistatic packaging)

Trigger: High ambient temperature
Failure: Increased dark current and reduced sensitivity, leading to signal errors
Mitigation: Select components rated for operational temperature range, add heat sinks or cooling

Industrial Ecosystem

Compatible With

Typical Suppliers & Equivalents

Compliance & Inspection

Tolerance

CTR tolerance typically ±20% to ±50% depending on grade; response time within ±10% of spec

Test Method

Testing per IEC 60747-5-5 for optoelectronic devices, including CTR measurement (input current vs. output current), isolation voltage test (hipot), and spectral response analysis

Procurement Evaluation Criteria

A practical evidence checklist for RFQ preparation and supplier evaluation.

Technical documentation

Request current drawings, revision history, and a signed specification sheet.

Manufacturing capability

Verify equipment lists, process limits, capacity, and representative production evidence.

Inspection readiness

Confirm test methods, calibrated equipment, sampling plans, and traceable reports.

Supplier transparency

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Manufacturers of Phototransistor

Manufacturer profiles associated with Phototransistor.

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Compiled and source-reviewed by the CNFX editorial team. Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review

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Frequently Asked Questions

What is the difference between a phototransistor and a photodiode?

A phototransistor provides internal gain (amplification) due to its transistor structure, offering higher sensitivity but slower response than a photodiode, which has no gain but faster speed.

How does a phototransistor ensure electrical isolation in optical couplers?

It uses light as the signal medium across a transparent isolation barrier (e.g., air or dielectric), preventing direct electrical connection and allowing high-voltage isolation between input and output circuits.

What factors affect the Current Transfer Ratio (CTR) of a phototransistor in couplers?

CTR depends on LED efficiency, phototransistor responsivity, alignment, aging (degradation over time), and temperature, typically decreasing with higher temperatures or prolonged use.

Data Basis

Editorial classification, named public sources where available, and source-reviewed manufacturer records. See the editorial policy.

Preliminary Technical Classification

This page supports structured research, RFQ preparation, and supplier evaluation. It does not replace buyer-led supplier qualification, standards review, or technical approval.

Phototransistor