What Are Phototransistors?

Phototransistors are semiconductor photodetectors that combine light detection with internal current amplification in a single device. A conventional transistor responds only to the electrical signal applied at its base; a phototransistor replaces the base current, wholly or in part, with photocurrent generated at an enlarged base-collector junction exposed to incident light. The transistor's inherent current gain then amplifies this photocurrent by a factor typically ranging from a few hundred to several thousand, producing an output signal substantially larger than a simple photodiode operating under the same illumination. This makes phototransistors well suited to applications requiring moderate sensitivity at low cost without external amplification circuits.

Like other optoelectronic sensors, phototransistors belong to the broader family of photodetectors that also includes photoconductors, photodiodes, and photovoltaic cells. Compared with photodiodes, phototransistors deliver higher responsivity but sacrifice speed and linearity, since the gain depends on the collector current and varies with illumination level. Compared with photoconductors, they are faster and more suitable for digital switching applications. The choice among sensor types depends on the specific requirements of bandwidth, sensitivity, and circuit complexity.

Bipolar Phototransistors

The bipolar phototransistor is the most prevalent form. It is typically packaged as a two-terminal or three-terminal device in a clear or filtered plastic case that allows light to reach the base-collector junction through a lens. When photons are absorbed in the depletion region of that junction, electron-hole pairs are generated. The minority carriers sweep across the junction, producing a photocurrent equivalent to the base current of a conventional npn or pnp bipolar transistor. The device then amplifies this current by its common-emitter current gain (hFE), so a small photocurrent produces a large collector current. A third-terminal connection to the base allows for bias adjustment or circuit control, but many applications leave the base unconnected.

The spectral response of silicon bipolar phototransistors spans roughly 400 to 1100 nm, with peak sensitivity near 800 to 900 nm in the near-infrared. The response time is limited by the transit time of carriers through the base and by the collector capacitance, resulting in bandwidths typically in the range of tens of kilohertz to a few megahertz. As described in a technical overview of phototransistors from RP Photonics, this bandwidth limitation is a defining trade-off compared with photodiodes, which can operate at gigahertz rates. Research on compound-semiconductor phototransistors, including III-nitride devices fabricated on LED epitaxial wafers, has demonstrated phototransistor operation at ultraviolet wavelengths, extending the technology beyond the silicon spectral window.

Photodarlingtons and Field-Effect Phototransistors

For applications demanding even higher sensitivity, the photodarlington integrates two bipolar transistors in a Darlington configuration within a single package. The photocurrent generated at the first transistor's junction is amplified by the combined gain of both stages, producing overall current gains of tens of thousands. This extreme sensitivity comes at the cost of slower response, with rise and fall times reaching tens to hundreds of microseconds, limiting photodarlingtons to slowly changing or quasi-static light levels.

Field-effect phototransistors (photoFETs) operate on a different principle: incident light modulates the gate potential of a field-effect transistor, either directly by generating a photovoltage or through a coupled photodiode structure. PhotoFETs are used in specialized applications such as infrared bolometric detection, where the gate is connected to a thermally sensitive element, and in optically addressed logic circuits. A survey of these device types and their application in optical isolation and sensing is provided in technical articles from IEEE Xplore on optoelectronic device design.

Applications

Phototransistors have applications in a wide range of disciplines, including:

• Optocoupler and opto-isolator circuits for galvanic isolation between high and low voltage stages
• Optical encoders for rotary and linear position sensing
• Infrared remote control receivers in consumer electronics
• Ambient light sensing for automatic display brightness adjustment
• Object detection and proximity sensing in industrial automation