# Photo transistor

A phototransistor is a semiconductor device that combines the light-sensing capabilities of a photodiode with the current-amplifying properties of a transistor, resulting in a highly sensitive light detector. Unlike regular bipolar junction transistors (BJTs), which use an electrical current at the base to control the current flow between the collector and emitter, a phototransistor utilizes light to control this current flow.
William Shockley first proposed the idea in 1951, it was Dr. John N. Shive who invented the phototransistor in 1948 while working on transistor-like devices. Bell Labs publicly announced the invention on March 30, 1950. Shive's phototransistor, initially referred to as an "electric eye," utilized a single collector wire with its tip resting in a small dimple on a germanium disk. Light focused on the opposite side of the disk controlled the current flow, demonstrating the device's light-sensing capabilities.
Early applications of the phototransistor included Bell Labs' development of an automated system for long-distance telephone dialing. Now a days phototransistors are used in consumer electronics for features such as automatic brightness adjustment and proximity sensing in the automotive industry for automatic headlight dimming, rain-sensing wipers, and adaptive lighting systems. Silicon phototransistors are used in power electronics applications, specifically in thyristors and triacs.

## History

The invention of the phototransistor followed closely on the heels of the invention of the point-contact transistor. John N. Shive is credited with inventing the phototransistor in 1948 while working at Bell Laboratories. Shive was part of a team investigating semiconductor technology. While working on transistor-like devices, he discovered that light could enable collector-emitter current to flow, leading to the development of the phototransistor. Bell Labs announced the invention on March 30, 1950.
Shive’s initial phototransistor design used a single collector wire on a germanium disk. The end of the wire rested in a small dimple ground into one side of the disk. Light was focused on the opposite side of the disk, and this light controlled the flow of current in the wire.
Shive filed a patent application for his phototransistor in 1949. The patent, US patent 2 560 606, was granted in 1951.

### Shives Design

Shive's work was deeply embedded within the vibrant research environment of Bell Laboratories during the early days of transistor development. This was a time of intense exploration into the properties and applications of semiconductors.
John N. Shive's original phototransistor, invented in 1948, utilized a simple yet effective design, as described in his 1951 patent (U.S. Patent 2,560,606). The device consisted of the following key elements:
● A Semiconductor Wafer: The core of the device was a thin wafer of semiconductive material. Shive specifically mentions using high back voltage N-type germanium, a material known for its ability to produce a transmitted photo effect, meaning that light shining on one part of the crystal can induce electrical changes in other, remote regions. While his patent mentions that other semiconductive materials like silicon could be used, germanium was the material of choice for early transistors and, therefore, likely for his initial phototransistor.
● A Recessed Area: A crucial feature of the design was a spherical depression ground into one face of the germanium wafer. The purpose of this recess was to create a thin portion of the wafer, measuring about 0.002 inches in thickness. This thin section was the active area of the device where light would be focused. By reducing the thickness of the germanium wafer in a localized area, the electric field produced by the reverse bias applied to the collector contact became concentrated in this thin region. This concentrated electric field played a crucial role in separating the electron-hole pairs generated by light absorption, effectively increasing the device's sensitivity to illumination.
● A Point Contact: A point contact, referred to as the collector, was positioned against the center of the recessed area. This contact was made of a metal that formed a rectifying junction with the germanium. (More Info : Point-contact transistor,Noble Lecture on point Contact)
● An Ohmic Connection: An ohmic connection, referred to as the base, was made to the peripheral surface of the germanium wafer. This connection provided a low-resistance electrical path to the semiconductor. Shive suggests using either a rhodium coating or a cured silver paste for the base connection.
● Light Focusing: A lens was used to concentrate light from a source onto the thin portion of the germanium wafer, opposite the point contact. This focusing ensured that light was directed to the active region of the device, maximizing its sensitivity to illumination.
A single point contact, typically made of a metal like phosphor bronze, served as the collector in Shive's design. This point contact pressed against the center of a small, spherical depression etched into one face of a germanium wafer. The collector was reverse biased—meaning it was held at a negative voltage relative to the base—creating a high electric field in the thin region of the germanium wafer beneath the contact. When light fell on the photosensitive region, the large electric field effectively lowered the resistance thus amplifying the current through the transistor. Unlike conventional bipolar transistors that have a separate emitter terminal, Shive's phototransistor used a beam of light as the emitter. Light, focused through a lens onto the opposite face of the germanium wafer, generated electron-hole pairs in the semiconductor. These holes were attracted to the negatively biased collector, increasing the collector current.
Shive’s phototransistor stood out for its high power output for a photoelectric device, a characteristic attributed to the amplifying nature of the transistor effect. It could deliver sufficient power to directly operate a switch in some cases, eliminating the need for preliminary amplification typically required by other photoelectric devices. This high power output opened up possibilities for direct control applications.

## Design

A phototransistor can be either a two-lead or a three-lead device. In the three-lead configuration, the base lead is brought out so that the device can be used as a conventional BJT with or without the additional light-sensitivity feature. In the two-lead configuration, the base is not electrically available, and the device can be used only with light as the input. In many applications, the phototransistor is used in the two-lead version.
The construction of the phototransistor is quite similar to the ordinary transistor. Earlier, the germanium and silicon are used for fabricating the phototransistor. The small hole is made on the surface of the collector-base junction for placing the lens. The lens focuses the light on the surface.

## Working

In a phototransistor the base current is produced when light strikes the photosensitive semiconductor base region. The collector-base pn junction is exposed to incident light through a lens opening in the transistor package. When there is no incident light, there is only a small thermally generated collector-to-emitter leakage current, $I_{ceo}$ this dark current is typically in the nA range. When light strikes the collector-base pn junction, a base current, $I_{\lambda }$, is produced that is directly proportional to the light intensity. This action produces a collector current that increases with Iλ. Except for the way base current is generated, the phototransistor behaves as a conventional BJT. In many cases, there is no electrical connection to the base.
* ↑ "Bell Labs announces the phototransistor, March 30, 1950 - EDN".
* ↑ "Shives Patent Document" (PDF).
* ↑ "The Phototransistor".
* ↑ "Phototransistor Principle".