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
[edit]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[1]. 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
[edit]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)[2]. 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. [3]
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
[edit]
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. [4]
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. Two types of illumination exist :[5]
- Rear Illumination
- Front Illumination
It can also be classified on the type of semiconductor material used into : [6]
- Homogeneous Phototransistor
- Heterogenous Phototransistor
Working
[edit]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, this dark current is typically in the nA range. When light strikes the collector-base pn junction, a base current, , 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.
Features
[edit]Absolute Maximum Ratings: This section outlines the maximum operating conditions that the phototransistor can withstand without damage.
Collector-Emitter Voltage (Vceo): The maximum allowable voltage that can be applied between the collector and emitter terminals.
Collector Current (Ic): The maximum continuous collector current the device can handle.
Power Dissipation (Pd): The maximum power that can be dissipated by the phototransistor.
Operating Temperature Range: The permissible ambient temperature range for safe operation.
Storage Temperature Range: The temperature range for safe storage of the device.
Electrical Characteristics: This section provides detailed electrical parameters and performance metrics.
Dark Current (Id): The current flowing through the device in the absence of light, typically measured at a specific collector-emitter voltage.
Collector-Emitter Breakdown Voltage (BVceo): The voltage at which the collector-emitter junction breaks down.
DC Current Gain (hFE): The ratio of the collector current to the base current, indicating the amplification factor.
Light Current (IL): The collector current when the device is illuminated at a specific light intensity.
Responsivity (Rλ): The ratio of the output current (photocurrent) to the incident light power at a given wavelength, usually expressed in amperes per watt (A/W).
Spectral Response: A graph depicting the responsivity of the phototransistor as a function of wavelength, showing the range of wavelengths to which the device is sensitive.
Response Time (tr, tf): The time it takes for the photocurrent to rise (tr) or fall (tf) to a certain percentage of its final value in response to a step change in illumination.
Optical Characteristics:
Peak Wavelength (λp): The wavelength at which the phototransistor exhibits its highest responsivity.
Angle of Half Sensitivity (φ): The angle from the optical axis at which the responsivity drops to half of its maximum value.
Active Area: The surface area of the phototransistor sensitive to light.
Applications
[edit]Direct distance dialing : Bell Laboratories developed a system that used punched cards and phototransistors. Information about call routing was encoded on metal cards using punched holes. When a call came in, the system would select the appropriate card for the desired destination. Light beams were then projected through the holes in the card onto a bank of phototransistors.
The phototransistors would detect the light passing through the holes and generate electrical signals. These signals would then activate switches to establish the necessary connections for the call. The punched card system, combined with the sensitivity and speed of phototransistors, allowed for a much more efficient and reliable way to route long-distance calls. The ease of updating the system by simply changing the punched cards was also a significant advantage.
Phototransistors for optical telecommunications : The phototransistor is the device which converts the optical signal into an electrical signal. It is situated at the input of the receiver, acting as an optical detector. In the transmitter, the light is modulated (directly or indirectly via an optical modulator) with the signal carrying the information. This signal can be of an analog or digital. After transport through the optical fiber, the optical signal is demodulated by the photodetector in order to recover the electrical signal. This signal is subject to noise and distortion, and so the receiving circuit may need to amplify and reconstruct the signal in order to extract the original information.
Galvanic isolation : Silicon phototransistors have been used for a long time in optical isolators. These devices may or may not have a base contact. For this application, all that is required is to connect a photodiode which takes care of the conversion of an input electrical signal into alight beam. This is in order to achieve a transfer of information which avoids any form of electrical connection. In this way, galvanic isolation between the controller and the receiver is ensured. The device providing the electron-photon conversion at the input is normally a light-emitting diode (LED) based on GaAs, whose emission line of around 850 nm is well suited to optical detection by the phototransistor or the photo-Darlington pair. The current conversion gain can reach 500%.[5]