Last Updated on October 22, 2023 by Kevin Chen
A phototransistor is a semiconductor device that converts light into electricity. It is made of three parts: the light-sensitive layer, the base, and the emitter.
The light-sensitive layer, which is usually silicon or germanium (Ge), consists of two parts: intrinsic semiconductor material and an extrinsic semiconductor material, which is usually p-type (positive charge carrier) or n-type (negative charge carrier).
The intrinsic semiconductor material absorbs photons while the extrinsic semiconductor material releases electrons, thus generating a current that flows through the base. The emitter releases electrons from the base, thus completing the circuit.
Phototransistors: Basic Functionality
A phototransistor is a semiconductor device that converts light into electricity and vice versa. In other words, a phototransistor is a photovoltaic device. This means that it has the ability to produce current when exposed to light.
The amount of current produced depends on the intensity of the light. This functionality is used in many applications where light needs to be converted into a usable form. For instance, it can be used to measure light intensity, control other devices based on light, or even act as an optical receiver.
Types of Phototransistors
There are two main types of phototransistors: NPN phototransistors and PNP phototransistor.
NPN phototransistors are voltage-operated devices that will produce a current when exposed to light. This current can then be used to control an electrical device, such as a motor. NPN phototransistors are used in applications that require current to be either stored or converted into voltage.
These devices have a higher resistance when in the dark, but this decreases as the intensity of light increases. NPN phototransistors are usually used in applications where light needs to be converted into voltage or current.
PNP phototransistors are current-operated devices that convert light into current when exposed to light. These devices are typically used in applications where current needs to be either stored or converted into voltage. PNP phototransistors have a higher resistance when in the dark, but this decreases as the intensity of light increases. PNP phototransistors are commonly used in applications where light needs to be converted into voltage or current.
How do Phototransistors Work?
To understand how phototransistors work, let’s first look at the parts and components of this device.
A typical phototransistor comprises of a photovoltaic cell, which is made up of a light-sensitive layer of material. This layer converts incident light into electric current. The photovoltaic cell is sandwiched between two electrodes: the base and collector electrodes. The base electrode is transparent and acts as a window for the light to reach the sensitive layer. The collector electrode collects the electric current that’s produced by the photovoltaic cell when exposed to light.
A phototransistor works by changing its electrical resistance based on how much light falls on it, thus allowing more or less current to flow through it, depending on how much light there is.
The sensitivity of a phototransistor varies depending on which type of semiconductor material is used in its construction. For instance, germanium-based devices are very sensitive to infrared (IR) radiation while silicon-based devices are more sensitive to visible red light.
From the structure, it can be seen that the base of a phototransistor is the same as that of a bipolar transistor. The collector region is typically wider than the emitter region, with two junctions between N-type and P-type layers. This structure is used because phototransistors are generally designed to be highly sensitive.
Understanding a phototransistor circuit
When light hits a phototransistor, it creates an upward flow of current. An increase in voltage will draw more current from the source, which can be used to control different devices. Therefore, a phototransistor will act as a switch, closing when light is present and opening when the light source is removed.
For example, a phototransistor can be used to control a motor by closing when light is detected and opening when light is no longer present. This simple circuit can be used to turn an LED on and off when a light is shined on it.
Key Differences Between Phototransistors and Photodiodes
Phototransistors and photodiodes are two devices that are often mixed up. What is the difference between them?
A phototransistor is a light-sensitive transistor that has a collector, base, and emitter. It is similar to a bipolar transistor and is made of the same materials.
A photodiode, on the other hand, is a semiconductor device that will only allow current to flow in one direction. It can be used as a light sensor but only when used in conjunction with a circuit to force current through it.
The main difference between the two devices is that they are made from different materials, have different structures, and will behave differently when exposed to light.
A phototransistor acts like a switch when exposed to light. A photodiode will only act as an amplifier if placed in conjunction with other circuitry that forces current through it.
A photodiode is made of silicon while a phototransistor is usually made of germanium.
The internal structures of a phototransistor and a photodiode are very different. A phototransistor contains an emitter, base and collector while the diode only has an anode and a cathode.
A transistor is able to amplify current while the diode will only allow current to flow in one direction.
When exposed to light, the resistance in a transistor decreases allowing current to flow through it. When exposed to light, the resistance in a diode increases greatly making it impossible for current to pass through it.
Technical Specifications of Phototransistors
– Operating Temperature – Phototransistors are typically designed for operation between -40 and +150 degrees Celsius. However, certain devices might be designed for a narrower range, so it’s important to check the specifications before purchasing any devices.
– Resistor Selection – Phototransistors have a much higher input resistance than photodiodes, so a much larger resistor must be used in the voltage divider. Most phototransistors have an input resistance between 10,000 and 100,000 ohms.
– Current Rating – Phototransistors are current-controlled devices, so they will have a maximum amount of current they can draw. This is measured in amperes and is typically very low—between 0.1 and 1 ampere.
– Sensitivity – Sensitivity is a measure of the change in resistance with respect to light. For example, a photocell with a sensitivity of 10k ohms/lux will have a resistance of 10k ohms when exposed to 100 lux (1 lux = 1 lumen per square meter). The smaller the sensitivity rating, the less light is required to drop the resistance by one-half.
Pros of Using a Phototransistor
Here are some advantages that come with using phototransistors in a circuit :
-Are highly sensitive to light: They can be used in low light conditions
-Are used for open and closed circuits: Phototransistors are used for both open and closed circuits with the help of a high gain amplifier.
-Require little external circuitry: They require very little external circuitry to work as compared to other sensors.
-Simple and affordable: Compared to other sensors, phototransistors are simple and affordable.
-Low power consumption: Phototransistors consume very little power as compared to other sensors.
-The photosensitive part is encapsulated in epoxy: The photosensitive part of a phototransistor is encapsulated in epoxy. This helps in protecting the sensitive part of the sensor from moisture and dirt.
Cons of Using a Phototransistor
Here are some disadvantages that come with using phototransistors in a circuit :
-Slower than photodiodes: Phototransistors are slower than photodiodes.
-Not as sensitive as photodiodes: Phototransistors are not as sensitive as photodiodes.
-Not suitable for high frequency applications: Phototransistors are not suitable for high frequency applications because of their slow response time.
-Low output voltage: Phototransistors have low output voltage. This makes it difficult to amplify the signal.
What are the applications of phototransistors?
Phototransistors have a wide range of applications which include:
-Lighting control: Lights are controlled by phototransistors. This is because they are able to control the amount of light that is emitted.
-Security systems: Phototransistors are used in security systems. They are able to detect people passing through the area. This is done by detecting the light that they emit.
-Card readers: Phototransistors are used in card readers. They are able to detect the magnetic strips on cards by sending out a signal.
-Medical equipment: Phototransistors can be found in medical equipment such as ophthalmoscopes and dental examination equipment.
-GPS systems: Phototransistors are used in GPS systems which help to determine the location of vehicles, ships, and aircraft.
Where to buy phototransistors
Are you looking for a phototransistor? You should be keen on where you buy them from. Ensure that you buy from reputable manufacturers and suppliers. This is the only way of ensuring that you get the best quality of phototransistor.
You should be careful when buying from the internet since you can never be sure of the quality of the products that you will be getting. If possible, it is best to buy through sourcing agents such as ICRFQ. They will help source quality phototransistors in China.
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