The transistor is a non-linear semiconductor three-terminal device. The transistor is considered to be one of the most important devices in the field of electronics. The transistor has transformed many aspects of man’s life. There are two main functions of transistors, to amplify input signals and to acts as solid-state switches. The transistor acts as a switch when operated either in saturation or cut-off region. Whereas it amplifies signals when used in the active region. It offers very high input resistance and very low output resistance.
Transistors are categorized into bipolar junction transistor and field effect transistor based on their construction.
Type of transistor:
Transistor symbol is represented as below.
transistor-symbol.jpgtransistor-symbol.jpgTransistor symbol
The most popular and commonly used transistors are BC547, 2N2222. Given below are a few common transistor packages:
transistor-packages.jpgtransistor-packages.jpgTransistor packages
The MOSFET (metal oxide semiconductor field-effect transistor) transistor is a semiconductor device that is different than bipolar junction transistor in terms of construction though the applications remain the same as switching and amplifying. It has four terminals such as drain, gate, source, and body. The body is shorted with a source terminal. The gate is insulated from the channel near an extremely thin layer of metal oxide. Due to which it offers very high resistance compared to BJT.
By controlling the gate voltage (VGS +ve/-ve) width of a channel along which charge carriers flow (electrons or holes) from source to drain can be controlled. The P-Channel MOSFET has a P-Channel region between the source and drain and for N-channel MOSFET has an N-channel region.
Advantages of MOSFET over BJT:
- Very high input resistance
- Low on-state resistance
- Low power loss
- High frequency of operations
transistor-operation.jpgtransistor-operation.jpgTransistor operation
Application of transistors (BJT/FET)
- Amplification of analog signals
- Used as switching devices in SMPS, microcontrollers, etc.
- Oscillators
- Over/under voltage protection
- Modulation circuits & demodulation of signals
- Power control in invertors and chargers (high-current power transistors)
Types of transistor packages
In terms of packaging BJT and MOSFET, transistors are available in through-hole (DIP) and SMD versions. e.g. DIP: TO-92, TO- 220 and SMD: SOT23, SOT223, TO-252, D2PAK.
Transistor selection parameters
While selecting a transistor in any circuit, the user needs to take care of the following parameters:
- Maximum collector current (Ic)
- Max collector voltage (Vce)
- VBE voltage
- Saturation Vce (sat) voltage
- Current gain, hfe/ß
- Input resistance
- Output resistance
- Reverse breakdown voltage
- Max reverse current
- Power dissipation
- Operating junction temperature
- Size
- Switching time/frequency
Manufacturers: Analog Devices, Rohm Semiconductor, Diodes Incorporated, On Semi, Texas Instrument, Panasonic, Infineon, Honeywell, etc.
The Different Types of Transistors
There are many, many types of transistors to become familiar with. Below is a brief description of a selection of them.
- Bipolar Junction Transistor, or BJT: These types of transistors are controlled by current and come in two different types PNP (Positive-Negative-Positive), or NPN (Negative-Positive-Negative). Depending on the type, the current will either flow from the collector to the emitter (NPN) or from the emitter to the collector (PNP).
- Diffusion Transistor: Diffusion transistors are a form of BJT transistors. These are formed by precisely diffusing impurities into a semiconductive material as a way of altering the electrical properties of the transistor.
- Avalanche Transistor: Another type of Bipolar Junction Transistors, avalanche transistors operate in avalanche breakdown region, meaning that the transistor has the ability to switch very high currents exceptionally rapidly, requiring less than a nanosecond to rise and fall.
- Schottky Transistor: This is a transistor that is combined with a Schottky diode, which prevents the saturation of the transistor by the diversion of extremely high input current.
- Darlington Transistor: Darlington transistors are made by combining two different transistors. In doing so, the Darlington transistor has a significantly greater ability to gain current.
- Field Effect Transistor: These transistors are controlled by voltage. They have high impedance, which tempers the amount of current that passes through them. Field effect transistors can not operate as an amplifier, unlike other BJTs.
- Multiple Emitter Transistor: In this type of transistor that is generally used at the inputs of integrated circuit TTL NAND logic gates, with input signals being applied to all emitters.