Introduction
The future challenges are the silicon solutions for switching applications in motor vehicles. A large range of low-voltage MOSFETs is offered by NEC which features lowest on-resistance and temperature stability up to 175°C.
A key component of all electronic and digital circuits today is transistors. Millions of transistors are used in computers and microprocessor based systems. Until 1970, bipolar junction transistors (BJT) were extensively used but were replaced by MOSFETs and power MOSFETs because of the advantages that MOSFETs offered over BJTs. There is different principle upon which this transistor works.
What is a MOSFET?
A Field Effect Transistor is a voltage-controlled device in the transistor family. This means that the output of the device is a function of the voltage applied across the device. MOSFETs are Metal Oxide Silicon Field Effect Transistors. There is a thin layer of silicon oxide on one side and a p-type material on the other side of MOSFETs.
A MOSFET has three terminals:
- Gate – this terminal is connected to the input device
- Drain – this terminal is connected to the positive node of the power source
- Source – this is the source of electrons required for conductivity
Between the gate and the channel, silicon dioxide is applied. The gate from the channel is insulated, which makes the MOSFET a device with very high resistance.
General specifications of MOSFETs
- The Forward transconductance is about 1 to 10 mA/V
- The input resistance is very high, about 10 to 12 W
- The output resistance is about 10 to 50 kilo-Ohms
- There are other specifications that must be considered before selecting a MOSFET which include breakdown voltage (250 to 1000 volts), switching characteristics, zero gate voltage, drain current, input capacitance etc. These specifications though depend on the type of application for which a MOSFET would be used
In applications with high operating frequency (greater that 200KHz), long duty cycles, wide load variations and lower output power (lesser that 50KW), MOSFETs are specifically used.
The largest applications of MOSFETs are in Switched mode power supplies (SMPS). Battery charging applications also use MOSFETs.
Some other applications of MOSFETs include:
- With a small input, MOSFETs provide large output current. In transducer drivers for high power devices like motors and light bulbs, this characteristic is used
- When configured in complimentary pairs,they can be used as Hi-Fi amplifiers. They produce less distortion because they are more linear than BJTs
- They can be made very compact, which is very useful in integrated circuits
Faster switching time than BJT (10 times), very small switching current, least effects of temperature, lower losses than BJT etc. are some o f the advantages of MOSFETs.
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