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Metal Oxide Field Effect Transistor (MOSFET)
Metal Oxide in the name MOSFET stands for the thin insulating layer of silicon oxide layer in the region between metal and semiconductor. The name field effect transistor itself hints the attributes of this device. Like BJT it is also capable of transferring a signal from high resistance to low resistance (Transfer resistor-transistor). The term field effect stands for the fact that the operation of the device mainly depends on the electric field applied between its terminals called Gate and source analogous to the base and collector in transistor to control the current through it.
History of MOSFET
The basic principle of this kind of transistor (JFET) was first patented by Julius Edgar Lilienfeld in 1925.In 1959, Dawon Kahng and Martin M. (John) Atalla at Bell Labs invented the metal oxide semiconductor field-effect transistor (MOSFET) as an offshoot to the patented FET design. The earliest microprocessors starting in 1970 were all MOS microprocessors i.e. fabricated entirely from PMOS logic or fabricated entirely from NMOSlogic. The most widely used device in integrated circuit technology, the MOSFET has profound impact on semiconductor electronics superseded JFET making JFET obsolete.
Types of Metal Oxide Semiconductor transistors
Metal Oxide Field Effect transistor’s are of mainly of two types based on constructional and operational features of the device.
- Enhancement type MOSFET
- N-channel Enhancement type
- P-channel Enhancement type
- Depletion type MOSFET
- N-channel Depletion type
- P-channel Depletion type
In Enhancement mode MOSFET channel is induced by applying gate voltage contrary to the depletion mode MOSFET in which already existing channel is modulated by applying gate voltage like JFET.
Differences and advantages between depletion and Enhancement MOSFET’s
- Enhancement type mode MOSFET will be off for gate to source 0V as there exists no channel to conduct. Depletion type MOSFET conducts at 0V has positive cut off gate voltage so less preferred. Depletion MOS also conducts at 0V therefore has less useful application.
- Since the logic operations of depletion MOSFET is the opposite to the enhancement MOSFET, the depletion MOSFET produces positive logic circuits, such as, buffer, AND, and OR.
- The depletion MOSFET is free from sub-threshold leakage current and gate oxide leakage current.
- As a enhancement MOSFET shrinking in size, there is no way to stop the sub threshold leakage current diffused across from source to drain because the drain and source terminals are closer physically. This is not the problem with depletion type MOSFET because a pinched channel stops the diffusion current completely.
Advantages and limitations of MOSFET over FET
- JFETs can only be operated in the depletion mode whereas MOSFETs can be operated in either depletion or in enhancement mode. MOSFETs have input impedance much higher than that of JFETs. This is due to negligibly small leakage current.
- JFETs have characteristic curves more flat than those of MOSFETs indicating a higher drain resistance due to low resistance offered by channel compared in JFET.
- When JFET is operated with a reverse bias on the junction, the gate current IG is larger than it would be in a comparable MOSFET. The current caused by minority carrier extraction across a reverse-biased junction is greater, per unit area, than the leakage current that is supported by the oxide layer in a MOSFET. Thus MOSFET devices are more useful in electro-meter applications than are the JFETs.
- MOSFETs are somewhat easier to manufacture, they are more widely used than are the JFETs.
- MOSFET has higher speed of operation compared to JFET’s.
Because the oxide layer is so thin, the MOSFET is susceptible to permanent damage by electrostatic charges. Even a small electrostatic build up can destroy a MOSFET permanently. But this can be avoided mostly by careful and intelligent design of the device.