Back to the basics of power MOSFETs
Get a refresher on the basics of a power MOSFET
By Dan Tulbure, Microsemi
Page 1 of 5
Power Management DesignLine
(02/21/2007 7:47 PM EST)
Get a refresher on the basics of a power MOSFET
By Dan Tulbure, Microsemi
Page 1 of 5
Power Management DesignLine
(02/21/2007 7:47 PM EST)
What is a power MOSFET?
We all know how to use a diode to implement a switch. But we can only switch with it, not gradually control the signal flow. Furthermore, a diode acts as a switch depending on the direction of signal flow; we can't program it to pass or block a signal. For such applications involving either "flow control" or programmable on/off switching we need a 3-terminal deviceand Bardeen & Brattain heard us and "invented" (almost by accident, like many other great discoveries!) the bipolar transistor. Structurally it is implemented with only two junctions back-to-back (no big deal; we were probably making common cathodes - same structure - long before Bardeen). But functionally it is a totally different device which acts like a "faucet" controlling the flow of emitter current - and the "hand" manipulating the faucet is the base current. A bipolar transistor is therefore a current
controlled device.
The Field Effect Transistor (FET), although structurally different, provides the same "faucet" function. The difference: the FET is voltage controlled; one doesn't need base current but voltage to exercise flow control.
We all know how to use a diode to implement a switch. But we can only switch with it, not gradually control the signal flow. Furthermore, a diode acts as a switch depending on the direction of signal flow; we can't program it to pass or block a signal. For such applications involving either "flow control" or programmable on/off switching we need a 3-terminal deviceand Bardeen & Brattain heard us and "invented" (almost by accident, like many other great discoveries!) the bipolar transistor. Structurally it is implemented with only two junctions back-to-back (no big deal; we were probably making common cathodes - same structure - long before Bardeen). But functionally it is a totally different device which acts like a "faucet" controlling the flow of emitter current - and the "hand" manipulating the faucet is the base current. A bipolar transistor is therefore a current
controlled device.
The Field Effect Transistor (FET), although structurally different, provides the same "faucet" function. The difference: the FET is voltage controlled; one doesn't need base current but voltage to exercise flow control.
