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NXP 7030AL - N-channel TrenchMOS logic level FET IRF640 Power Mosfet die. The power MOSFET is the most widely used power semiconductor device in the world. [3] As of 2010, the power MOSFET accounts for 53% of the power transistor market, ahead of the insulated-gate bipolar transistor (27%), RF power amplifier (11%) and bipolar junction transistor (9%). [24]
The MOSFETs are n-type enhancement mode transistors, arranged in a so-called "pull-down network" (PDN) between the logic gate output and negative supply voltage (typically the ground). A pull up (i.e. a "load" that can be thought of as a resistor, see below) is placed between the positive supply voltage and each logic gate output.
where the combination V ov = V GS − V th is called the overdrive voltage, [41] and where V DSsat = V GS − V th accounts for a small discontinuity in which would otherwise appear at the transition between the triode and saturation regions. Another key design parameter is the MOSFET output resistance r out given by:
MOSFET drain current vs. drain-to-source voltage for several values of the overdrive voltage, ; the boundary between linear (ohmic) and saturation (active) modes is indicated by the upward curving parabola.
Like other MOSFETs, PMOS transistors have four modes of operation: cut-off (or subthreshold), triode, saturation (sometimes called active), and velocity saturation. While PMOS logic is easy to design and manufacture (a MOSFET can be made to operate as a resistor, so the whole circuit can be made with PMOS FETs), it has several shortcomings as well.
A fundamental limitation of voltage-controlled resistors is that input signal must be kept below the linearization voltage (approximately the point when the JFET enters saturation). If the linearization voltage is exceeded, the voltage control resistor value will change both with the level of the input voltage signal and the gate-to-source voltage.