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0.4 Applications: The MOSFET


MOSFET application Almost all electronics and appliances, including personal computers, contain millions of silicon MOSFETs on a thumbnail sized chip

Heatsink and cooling Within a computer most MOSFETs are located on the microprocessor chip, mounted on the motherboard and conspicuously cooled by its own heat sink and cooling fan

Microprocessor chip The microprocessor chip itself is mounted in an electronic package with hundreds of interconnecting pins and connected to the chip by hundreds of tiny bond wires

Chip cross-section A cross-section of the chip reveals multiple layers of tiny wires above the MOSFETs, which are embedded in the silicon substrate

MOSFET structure The MOSFET consists of two conducting regions, called source and drain, on each side of the channel that is controlled by the gate electrode

MOSFET biasing The MOSFET is biased by applying a voltage to the drain through a current-limiting resistive load and by applying a variable gate voltage.

Load line The load line represents the drain current versus the drain voltage as determined by the resistive load. The intersection between the load line and the MOSFET characteristics provide the operating points for each gate voltage

1 Volt gate voltage As a gate voltage larger than the threshold voltage is applied, the charge on the gate attracts an equal and opposite charge in the channel

2 Volt gate voltage A larger gate voltage results in a larger charge on the gate and in the channel, resulting in a larger drain current

3 Volt gate voltage The larger drain current in turn results in a larger voltage drop across the resistive load and hence a lower drain voltage as can be observed on the load line

4 Volt gate voltage The lower drain voltage results in a smaller depletion layer width between the drain and the substrate

5 Volt gate voltage An even larger gate voltage eventually causes the MOSFET to operate in the linear region