54-mosfet-transistor

MOSFET Transistor is electronic component with field effect, similarly as JFET Transistor. It has three terminals: source, drain and gate, but amplification is achieved not via reverse polarized PN junction, but with influence of the electric field formed within capacitor structure of conductive gate, metal-oxide insulator and conductive channel.

In MOSFET transistor, small arrow symbolizes direction of source current flow. In figure bellow, on the left hand side there is symbol of N channel MOSFET, and on the right hand side there is symbol of P channel MOSFET.

mosfet-symbols

Amplification is achieved with electric field effect. In N channel MOSFET, gate has to be on positive potential in order to make MOSFET conductive. Capacitor structure makes that positive potential of the gate attracts negative charge into N channel and makes it conductive. Similarly, in P channel MOSFET, gate has to be on negative potential referring to channel and so to attract positive carriers into the channel and makes it conductive. Depending on if zero voltage is enough to switch off MOSFET, MOSFETS can be divided into:

  • Depleted MOSFETS (with integrated channel) and for these, if MOSFET is N type, voltage VGS, has to be negative to switch MOSFET off (if VGS=0, MOSFET is still conductive)
  • Enhanced MOSFETS (with induced channel) and for these, if MOSFET is N type, voltage VGS, of zero volts is enough to switch MOSFET off (if VGS=0, MOSFET is switched off)

Transfer function of Enhanced N channel and P channel MOSFET is given respectively in Figure bellow.

transfer-mosfet

If MOSFET is used to process power, it needs to sustain high voltage while cut-off and to conduct high current while in ohmic regime. These demanding tasks are fulfilled with special construction – VMOS (vertical MOS) transistor, given in Figure bellow.

vmos-structure

Serial connection of P channel MOSFET with induced channel and N channel MOSFET with induced channel is called complementary MOSFEET or CMOS inverter, shown in Figure bellow.

cmos-inverter-symbol

CMOS is technology for logic circuits and can also be used for timer circuits and non-standard amplifiers. Logic circuits in CMOS technology are 40 families. These circuits operate with supply voltage in range from 3V to 18V. Operating frequency is up to 3MHz. For higher speeds in digital signal processing, some other technologies are used such as TTL, High Speed CMOS and ECL.

MOSFET transistors are widely used in many schemes, such as:

  • MOSFET as the Switch: Tutorial about R-L Circuit with MOSFET as the Switch and Feewheel Diode – Simulate R-L Circuit with MOSFET as the Switch and Feewheel Diode

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