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Basic MOSFET structure
- 1. Government Engineering College, Bharuch VLSI Presentation
- 2. Basic MOSFET Structure Preparedby: Xyz 6th SemE.C.(A) Electronicsand Comm.Dept.
- 3. MOSFET Structure p-Si n+ L Source Gate Drain Field Oxide Gate Oxide Bulk (Substrate)
- 4. • Importance for LSI/VLSI –Low fabrication cost –Small size –Low power consumption • Applications –Microprocessors –Memories –Power Devices
- 5. i. Basic Properties i. Unipolar device ii. Very high input impedance iii. Capable of power gain iv. Two possible device types: enhancement mode; depletion mode v. Two possible channel types: n-channel; p-channel
- 6. Symbols G D S B G D S B p Channel MOSFET n Channel MOSFET
- 10. Triode Region A voltage-controlled resistor @small VDS G p n+n+ metal S DB oxide +- +++ +++ - - - - VGS1>Vt p n+n+ metal S DB oxide +- +++ +++ +++ - - - - - - VGS2>VGS1 p n+n+ metal S DB oxide +- +++ +++ +++ - - - - - - - - - VGS3>VGS2 +++ ID VDS 0.1 v increasing VGS Increasing VGS puts more charge in the channel, allowing more drain current to flow cut-off
- 11. Saturation Region occurs at large VDS p n+n+ metal source S gate G drain D body B oxide + - +++ +++ +++ VDS large As the drain voltage increases, the difference in voltage between the drain and the gate becomes smaller. At some point, the difference is too small to maintain the channel near the drain pinch-off
- 12. Simplified MOSFET I-V Equations Cut-off: VGS< VT ID = IS = 0 Triode: VGS>VT and VDS < VGS-VT ID = kn ’(W/L)[(VGS-VT)VDS - 1/2VDS 2] Saturation: VGS>VT and VDS > VGS-VT ID = 1/2kn’(W/L)(VGS-VT)2 where kn ’= (electron mobility)x(gate capacitance) = mn(eox/tox) …electron velocity = mnE and VT depends on the doping concentration and gate material used (…more details later)
- 13. ThankYou