Basic MOSFET structure

Metal oxide semiconductor field emitting transistor is a unipolar high input impedance device usually operated in saturation region for applications like microprocessor, power devices, memories etc.

1. Government Engineering College, Bharuch VLSI Presentation

2. Basic MOSFET Structure Prepared by: Xyz 6 th Sem E.C.(A) Electronics and 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. Basic Properties Unipolar device Very high input impedance Capable of power gain Two possible device types: enhancement mode; depletion mode Two possible channel types: n-channel; p-channel

6. Symbols G D S B G D S B p Channel MOSFET n Channel MOSFET

7. Current-Voltage Characteristic A B C D I DS V DS

8. Channel Formation p-Si B V G +V DS n-Channel S D

9. Increased V DS p-Si B V G +V DS n-Channel S D

10. Triode Region A voltage-controlled resistor @small V DS G p n+ n+ metal S D B oxide + - +++ +++ - - - - V GS1 >V t p n+ n+ metal S D B oxide + - +++ +++ +++ - - - - - - V GS2 >V GS1 p n+ n+ metal S D B oxide + - +++ +++ +++ - - - - - - - - - V GS3 >V GS2 +++ I D V DS 0.1 v increasing V GS Increasing V GS puts more charge in the channel, allowing more drain current to flow cut-off

11. Saturation Region occurs at large V DS p n+ n+ metal source S gate G drain D body B oxide + - +++ +++ +++ V DS 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: V GS < V T I D = I S = 0 Triode: V GS >V T and V DS < V GS -V T I D = k n ’ (W/L)[(V GS -V T )V DS - 1 / 2 V DS 2 ] Saturation: V GS >V T and V DS > V GS -V T I D = 1 / 2 k n ’(W/L)(V GS -V T ) 2 where k n ’ = (electron mobility)x(gate capacitance) = m n ( e ox / t ox ) … electron velocity = m n E and V T depends on the doping concentration and gate material used (…more details later)

13. Thank You