Metal-Oxide-Semiconductor Fields Effect Transistors (MOSFETs)

1. Metal-Oxide-SemiconductorFields Effect Transistors(MOSFETs) From Prof. J. Hopwood

2. gate G body B source S drain D metal oxide p n+ n+ W L Structure: n-channel MOSFET(NMOS) IG=0 ID=IS IS

3. D G B (IB=0, should be reverse biased) S Circuit Symbol (NMOS) ID= IS IG= 0 IS

4. gate G body B source S drain D + - metal oxide p n+ n+ W L VGS = 0 n+pn+ structure  ID = 0 VD>Vs

5. gate G body B source S drain D + - metal oxide p n+ n+ W L 0 < VGS < Vtn+-depletion-n+ structure  ID = 0 VD>Vs +++

6. gate G body B source S drain D + - VD>Vs +++ +++ +++ metal oxide p n+ n+ - - - - - W L VGS > Vtn+-n-n+ structure  ID > 0

7. Summary • Vt is the threshold voltage • If VGS < Vt, then there is insufficient positive charge on the gate to invert the p-type region • This is called “cut-off” • If VGS> Vt, then there is sufficient charge on the gate to attract electrons and invert the p-type region, creating an n-channel between the source and drain • The MOSFET is now “on” • 2 modes of operation: triode and saturation

8. B S D + - +++ VGS1>Vt +++ metal oxide p n+ - - - - n+ Triode Region A voltage-controlled resistor @small VDS ID increasing VGS B S D - + +++ VGS2>VGS1 +++ +++ G metal oxide p n+ n+ - - - - - - VDS cut-off B 0.1 v S D + - +++ VGS3>VGS2 +++ +++ Increasing VGS puts more charge in the channel, allowing more drain current to flow +++ metal oxide p n+ - - - - - - - - - n+

9. gate G body B source S + drain D - VD>>Vs +++ +++ +++ metal oxide p n+ n+ Saturation Regionoccurs at large VDS 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

10. gate G body B source S + drain D - VD>>Vs +++ +++ +++ metal oxide p n+ n+ Saturation Regionoccurs at large VDS The saturation region is when the MOSFET experiences pinch-off. Pinch-off occurs when VG - VD is less than Vt.

11. gate G body B source S + drain D - VD>>Vs +++ +++ +++ metal oxide p n+ n+ Saturation Regionoccurs at large VDS VG - VD < Vt … VGS - VDS < Vt … VDS > VGS - Vt

12. Saturation Regiononce pinch-off occurs, there is no further increase in drain current saturation ID triode VDS>VGS-Vt increasing VGS VDS<VGS-Vt VDS 0.1 v

13. 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 - 1/2vDS2] 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

14. Electrostatic Discharge (ESD) • The gate oxide is very thin • tox < 10 nm (10x10-9 m) • It is very easy for static electricity to destroy this very thin insulating layer • Must practice precautions, such as wrist straps and static free work areas

15. Parasitic Capacitance • Notice that the entire gate structure looks exactly like a capacitor (metal-insulator-semiconductor) • This parasitic capacitance at the gate allows current to flow at high frequencies! iG > 0 as frequency increases and, just like other semiconductor devices, the parasitic capacitance limits the speed of the device (turning the MOSFET “on” requires charging the gate capacitance). The RsigCgatetime constant tells us the signal delay for digital circuits and the upper cut-off frequency for analog circuits.

16. Conclusion • For the remainder of the class, we will look at the behavior of semiconductor devices in much more detail • Occasionally, you might get caught-up in the details! Please refer back to this overview to see how it all fits together.