Biological Implants: Technology and Implicationssrout01/ee12-2008/pdfs/lecture15... · 2008-02-21 · 3 EE12: MOSFET PRIMER Sameer Sonkusale Quick Look At MOS Vs BJT Structure In

MOSFET PRIMER

Sameer Sonkusalehttp://nanolab.ece.tufts.edu

2EE12: MOSFET PRIMERSameer Sonkusale

MOSFET

Metal Oxide Semiconductor Field Effect Transistor

Gate electrode is used to control the electric field in the channel region – which in turn controls the flow of charges between source and drain


Quick Look At MOS Vs BJT Structure

In BJT, current conduction is due to both holes and electrons – hence the nameBipolar Transistor

In MOSFET, current conduction is due to one carrier – either holes or electrons


Dissecting the MOSFET Structure

• Source-Drain are either p-type or n-type

• Gate is metal or polysilicon


Views and Types of MOSFET

N-channel MOSFETNMOS

P-channel MOSFETPMOS


NMOS as a switch


PMOS as a switch


N-channel MOSFET and P-channel MOSFET on same Substrate


Array of FETs in an Integrated Circuit Environment

FOX – Field oxideN-well process example is shown above

Next problem : how do you connect these transistors ?- metal layers


Metal layers and connections


Side view showing metal interconnect layers in an IC environment


Studying NMOS Regions of Operation


Control the channel in NMOS


Linear Region (Vgs > Vt and Vds small)


MOSFET in linear region resembles a resistor (small Vds)


As Vds is increased, channel tapers


Pinch-Off

When pinch-off is reached, the amount of charge in the channel remains fixed* and therefore the current saturates.


I-V characteristics of the MOSFET


ID vs VGS

Square Law


The iD–vDS characteristics for a device with k’n (W/L) = 1.0 mA/V2.

I-V characteristics of the MOSFET


Figure 4.15 Increasing vDS beyond vDSsat causes the channel pinch-off point to move slightly away from the drain, thus reducing the effective channel length (by DL).

Actually Pinch-off point moves as Vds increases


I-V characteristics showing finite output impedance


Equations

Cut-off: VGS < VT ID = 0


Key Observations

Control the ratio of Width and Length of the transistor to control transistor properties

It is a square law device. BJT was an exponential device

Terminologies for Triode and Saturation for MOSFETs are different than in BJT


Symbol for NMOS


Large Signal Model For MOSFET


PMOS

OFF

ON


Table 4.1

Large Signal Models Revisited

Biological Implants: Technology and Implicationssrout01/ee12-2008/pdfs/lecture15... · 2008-02-21 · 3 EE12: MOSFET PRIMER Sameer Sonkusale Quick Look At MOS Vs BJT Structure In

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