Qualitative Discussion of MOS Transistors. Big Picture ES220 (Electric Circuits) – R, L, C, transformer, op-amp ES230 (Electronics I) – Diodes – BJT –

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Presentation transcript:

Qualitative Discussion of MOS Transistors

Big Picture ES220 (Electric Circuits) – R, L, C, transformer, op-amp ES230 (Electronics I) – Diodes – BJT – Complementary Metal Oxide Semiconductor (CMOS) Field Effect Transistor (FET) Applications in Digital Integrated Circuits ES330 (Electronics II) – Explore Applications of CMOS in Analog Integrated Circuits

Topics Covered in ES330 Small Signal Model Body Effect

A Crude Metal Oxide Semiconductor (MOS) Device P-Type Silicon is slightly conductive. Positive charge attract negative charges to interface between insulator and silicon. A conductive path is created If the density of electrons is sufficiently high. Q=CV. V2 causes movement of negative charges, thus current. V1 can control the resistivity of the channel. The gate draws no current!

An Improved MOS Transistor n+ diffusion allows electrons move through silicon. (provide electrons)(drain electrons)

Typical Dimensions of MOSFETs These diode must be reversed biased. tox is made really thin to increase C, therefore, create a strong control of Q by V.

A Closer Look at the Channel Formulation Need to tie substrate to GND to avoid current through PN diode. Positive charges repel the holes creating a depletion region, a region free of holes. Free electrons appear at VG=VTH. VTH=300mV to 500 mV (OFF)(ON)

MOSFET as a Variable Resistor As VG increases, the density of electrons increases, the value of channel resistance changes with gate voltage. You can build an attenuator circuit. (i.e. a voltage divider)

Change Drain Voltage Resistance determined by VG.

Change Gate Voltage Higher VG leads to a lower channel resistance, therefore larger slope.

Length Dependence The resistance of a conductor is proportional to the length. fixed V D fixed V G

Dependence on Oxide Thickness Q=CV C is inversely proportional to 1/tox. Lower Q implies higher channel resitsance. fixed V D fixed V G

Width Dependence The resistance of a conductor is inversely proportional to the cross section area. A larger device also has a larger capacitance!

Channel Pinch Off Q=CV – V=VG-V OXIDE-Silicon V OXIDE-Silicon can change along the channel! Low V OXIDE-Silicon implies less Q.

VG-VD is sufficiently large to produce a channel VG-VD is NOT sufficiently large to produce a channel No channel Electrons are swept by E to drain. Drain can no longer affect the drain current!

Regions No channel (No Dependence on VDS)

Determination of Region How do you know whether a transistor is in the linear region or saturation region? – If VDS>(VGS-VTH) and VGS>VTH, then the device is in the saturation region. – If VDS VTH, then the device is in the linear region.

Graphical Illustration

Limited VDS Dependence During Saturation As VDS increase, effective L decreases, therefore, ID increases.

Pronounced Channel Length Modulation in small L

PMOS Transistor

IV Characteristics of a PMOS