Qualitative Discussion of MOS Transistors

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Presentation transcript:

Qualitative Discussion of MOS Transistors

Big Picture ES230 Diodes BJT Op-Amps ES330 Applications of Op-Amps CMOS Analog applications Digital applications

A Crude Metal Oxide Semiconductor (MOS) Device V2 causes movement of negative charges, thus current. V1 can control the resistivity of the channel. 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. P-Type Silicon is slightly conductive. The gate draws no current!

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

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. VTH=300mV to 500 mV (OFF) (ON) Positive charges repel the holes creating a depletion region, a region free of holes. Free electrons appear at VG=VTH.

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.

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

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-VOXIDE-Silicon VOXIDE-Silicon can change along the channel! Low VOXIDE-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 Dependence on VDS) No channel

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<(VGS-VTH) and VGS>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

Transconductance As a voltage-controlled current source, a MOS transistor can be characterized by its transconductance: It is important to know that

What Happens to gm/ID when W and ID are doubled?

Body Effect The threshold voltage will change when VSB=0!

Experimental Data of Body Effect The threshold voltage will increase when VSB increases.

What if we drive the base with a small signal?

Input and Output Vout, m=46 mV Vin, m=1 mV

Replace the transistor by its small signal equivalent circuit Comparision: ADS Simulation: 46 EQ 5.157: 49.33

Small Signal Model for NMOS Transistor

PMOS Transistor

IV Characteristics of a PMOS

Small Signal Model of PMOS

Small Signal Model of NMOS