Qualitative Discussion of MOS Transistors
31
Qualitative Discussion of MOS Transistors
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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 - PowerPoint PPT Presentation
Transcript of Qualitative Discussion of MOS Transistors
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
P-Type Silicon is slightly conductive.
Positive charge attractnegative chargesto interface between insulator and silicon.
A conductive path is createdIf 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 allowselectrons movethrough silicon.
(provide electrons) (drain electrons)
Typical Dimensions of MOSFETs
These diode mustbe 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 GNDto avoid current through PN diode.
Positive charges repel the holescreating 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 ofchannel resistance changes with gate voltage.
You can build anattenuator 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=CVC is inversely proportional to 1/tox.
Lower Q implies higher channel resitsance.
Width Dependence
The resistance of a conductor is inversely proportional to the crosssection 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 largeto produce a channel
VG-VD is NOT sufficiently largeto produce a channel
No channel
Electronsare sweptby 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<(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 mVVin, m=1 mV
Replace the transistor by its small signal equivalent circuit
(EQ 5.157)
Comparision:ADS Simulation: 46EQ 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
