Introduction to MOSFETsIntroduction to MOSFETsD. W. ParentNMOS Cross SectionNMOS Cross SectionPoly-SiViSiO2Poly SiGaten+Sourcen+Drainp+BodyField OxideField OxideViaVia ViaInverted ChannelPWELL2The MOS Field‐Effect TransistorThe MOS FieldEffect TransistorV++++++++SiO2VGs++++++++d----------------------D(n+) S(n+)InversionDepletionDepletion3The MOS Field‐Effect TransistorThe MOS FieldEffect Transistor:RegionLinear TGDSAT:)V-V(VRegion Saturation =⎤⎡=Dinm1ZVCLZggμ2DSATinm(sat)ZVCμLZg =()⎥⎦⎤⎢⎣⎡−−=2inD21CLZIDDTGVVVVμ2DSATinD(sat)VCμLZI=IDPinch-offVD (Sat.)VG = +3V+1VG = +24VG= +1VDBasic MOSFET operationBasic MOSFET operation•PMOSPMOS– a p‐type channel is created in a n‐type substrate.•NMOS•NMOS – an n‐type channel is created in a p‐type substrate.5Basic MOSFET operationBasic MOSFET operationIDPinch-offVD (Sat.)VG= +3VG=+1VG = +2VG 1VDICIB6VCEThe MOS Field‐Effect TransistorThe MOS FieldEffect TransistorVVxVDdVxXL0xxx +dx7The MOS Field‐Effect TransistorThe MOS FieldEffect Transistor:RegionLinear TGDSAT:)V-V(VRegion Saturation =⎤⎡=Dinm1ZVCLZggμ2DSATinm(sat)ZVCμLZg =()⎥⎦⎤⎢⎣⎡−−=2inD21CLZIDDTGVVVVμ2DSATinD(sat)VCμLZI=IDPinch-offVD (Sat.)VG = +3V+1VG = +28VG= +1VDWhat is“Pinch off”What is Pinch off•For a given VGyou have a maximum amountFor a given VG, you have a maximum amount of current you can flow through the channel (regardless of VD)(regardless of VD).• When VD=VG‐VT the channel is flowing as much current as it can so we call it pinched offmuch current as it can so we call it pinched off (even though current continues to flow).9MOSFET can be used in discrete form (2N7000) t it td iit(2N7000) or as part on an integrated circuit• Discrete (EE122)– Amplifiers– Switches–Designer chooses the part and how it is connected on theDesigner chooses the part and how it is connected on the PCB board.• IC (EE125, 166, 223, 224, 227)Diff ti l i–Differential pairs– Current mirrors– Digital Logic–Create OPAMPS– Designer chooses all the dimensions of the transistor and how they are connected on the IC.yhttp://cds.linear.com/docs/Application/df%20Note/an89.pdfNeural InterfacingPossibleSqrtFunction?Possible SqrtFunction?N MOSFETS are non‐linear DevicesCut Off: For our discrete devices we can assume that if VGS is less than or equal to the threshold voltage (VT)no current will flow (no channel), regardless of theto the threshold voltage (VT)no current will flow (no channel), regardless of the voltage from drain to source.Linear: if VGS is greater than the threshold and a potential exists from drain to source of the transistor that is less than (VGS‐VT). It is called linear because if Vdsis small the parabolic term drops out. Once can make resistors this way.KN is theVT is the thresholdKN is the transconductance. You have to extract it (A/V2)VT is the threshold voltage. You have to extract it (V)Saturation: if VGSis greater than the threshold voltage current and a potential exists from drain to source of the transistor that is greater than or equal to (VGS‐VT). Amplifier design is done in saturation.P MOSFETS are non‐linear DevicesCut Off: For our discrete devices we can assume that if VGS is greater than or equal to the threshold voltage (VT)no current will flow (no channel), regardlessequal to the threshold voltage (VT)no current will flow (no channel), regardless of the voltage from drain to source.Linear: if VGS is less than the threshold and a potential exists from drain to source of the transistor that is greater than (VGS‐VT). It is called linear because if Vds is small the parabolic term drops out. Once can make resistors this way.KP istheVT is the thresholdKP is the transconductance. You have to extract it (A/V2)VT is the threshold voltage. You have to extract it (V)Saturation: if VGSis less than the threshold voltage current and a potential exists from drain to source of the transistor that is less than or equal to (VGS‐VT). Amplifier design is done in saturation.In order to model the current voltage relationships in a MOSFET, one needs to know whether it is in cut‐off, ,,sub‐threshold linear or saturation•Spice does this automaticallySpice does this automatically• In an “hand analysis” we make an educated guess and then calculate the current voltageguess and then calculate the current voltage– One must check the educated guess• The most basic skill is to tell based on drain current and gate, drain and source voltages hh hwhich mode the device is in.A Behavioral Current source can easily dlh hiifmodel the DC characteristics of a MOSFETWhich Mode is the MOSFET in?Which Mode is the MOSFET in?VG 3VBVG=3VIDVS=0VVT=1VVG=2VVG 2VAECVG=0, 1 VVDSEDFWhich Mode? Point ALinearWhich Mode? Point BSaturationWhich Mode? Point CSaturationWhich Mode? Point DffCut‐offGroup Quiz: What mode is point E?h dWhat mode is point F?PMOS transistors are better modeled like thislike this.There are NMOS transistors with negative d i ih iiVTs and PMOS transistor s with positive VTsThese are called depletion MOS or“normally on”These are called depletion MOS, or normally on.The definition on normally on is a transistor that has a conducting channel when VGS is zero Volts. (NMOS or PMOS).VT=‐1VThere are NMOS transistors with negative d i ih iiVTs and PMOS transistor s with positive VTsThese are called depletion MOS or“normally on”These are called depletion MOS, or normally on.The definition on normally on is a transistor that has a conducting channel when VGS is zero Volts. (NMOS or