Transistor

Overview ofSemiconductor Devices(Transistor)

Overview of Semiconductor Devices

Semiconductor DevicesResistorsCapacitorsDiodesTransistors


Electrical FundamentalsChargePositive (hole) or negative (electron) particleUnits: CoulombsAnalogy: One molecule of waterCurrentAmount of charge flowing per time periodUnits: Amps (Coulombs/sec)Analogy: Water flow rate (ex. Gallons per hour)VoltageForce of a given charge to moveUnits: VoltsAnalogy: Water pressure (ex. Foot-pounds)


Silicon Dopants


Electrons in N-Type Silicon


Conduction in n-Type Silicon


Holes in p-Type Silicon


Conduction in p-Type Silicon


TransistorConducts or restricts flow based on inputAnalogy: Switched water valve


How a Transistor WorksOFFON


How a Transistor WorksEngineerFoodIslands are made of Sand and Water?Oceans are made of Water and Sand?Its all a matter of concentration!


How a Transistor WorksSand Magnet


NMOS Transistor ConstructionN-typeN-typeP-typeSourceDrainGateSourceDrainGate


NMOS Transistor ConstructionN-typeP-typeSourceDrainGateSourceDrain+ + + +- - - -+VN-type


How an NMOS Transistor WorksN+N+S=0VD=5VG=0VP-P+W=0VTransistor in OFF state5V0V5V5VS=5VD=5VG=5VP-P+W=0VTransistor in ON stateNN+N+5V0V


Sheet1

GateDrainSourceWell

0 V5V0V0V

How a PMOS Transistor WorksP+P+D=5VS=0VG=5VN-N+W=5VTransistor in OFF state0V5V5VD=0VS=0VG=0VN-N+W=5VTransistor in ON statePP+P+5V5V


Sheet1

GateDrainSourceWell

5V5V0V5V

Sheet1

GateDrainSourceWell

0V0V0V5V

How a Transistor WorksGateSourceDrain


NMOS I-V Curve ExamplesIDS vs VDSVAYXY2VIndexA


Setting the Threshold VoltageP-Light Well ImplantWith Heavier (shallow) Vth ImplantN+N+PN+N+P-N+N+Vth=500mVVth=800mVLight Well ImplantHeavier Well ImplantVth=800mVP-N+N+Vth=800mVLight Well ImplantWith Thicker Gate OxideThreshold Voltage is dependent on the dopant concentration in the channel and the gate oxide thickness


Smaller Transistors/Short Channel Effects - HCIN+P-+VGSN++VDSDepletion Region---HotCarrierInjectionDepletion Region+VDSGND


Short Channel Effects - HCI+VDSEffect: Alters capacitance of gate ox over time.

Fix Options:Lower VDSWiden depletion regionMove depletion from under gate


Smaller Transistors/Short Channel Effects Punch ThruEffect: Depletion regions touch. Device is always on

Fix Options:Lower VDSNarrow depletion regionN+P-+VGSN++VDSDepletionRegionPunch ThruDepletionRegion


Smaller Transistors/Short Channel Effects: Fix Options


Evaluating Fix OptionsLower V DSLowering V DS may may compromise reliability of signal


Evaluating Fix Options Move depletion region from under gateSpread the S/Ds out from the gateMay lead to always open transistorNo Gate Control


Evaluating Fix Options Change Dopant LevelsDepletion RegionP-N+Depletion RegionN- Change Well to P+Would increase Threshold Voltage Change Drain Implant to N-Would increase Contact Resistance


Short Channel Effects SolutionDepletion RegionP-N+N+P-N-Fix HCI and Punch-ThruWITHOUT Narrowing itWITHOUT moving the S/DsWITHOUT increasing VthWITHOUT increasing Contact RsPunch Thru ImplantLightly Doped Drain (LDD)


Example G12 Transistor Types


Sheet1

TransistorCategoryTransistorTypeThresholdVoltageCriticalDimensionOperatingVoltage

HPNMOS300mV0.18um1.8V

(High Perf)PMOS-300mV0.18um1.8V

LLNMOS450mV0.18um1.8V

(Low Leakage)PMOS-450mV0.18um1.8V

TONMOS650mV0.24um2.5V

(Thick Oxide)PMOS-550mV0.24um2.5V

ATONMOS600mV0.36um3.6V or 5V

(Analog TO)PMOS-600mV0.36um3.6V or 5V

ATONNMOS50mV0.75um3.6V

(Native ATO)PMOSNANANA

TONNMOS180mV0.75um3.6V

(Native TO)PMOSNANANA

Process Variations in G12 TransistorsHP: Nominal Well and threshold voltage implantsLL: 2 extra masks and implantsNMOS and PMOS Vt adjustments to raise VtHigher Vt transistors have less performance but better leakageTO: 1 extra maskBlock Nitrogen implant in thin gate areas that retards growth rateThicker gate oxide for higher voltageA(nalog): 2 extra masks and implantsNominal punch-thru and LDD implants blockedSpecial lighter NMOS and PMOS LDD implantsProvides less on-resistance and higher HCI protectionN(ative): 1 extra mask and implantNo nominal p-well or Vt adjust implantsSpecial very light p-well implant


A short review of chemistrys periodic table of the elements shows the location of silicon with respect to its neighboring elements. Silicon is a member of the group 4 elements. These are elements that have four valence electrons and form covalent bonds very much like in the preceding diagram. Elements in groups 3 and 5 serve as doping materials. These are materials that are used to deliberately change the electrical conductivity of silicon. The materials are sometimes referred to as impurities or dopants. Impurities are not to be confused with contaminants that result in defects in semiconductor products.Group 5 elements such as phosphorus have one more free electron than the silicon atom. When phosphorus is added to a crystal of silicon and then heated, phosphorus atoms become bonded with silicon atoms. The phosphorus atoms become part of the crystal lattice structure. Now, if we look closely around the phosphorus (P) atoms, well see one additional electron in the lattice. These are free electrons that are available to carry current flow through the silicon crystal. Because this type of silicon crystal has an excess of negative electron charges, it is commonly referred to as n-type silicon.Group 3 elements such as boron have one less free electron than the silicon atom. When boron is added to a crystal of silicon and then heated, boron atoms become bonded with silicon atoms. The boron atoms become part of the crystal lattice structure. Looking closely around the boron (B) atoms, well see there are vacancies in the lattice. These vacancies are called holes. Holes have a positive charge and are also available to carry current flow through the silicon crystal. Because this type of silicon crystal has predominantly positive hole charges, it is commonly referred to as p-type silicon.

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Transistor