Basic Devices, Functions, and CharacterizationFlexible Electronics CourseSept. 25, 2008Michael ThompsonMuseum Stop 11st TransistorPoint ContactGe Slab, Au LeadsPlastic Triangle Paper ClipBell LabsBardeen, Shockley,Brattain 1947Museum Stop 2First ICMesa Transistor BarsResistors, CapacitorsAssembled Solid StateOscillator FunctionKilby 1958Texas InstrumentsMuseum Stop 31st Monolithic BJT ICRTL TechnologyMicroLogic Family4 BJT, 2RFlip-Flop FunctionCommercial Fairchild 1961Museum Stop 44 MOS ICBCD-BinaryConverter (TL);Dual 20 BitShift Reg (TR);Dual 4-InputGate (BL);Dual J-K Flip-Flop (BR)General Micro-electronics1964 Electronics 4, 1980Museum Stop 51st 8-Bit µPSi-Gate PMOS2900 Devices50K Instruct/s0.05 MIPSIntel 1971Museum Stop 6Pentium IIIMicroprocessorCMOS180 nm MFS700 MHzIntel 1999Outline• Basic electrical conduction• Fundamental semiconductor equations– Fermi level / carrier concentrations• Diodes– Junction– Schottky Barrier• Field Effect Transistors– JFET– MOSFET• FET characteristics– Key behavior / Parameter extraction• FET architectures– Top/bottom gate– Top/bottom contactBasic Electrical Conduction• Potential across contacts creates an electric field• Carriers move in response to the field with drift velocity– vd= µΕ• Total current flux product of carrier concentration and velocity– J = nevd= neµΕ = σΕ• Define electrical conductivity σ = neµ ρ = 1/neµ• Critical properties for transport– n – carrier density µ – carrier mobilityHall Effect: Determining n and µ• Carriers deflected by an applied magnetic field perpendicular to carrier flow induced by EX• Displacement force proportional to the carrier velocity (Lorentz Force) establishes a lateral field EYto balance– FY= q (v x B)– EY = v B = µ EXB• Simultaneous measurement of the sample conductivity gives the product  σ = n e µ• Combined gives independent way to determine both the effective mobility and carrier concentrationMeasuring conductivity of thin samples – 4-pt probes• Probes typically uniformly spaced– 1 mm to 10 µm• Current injected outer pins and voltage measured on inner pins– Eliminates problem of voltage drops across contacts between probe and thin film• Voltage measured on inner pins• Measures sheet resistance ρ□= 4.53 V/I = σ/t– Referred to as ohms/square for thin film“Insulating” substrateConducting thin filmMobilities of organics versus SiBand Gaps – Electrons and Holes• Characterize materials by the size of the gap between empty and filled electron states– Valance band– Conduction band• For organic materials, equivalent terms are– HOMO– LUMO• Also have direct and indirect gap, though these do not impact typical device (our) device characteristicsTypical bandgaps1.344InP0.354InAs~1.71a-Si0.661GaSb5.46C (diamond)2.26GaP0.661Ge1.41GaAs1.12SiFermi Level / Carrier concentrationChemical potential of the electrons (Fermi Level) determines the occupancy of the conduction and valence bands – hence the number of carriersExtrinsic doping – adjusting Fermi Level• Normally carrier concentrations set by extrinsic dopants (B for p-doping or As for n-doping in Si) and the Fermi Energy adjusts to match• Fundamental equations• However, the carrier concentrations are set by the Fermi level even in non-equilibrium2/322/32/)(/)(2222====−−−−hkTmNhkTmNeNpeNnhVeCkTEEVkTEECVFFcππThe Junction DiodeElectrostatics of joining junctionsFormation of the depletion zoneDepletionZone!Dynamic / Applied Bias• Barrier to carriers crossing the depletion zone is either decreased or increased with applied voltage• Forward bias – barrier decreased and current increases exponentially• Reverse bias – barrier increased and current limited to reverse leakage current• Currents actually established by diffusion of carriers from junction and recombination as minority carriers()+=−=pnnnPPkTqVnLDpLDqJeJJ0/01Current-Voltage Diode CharacteristicsLimits – Reverse Breakdown• Two normal limits• Zener Effect– Direct Tunningbetween conduction and valance band states across the depletion zone• Avalanche Breakdown– Most common– Field sufficient to accelerate carriers to velocities that generate additional carriers in the depletion zone• Both set by doping characteristicsAlternative Structure – Schottky BarrierVaIMS/CIVa> 0 RevFwdIoRunyanBeanFig. 10.20p. 543PtSi/SiCr/Si M/S SchottkyDiodesForwardSchottky Diode Forward I-VQualitative TheoryJo= ConstantQuantitative Theoryφφφφi= Built-In VoltageφφφφB= Barrier Height−== 1expkTaqVoJAIJ−−=−=kTBsaViqkTCNnDqoJkTaqVoJJφεφexp2/1221expMIS Capacitor Structure• Prototype for gate of a MOSFET structure• Application of potential moves the electron chemical potential modifying the carrier concentrations• Sufficient field will “invert” the near surface of the semiconductorMetalOxide (SiO2)Semiconductor(assume n-type)Band Diagram across MISBand diagram at onset of InversionDefinitions of weak/strong inversionInversion threshold in MISMetalOxide (SiO2)Semiconductor(assume n-type)( ) ( )TToxVVtKVVCQ −=−=0εKey types of FETs• MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor)– Utilizes an insulation (usually SiO2) between the gate and body• JFET (Junction Field Effect Transistor)– Uses reverse biased p-n junction to separate the gate from the body• MESFET (Metal-Semiconductor Field Effect Transistor)– Substitutes p-n junction with a Schottky barrier diode– Used primarily in GaAs and other III-V semiconductor materials• Subdivided by carrier type in the substrate– N-type MOSFET / P-type MOSFET• Subdivided again by whether 0-bias already inverted– Accumulation mode devices (normally off)– Depletion mode devices (normally on)Basic Operation of a MOSFETn+ S/Dn+ S/Dp- substrateDynamic characteristics• Depletion zone established between the n+ S/D and the p-type substrate, dependent on potential on S/D sides• Gate voltage behaves as spatially dependent MIS structure with inversion channel established for conduction• Charge in channel set by (V-VT) – as in MIS•