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ChemFET Sensors: Combination of p-n Junctions in Bipolar Transistor and Field Effect Transistor (FET)

This lecture discusses the combination of two back-to-back p-n junctions in a bipolar transistor and the use of a field effect transistor (FET) in ChemFET sensors. Topics covered include electron tunneling, electron transmission, potential energy of barriers, voltage-controlled resistor inversion zone, Poisson's equation, metal contacts, boundary conditions, inversion layer, Fermi energy, gas adsorption and intercalation, semi-conducting oxide sensors, gate materials, non-stoichiometric dielectrics, and extrinsic conductivity.

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ChemFET Sensors: Combination of p-n Junctions in Bipolar Transistor and Field Effect Transistor (FET)

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  1. Lecture 7.2 ChemFET Sensors

  2. Combination of two back-to-back p-n junctions P-N-P or N-P-N Bipolar Transistor

  3. Bipolar Transistor

  4. Field Effect Transistor (FET)

  5. Electron Tunneling • Electron Transmission, T, through thickness, δ. • U=Potential Energy of Barrier • E=Total Energy of Electron

  6. Voltage Controlled Resistor

  7. Inversion Zone - Poisson’s Eq. • 2U = -/( o ) • Metal on • N Zone P Zone • n= - e Nd -p=+ e Na • Boundary Conditions • U=Uo at x=0 • U=0 V at x=

  8. Inversion Layer

  9. Fermi Energy is related to the Work Function for the Metal Work Function is changed by gas adsorption or intercalation Different Metals have different Fermi Energies

  10. Metal Pd H2 2 H (intercalated in Pd) Alters Pd work function and Ef Hydrogen Sensor Semi-Conducting Oxide SnO2_ Reducing Gas Alters Oxygen Vacancy Alters Ef and Conductivity Alcohol Sensor Formaldehyde Sensor CO sensor Gate Material

  11. Non-Stoichiometric Dielectrics • Metal Excess • Metal with Multiple valence • Metal Deficiency +4 +3 +2 +3

  12. Density Change with Po2 SrTi1-xO3

  13. Non-Stoichiometric Dielectrics Ki=[h+][e-] K”F=[O”i][V”O] Conductivity =f(Po2 ) Density =f(Po2 )

  14. Non-Stoichiometric Dielectrics Excess M1+x O Deficient M1-x O

  15. Dielectric Conduction due to Non-stoichiometry • N-type P-type

  16. Dielectric Conduction due to Non-stoichiometry • N-type P-type + h + h Excess Zn1+xO Deficient Cu2-xO

  17. Extrinsic Conductivity • Donor Doping Acceptor Doping • n-type p-type Ed = -m*e e4/(8 (o)2 h2) Ef=Eg-Ed/2 Ef=Eg+Ea/2

  18. Inversion Layer

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