lecture20_March2.ppt

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  • Look for another threshold band diagram without interface charges! Note here the electric field in the semiconductor is too large!
  • Look for another threshold band diagram without interface charges! Note here the electric field in the semiconductor is too large!
  • lecture20_March2.ppt

    1. 1. EE 30357: Semiconductors II: Devices Lecture Note #20 (03/02/09) MOS Field Effect Transistors Grace Xing <ul><li>Outline: </li></ul><ul><ul><li>Last class: Compound semiconductor based devices </li></ul></ul><ul><ul><li>Quick revisit of MOS capacitors and FETs (read on your own how DRAM, CCD, flash memory etc. work, they are all based on MOS structures!) </li></ul></ul><ul><ul><ul><li>Flat band voltage </li></ul></ul></ul><ul><ul><ul><li>Effects of oxide charges (interface charges, fixed charges) </li></ul></ul></ul>Grace Xing---EE30357 (Semiconductors II: Devices)
    2. 2. Grace Xing---EE30357 (Semiconductors II: Devices) Oxide thickness How to extract doping concentration from C-V measurements V T Weak inversion is “weakly” defined term. The difference between high f and low f is true for MOS (gate-back ohmic contact) capacitors However, there is no difference between high f and low f for MOS (gate-S/D ohmic contact) capacitors since the minority charges are supplied from the top ohmic contacts and the majority carrier are still supplied from the substrate ohmic contacts. Gate Oxide p- substrate In a MOSFET, n+ ohmic contacts can be grounded. p+ n+ n+ Depletion Weak inversion Gate Oxide p- substrate Ohmic contacts p+ In a MOS-CAP, p+ ohmic contact is grounded.
    3. 3. Grace Xing---EE30357 (Semiconductors II: Devices) When the semiconductor energy band is flat, we call it flat band condition and the voltage (V GS ) needed the flat band voltage V FB . Q: if there is no charge in the oxide or at the oxide-semiconductor interface, i.e. an ideal MOS capacitor, what is the electric field in the oxide at Flat Band? What is V FB ? A: zero since there is no charge anywhere; V FB = V bi =  ms /q Q: what if oxide charge is not zero? Flatband Voltage
    4. 4. Grace Xing---EE30357 (Semiconductors II: Devices) Donor-like traps: Neutral when filled Positive when empty Gate Oxide Channel
    5. 5. Grace Xing---EE30357 (Semiconductors II: Devices) E vac Flatband Condition (  s = 0) Can you reason for V FB then? Concept-Graph-Equation Q B =0 Area enclosed by  is V bi +V FB not V bi ! Do not confuse V FB (the external applied bias) with the potential drop in the MOS! q(V FB +V bi ) q  ox  m  S
    6. 6. Grace Xing---EE30357 (Semiconductors II: Devices) E vac Threshold Condition: I (w/o oxide & interface charges) (  s =2  f ) Induced mobile charges (electrons in this example) << ionized dopants (acceptors here)  Can be ignored Concept-Graph-Equation Q B (2  f ) Inaccuracy: the slope of Evac should be 1/3 smaller in Si than in SiO2 Area enclosed by  is V bi +V T not V bi ! q(V T +V bi ) q(2  f ) q  ox Φ m Φ S
    7. 7. Grace Xing---EE30357 (Semiconductors II: Devices) Donor-like traps: Neutral when filled and Positive when empty q(V T +V bi ) q(2  f ) q  ox Φ m Φ S E vac Threshold Condition: II (with oxide/interface charges) (Still true:  s =2  f ) Concept-Graph-Equation Q B (2  f ) Positive oxide charges or interfacial charges  Smaller charger at the gate  Smaller field inside the oxide  Smaller total band bending in E vac Area enclosed by  is V bi +V T not V bi !
    8. 8. Grace Xing---EE30357 (Semiconductors II: Devices)
    9. 9. Grace Xing---EE30357 (Semiconductors II: Devices) Boron – acceptors: ionized acceptors are negatively charged Threshold voltage control <ul><li>There is a n-channel Si MOSFET. Due to some mishaps during fabrication, it ended up being a depletion mode FET with Vth = -0.1V. Our target Vth is 0.5V. </li></ul><ul><li>What type of interface charges can help us tune it to the right Vth? (negative) </li></ul><ul><li>How many charges are necessary if C ox’ = 0.8uF/cm 2 ? (the grey area = 0.5 – (-0.1) = 0.6V = Q ii /C ox ’) </li></ul><ul><li>Does this process change V bi of the device? (No) </li></ul>Q B (>2  f ) Q B (<2  f ) <ul><li>Is E-field in Si equal to 1/3 of E-field in SiO 2 ? </li></ul><ul><li>Why not? </li></ul>Q B (=2  f ) 0 V 0 V 0.5 V Q ii
    10. 10. Grace Xing---EE30357 (Semiconductors II: Devices) C i = C ox C d = C B Also see Fig. S3.14 in Anderson
    11. 11. Grace Xing---EE30357 (Semiconductors II: Devices) Field effect transistors – Voltage controlled barrier for current flow Input current (DC) = zero Capacitive actions – Need two plates of charges separated by an insulating layer Choices of insulating layer: oxide and depletion region Compare with current controlled barrier devices - BJTs
    12. 12. Grace Xing---EE30357 (Semiconductors II: Devices)
    13. 13. Grace Xing---EE30357 (Semiconductors II: Devices) Source Drain During writing operations,
    14. 14. Grace Xing---EE30357 (Semiconductors II: Devices) Source Drain During reading operations,

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