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Lecture #33

OUTLINE The MOS Capacitor: C-V examples Impact of oxide charges Reading: Chapter 18.1, 18.2. Lecture #33. C. QS. C ox. HF-Capacitor. V G. V T. V FB. Examples: C - V Characteristics. Does the QS or the HF-capacitor C-V characteristic apply? MOS capacitor, f=10kHz.

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Lecture #33

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  1. OUTLINE The MOS Capacitor: C-V examples Impact of oxide charges Reading: Chapter 18.1, 18.2 Lecture #33 EE130 Lecture 33, Slide 1

  2. C QS Cox HF-Capacitor VG VT VFB Examples: C-V Characteristics • Does the QS or the HF-capacitor C-V characteristic apply? • MOS capacitor, f=10kHz. • MOS transistor, f=1MHz. • MOS capacitor, slow VG ramp. • MOS transistor, slow VG ramp. EE130 Lecture 33, Slide 2

  3. Example: Effect of Doping C/Cox • How would C-V characteristic change if substrate doping NA were increased? • VFB • VT • Cmin 1 VG VT VFB EE130 Lecture 33, Slide 3

  4. Example: Effect of Oxide Thickness C/Cox • How would C-V characteristic change if oxide thickness xo were decreased? • VFB • VT • Cmin 1 VG VT VFB EE130 Lecture 33, Slide 4

  5. In the oxide: Trapped charge Qot High-energy electrons and/or holes injected into oxide Mobile charge QM Alkali-metal ions, which have sufficient mobility to drift in oxide under an applied electric field At the interface: Fixed charge QF Excess Si (?) Trapped charge QIT Dangling bonds Oxide Charges In real MOS devices, there is always some charge in the oxide and at the Si/oxide interface. EE130 Lecture 33, Slide 5

  6. In general, charges in the oxide cause a shift in the gate voltage required to reach the threshold condition: (x defined to be 0 at metal-oxide interface) In addition, they may alter the field-effect mobility of mobile carriers (in a MOSFET) due to Coulombic scattering Effect of Oxide Charges EE130 Lecture 33, Slide 6

  7. Fixed Oxide Charge QF M O S qQF / Cox 3.1 eV Ec= EFM |qVFB| Ev Ec EFS Ev 4.8 eV EE130 Lecture 33, Slide 7

  8. Parameter Extraction from C-V From a single C-V measurement, we can extract much information about the MOS device. • Suppose we know that the gate-electrode material is heavily doped n-type poly-Si (FM=4.05eV), and that the gate dielectric is SiO2 (er=3.9): • From Cmax = Cox we determine the oxide thickness xo • From Cmin and Cox we determine substrate doping (by iteration) • From substrate doping and Cox we calculate the flat-band capacitance CFB • From the C-V curve, we can find • From FM, FS, Cox, and VFB we can determine Qf EE130 Lecture 33, Slide 8

  9. Determination of FM and QF Measure C-V characteristics of capacitors with different oxide thicknesses. Plot VFB as a function of xo: V FB 10nm 20nm 30nm xo 0 –0.15V ´ ´ –0.3V ´ EE130 Lecture 33, Slide 9

  10. Odd shifts in C-V characteristics were once a mystery: Source of problem: Mobile charge moving to/away from interface, changing charge centroid Mobile Ions EE130 Lecture 33, Slide 10

  11. Interface Traps Traps cause “sloppy” C-V and also greatly degrade mobility in channel EE130 Lecture 33, Slide 11

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