Lecture #33Slide 2Examples: C-V CharacteristicsExample: Effect of DopingExample: Effect of Oxide ThicknessOxide ChargesEffect of Oxide ChargesFixed Oxide Charge QFParameter Extraction from C-VDetermination of FM and QFMobile IonsInterface TrapsEE130 Lecture 33, Slide 1Spring 2007Lecture #33OUTLINEThe MOS Capacitor:•C-V examples•Impact of oxide chargesReading: Chapter 18.1, 18.2EE130 Lecture 33, Slide 2Spring 2007EE130 Lecture 33, Slide 3Spring 2007Does the QS or the HF-capacitor C-V characteristic apply?(1) MOS capacitor, f=10kHz.(2) MOS transistor, f=1MHz.(3) MOS capacitor, slow VG ramp.(4) MOS transistor, slow VG ramp.Examples: C-V CharacteristicsVGVFBVTCCoxQSHF-CapacitorEE130 Lecture 33, Slide 4Spring 2007Example: Effect of Doping•How would C-V characteristic change if substrate doping NA were increased?–VFB–VT–CminVFBVTC/Cox1EE130 Lecture 33, Slide 5Spring 2007Example: Effect of Oxide Thickness•How would C-V characteristic change if oxide thickness xo were decreased?–VFB–VT–CminVGVFBVT1C/CoxEE130 Lecture 33, Slide 6Spring 2007Oxide Charges•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 bondsIn real MOS devices, there is always some charge in the oxide and at the Si/oxide interface.EE130 Lecture 33, Slide 7Spring 2007Effect of Oxide Charges•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 scatteringoxoxSiOTdxxxV0)(12EE130 Lecture 33, Slide 8Spring 2007oxFMSFBCQV Fixed Oxide Charge QFEcEFSEvEc= EFMEvM O S3.1 eV4.8 eV|qVFB |qQF / CoxEE130 Lecture 33, Slide 9Spring 2007Parameter Extraction from C-VFrom a single C-V measurement, we can extract muchinformation about the MOS device.•Suppose we know that the gate-electrode material is heavily doped n-type poly-Si (M=4.05eV), and that the gate dielectric is SiO2 (r=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 M, S, Cox, and VFB we can determine QfFBCCGFBVVEE130 Lecture 33, Slide 10Spring 2007FSiOoMSFBQxV20–0.15V–0.3VxoVFB10nm 20nm 30nmDetermination of M and QFMeasure C-V characteristics of capacitors with different oxide thicknesses. Plot VFB as a function of xo:EE130 Lecture 33, Slide 11Spring 2007Mobile Ions•Odd shifts in C-V characteristics were once a mystery:•Source of problem: Mobile charge moving to/away from interface, changing charge centroidoxMFBCQV EE130 Lecture 33, Slide 12Spring 2007Interface TrapsTraps cause “sloppy” C-V and also greatly degrade mobility in
View Full Document
Unlocking...