Spring 2007EE130 Lecture 33, Slide 1 Lecture #33 OUTLINE The MOS Capacitor: C-V examples Impact of oxide charges Reading: Chapter 18.1, 18.2

Spring 2007EE130 Lecture 33, Slide 2 Does the QS or the HF-capacitor C-V characteristic apply? (1)MOS capacitor, f=10kHz. (2)MOS transistor, f=1MHz. (3)MOS capacitor, slow V G ramp. (4)MOS transistor, slow V G ramp. Examples: C-V Characteristics VGVG V FB VTVT C C ox QS HF-Capacitor

Spring 2007EE130 Lecture 33, Slide 3 Example: Effect of Doping How would C-V characteristic change if substrate doping N A were increased? –V FB –V T –C min VGVG V FB VTVT C/C ox 1

Spring 2007EE130 Lecture 33, Slide 4 Example: Effect of Oxide Thickness How would C-V characteristic change if oxide thickness x o were decreased? –V FB –V T –C min VGVG V FB VTVT 1 C/C ox

Spring 2007EE130 Lecture 33, Slide 5 Oxide Charges In the oxide: –Trapped charge Q ot High-energy electrons and/or holes injected into oxide –Mobile charge Q M Alkali-metal ions, which have sufficient mobility to drift in oxide under an applied electric field At the interface: –Fixed charge Q F Excess Si (?) –Trapped charge Q IT Dangling bonds In real MOS devices, there is always some charge in the oxide and at the Si/oxide interface.

Spring 2007EE130 Lecture 33, Slide 6 Effect 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 scattering

Spring 2007EE130 Lecture 33, Slide 7 Fixed Oxide Charge Q F EcEc E FS EvEv E c = E FM EvEv MOS 3.1 eV 4.8 eV |qV FB | qQ F / C ox

Spring 2007EE130 Lecture 33, Slide 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 (  M =4.05eV), and that the gate dielectric is SiO 2 (  r =3.9): –From C max = C ox we determine the oxide thickness x o –From C min and C ox we determine substrate doping (by iteration) –From substrate doping and C ox we calculate the flat-band capacitance C FB –From the C-V curve, we can find –From  M,  S, C ox, and V FB we can determine Q f

Spring 2007EE130 Lecture 33, Slide 9 0 –0.15V –0.3V xoxo V FB 10nm20nm30nm    Determination of  M and Q F Measure C-V characteristics of capacitors with different oxide thicknesses. Plot V FB as a function of x o :

Spring 2007EE130 Lecture 33, Slide 10 Mobile Ions Odd shifts in C-V characteristics were once a mystery: Source of problem: Mobile charge moving to/away from interface, changing charge centroid

Spring 2007EE130 Lecture 33, Slide 11 Interface Traps Traps cause “sloppy” C-V and also greatly degrade mobility in channel