Lecture 33 OUTLINE The MOS Capacitor CV examples

Lecture #33 OUTLINE The MOS Capacitor: • C-V examples • Impact of oxide charges Reading: Chapter 18. 1, 18. 2 1 Spring 2007 EE 130 Lecture 33, Slide 1

Examples: C-V Characteristics C QS Cox HF-Capacitor VFB VT VG Does the QS or the HF-capacitor C-V characteristic apply? (1) MOS capacitor, f=10 k. Hz. (2) MOS transistor, f=1 MHz. (3) MOS capacitor, slow VG ramp. (4) MOS transistor, slow VG ramp. 2 Spring 2007 EE 130 Lecture 33, Slide 2

Example: Effect of Doping C/Cox 1 VFB VT VG • How would C-V characteristic change if substrate doping NA were increased? – VFB – VT – Cmin 3 Spring 2007 EE 130 Lecture 33, Slide 3

Example: Effect of Oxide Thickness C/Cox 1 VFB VT VG • How would C-V characteristic change if oxide thickness xo were decreased? – VFB – VT – Cmin 4 Spring 2007 EE 130 Lecture 33, Slide 4

Oxide Charges In real MOS devices, there is always some charge in the oxide and at the Si/oxide interface. • 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 5 Spring 2007 EE 130 Lecture 33, Slide 5

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 6 Spring 2007 EE 130 Lecture 33, Slide 6

Fixed Oxide Charge QF M 3. 1 e. V O S q. QF / Cox Ec= EFM |q. VFB | Ev Ec EFS Ev 4. 8 e. V 7 Spring 2007 EE 130 Lecture 33, Slide 7

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. 05 e. V), and that the gate dielectric is Si. O 2 (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 8 Spring 2007 EE 130 Lecture 33, Slide 8

Determination of FM and QF Measure C-V characteristics of capacitors with different oxide thicknesses. Plot VFB as a function of xo: VFB 10 nm 20 nm 30 nm 0 xo – 0. 15 V ´ ´ – 0. 3 V ´ 9 Spring 2007 EE 130 Lecture 33, Slide 9

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 10 Spring 2007 EE 130 Lecture 33, Slide 10

Interface Traps cause “sloppy” C-V and also greatly degrade mobility in channel 11 Spring 2007 EE 130 Lecture 33, Slide 11