Advanced MOS Devices Chapter 6 Long Channel MOSFET Cullen College of Engineering Department of Electrical and Computer Engineering Dr Len Trombetta Spring 2011 The MOSFET An appropriate voltage applied to the gate the threshold voltage allows a current to flow from drain to source Gated Diode A simpler device the gated diode shows some important characteristics of the MOSFET drain region MOS capacitor an annulus Diode Guard ring for isolation We can look at the CV curve Diode is putting additional charge into the inversion layer Diode is pulling charge away from inversion layer The IV characteristics tell us something about generation recombination processes Surface carrier concentration in inversion ns ni e q S F kT q F q s q S F w wT SiO2 Si Ei EF Enhancement Mode Application of VG VT turns on the FET by enhancing the channel conductivity Depletion Mode DMOS VT 0 Application of VG VT turns off the FET by depleting the channel charge pFET p channel Substrate is n type and channel is p type nFET n channel Substrate is p type and channel is n type Uyemura Fundamentals of MOS Digital Integrated Circuits Addison Wesley 1988 Uyemura Fundamentals of MOS Digital Integrated Circuits Addison Wesley 1988 Uyemura Fundamentals of MOS Digital Integrated Circuits Addison Wesley 1988 Uyemura Fundamentals of MOS Digital Integrated Circuits Addison Wesley 1988 Geometry for basic current voltage analysis VGB VSB Assumptions Ignore depletion regions around source drain Current is entirely due to drift no diffusion current Channel charge is determined by MOS capacitor formulae Channel mobility is lower than bulk but constant across the channel n VDB VSB n E ID y L x y 0 p Si substrate B Illustration of w y used for bulk charge calculation w y y Schematic illustration of variation in potential across the channel V y VD y Square Law and Bulk Charge Comparison 5 Drain Current ID mA 4 3 2 1 0 0 1 2 3 Drain Voltage VD V 4 5 Square Law VG 5 V Bulk Charge VG 5 V Square Law VG 3 V Bulk Charge VG 3 V Comparison of Square Law and Bulk Charge models for dox 200 NB 2 x 1016 cm 3 VT 1 V W L 5 meff 500 cm2 V s Bulk charge model gives smaller ID because for a given VG channel charge QI is less Generalized vs Bulk Charge Models 5 Drain Current ID mA 4 3 2 1 0 0 1 2 3 Drain Voltage VD V 4 5 Generalized VG 5 V Bulk Charge VG 5 V Generalized VG 3 V Bulk Charge VG 3 V Comparison of Generalized IV and Bulk Charge models for d ox 200 NB 2 x 1016 cm 3 VT 1 V W L 5 meff 500 cm2 V s ID Strong Inversion Approximation If we take the approximations so 2 F VSB sL 2 F VDB we arrive at 1 2 2 V V 2 V V V V GB FB F DB SB DB SB 2 W I D1 m Cox L 2 2 V 3 2 2 V 3 2 F DB F SB for the drift current component of ID which is the dominant component in inversion This result is equivalent to the one obtained using the Bulk Charge Model MOSFET Surface Potential Surface Pot ential vs Channel Len gt h 5 Surface Potential V 4 3 2 1 0 0 0 2 0 4 0 6 Normalized Channel Length 0 8 Surface potential vs distance along channel L x L MOSFET parameters are the same as for the I V calculations 1 Electric Field Along Channel Lateral Electric Field V m 40 30 20 10 0 0 0 2 0 4 0 6 Normalized Channel Length 0 8 Electric field vs distance along channel L x L MOSFET parameters are the same as for the I V calculations The rapid rise in field near the drain is reproduced by more sophisticated calculations 2D simulation and is a problem for device reliability 1 Subthreshold Current Calculation ITotal VG 1 V 0 V ITotal VG 0 2 V 0 V 1 10 3 1 10 4 1 10 5 1 10 6 1 10 7 1 10 8 1 10 9 1 10 10 1 10 11 1 10 12 1 10 13 0 0 5 1 1 5 2 2 5 3 VG Subthreshold current calculated from Generalized Model MOSFET parameters are the same as for previous I V calculations Drain voltage is either 1 V red curve or 0 2 V blue curve We can extract threshold voltage from ID1 2 vs VDS The FET will be in saturation if the gate and drain voltages are equal since VDS will always be greater than VGS VT Notation VT0 is the threshold voltage with no body bias VT1 VT2 etc are shifted because of the application of body bias Also VSB VBS MOSFET ID VDS Data 1 2E 03 1 0E 03 ID A 8 0E 04 VG 1 5 V VG 2 V 6 0E 04 VG 3 V VG 2 5 V 4 0E 04 2 0E 04 0 0E 00 0 0 5 1 1 5 2 2 5 3 VDS V This data was taken on a ON Semiconductor MC14007UB MOSFET chip IDS A Subthreshold Current Data from MC14007UB 1 0E 01 1 0E 02 1 0E 03 1 0E 04 1 0E 05 1 0E 06 1 0E 07 1 0E 08 1 0E 09 1 0E 10 1 0E 11 1 0E 12 VD 1 5 V VD 0 5V 0 0 5 1 1 5 2 2 5 3 3 5 VGS V The subthreshold current should not be a function of drain voltage dependence on V D is evidence of short channel effects The subthreshold current in this device appears to be independent of VD Saturation Current and Body Bias Data from MC14007UB 4 0E 02 3 5E 02 SQRT ID A 3 0E 02 VBS 0 2 5E 02 VBS 0 5 V 2 0E 02 VBS 1 0 V 1 5E 02 VBS 1 5 V 1 0E 02 5 0E 03 0 0E 00 0 1 2 3 VGS VDS V 4 5 Silvaco Virtual Wafer Fab Simulations L 0 5 mm tox 100 0 008 0 007 Mobility tends to drop with increasing gate voltage so ID rolls over which complicates the extraction of the threshold voltage from these curves 0 005 0 004 0 003 0 002 0 001 0 000 0 0 5 1 1 5 2 2 5 3 3 5 Gate Voltage V 0 2 4 Same data as above plotted as log ID vs VG Subthreshold slop is 100 mV dec Log ID Sqrt ID 0 006 6 8 10 12 14 0 0 2 0 4 0 6 0 8 1 Gate Voltage V 1 2 1 4 1 6 Parasitic source and drain resistance From Tsividis Yuan Taur et al Shift and Ratio Method for MOSFET Channel Length Extraction IEEE El Device Letters 13 5 267 1992 From Uyemura Sign of VT Terms VT MS Qox …