Comparison of the Behavior of MOSFET Transistors Described in Hardware Description Languages Aravind Gurumurthy M.S Thesis Defense Presentation Committee Chair: Dr. Carla Purdy

Goals Choose the correct transistor model appropriate for a given application Compare simulation results of Verilog-AMS models with results from SPICE Improve simulation time and accuracy

Thesis outline MOSFET transistor MOSFET modeling Different generations of MOS models Experimental setup and models used Results Conclusions and future work

MOSFET transistor MOSFET is a majority carrier device Can be of two types – NMOS or PMOS Fig 1 Top view of NMOS transistor, W= Channel width, L= Channel length In NMOS transistor as shown, current carried from source to drain by electrons through the n-type channel

MOSFET parameters The table 1 shows the most frequently used MOS parameters Table 1 MOS parameters

Modes of operation Cutoff region : V GS < V T Resistive region: V GS – V T > V DS I D = (k n ’. W)/(L)*[(V GS - V T ) 2 – V DS 2 /2] Saturation region: V T < V GS and V GS – V T < V DS I D = (k n ’. W)/(L)*[(V GS - V T ) 2 /2]

MOSFET history MOSFET modeling dates back 35 years Initially based in V-I and V-C characteristics Modeling has become more complex Current models include effects from short channel and long field strengths Steady increase in the number of model parameters over almost four decades

MOSFET history Source : Fig 1 Number of model parameters Vs time

MOS modeling Modeling can be defined as “The method of finding the parameter values for fixed simulator model equations” MOS modeling -Writing a set of equations that link voltages and currents Behavior of the device can be simulated and predicted Basic MOS model components 1.Equations describing I ds (V ds ) and I ds (V gs ) 2.Parameters that link the technology being used for fabrication

Requirements of good MOS model Good I-V characteristic accuracy Meet charge conservation requirement Correct values of small-signal quantities Good prediction for white and 1/f noise Ability to provide results even when device operation is quasi static Ability to include all physical mechanisms for sub-micron devices

Benchmark tests Benchmark tests used to examine the accuracy For simple circuits unveils the problem areas for a given model General benchmark tests that can be performed are: – DC tests – Small signal tests – Noise tests – Frequency test

Flowchart of the operation Fig 2 Flowchart for choosing the correct MOS model

Generation of MOS models Generation 1 – MOS 1 (LEVEL 1) **** – MOS 2 (LEVEL 2) – MOS 3 (LEVEL 3) Generation 2 – BSIM 1 (LEVEL 13) – MODIFIED BSIM (LEVEL 28) – BSIM 2 (LEVEL 39) Generation 3 – BSIM 3 (LEVEL 47) **** – MOS 9 (LEVEL 50) – BSIM 4 (LEVEL 54) – EKV (LEVEL 55) **** – BSIM3-SOI (LEVEL 59) – MOS 11(LEVEL 63) ****

Generation I Focuses on analytical expressions and extraction of parameters Source: Kriplani, N., Transistor modeling using Advanced Circuit Simulator Technology Fig 3 Schematic of LEVEL 1, 2 and 3 MOSFET models

MOS 1 model Model equations simple Implements the Shichman-Hodges model Based on gradual channel approximation and square law for saturated drain current Advantages – Can be used for preliminary circuit simulations – Appropriate for long channel and uniform-doping devices

Generation II Focus on mathematical conditioning and robust circuit simulation Focus less on developing exact analytical models Binning concept introduced – Process of modifying the model parameters for different values of drawn channel length and width HSPICE binning uses multiple model statements modeling a range of different lengths and widths

Generation III - BSIM 3 Model derived from research of General Electric and Intersil Enhanced version of I ds equation from LEVEL 2 model Varies from LEVEL 2 model in the area of – – Substrate doping – Threshold voltage – Effective mobility – Channel length modulation – Sub-threshold current

MOS 11 model Symmetrical, surface potential based model Provides accurate physical description of transition from weak to strong inversion Simple parameter extraction Appropriate for digital, analog and RF design

EKV model Physics based MOSFET model Has less than twenty intrinsic model parameters Specifically geared towards analog circuit simulation Useful for statistical modeling tasks Models available for all major circuit simulators

Overview of experiments Goal is to show quantitative results proving that by choosing the correct model the results are improved Hardware description language Verilog-AMS used to compare the results against the results obtained in SPICE For simulations, HSPICE (Version: hspice-X SP1) from Synopsys and Verilog-AMS (Version: IUS version 5.6) from Cadence were used Used 4 X 336-Mhz UltraSPARC-II processors, 1.3GB of memory and the Solaris 9 operating system for simulations

HSPICE Analog simulator Capable of performing transient, steady state and frequency domain analysis Capable of simulating up to 100,000 transistors HSPICE program contains four parts – Title line – Element declaration – Control commands –.END

HSPICE (Contd.) To view results from transient analysis, Avanwaves was used Avanwaves is a point and click interface with bult-in math functions for users

Hardware description languages (HDLs) By definition “HDL is a programming language for developing executable simulation models of hardware systems” HDLs describe circuit’s operation and design and also have tests to verify the circuit’s functionality by simulation HDLs can be used to design dedicated IC even before the actual circuit is built The HDL that was used for simulations in this thesis was Verilog-AMS

Verilog-AMS Supports both analog and digital component description Description in analog components is done by Verilog-A and digital components by Verilog-HDL Has SPICE compatibility with SPICE netlist by defining: – Primitive names – Parameter names – Port names Facilitates code reuse and ease of design

Verilog-AMS (Contd.) Unique feature of this language is the possibility of interconnecting instances of Verilog HDL, Verilog-A and Verilog-AMS with their netlists in a single module Both electrical and non-electrical models can be described in Verilog-AMS

Available HDL transistor model descriptions Table 2 Different MOS models available

Description of experiments In this thesis, simulations performed initially for CMOS inverter using a specific MOS model Then the same MOS model was used in a op-amp circuit to compare the performance when different MOS models are used Structural level of MOS is used for all simulations Three MOS models were studied – MOS 1, MOS 11 and EKVMOS

CMOS inverter Consists of one PMOS and one NMOS as shown Fig 4 Schematic of CMOS inverter Four terminals – gate, drain, source and bulk Specific components were instantiated in Verilog-AMS like the type of transistor, transistor length, width and terminals of each transistor Test bench consisted of applying 0 to 5V in steps of 0.1v

CMOS inverter (Contd.) For spice simulation, a basic inverter was constructed using 0.8 μ technology and used level 49 BSIM v3 model The output was observed for all these simulations by varying the W/L ratio of both PMOS and NMOS transistors The output observed was the inverted form of the input

Results Two types of experiments were performed for CMOS inverter – Constant supply voltage with varying transistor dimensions – Constant W/L ratio (2) with varying supply voltage

Simulations

Simulations--Errors Fig 4.4 Relative error between the MOS 1 and SPICE model (Inverter) Fig 4.5 Relative error between the MOS 11 and SPICE model (Inverter) Fig 4.6 Relative error between the EKV and SPICE model (Inverter)

Operational amplifier Two inputs that operate on dual DC power supply and has a high open-loop gain On feedback, the closed-loop gain is determined by the feedback network Fig 6 Op-amp schematics

CMOS Structure of op-amp Source : R. Jacob Baker, Harry W.Li & David E. Boyce, CMOS Circuit Design, Layout and Simulation Fig 7 Schematic of op-amp with W/L values

Op-amp parameters Considered three cases – Original dimensions – 75% original dimensions – 50% original dimensions Dimensions of the transistor changed only for the differential stage not the output stage Simulations performed with 5V dc supply and ramp input given to MOS models had a sweep from -2.5V to + 2.5V

Unit step function Source: Simon Foo, Lisa Anderson & Yoshiyasu Takefuji Analog Components for the VLSI of Neural Networks IEEE, 1990 Fig 8 Structural model of unit step function with test parameters The value of output follows the function f(x)=0 for x <= 1 1 for x > 1

Simulations

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Linear function (Fixed threshold) Source: Simon Foo, Lisa Anderson & Yoshiyasu Takefuji Analog Components for the VLSI of Neural Networks IEEE, 1990 Fig 9 Structural model of Linear function with test parameters

Simulations

% Error

Conclusion Different Verilog-AMS MOS models were successfully tested in inverters and also op-amps Output voltages for different MOS models for both inverter and op-amp were compared Assumption - More advanced MOS models have better accuracy and timing CHANGE: Of all MOS models that were used, MOS 1 is the most accurate model, matching closely with the SPICE values

Conclusion (Contd.) MOS 11 and EKVMOS models available for use in this thesis don’t represent the complete models Verilog-AMS models currently available not mature enough to get results as expected

Future work To improve accuracy, improve the current models Open source library of models can be developed

THANK YOU

Questions??

Supplementary Slides

MOS 2 model (Generation 1, Level II) Geometry based model Advantages – Takes into account velocity saturation, mobility degradation and DIBL Disadvantages – Not accurate for models with submicron geometries – Has convergence problems – Slower

MOS 3 model (Level III) Has semi-empirical parameters to model short channel effects Uses measured data to determine its main parameters Works well for channel lengths less than 1µm More accurate than LEVEL 1 and LEVEL 2 models Advantages – Simple – Operational reliability Disadvantages – Abrupt change from linear to saturation region – Poor fit of data

BSIM 1 (Level ???) Same as LEVEL 2 model with the following exceptions – Doesn’t have narrow width effects – No short-channel effects – Model parameter TPG defaults to zero for aluminum gate and for other levels, it defaults to one – Value of V T0 is computed using this parameter

BSIM 2 (Level XIII) Two modes of operation – Enhancement – Depletion If the mode parameter ZENH value is 1, then its the enhancement model else it’s the depletion model

HSPICE 28 (Level ???) Model binning can be accomplished Model parameter is set empirically Geared towards analog design

BSIM 4 (Level ???) Enhanced version of BSIM3 Accounts for the physical effects when the 100nm regime is reached Accurate model of intrinsic input impedance for analog, digital and RF applications Accurate model for induced gate noise and thermal noise

MOS 9 model (Level ???) Physics based model specifically geared towards analog simulation developed by Philips Very good description of electrical characteristics for all regions of transistor operation Even using one parameter set, behavior of the model over a wide range of lengths and widths Appropriate not only for circuit design, process technology but also in CAD tool development