1. Thermal Modeling for Modern VLSI Circuits
Dr. D. Nagchoudhuri
Professor
EE Department, IIT Delhi

2. Lecture Plan What are Thermal Issues ? Why today and not earlier?
The Problems of Simulation
Thermal Modeling of the MOSFET
Thermal Modeling of Circuits
Conclusions

3. Trends Today Modern Circuits use Analog Interfaces
BiCMOS replacing CMOS
Bipolar Drive Circuitry Consumes Power
Short Channel Effects cause Degradation
Dimensional shrinkage continues

4. Thermal Issues Temperature Rise Heat Conduction
Temperature of neighbors
Heat dissipation in Device
Heat Conduction
Radiation
Conduction

5. Why today and not earlier?
Increasing Density
Increasing proximity – effect of neighbors
Increasing number
Increasing SoC => increasing Analog components
Increasing Dopings for scaling
Smaller Depletion widths
Larger Fields
Larger Leakage currents

6. Effect of Temperature Rise
Temperature dependent Semiconductor Parameters
Mobility
Threshold Voltage
Intrinsic Carrier Concentration
Band-Gap
Only first two parameters being considered!!
Found dominant in many cases!

7. Effect of Semiconductor Parameters
, VT
Changes IDS
IDS changes gm
gm affects
Gain
Bandwidth
Pole locations

8. The Problems of Simulation
Cost of Error in prediction enormous
SPICE Simulator –Tried and Tested
Industry Standard
SPICE simulates chip at single temperature.
Cannot show dynamic variation with temperature.
Modern Chips have devices at different Temperature.

9. Approach Power Dissipation => Temperature
Temperature => Device Parameters
Device Parameters => Circuit performance
Circuit performance
Add thermal circuit
Simulate by SPICE

10. Device Equations ID = 1/2  Cox (W/L) [VGS - VTh]2
 ID/  T = ( ID/  )(  / T )
 ID/ VTh =  Cox (W/L) [VGS - VTh]
ID = ( ID/   )  +( ID/ VTh) VTh

11. Temperature Dependence of Parameters
VTh(T) = VTh(Tref) - k1(T - Tref)
k1 = - VTh/ T => mv/K
(T) = (Tref) [T/Tref]-k2
  / T = - k2 (Tref)/T
(T) and VTh(T) are decreasing functions of temperature.

12. The Transistor with Thermal CircuitG D
CTh
RTH N0
Self-Heating Node
V2,V3, …Vn B S

13. The Feedback Loop
IDS
Power
Temperature
Device Parameters

14. Sensing Current iDS
iDS H
CCVS =>
v0 = KiDS

15. Self Heating Power vDSiDS
V0 => N0 E1 V2
VCVS =>
V0 = KV1V2 = vDSiDS = P0
V1 = K iDS
V2 = vDS

16. Power => Temperature
Temperature rise proportional to Power dissipation
Rate of Temperature Rise determined by thermal time constant
Rth => Thermal Resistance
Cth => Thermal Capacity

17. RTh
Tout
CTh
Thermal Resistance and Capacity

18. Computing Temperature = Tavg
V0 => Tavg E2
V2,V3, …Vn
VCVS => V0 = K1 V1 + {V2,..Vn}
V1 = Self Heating
{V2,..Vn} = Neighbor Node Temperatures

19. Threshold Voltage Dependence
VCVS => V0 = KV1 + K2 V2
V2 = K3Tavg ; V1 = Vin - VT
V0 =  VTh

20. Mobility Variation with TemperatureG
VCCS => V0 = G V1
V1 = Tavg
V0 = I

21. The Model with Controlled SourcesG E3 G E2
N1..Nn S B

22. IDS vs VDS w/o Correction

23. IDS vs VDD with correction

24. Conclusions Digital CMOS Circuit Power Dissipation
Leakage Current
Switching Power
Conduction during transition
Increasing Speed => Digital activity
Power = CV2f
Reducing Dimensions
Leakage
Short Channel Effects
Affects proximate analog circuits

25. Issues Predict temperature from power dissipation
=> 3D problem
Thermal Layout vs Area minimization
Predict Thermal Degradation
=> Strong Layout Dependence
Effect of Temperature Gradients