1. Semiconductor devices and physics
Zhejiang university
zengjie
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2. Introduction to MOSFET
CONTENT
3.1 MOSFET structure
3.2 MOSFET characteristics
Introduction to textbook
Chapter MOS Transistor Structure and Operation
Chapter Threshold Voltage
Chapter Modeling Hot-carrier Effects
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3. Cross view of CMOS field effect transistors
The thickness of the insulating oxide is
The two heavily doped region of depth
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4. Four types of MOSFETs Depletion n-MOS Depletion p-MOS
Enhancement n-MOS
Enhancement p-MOS
In fact , the operation of these MOSFETs is all the same , but the threshold voltage is different
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5. The operation of depletion MOSFET
When , the depletion MOSFET can conduct currents , which is different from enhancement MOSFET . The depletion MOSFET has two operation mode , namely depletion mode and enhancement mode
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6. All kinds of MOSFET circuit symbols
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7. The operation characteristics of MOSFET
Four operation regions
Cut off region Breakdown region
Linear region Saturation region
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8. When the increases further , the drain current suddenly increases
Linear region
Saturation region
Breakdown region
When the increases further , the drain current suddenly increases
In fact , because the parasitic bipolar transistor turns on. Particularly in short-channel devices , due to the high electric field at the drain end, the device breakdown. This is so-called hot carrier effect.
Reference to chapter 8 for detail
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9. there is no current, since there is no channel formed
Cut-off region
This is the region in which In circuit design , we always assume in this region , however , the fact is different .
For
there is no current, since there is no channel formed
For
There is a weak inversion channel existing . This is so-called subthreshold region .
In this region , the electric field in the channel is weak , so the channel current is mainly due to the diffusion of carriers and the drift current is negligeable.
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10. The diffusion current is (1)
(2)
Where
is the channel charge density.
Substituting (2) into (1) and integrating , we can get
(3)
Equation (3) has something different from equation (3.6) in the textbook
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11. From the equation (3) we developed for the subthreshold region , we can draw the I-V characteristics as below
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12. Depletion region in the source and drain
The source and drain has different doping type from substrate , so there forms the p-n junction between them. The width of the depletion region is expressed as below:
(4)
Where is the built-in potential given by
(5)
is the voltage applied at the source or drain
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13. This is the so-called DIBL (Drain induced barrier lower) effect
From the p-n junction theory , as the voltage applied to source or drain increases , so does the width of depletion region. It has a big influence on the threshold of MOSFET.
This is the so-called DIBL (Drain induced barrier lower) effect
Reference to chapter 5 for detail
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14. Effect of substrate bias
When bias applied to the substrate , modulation of the channel conductance results. Because the substrate voltage mainly affect the inversion layer charge
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15. Channel length modulation
As the value of increases , the effective channel length L decreases causing the current increases.
(6)
This is the equation which we are familiar with to describe channel length modulation
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16. The equation which is mentioned previously is in the view of terminal voltage and current. Now we transfer the view to physical mechanism.
The saturation current is described as below:
(7)
How to develop the ?
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17. Solve the Possion equation in the depletion region near the drain
Method 1
Solve the Possion equation in the depletion region near the drain
(8)
Problem: the conductance is too high and the electric field is discontiuity
Method 2
Assuming , when
This is given by Baul-Beneking
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18. (9)
where
(10)
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19. Experiential model given by Frohman-Benchkowsky and Grove
Problem: the conductance and the electric field are not so accurate , since ignoring the oxide electric field
Method 3
Experiential model given by Frohman-Benchkowsky and Grove
(11)
Problem: the conductance is still not so accurate
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20. This method is so complex!
According to the shape of drain , modify the model developed in method 3, given by Elmans
This method is so complex!
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21. MOSFET latch-up Silicon Controlled Rectifier 10/26/2011
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22. SUMMARY Introduction to the structure of MOSFET and the all types
of MOSFETs
Introduction to the MOSFET operation mechanism which
has four operation regions
Introduction to the depletion region of source and
drain , effect of substrate bias , channel length
modulation and latch-up
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23. Thank you for your attention!
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