Semiconductor devices and physics Zhejiang university zengjie 10/26/2011 TOTAL PAGES : 23

Introduction to MOSFET CONTENT 3.1 MOSFET structure 3.2 MOSFET characteristics Introduction to textbook Chapter 3 MOS Transistor Structure and Operation Chapter 5 Threshold Voltage Chapter8 Modeling Hot-carrier Effects 10/26/2011 TOTAL PAGES : 23

Cross view of CMOS field effect transistors The thickness of the insulating oxide is The two heavily doped region of depth 10/26/2011 TOTAL PAGES : 23

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 10/26/2011 TOTAL PAGES : 23

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 10/26/2011 TOTAL PAGES : 23

All kinds of MOSFET circuit symbols 10/26/2011 TOTAL PAGES : 23

The operation characteristics of MOSFET Four operation regions Cut off region Breakdown region Linear region Saturation region 10/26/2011 TOTAL PAGES : 23

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 10/26/2011 TOTAL PAGES : 23

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. 10/26/2011 TOTAL PAGES : 23

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 10/26/2011 TOTAL PAGES : 23

From the equation (3) we developed for the subthreshold region , we can draw the I-V characteristics as below 10/26/2011 TOTAL PAGES : 23

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 10/26/2011 TOTAL PAGES : 23

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 10/26/2011 TOTAL PAGES : 23

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 10/26/2011 TOTAL PAGES : 23

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 10/26/2011 TOTAL PAGES : 23

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 ? 10/26/2011 TOTAL PAGES : 23

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 10/26/2011 TOTAL PAGES : 23

(9) where (10) 10/26/2011 TOTAL PAGES : 23

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 10/26/2011 TOTAL PAGES : 23

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! 10/26/2011 TOTAL PAGES : 23

MOSFET latch-up Silicon Controlled Rectifier 10/26/2011 TOTAL PAGES : 23

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 10/26/2011 TOTAL PAGES : 23

Thank you for your attention! 10/26/2011 TOTAL PAGES : 23