Bologna / 06_TCAD_laboratory_MOSFET_GBB_20150223H1655

Drain current vs. Voltage chart (2): turn-on characteristics

Considering NMOS devices.

Turn-on characteristics= vs. , at fixed > (saturation condition)

saturation, ~ 2

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The MOSFET: threshold voltage

• The turn-on phenomenon is a gradual function of the gate voltage. In semiconductor physics, the threshold voltage is usually defined as the gate voltage for which at the semiconductor-oxide interface the concentration of the inversion charge is higher that the concentration of the substrate doping, i.e. > − for NMOSFET, and > + for PMOSFET.

• It can be shown that, for NMOSFET = Φ + 2Φ +

where

Φ is the workfunction difference between the polysilicon gate and the silicon substrate

•Therefore, increases if increases. From a qualitative point of view it is explained since ifis higher we have more holes at the surface and we need to increase the applied voltage to attract a sufficient number of electrons in such a way that electrons concentration will be higher than holes one.

•Then, increases if decreases. A lower means a lower electrostatic control on the channel (i.e. lower electric field), therefore, in order to get inversion we need to apply a higher .

•is pretty much independent on channel length.

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The MOSFET: basic operation regimes (3)

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The structure

G

Only half of the device geometry can be defined, then the device structure is mirrored around the vertical axis

Geometrical definitions / MOSFET

Xg= Lg/2

zoom

Xsp

Ypol

Lreox

y

oxide: extending y from 0 to –Tox=Ygox (very small, 2.5 nm)

SWB: project tools & parameters (1)

OPEN SWB FROM THE LINUX COMMAND LINE swb &

STARTING (AND SAVING) A NEW SWB PROJECT

Project  New  New Project

Project Save as  Project  MOSFET

ADD TOOLS

left click on No tools  right click  Add  Name, scroll for Sde  select Batch  Ok left click on Sde  right click  Add  Name  scroll for Sdevice  Ok

ADD ANOTHER Sdevice TOOL

left click on Sdevice  right click  Add  Name  scroll for Sdevice  Ok

Double click on first Sdevice tool  Input Files  change name (double click on the Master field) in the Commands field to turn_on_des.cmd

Double click on second Sdevice tool  Input Files  change name in the Commands field to output_des.cmd

ADD PARAMETERS (WITH THEIR DEFAULT VALUES): SDE /GEOMETRY AND DOPING

Parameter Add  Parameter Lg  default value  0.13  Ok Right click on Lg  Add  Parameter Tox  default value  10

Right click on Tox  Add  Parameter  SubDop  default value  1e18

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SWB: project tools & parameters (2)

ADD PARAMETERS (WITH THEIR DEFAULT VALUES): Sdevice /MODELS AND VOLTAGES

Turn-on characteristics

We first increase VDS, that is we fix a VDS, than we sweep VGS from 0 to goal: right click on the cell directly below the first Sdevice instance

Add  Parameter vds_fixed  default value  1.5  Ok

Right click on vds_fixed  Add  Parameter  vgs_goal  default value  1.5

Output characteristics

We first increase VGS, that is we fix a VGS, than we sweep VDS from 0 to goal: right click on the cell directly below the second Sdevice instance

Add  Parameter vgs_fixed  default value  1.5  Ok

Right click on vgs_fixed  Add  Parameter  vds_goal  default value  1.5

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SWB: project tools & parameters (3)

EXPERIMENT RAMIFICATION

Left click just below the box Lg of first line Nodes  Extend Selection To Experiments  Experiments  Add Values  Parameter  Lg  Min. Value: 0.13  Step: 0.1  Number of values: 3  Apply  Ok

Then change the values to more suitable ones: F6 on n11 and write 0.250 and F6 on n20 and write 0.500

Left click just below the box Tox of first line Nodes  Extend Selection To Experiments  Experiments  Add Values  Parameter  Tox  Min. Value: 2.5e-3  Step: 1e-3  Number of values: 3  Apply  Ok

Then change the values to more suitable ones: F6 on n29 and write 10e-3 and F6 on n37 and write 20e-3

Left click just below the box SubDop of first line Nodes  Extend Selection To Experiments  Experiments  Add Values  Parameter  SubDop  Min. Value: 1e18  Step: 1e18  Number of values: 3  Apply  Ok

Then change the values to more suitable ones: F6 on n45 and write 1e15 and F6 on n52 and write 1e13

 DONE SWB PART

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SDE command file (2)

;polysilicon-gate thickness (define Hpol 0.2)

;spacer rounding

(define fillet-radius 0.08) ; rounding radius

;pn-junction mesh resolution (define Gpn 0.005)

;substrate doping level (define SubDop @SubDop@)

;junction depth

(define Xj 0.1 )

(define Xsub (/ Ltot 2.0))

; define

(define Xg (/ Lg 2.0))

; define spacer x-coordinate

(define Xsp (+ Xg Lsp))

(define Ysub Hsub)

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