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Design Languages >> Verilog-AMS >> What is $Vt for a ordinary diode in veriloga code
https://designers-guide.org/forum/YaBB.pl?num=1158080527

Message started by gmadhavareddy on Sep 12^th, 2006, 10:02am

Title: What is $Vt for a ordinary diode in veriloga code
Post by gmadhavareddy on Sep 12^th, 2006, 10:02am

Hi,
I want to change the model of Schottky diode which is avilable in Cadence ahdlLib.
I want to know the $Vt and what $ indicates in veriloga code?

Vd = V(vanode, vcathode);
`ifdef __VAMS_ENABLE__
Id = I(<vanode>);
`else
Id = I(vanode, vanode);
`endif
// intrinsic diode.
if (Vd < -5*n*$vt) begin
if (Vd == -bv) begin
I(vanode, vcathode) <+ -area*ibv;
end else if (Vd > -bv) begin
I(vanode, vcathode) <+ -area*is;
end else begin
I(vanode, vcathode) <+ -area*is*(exp(-(bv + Vd)/$vt) -
1 + bv/$vt);
end
end else begin
I(vanode, vcathode) <+ area*is*(exp((Vd - rs*Id)/(n*$vt)) - 1);
end

I(vanode, vcathode) <+ gmin * Vd;
// capacitance (junction and diffusion).
if (Vd <= fcp) begin
Qd = tt*Id + area*cjo*phi*(1 - pow((1 - Vd/phi), (1 - m)))/(1 - m);
end else begin
Qd = tt*Id + area*cjo*(f1 + (1/f2)*(f3*(Vd - fcp) + (0.5*m/phi)*
(Vd*Vd - fcp*fcp)));
end
I(vanode, vcathode) <+ ddt(Qd);

Title: Re: What is $Vt for a ordinary diode in veriloga c
Post by Ken Kundert on Sep 12^th, 2006, 10:04am

$vt is kT/q.

Title: Re: What is $Vt for a ordinary diode in veriloga c
Post by Geoffrey_Coram on Sep 12^th, 2006, 12:33pm

The $ (dollar sign) is usually used for "system functions and tasks," ie, things the simulator handles: $strobe, $fopen, etc. Also $temperature, which is the simulation temperature.

Title: Re: What is $Vt for a ordinary diode in veriloga c
Post by gmadhavareddy on Sep 28^th, 2006, 8:11am

Thank you.

I have some more doubts to clarify clearly.
1) Is $vt represents threshold voltage or not?
2) If yes, here in this verilog model, they used $vt same for both ordinary diode and schottky diode.

/Madhav

Title: Re: What is $Vt for a ordinary diode in veriloga c
Post by ACWWong on Sep 28^th, 2006, 8:35am

gmadhavareddy wrote on Sep 28^th, 2006, 8:11am:

1) As ken said, $vt is kT/q, the thermal voltage. so NO $vt does not reperesent threshold voltage
2) .

Title: Re: What is $Vt for a ordinary diode in veriloga c
Post by jbdavid on Sep 30^th, 2006, 1:52am

$vt represents V_t a "thermal voltage"
as most diodes have a forward Current

I_d = I_s exp(-n⋅Vd/Vt)

which of course comes from the carrier density equation (see "Physics of Semiconductor Physics" by S. M. Sze chapter 1)

kT/q shows up so often in device equations because of this that the early users decided this needed to be a "built-in" function.

because it depends on temperature it cannot be a "constant"

Jbd