About MOSFET gate resistance!!

[guamak_menanak]

gate resistance

Can anybody show me how to calculate the gate resistance(Rg) in a MOSFET? It's better to give some example on calculation or simulation on that matter!

From 'Impact of Distributed Gate Resistance on The Performance of MOS Devices' by B.Razavi, R.Hong Yan and Kwing F.Lee it shows that there is a relation between Rg/3 and 1/gm.

8O

 

[lunren]

mosfet gate resistance

Mabey you can try small signal circuit calculation.

 

[alphi]

gate resistance mosfet

I think you also need consider gate capcitor,gate resistor is relate about gate capcitor!

 

[alphi]

fet gate resistance

you also reference this application:

 

[Humungus]

mosfet gate resistor calculation

gate resistance refers to the resistance of the gate material, that combined with the gate capacitance give a distributed trnasimision line. This affects the transistor frequency response. Calculate gate resistance as a sinple poly stripe. Gate resistance depends on if you connect the gate from only one side, or is you do it from both sides.

 

[petarpav]

gate resistor mosfet

Look at datasheet of MOSFET transistor you plat to use, they reccomed gate resistor.

Best Regards.

 

[guamak_menanak]

calculating mosfet gate resistance

Actually, why I'm asking all of you about gate resistance is due to calculate the input impedance of my LNA design!

As you can see in the attachment, the circuit and the input impedance equation are as depicted!

R1 = is the Lg and Ls paracitic resistance

Rg = is the effective gate resistance
= R0*W/3*n*n*L

- R0=sheet resistance (as RSH in SPICE parameter)
- n = number of fingers

Therefore, to get Zin=50, I have to include Rg and R1 to my calculation.
Is the Rg equation can be used??

 

[MoonShine]

gate resistance calculation

Hello,

As previously mentioned, you need to know the layout used when creating the gate process. I'm not sure why there's an n squared in the denominator; I was only expecting n. You hab=ve the factor of 3 in the denominator to account for the distributed nature of the gate transmission properties if it's a single ended connection, however a factor of 12 is required if it is doubly connected. If you look at some of the BSIM3 manuals, the Zin is discussed at great length and some finer corrections are included to improve the accuracy.

Regards,
MoonShine

 

[electronrancher]

calculating gate resistance

for an experimental circuit, I guess you could use gate resistance to get Zin=50Ohm but I would suggest a real impedance matching front end next time. Zin depends on more than just Rgate..

But anyway, you can calculate gate resistance just by calculating the number of squares of gate material * Rsh-gate..

So for example, 1um long gate *100um wide * 50mOhm per square gives you 5 ohms per gate finger. Five gates in parallel gives you 1Ohm.


Now that I think about it, if you make a 50Ohm gate it will be so wide you will get travelling waves - the contact side will turn on way before the other end. Could you try some sort of common-base impedance matching circuit? Or actually draw a 50Ohm resistor to take the input across (will also vary 20%) but then you can build a lower Rgate (and therefore much faster) input pair. Just my opinion, though.. Good Luck!

 

[danda821]

calculate gate resistor

Thomas LEE has a very good paper about Rg effect on LNA design in JSSC. You can check that.
IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 32, NO. 5, MAY 1997
"A 1.5-V, 1.5-GHz CMOS Low Noise Amplifier"

 

[guamak_menanak]

mosfet rg

I'm using inductive source degenerated as my input matching(depicted in my previous post), so...Rg is not my only point a matching component.

By electronrancher:
"So for example, 1um long gate *100um wide * 50mOhm per square gives you 5 ohms per gate finger. Five gates in parallel gives you 1Ohm.

Con you explain in detail on that matter because I'm a new in using fingers in my design & layout. How can in 5 parallel with 5 ohms per gate finger we can get 10ohm??

Very courious... :roll:

 

[electronrancher]

mos gate finger and gate capacitance

that's just ohm's law for parallel resistors. 5||5||5||5||5 = 1

 

[suiwinder]

great.

so many sages.