why do transistors with same W/L have different performance?

[katrin]

channel w/l

Now I am using CMOS 0.35um technology
when I am using the transistors with different Width, but the same W/L ratio
for example, transistorA:W/L=0.5um/0.5um, transistorB: W/L=2um/2um
I find their performances are very different

but on the other hand, it is said the performance of MOS transistors are mainly determined by its W/L. thus these two transistors should have the same output characteristic

who could help me to explain this?

 

[deepa]

Re: why do transistors with same W/L have different performa

what characteristics have you observed and both will be different because the short channel effects will deviate the behaviour from the predicted long channel behaviour,

 

[katrin]

Re: why do transistors with same W/L have different performa

what characteristics have you observed and both will be different because the short channel effects will deviate the behaviour from the predicted long channel behaviour,

for example, when I simulate the output characteristic of these two transistors, they are significantly different. The larger Width is, the larger the current which is flowing through the transistor, despite they have the same W/L

is the short channel effect so significant?

 

[cmos_dude]

To add on to what Deepa has mentioned, there will also be a loads of parasitic capacitances acting in your design whose value will depend on the W and L and not the ratio W/L.

--Cmos_Dude

 

[gingerjiang]

Re: why do transistors with same W/L have different performa

for small length, there are short channel effect, larger Lambda, i.e. smaller intrinsic resistance, relative smaller effective length
long length transistor has larger parasitic capacitance

 

[demodb]

Re: why do transistors with same W/L have different performa

your width of transistor B is 4 times larger than transistor A. The gate-source capacitance is proportional to the width, therefore also 4 times larger. The decreases you unity gain bandwidth of your transistor with a factor 4. (not taken into account the length of the transistor)

 

[hung_wai_ming@hotmail.com]

it is quite a good observation that I seldom take into account. Thanks for reminder.

 

[IBO]

Re: why do transistors with same W/L have different performa

having a look at equations cud justiify they are similar. But these arent ac0
curate enough when transistor are scaled down in size (other effects are profound)

a devices course is required

 

[scottieman]

Re: why do transistors with same W/L have different performa

what characteristics have you observed and both will be different because the short channel effects will deviate the behaviour from the predicted long channel behaviour,

for example, when I simulate the output characteristic of these two transistors, they are significantly different. The larger Width is, the larger the current which is flowing through the transistor, despite they have the same W/L

is the short channel effect so significant?

You are refeering to the classical or somebody called "long channel" I-V equation to predict the simulation results. However, I-V equation implemented in the model (e.g. BSIM3v3) is a more sophisticated one including velocity saturation and modility degradation (you can read BSIM3v3 mannual for more). Of course, short-channel effects should as DIBL are all included in the models you are using for simulations.

Hope it helps
Scottie

 

[pankajdudulwar]

Please refer to the governing equations for long channel and short channel. This will sort out your problem also the parasitic components play a bigger role like gamma which has different values in short channel and long channel.
Also thanks for reminding the concept.

 

[jhan_wzx]

dear all,
we should know what the characteristic is. if it is for dc, i think the parasitic cap does not effect the result. even for the same w/l, the dc characteristic differ largerly due to different size especially when scalled down to smaller size.
i hope it helps.

 

[shaikhsarfraz]

Re: why do transistors with same W/L have different performa

these effects are basically because of two effects

Wide channel or long channel effects (90%)
Haloe effect (10%)

regards
Sarfraz