Why is it that BJT is faster than CMOS?

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dineshbabumm

Joined: 07 Dec 2005
Posts: 125
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12 Mar 2007 15:22 vertical bjt

Its a known fact that BJT is more faster than CMOS.. Can any one make clear why it is so? Both of the has their own capacitances.. My friends told me its perhaps due to their transconductance.. Anyways can any one give a clear picture with substantiate answer please??

dkace

Joined: 13 Jun 2002
Posts: 365
Helped: 24
Location: Greece

12 Mar 2007 15:29 limitations of bjt

BJT faster than CMOS. In what issue? Faster switvhing on/off? Faster regarding the time that the output will be appeared after input is applied?
D.

dineshbabumm

Joined: 07 Dec 2005
Posts: 125
Helped: 8

12 Mar 2007 15:54 bjt faster cmos

dkace wrote:

BJT faster than CMOS. In what issue? Faster switvhing on/off? Faster regarding the time that the output will be appeared after input is applied?
D.

I guess BJT is faster than MOS usually in all aspects... But generally people refer BJT is useful in switching at high frequency than MOS.. Can u plz make clear why is it so? Also isnt BJT faster than MOS in all aspects?

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Yahia Muhammad

Joined: 30 Mar 2006
Posts: 91
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12 Mar 2007 16:10 bjt dimensions

if we are talking about higher ft

the gate in the CMOS is lateral and the base in BJT is vertical
technology wise we can control vertical dimensions more than lateral dimensions during manufacturing so we can scale down the base width more basically base width now is in the range of 35 nm as the base width decreases the base transit time decreases so ft increases

dineshbabumm

Joined: 07 Dec 2005
Posts: 125
Helped: 8

12 Mar 2007 16:14 nmos faster than cmos

Yahia Muhammad wrote:

"we can control vertical dimensions more than lateral dimensions "

Why is it so? Can u plz substantiate the statement?

"during manufacturing so we can scale down the base width more basically base width now is in the range of 35 nm as the base width decreases the base transit time decreases so ft increases"

But MOS occupies much smaller area than BJT and thats the reason we use MOS in ICs generally right? How does this justifies the answer?

Yahia Muhammad

Joined: 30 Mar 2006
Posts: 91
Helped: 5

12 Mar 2007 17:13 compare vertical lateral bjt cmos

the lateral directions are less controlled due to the diffraction of light used in photolithography this is a factor that can affect the dimensions
but vertical component are not affected by this factor

yes the MOS takes smaller area than CMOS but the base width is the smallest we tend to make the base very narrow

Added after 52 minutes:

also from the parasitics point of view the BJT has only two capacitances but the mosfet we have 6 (the 5 shown and the oxide capacitance ) capacitances as we expect a capacitance between each of the four ports so it takes time to charge these capacitances ( the mosfet is a self loaded device )

Why is it that BJT is faster than CMOS?

barath_87

Joined: 07 Feb 2006
Posts: 171
Helped: 10

14 Mar 2007 2:22 lateral vertical bjt

Think about the frequency response of a diode , it is a very fast device that can be used to operate at high frequency similarly in a BJT you have two semiconductor junctions... in a MOS the charge carrier has to travell along the complete length of the channel (source to drain) under the influence of a vertical field... so BJT's are much faster than CMOS amd are used in high frequency application.

SkyHigh

Joined: 13 Jan 2005
Posts: 376
Helped: 51

14 Mar 2007 3:16 what resistances make nmos faster than the cmos

Sorry to comment, but I think none of you answered his question.
Maybe none of you knows why BJT is faster than MOS, although many of you tried, but your understanding is not even close.

In general, when comparing a monolithic BJT and a monolithic UJT such as MOS:

BJT has a base, intended for hole replacement. This is like a minority carrier buffer for electrons. Under the high electric field strength at the Collector, most of electrons are accelerated. Such acceleration depends on Vce and HFE.

MOS has no buffer. MOS depends on inversion (regardless weak or strong) to conduct between source and drain, thus the channel poses a considerable resistance (Ron).
As a device operates over a longer period of time, heat causes to Ron increases, this reduces maximum bandwidth.

Parasitic caps on BJT is relatively less significant than in MOS because such caps mainly between the nodes to emitter. There parasitic caps pose little limitations to BJT. However, parasitic caps in MOS exhibit influence inside the device lateral structure, inter-referencing source, gate and drain. Some are ignorable at high-frequency model, but still the inherent Cgs, Cgd are Cds are forever there!

However, MOS has evolved from long-channel to short-channel, to HEMT, FinFet and even to the extend of using SOI. The gap is closing.

jinnose

Joined: 24 Feb 2007
Posts: 20
Helped: 1

14 Mar 2007 5:37 gm bjt vs cmos

in terms of gm... for the same bias current gm of BJT will be 4-10X higher than gm of MOSFET.

dkace

Joined: 13 Jun 2002
Posts: 365
Helped: 24
Location: Greece

14 Mar 2007 8:40 gm bjt

I agree completely with SkyHigh. There is no mystic development on microelectronics and all paracitics can be easily found. Try to go into the physics of the device not to the outcome observed!

D.