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Modeling >> Semiconductor Devices >> BJT vs MOSFET
https://designers-guide.org/forum/YaBB.pl?num=1321122600

Message started by summi on Nov 12^th, 2011, 10:30am

Title: BJT vs MOSFET
Post by summi on Nov 12^th, 2011, 10:30am

Dear Forum,
Can any one kindly explain why BJT is much faster than MOSFET?

Br,
Summi.

Title: Re: BJT vs MOSFET
Post by raja.cedt on Nov 12^th, 2011, 10:38am

hello,
Bjt speed depends on the base width, Mosfet speed depends on Channel lenght. In verical BJT's width defined by diffusion where as Channel lenght defined by Lithography. With diffusion we can finer dimentions compared to lithography, hence BJT is faster than mosfet.

Thanks,
Raj.

Title: Re: BJT vs MOSFET
Post by loose-electron on Nov 14^th, 2011, 10:18am

Hm... oversimplification, the mobility of electrons, thickness of the base region and other things also play into it.

Title: Re: BJT vs MOSFET
Post by vivkr on Nov 15^th, 2011, 3:06am

agree with Jerry fully. You need to compare apples to apples. What does it mean when someone says that a BJT is faster than a MOSFET? For any realistic comparison, you need to look at BJTs and MOSFETs in the same or similar technology nodes.

1. BJTs as bulk devices will have higher carrier mobility. MOS as surface device will have lower mobility.

2. Intrinsic gm/Id is higher for BJTs than for MOSFETs. SOI MOS will tend to fare better than a bulk MOS.

3. Depending on make, BJTs have some speed limitation due to base charge recombination time. I think that modern SiGe doped HBTs don't suffer from this limitation anymore.

4. On the other hand, MOS dimensions can be shrunk quite aggressively, and the intrinsic drift time may thus be still not be so far from that of a BJT despite lower mobility.

Again, there are too many variables in question for you to make a clear statement. The fact however is that BJTs are HUGE in size and much more expensive to manufacture in MOS process because they come require additional steps.

So nobody would make or use BJTs in modern processes unless they could make them significantly faster than the available MOS devices. The textbook explanation that a BJT is always faster than a MOS is as banal and missing the point as quoting Moore's Law to say that transistor density will keep doubling over time. Someone has to actually work to make those BJTs faster than the best available MOS in each process generation! Until the introduction of commercial SiGe HBTs and vertical scaling of base width a decade or so ago, it was not self-evident that the BJT would survive for long.

Vivek

Title: Re: BJT vs MOSFET
Post by Kevin Aylward on Aug 4^th, 2013, 2:39am

vivkr wrote on Nov 15^th, 2011, 3:06am:

agree with Jerry fully.
So nobody would make or use BJTs in modern processes unless they could make them significantly faster than the available MOS devices.

Vivek

I Disagree. Bipolars are extremely useful, irrespective of speed. For low frequency noise, it’s a no contest. Mos are almost useless for low frequency noise design. It is they that must be huge relative to bipolars to reduce their 1/f noise. Matching is also way better for bipolars. Bipolar use is also a no contest for high performance bandgap design. Or even for wideband noise, an emitter follower buffer has much lower noise than an equivalent mos design. Given the option, I would always chose a BiCmos process.

Title: Re: BJT vs MOSFET
Post by tm123 on Aug 5^th, 2013, 9:52am

How do you define 'modern processes'? No one is making BJTs in a 28nm process, but in >0.13um BJTs are a much better choice if you are designing for high performance and/or low power. Of course it comes at the expense of higher cost from more complex fabrication. Also, BJTs offer much better 1/f noise performance and device matching as Kevin described. If you want to know what determines the speed of a device, have a look at the ft equations for BJT and MOS. The Gray/Meyer text has a nice section on device operation without getting too heavy into equations.