Why FET has such high input impedance?

hi there.. please tell me, what makes the FET have the high input impedance?

rf fet and impedance match

zhi_yi said:

hi there.. please tell me, what makes the FET have the high input impedance?

Click to expand...

Insulator

Scottie

Re: fet input impedance

Between the gate and the substrate there is a thin layer of something like SiO2 which is an insulator that doesn't conduct.

Re: fet input impedance

zhi_yi

hi there.. please tell me, what makes the FET have the high input impedance?

Click to expand...

If you are refering to general FET, it is the insulated gate technology, except two special classes of FET called MESFET and MODFET (HEMT) which don't have an insulated gate.

What others have said in the earlier posts is only one common class of FET called MOS or MOSFET. The insulated gate technology is such that a thin oxide layer (usually called thin film oxide or thin oxide made up of SiO2 or Silicon Dioxide with dielectric constant K, sometimes called relative permittivity of silicon dioxide εr, of about 4.5) is sandwiched between the polysilicon gate (in the older days, metal like aluminum or copper) and the substrate or bulk, thus created a MOS capacitor which gives high impedance only in DC and low frequency.

High impedance doesn't hold at frequencies above the maximum frequency Ft.


PhD MSc DIC (Imperial College)

Re: fet input impedance

SkyHigh said:

zhi_yi

hi there.. please tell me, what makes the FET have the high input impedance?

Click to expand...

If you are refering to general FET, it is the insulated gate technology, except two special classes of FET called MESFET and MODFET (HEMT) which don't have an insulated gate.

What others have said in the earlier posts is only one common class of FET called MOS or MOSFET. The insulated gate technology is such that a thin oxide layer (usually called thin film oxide or thin oxide made up of SiO2 or Silicon Dioxide with dielectric constant K, sometimes called relative permittivity of silicon dioxide εr, of about 4.5) is sandwiched between the polysilicon gate (in the older days, metal like aluminum or copper) and the substrate or bulk, thus created a MOS capacitor which gives high impedance only in DC and low frequency.

High impedance doesn't hold at frequencies above the maximum frequency Ft.


PhD MSc DIC (Imperial College)

Click to expand...

Thanks for your detail explaination, and even cover Junction FET (JFET).

Scottie

Re: fet input impedance

You're welcome.

Re: fet input impedance

oo.. okay.. thank you

what is mesfet?? and what is modfet?? and what is HEMT?

how to determine in what condition we use fets that will more appropriate than use bjt? or use bjt that will more appropriate than fets?

is it a link in internet that tell us about the code of bjt or fets? for example like A = germanium, B = silicon, C= arsenicum sianide, the second letter A = low power audio frequency, etc, please help me

thank you ^^

Re: fet input impedance

MESFET = Metal-Semiconductor FET
MODFET = Modulation-Doped FET
HEMT = High-Electron Mobility Transistor
TEGFET = Two-dimensional Electron Gas FET

MODFET=HEMT=TEGFET

MESFET, MODFET, HEMT and TEGFET, all use 2-dimensional electron gas as confined planar carrier buffer for high-velocity carrier transportation or charge transfer.

BJT for low-noise, high-speed, good impedence matching. Mainly RF, Analog and Power.
FET, refering to MOS for good linearity, low-power and low-cost. Mainly Digital, using CMOS Technology.

Special case 1: High-speed FET such as HEMT and MESFET for Microwave-RF.

Special case 2: FET such as SiGe MOS for low-power, high-speed, low noise, good linearity and low-cost. Mainly in RF-Analog.

There are abundant of information on the Internet. More than you can read for a year.

fet input impedance

The high input resistance of an ordinary JFET is due to its reverse bias. No SiO2 there.

Re: fet input impedance

oo.. okeh.. thank you

how about these question :

how to determine in what condition we use fets that will more appropriate than use bjt? or use bjt that will more appropriate than fets?

is it a link in internet that tell us about the code of bjt or fets? for example like A = germanium, B = silicon, C= arsenicum sianide, the second letter A = low power
audio frequency, etc, please help me

thank you ^^

Re: fet input impedance

hi there, i've got an answer for this question, please correct me if i am wrong

how to determine in what condition we use fets that will more appropriate than use bjt? or use bjt that will more appropriate than fets?

use jfets for low noise amplifiers, and use bjt for power amplifier, the jfet generates low noise than others but jfets just for low power, and in switching, jfet can switching faster than bjt because it don't have charge in base when it is operate in saturated condition, but the bjt have it. and then i have a question, what makes the jfet generates low noise than others?


thank you

Re: fet input impedance

In general, I think what you have said are too superficial. Depending on applications, whether to use BJT or FET can become an endless topic.

LNA can be designed in BJT or FET depending on what are the attributes required.

At superficial level, most people with good background of analog designs will point out two key attributes - linearity and low noise.

For low noise and high speed, BJT is better, but at the expense of higher cost and power consumption.

For linearity, ease of design by geometries and fabrication, FET is better.

At RF entry level, attributes like IIP3, gain, NF, I/O Impedance matching and Loss, Isolation etc are considered.

At RF veteran level, maximum frequency and circuit topology are considered for LNA design. Certain RF subsystems or circuit topologies are better done in FET whereas some are better in BJT.

A designer would first carefully consider if he can implement the design in FET as long as other attributes are not compromised, in order to reduce power consumption and cost, and also benefit from ease of design. Else Bipolar comes into the picture if FET cannot meet the design specifications targeted for a known application.

For PA, it is not so much about BJT or FET. It is more on the efficiency depending on the class of amplification chosen. In today's intelligent digital wireless network that monitors and control transceiver power levels such as "cell-breathing" used in W-CDMA, low power consumption is more important in portable electronics. FET gets more favour in this aspect.

I wonder where you got your information about FET generates lesser noise than BJT. This is a worrying claim made!

At low frequencies, FET is definitely noisy due to 1/f noise. Not so much if FET is used at high frequencies.
Both BJT and FET suffers from device thermal noise and shot noise.

Having replacement holes in the base of BJT only speeds up energy transfer, not slowing it.

Where did you learn all your principles? Are you a self-learner or your lecturers at college or university taught you all that said?

You have got many things confused.

Re: fet input impedance

oo.. okay.. thank you

LNA can be designed in BJT or FET depending on what are the attributes required. <-- what is LNA??

For low noise and high speed, BJT is better, but at the expense of higher cost and power consumption <-- why the bjt can be better for low noise and high speed ?

At RF entry level, attributes like IIP3, gain, NF, I/O Impedance matching and Loss, Isolation etc are considered. <-- what is IIP3?? NF??

I wonder where you got your information about FET generates lesser noise than BJT. This is a worrying claim made! <-- i am sorry, i got the informations from someone in the yahoo messenger chat room "computer & electronics"

At low frequencies, FET is definitely noisy due to 1/f noise. Not so much if FET is used at high frequencies. <-- what is 1/f?? is it means that fet is good for high frequency??

Where did you learn all your principles? Are you a self-learner or your lecturers at college or university taught you all that said? <-- i learn it from many tutorials, and asked it in the yahoo messenger chat room, yes, i am a self learner

You have got many things confused. <-- thank you for corrected me ^^

Re: fet input impedance

LNA = Low-Noise Amplifier, a linear Class A amplifier

why the bjt can be better for low noise and high speed

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Dominant noise in BJT is Shot noise, but suppressed by high Gm. Hence resulting low noise.
Also due to high Gm, the Miller Effect is less pronounced in BJT than in MOS. Hence higher maximum frequency and larger operating bandwidth than MOS.

what is IIP3?? NF??

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IP³ = Third Intercept Point, a parameter measured by 2-tone test
NF = Noise Figure, a ratio of SNRin/SNRout. If NF = 1, the system is noiseless.

what is 1/f?? is it means that fet is good for high frequency

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1/f noise is a common name for Flicker noise, due to its mathematical representation of: Mean Vn² = (1/f).K/Cox.W.L
From the 1/f ratio, this means that increasing the frequency, the lower the influence of 1/f noise on FET.
Depending on threshold voltage, Miller Effect and capacitive load, speed of FET is subjected.

Re: fet input impedance

oo.. okay, thank you, is there any tutorials about noise? what is flicker noise? why there so many name for noise? flicker noise, thermal noise, shot noise, johnson white noise, etc.. and is there any good tutorial that explain about BJY, JFETs, and MOSFET? and also about the class of amplifier?

thank you very much

Re: fet input impedance

Flicker noise is caused by several mechanisms. One such mechanism is the trap/release of carriers at the oxide-substrate interface, occuring at low frequencies and DC, given by the formula: Mean Vn² = (1/f).K/Cox.W.L

From the questions you asked, you are confined by a problem.

You need to receive formal education from a technical college or university in Electrical/Electronic Engineering to learn the fundamentals. This is not enough. You will need to buy or borrow books on Electronics to strengthen your understandings. You will need to know the theories well enough to start by exploring with experiments and doing some projects to further improve your understandings.

If you want to learn the hard way by asking many questions, you will only get simple and basic answers, sometimes bad answers from people who understand badly.

I can only help by offering straightforward answers. I can't tell you everything.