# BJT, JFET and MOSFET Functionality?

#### pinkyponky

Joined Nov 28, 2019
22
Hello All,

I have been spending more time for watching and reading the tutorials to understand the functionality of BJT (NPN and PNP) transistors, JFET (N-channel and P-channel) and MOSFET (N-channel and P-channel for both Depletion and Enhancement modes), but I got confused and not understand. I would like to know only that when these BJT's and FET's will be turn ON and OFF?. Please could you clearly explain with examples (If possible)?.

#### Jony130

Joined Feb 17, 2009
5,211

Also as for the BJT's. But does not be confused by these examples and assumptions that the base current is being somehow magically magnified to form the collector current. This is not the case (not true). What is happening is that the base current is controlling the amount of current that collector-emitter drawn from a supply source.
As is shown here in the "water model".

All transistors work in a very similar way to the tap water valve.
A water valve is always used as a device to control the flow of water. Similarly, always think of a transistor as a device used to control electric current flow with the assistance of a middle "terminal". The base in BJT's and the gate in FET or MOSFET.

And some pictures with the MOSFET's

Thus, in short, to "ON" the N-channel MOSFET the voltage between the gate and a source must be larger than Vgs(th) voltage. And to "ON" P-channel MOSFET the gate voltage must be lower than Vgs(th) comper to the source voltage.

Any questions?

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#### pinkyponky

Joined Nov 28, 2019
22

Also as for the BJT's. But does not be confused by these examples and assumptions that the base current is being somehow magically magnified to form the collector current. This is not the case (not true). What is happening is that the base current is controlling the amount of current that collector-emitter drawn from a supply source.
As is shown here in the "water model".

View attachment 226364

All transistors work in a very similar way to the tap water valve.
A water valve is always used as a device to control the flow of water. Similarly, always think of a transistor as a device used to control electric current flow with the assistance of a middle "terminal". The base in BJT's and the gate in FET or MOSFET.

Any questions?
Thank you Jony.

I have read the functionality of NPN and PNP transistors through the link that you provided and I understand, but I have seen in the another tutorial saying that NPN transistor initially is OFF and PNP transistor initially is ON (without any voltage applying to the base or/and the collector). But, in the above link (that you provided) saying that the PNP transistor is ON when the base voltage 0.6 lower than the emitter voltage, which means that the the PNP transistor is OFF initially, Right?.

So, I'm confusing, please could you clear my question?.

#### Jony130

Joined Feb 17, 2009
5,211
but I have seen in the another tutorial saying that NPN transistor initially is OFF and PNP transistor initially is ON (without any voltage applying to the base or/and the collector).
Can you show me a link to such a strange statement?

saying that the PNP transistor is ON when the base voltage 0.6 lower than the emitter voltage, which means that the PNP transistor is OFF initially, Right?.
Yes, to "ON" an PNP transistor the emitter voltage must be "tight" to 0.6V higher potential than the base terminal. If you for example short the base terminal together with the emitter the BJT will be OFF.

Try to analyze this example:

#### atferrari

Joined Jan 6, 2004
4,166
Can you show me a link to such a strange statement?

Yes, to "ON" an PNP transistor the emitter voltage must be "tight" to 0.6V higher potential than the base terminal. If you for example short the base terminal together with the emitter the BJT will be OFF.

Try to analyze this example:
I believe the problem of the of the OP lies in trying to force the use of "initially".
For the components in question, "initial" conditions are given by the circuit they are in.

#### pinkyponky

Joined Nov 28, 2019
22
Can you show me a link to such a strange statement?

Yes, to "ON" an PNP transistor the emitter voltage must be "tight" to 0.6V higher potential than the base terminal. If you for example short the base terminal together with the emitter the BJT will be OFF.

Try to analyze this example:
" Can you show me a link to such a strange statement?"

See in the below link saying that PNP transistor is ON fully when there is no base current.
I was confused and I said that no voltage to the base and collector, instead of saying above statement. I do apologies for this.
But, In that link saying that the the device is normally on, this statement I'm not understanding.

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#### Jony130

Joined Feb 17, 2009
5,211
OMG, What a confusing answer. You should stop reading the page immediately.

The main difference between NPN and the PNP is the current direction and the voltage "polarity" needed to ON/OFF the device.

See the real-world example:

As you can see NPN transistor (N- channel MOSFET as well) is a "low-side" device. And the current enters the base terminal. On the other hand, the PNP is a "high-side" device and thus, the current is now leaving the base terminal. And in both cases, if you remove the base resistor (Rb1 and Rb2), then no base current will flow thus, both NPN and PNP will be completely OFF.

#### crutschow

Joined Mar 14, 2008
25,980
See in the below link saying that PNP transistor is ON fully when there is no base current.
That is absolutely incorrect.
A PNP is not a "normally on" device as it acts the same as an NPN, just with opposite voltage polarities.
In either case they conduct no collector-emitter current (other than leakage) when there is no base current.

#### dl324

Joined Mar 30, 2015
12,210
See in the below link saying that PNP transistor is ON fully when there is no base current.
I was confused and I said that no voltage to the base and collector, instead of saying above statement. I do apologies for this.
But, In that link saying that the the device is normally on, this statement I'm not understanding.
WOW! You'd think a site called learningaboutelectronics would have more accurate information. That page is garbage.

With no base current, all you'll have is collector leakage current. This is from a Fairchild 2N3906 datasheet:

I sent them feedback about the incorrect information. Original content for posterity.

#### sparky 1

Joined Nov 3, 2018
401
You can also use a simulator for this exercise. It is assumed that you completed the textbook practical examples
and are prepared to complete the lab exersize.

Here is a video of a school electronics lab in India. The students can achieve a level of understanding
in class then perform an evaluation shown: the input and output characteristics of a BJT transistor.

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#### kaindub

Joined Oct 28, 2019
45
Since the major adoption of microcontrollers to electronics, design engineers have been treating all froms of transistors as just switches. But that is far from the reality.
It is easier to understand the concept of transistors if you remember that they are in fact amplifiers.
Bipolar transistors amplify the base CURRENT (the current flowing from Base to Emitter) and amplify it into a CURRENT flowing from the Collector to the emitter. And if you use this simple explanation you dont even have to make an exception whether it is a PNP or an NPN transistor.
Mosfets take the VOLTAGE applied between the gate and the source amplify it into a CURRENT flowing from the drain to source (I find this naming confusing but that the way it is). Again dont need to know whether its a P or N channel fet to apply this principle.
At some point the Bipolar transistor or the FET reaches a point where the input cannot be amplied any further. Thats when the device becomes a switch.
Depletion modes FETs are a little different. In there natural state they conduct between the drain and the source. Measure it and you will find a resistance of a few hundred ohms. Applying a voltage between the gate and source causes less current to flow between the drain and source. Its sort of like an UNamplifier. Depletion mode FETs are the solid state equivalent of thermionic valves. But Delpetion mode Fets are rarely used these days and harder to come by. They are also rarely used as switches .

#### crutschow

Joined Mar 14, 2008
25,980
It is easier to understand the concept of transistors if you remember that they are in fact amplifiers.
That word can be somewhat ambiguous, as they can be used to build an amplifier (signal power gain greater than 1) circuit, but do not generally do that by themselves.
It would be more accurate to say that the transistor acts as a "valve" (the British term used for vacuum tubes) in that its base or gate controls the flow of current in the device with a small current or voltage controlling a much larger current (and voltage).
This can then be readily used to build a circuit to amplify a signal.
But Delpetion mode Fets are rarely used these days and harder to come by.
There are few depletion mode MOSFETs but all JFETS are depletion mode, which are still used in some applications.

#### neonstrobe

Joined May 15, 2009
126
It is sometimes more helpful to see how transistors work by considering the actual flow of electrons and holes.
In any junction diode an N-type semiconductor is formed in the same crystal as a P-type. A barrier called a depletion region forms between the two. With no applied voltage the diode is more like a capacitor because of the depletion region, which fits the description: two charged plates (the N and P) separated by a high impedance (almost insulating) layer. Forward biasing the junction causes electrons to flow from the N into the P and holes from the P into the N because the electrostatic potential barrier is reduced.
In a BJT the emitter is more heavily doped than the base so that the electron current is much larger than the hole current in forward bias. Most of the electrons injected into the base flow across into the collector. The base current, by comparison, is much smaller - giving rise to the gain or Ic/Ib ratio which is about 100, typically.
If the device is NPN forward biasing the base means making it more positive compared with the emitter. The collector is connected throuh some load, or resistors, to a positive supply (or more positive than the emitter anyway). For a PNP the base has to be more negative than the emitter to turn it on. So the emitter of a PNP is connected to a positive (or more positive) supply compared with the collector.
Neither device is "normally on" with zero base-emitter voltage (and only conducts a little leakage current, possibly).
MOSFETs can be surface type as in an I.C. or a sort of 2D effect with a top source and bottom drain (as developed by IR with their "Hexfet"). Both work primarily by having two surface N regions separated by a P (for NMOS) but on top of which is an insulating oxide and on top of that a conducting electrode called the gate (because it gates the current). For a PMOS there are two P regions separated by an N.
These devices are usually "normally off" too, because no gate bias provides no link between the other two (source and drain) regions. For NMOS a positive gate voltage attracts electrons underneath the oxide, which joins the two N regions and allows the device to conduct. In the power device the drain N region is then formed as a vertical channel connected underneath.
For PMOS, a negative gate voltage attracts holes underneath which then allows current to flow between the two P regions.
JFETS differ in that the two N regions (in an NJFET) are connected by a lightly doped N layer, so normally look like a resistor-that is, it conducts. The p region forms a junction between the two ends and usually from both the top and bottom. When a reverse bias is applied (negative for NJFET) the depletion region of the junction widens and squeezes the conducting area, reducing the current flow. A PJFET works but with a P-to-P-to-P layer surrounded by the N diode junction in the middle section, and works with a positive source and negative drain, and needs a positive gate voltage to cut down the current.
"Normally on" MOSFETS are a bit like a cross between the JFET and MOSFET: they are MOSFET in construction (i.e. source and drain with a gate over the top) but also have a lightly doped connecting region which connects the source and drain with zero bias, and can increase or decrease the current for a positive or negative gate bias (for NMOS n/on FET). If negative bias the channel between the source and drain is squeezed, but if positive, additional electrons pile up under the gate and increase the current.
These devices are usually rarer these days as power switching applications - and amplifier circuits- are usually easier to design when devices are initially off (i.e when the circuit is unbiased or voltages are set to zero). JFETS are commonly used for switching analogue signals as they can conduct in either direction and are reasonably linear at low drain to source voltages, or high input impedances in amplifier front ends (especially in bipolar IC circuits because the JFET can be relatively easily included).

#### Ian0

Joined Aug 7, 2020
1,097
Think of them all as being voltage controlled, and you won't be far wrong.

For a NPN transistor, N-channel JFET or N-channel MOSFET - gate/base voltage more positive with respect to source/emitter turns it on.
For a PNP transistor, P-channel JFET or P-channel MOSFET - gate/base voltage more negative with respect to source/emitter turns it on.

What's different is the voltage at which it swaps from OFF to ON:

In a bipolar transistor and a JFET there is a diode between base/gate and emitter/source which conducts just like any other diode does, starting at about 0.6V.
No current flows between gate and source in a MOSFET.

#### kaindub

Joined Oct 28, 2019
45
Ian0, sad tp say you are way off.
transistors are current controlled devices.
Fets are voltage controlled devices.
Transistors have an initial 0.6 v to over come due to the on junction at the base. They don’t switch at 0.6 v they start to conduct in the linear region.
Fets require a higher voltage to begin to conduct and once over that initial voltage (a few volts depending on the construction) they conduct linearly.

#### Ian0

Joined Aug 7, 2020
1,097
You think so?
Ebers-Moll gives the emitter current as Ie = Ies * exp(Vbe/Vt -1).
I see the base-emitter voltage in the equation, but there's no mention of the base current!

The TS was interested in the devices' being either ON or OFF. He can be introduced to the linear region at his leisure!

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#### neonstrobe

Joined May 15, 2009
126
Transistors are voltage controlled!!!
The voltage controls the potentials across the junctions (or gate oxide) which determines the current flows.
It is often written that bipolar transistors are current controlled. Fundamentally that is not the case, but if you put a current into the base of a transistor its potentials will adjust so that the current flow equates. The potential then sets the collector current in the device, not the current per se.
Ian0 - that is the collector current, not emitter current. Ic=IS*(exp(vbe/vth)-1) (IS= "saturation" current, really the zero bias current flux.)
But if you current drive the transistor Vbe=vth*log(Ic/IS) (approximately). But it is Vbe which is important inside the device.

#### Ian0

Joined Aug 7, 2020
1,097
Ian0 - that is the collector current, not emitter current. Ic=IS*(exp(vbe/vth)-1)
Well, Its 99% right (differs only by 1/beta), and I admit that I looked it up on Wikipedia instead of going through my university notes from 1986!
I think we both agree on the basic principle (and so does Doug Self).
Ibe also varies with exp(Vbe) in a similar fashion to Ic and Ie, and that gives a proportionality between Ib and Ic which is more coincidence than physics.

#### Ian0

Joined Aug 7, 2020
1,097
What is happening is that the base current is controlling the amount of current that collector-emitter drawn from a supply source.
As is shown here in the "water model".

All transistors work in a very similar way to the tap water valve.
Acutally, it's the pressure of the water on the small gate (the analogue of the Base voltage) which causes the small gate to move and allow a large collector current to flow, at the same time as allowing a small base current to flow.