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Old 14th January 2006, 09:48 PM   #1
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Default JFETS, IGFET, MOSFETS and silicon type

I've been trying to hot up my understanding of solid state and have been reading through texts about transistors.

One thing that puzzles me at the moment is why MOSFETs have areas of the opposite doping under the source and drain terminals.

In a JFET you have a continuous channel that's depleted as you increase the gate voltage, a depletion mode device. But you have a bare gate that's easier to toast and has reverse leakage current.

Enter the IGFET. Add some insulation to protect the gate.

The explaination of MOSFETs I've read, and the term MOSFET it's self, doesn't explain how they are any different from IGFETs other than the Metal Oxide bit. Which isn't actually correct any more since the gate is usually a semiconductor and the oxide will be formed on the channel during deposition.

What is majorly different when you look at the diagram of a MOSFET are the two regions under the drain and source that are doped to be opposites of the channel, causing the device to have a threshold voltage and to operate in depletion or enhancement modes. Explainations don't mention why these are here or what's important about them.

As far as I can tell, this property of the device would make it useful for CMOS, unidirectional flow and other digital applications where an on / off mode is necessary; even though MOSFETs 'leak'. But if all that's needed is a controllable diode like device, why create two regions? One under the drain and the source? Wouldn't just one would be enough, a diode with a gate over one of the dopings to create a depletion / enhancement layer through it?

I'm going to take a guess that the second opposite doping region is formed just as a byproduct of how the devices are made.

Can anyone clear this up a bit?
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Old 14th January 2006, 10:33 PM   #2
poobah is offline poobah  United States
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Your guess is correct...
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Old 14th January 2006, 10:42 PM   #3
lineup is offline lineup  Sweden
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I am afraid I cant help you, eeka chu.

I know, like most diy-ers, only the outside, how to read some data
and connect them pins.
I do not find it necessary or interesting to go any further.

Some develope and construct cars - others just want to use them.
We want them never to break down.
If we have to pick them into pieces and start looking for faults
we find it is a horrible pain.

Hope some of those that have gone deeper into the internal mysteries of semiconductors
will discuss and try to answer your questions.
I wouldnt be surprised, if among many members of diyaudio,
there are people who are involved in designing chips!

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Old 15th January 2006, 03:22 AM   #4
BWRX is offline BWRX  United States
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Quote:
What is majorly different when you look at the diagram of a MOSFET are the two regions under the drain and source that are doped to be opposites of the channel, causing the device to have a threshold voltage and to operate in depletion or enhancement modes. Explainations don't mention why these are here or what's important about them.
When you say "two regions" are you referring to the p-type body regions between the drain and source of the (n+)(p)(n-)(n+) structure separated by the n- drift region? That overlapping of the gate conductor with the n- drift region is like that for a couple of reasons (these device designers really do know what they're doing!).

Quoting from one of my texts, the overlapping "tends to enhance the conductivity of the drift region at the (n-)-SiO2 interface by forming an accumulation layer, helping to minimize the on-state resistance." So, as the gate reaches a higher voltage than the source, an accumulation layer of electrons builds up at that interface reducing the on-state resistance.

The overlapping also "tends to act as a field plate when the mosfet is off that keeps the radius of curvature of the depletion region of the drain-body pn junction from getting too small and thus reducing the breakdown voltage of the device." Basically, when the gate is at the same voltage as the source the electrons are now forced out of the drift layer close to the gate oxide, which increases the breakdown voltage. Without the overlapping layer the depletion layer would be much smaller and devices would have lower breakdown voltage ratings.

Quote:
But if all that's needed is a controllable diode like device, why create two regions?
There are devices called thyristors or SCRs (silicon controlled rectifiers) if you need a controllable diode-like device. The two regions are there because of what was stated above and partly because of the way devices are actually made. Many of those "cells" are connected in parallel in a power mosfet.
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Old 15th January 2006, 03:42 AM   #5
mirlo is offline mirlo  United States
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It isn't the drain and source regions that create the threshold voltage, but the channel doping itself. The drain and source regions are there to allow an ohmic connection to the channel.

Once the gate of an NMOS transistor is pulled high, the previously lightly p doped region under the gate acts as if it had an n doping. To make the ohmic connection to the n channel, you need n doped drain and source regions.

I believe that IGFET is just an old term for MOSFET. Means exactly the same thing.
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Old 15th January 2006, 03:45 AM   #6
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Is there any difference between IGFET and MOSFET? I have always though that IGFET is just an older synonym. I have an old electronics book, where the chapter about these devices starts out writing "IGFET (MOSFET)" a cople of times, and the writes only IGFET. My book in semicondutor physics does not mention the term IGFET anywhere.

Edit:

Mirlo,
you posted while I was writing this post.
We seem to have the same understanding, though.
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Old 15th January 2006, 03:53 AM   #7
poobah is offline poobah  United States
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Metal Oxide Semi,

same thing... different words

the structure changed to create an "enhancement" device.

The real reason was to create a LARGE device... "enhancement" came along free.
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Old 15th January 2006, 08:43 AM   #8
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Quote:
Originally posted by Christer
Is there any difference between IGFET and MOSFET? I have always though that IGFET is just an older synonym. I have an old electronics book, where the chapter about these devices starts out writing "IGFET (MOSFET)" a cople of times, and the writes only IGFET. My book in semicondutor physics does not mention the term IGFET anywhere.
You're right...
MOS=
METAL(gate)
OXIDE(insulator)
SEMICONDUCTOR(channel)
The insulator is typically made of an oxide such as silicon dioxide, SiO2 (glass)
citation:
" Another common name for the device, derived from the construction method, is the insulated gate field effect transistor (IGFET)"
description p1
description p2
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Old 15th January 2006, 10:03 PM   #9
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Thanks for the replies everyone,

I believe the term IGFET is pretty much synomonous with MOSFET now, as MOSFET is synomonous with devices that don't actually have any metal oxide anywhere in them. I guess if you wanted to get picky, an IGFET would be a JFET with an insulated gate, a 'MOSFET' is slightly more complex; it has some extra doping under the source and drain. But then, MOSFET doesn't really fully explain what a MOSFET is; no mention of additional doping.

I didn't expect so many replies and my next question to Poobah was going to be, since these guys tweak the silicon until they've tweaked it 100%, and then tweak it some more, why wouldn't they have tweaked the extra region away since it's responsible for things like channel pinch off as Vds increases? But BWRX preempted that. So the regions are basically there to shape the channel.

They can't be there purely as 'contact points' because a JFET doesn't need them; the contact go directly to the substrate / channel material.

I'm not so sure that the channel is responsible for the threshold voltage on it's own. The threshold is a discrete 'on' point, whereas the field from the gate should be allow current flow as soon as both ends of it are connected to the terminals. If you have a uniform field, no pinching off at the drain, that would be as soon as you applied any voltage what so ever to the gate. I think the threshold point emerges as a result of the junction between the substrate / channel and the doping points around drain / source the contacts; you have two opposite dopings so there'll be a depletion layer between them, like in a normal diode or BPJ transistor, you have to overcome or bias before the channel opens.

BWRX, what texts are your quoting from? They sound quite involved. I'd like to fully understand the benefits of changing the doping around the source and drain. Like you say, thyristors and SCRs are examples of simple 'MOSFETs', composed of a diode and a gate over the opposite doping. They've obviously found that adding an extra doping region around the other terminal is beneficial for performance, provided you avoid pinch off conditions.

I also noticed the funny shapes they apply to the source / drain dopings, each one is like two blobs sat on top of each other, undoubtely because this shape helps enhance the device performance.

Dopant + = heavily doped?
Dopant - = lightly doped?
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Old 15th January 2006, 10:04 PM   #10
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mosfet's need two dopings, especially in IC's because a mosfet can be symettric (transmission gate digital).

it needs to a junction for very obvious reasons! if you wanted an ohmic connection you could make a P+ P P+ structure, but the substrate would be connected to some voltage and well, you've got a short across the entire "device". no good.

with a N P N structure you have reversed biases parasitic diodes.

if you did N P P+ like you suggested, you would have the substrate connected to the same potential as the substrate. not too useful.

MOSFETs really are 4 terminal devices. its sometimes hard to remember that.

an excellent old book for this is: Solid State Electronic Devices by Ben Streetman, Sanjay Banerjee . i've seen the 3rd and 5th editions. the 5th edition adds information about IGBTs IIRC, but nothing really major. after reading streetman you should be able to get the IGBT stuff from IRF.
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