Doped semiconductors in electronic devices & main principles for the operation of transistors / Discussion

This is the discussion related to the wiki page Doped semiconductors in electronic devices & main principles for the operation of transistors.

I-V plot

Dharma Maddala 28 Apr 2009 03:35

good job Maria. I have cited your work in my term paper on work function.
Its good that you mentioned the carrier density is small and only a very small reverse saturation current flows in (P negative with respect to N) reverse bias and in forward bias (P positive with respect to N) the carrier density is large (it varies exponentially with the applied bias voltage), making the junction conductive and allowing a large forward current. I think, this will get a complete picture if you show an I-V plot like the below one to show these effects.

Re: I-V plot

mariael84 30 Apr 2009 16:14

Dharma, thank you for your comment. You are right about the graph and I appreciate you added it.
In fact, for the case of MOSFETS particularly the curvature corresponding to forward bias is a result of application of different voltages to the gate as shown in the following graph for voltages VG = 5 V (top curve), 4 V, 3 V and 2 V (bottom curve).

The gate controls the flow of electrons from the source to the drain. A positive voltage applied to the gate attracts electrons to the interface between the gate dielectric and the semiconductor. These electrons form a conducting channel called the inversion layer. No gate current is required to maintain the inversion layer at the interface since the gate oxide blocks any carrier flow. The net result is that the applied gate voltage controls the current between drain and source.

Last edited on 30 Apr 2009 16:15 by mariael84 Show more

Re: I-V plot

Gokhan Bakan 05 May 2009 13:43

Hi Dharma,

I want to make a comment on yours: reverse bias current of a diode does not continue constant as reverse bias increasing negatively. After a certain point, diode breaks and current blows up due to two main mechanisms:

- impact ionization due to high energy (hot) electron diffused from p-side into the depletion region. These electrons have capability of knocking down more electrons and this process can yields an avalanche breakdown [Maria shows reversed biased diode in the paper] —> this process might damage the device.

- electrons in the p-side see a lower energy on the n-side's conduction band and band-to-band tunneling occurs (Zener breakdown). Since breakdown voltage for this case can be large (in the order of bandgap of the material used), this diode can be used in rectifier circuits in reverse bias.

Thanks.

MOSFET overlap

Gokhan Bakan 28 Apr 2009 23:01

Hi Maria,

I will be doing conventional and quantum mechanical analysis of MOS capacitors for my second term paper. To introduce MOS capacitors, I want to talk on MOSFETs first. I have seen that you have already discussed here, too. I will refer to this paper instead of explaining how a MOSFET works. I already included some energy-band diagrams of a MOSFET in accumulation, flat-band, depletion and inversion which explains working principles in another way. Although MOSFET part of my paper looks like overlapping with yours, I will focus on MOS capacitor and solving shcrodinger equation for triangular well which appears at Si-SiO2 interface (you also show a triangular well for AlGaAs-GaAs structure).

Please let me know, if you are planning to add anything to MOSFET part of this paper, or if you have anything that you want me to mention in my paper.

Best regards.

Gokhan.

Last edited on 29 Apr 2009 13:14 by Gokhan Bakan Show more

Manufaturing MOSFET

satyesh 30 Apr 2009 18:22

Good work..
you can probably talk some thing about how MOSFET is manufactured.

Re: Manufaturing MOSFET

mariael84 30 Apr 2009 19:23

Thanks Satyesh.

There is a very nice demonstration on fabrication of a N-MOSFET in this link:
http://www.ee.byu.edu/cleanroom/virtual_cleanroom.parts/MOSFETProcess.html

I'd rather not copy it here, because it is a presentation that also contains motion, but you can easily find it in the above-mentioned link.

A-Rivera 04 May 2009 23:09

Hi Maria,

You got a nice picture of Silicon doped with P (n-type material), but I'm not sure how can you relate this picture with the conduction and balance band diagram? I am trying to visualize in a real model, what happen once the bond is broke, and the electron become an conduction electron. Is hard for me to match this concept in 3d and if you add the phonon as the temperature increase, the picture get worse. Any idea?

Thanks, Abdiel

Re:

mariael84 05 May 2009 16:30

N-type and p-type doping gives the following different energy band structures. In the case of a donor atom in the lattice (n-type doping), there will be creation of extra states located close enough to the conduction band, so that the electrons which they contain are easily excited up into the conduction band.

On the other hand, in the presence of an acceptor (p-type case), the absence of an electron will create a new set of states very close to the valence band. Electrons in the valence band can be thermally excited up into these new allowed levels, creating empty states, or holes, in the valence band. The excited electrons are stuck at the boron atom sites and act as fixed negative charges, localized there.

The images are oversimplified but, still, I believe they give a very vivid representation of each case.

Last edited on 05 May 2009 16:40 by mariael84 Show more

HEMT

yuan xue 05 May 2009 03:05

Hi, Maria,
Well done! Your paper gives me the particular knowledge about p-n junction. I am interested in HEMT, is HEMT used widely as MOSFET?

Re: HEMT

mariael84 05 May 2009 16:01

Nowadays numerous companies worldwide develop and manufacture HEMT-based devices. These devices are involved in any application where high gain and low noise at high frequencies are required. Thus, they are found in many types of equipment ranging from cellphones and DBS receivers to electronic warfare systems such as radar and for radio astronomy.

Re: HEMT

yuan xue 05 May 2009 19:13

Thank you Maria, it helps me to understand HEMT!