Search Engines

Search Engines

It is the search engines that finally bring your website to the notice of the prospective customers. Hence it is better to know how these search engines actually work and how they present information to the customer initiating a search.

There are basically two types of search engines. The first is by robots called crawlers or spiders.

Search Engines use spiders to index websites. When you submit your website pages to a search engine by completing their required submission page, the search engine spider will index your entire site. A ‘spider’ is an automated program that is run by the search engine system. Spider visits a web site, read the content on the actual site, the site's Meta tags and also follow the links that the site connects. The spider then returns all that information back to a central depository, where the data is indexed. It will visit each link you have on your website and index those sites as well. Some spiders will only index a certain number of pages on your site, so don’t create a site with 500 pages!

The spider will periodically return to the sites to check for any information that has changed. The frequency with which this happens is determined by the moderators of the search engine.

A spider is almost like a book where it contains the table of contents, the actual content and the links and references for all the websites it finds during its search, and it may index up to a million pages a day.

Example: Excite, Lycos, AltaVista and Google.

When you ask a search engine to locate information, it is actually searching through the index which it has created and not actually searching the Web. Different search engines produce different rankings because not every search engine uses the same algorithm to search through the indices.

One of the things that a search engine algorithm scans for is the frequency and location of keywords on a web page, but it can also detect artificial keyword stuffing or spamdexing. Then the algorithms analyze the way that pages link to other pages in the Web. By checking how pages link to each other, an engine can both determine what a page is about, if the keywords of the linked pages are similar to the keywords on the original page.

Keyword Density

Keyword Density

Keyword density is an indicator of the number of times the selected keyword appears in the web page. But mind you, keywords shouldn’t be over used, but should be just sufficient enough to appear at important places.

If you repeat your keywords with every other word on every line, then your site will probably be rejected as an artificial site or spam site.

Keyword density is always expressed as a percentage of the total word content on a given web page.

Suppose you have 100 words on your webpage (not including HMTL code used for writing the web page), and you use a certain keyword for five times in the content. The keyword density on that page is got by simply dividing the total number of keywords, by the total number of words that appear on your web page. So here it is 5 divided by 100 = .05. Because keyword density is a percentage of the total word count on the page, multiply the above by 100, that is 0.05 x 100 = 5%

The accepted standard for a keyword density is between 3% and 5%, to get recognized by the search engines and you should never exceed it.

Remember, that this rule applies to every page on your site. It also applies to not just to one keyword but also a set of keywords that relates to a different product or service. The keyword density should always be between 3% and 5%.

Simple steps to check the density:

Copy and paste the content from an individual web page into a word-processing software program like Word or Word Perfect.

Go to the ‘Edit’ menu and click ‘Select All’. Now go to the ‘Tools’ menu and select ‘Word Count’. Write down the total number of words in the page.

Now select the ‘Find’ function on the ‘Edit’ menu. Go to the ‘Replace’ tab and type in the keyword you want to find. ‘Replace’ that word with the same word, so you don’t change the text.

When you complete the replace function, the system will provide a count of the words you replaced. That gives the number of times you have used the keyword in that page.

Using the total word count for the page and the total number of keywords you can now calculate the keyword density.

Securing Your Computer System

Securing Your Computer System

Today, more and more people are using their computers for everything from communication to online banking and investing to shopping. As we do these things on a more regular basis, we open ourselves up to potential hackers, attackers and crackers. While some may be looking to phish your personal information and identity for resale, others simply just want to use your computer as a platform from which to attack other unknowing targets. Below are a few easy, cost-effective steps you can take to make your computer more secure.

• Always make backups of important information and store in a safe place separate from your computer.
• Update and patch your operating system, web browser and software frequently. If you have a Windows operating system, start by going to www.windowsupdate.microsoft.com and running the update wizard. This program will help you find the latest patches for your Windows computer. Also go to www.officeupdate.microsoft.com to locate possible patches for your Office programs.
• Install a firewall. Without a good firewall, viruses, worms, Trojans, malware and adware can all easily access your computer from the Internet. Consideration should be given to the benefits and differences between hardware and software based firewall programs.
• Review your browser and email settings for optimum security. Why should you do this? Active-X and JavaScript are often used by hackers to plant malicious programs into your computers. While cookies are relatively harmless in terms of security concerns, they do still track your movements on the Internet to build a profile of you. At a minimum set your security setting for the “internet zone” to High, and your “trusted sites zone” to Medium Low.
• Install antivirus software and set for automatic updates so that you receive the most current versions.
• Do not open unknown email attachments. It is simply not enough that you may recognize the address from which it originates because many viruses can spread from a familiar address.
• Do not run programs from unknown origins. Also, do not send these types of programs to friends and coworkers because they contain funny or amusing stories or jokes. They may contain a Trojans horse waiting to infect a computer.
• Disable hidden filename extensions. By default, the Windows operating system is set to “hide file extensions for known file types”. Disable this option so that file extensions display in Windows. Some file extensions will, by default, continue to remain hidden, but you are more likely to see any unusual file extensions that do not belong.
• Turn off your computer and disconnect from the network when not using the computer. A hacker can not attack your computer when you are disconnected from the network or the computer is off.
• Consider making a boot disk on a floppy disk in case your computer is damaged or compromised by a malicious program. Obviously, you need to take this step before you experience a hostile breach of your system.

Fighting off Viruses

Fighting off Viruses:

Protecting your computer from a virus is getting harder and harder each day. While it may border on the paranoid, it goes without saying that you can’t leave your guard down for one second. Even corporate giant Microsoft has found its own systems compromised on more than one occasion.

Remember the “good old days”, before the advent of the Internet and downloadable programs? Life was simple then in terms of computer viruses. With the primary way in which a virus could be transmitted being limited to floppy disks, the ability to catch and eradicate the virus was a lot easier. By today’s standards, it used to take quite a while before a virus was able to infect a computer and slow down the system. The antivirus software of that time was typically able to identify and eradicate viruses before they caused too much damage. Additionally, computer users were pretty savvy on how to protect themselves in terms of scanning all floppy disks before copying them to our desktop.

The Internet helped change all that. The Internet provided a conduit by which viruses could move from host to host with lightening speed. No longer could a computer user just worry about floppy disks as points of entry, but they now had to worry about email, email attachments, peer-to-peer file sharing, instant messaging, and software downloads. Today’s viruses can attack through multiple entry points, spread without human intervention, and take full advantage of vulnerabilities within a system or program. With technology advancing everyday, and the convergence of computers with other mobile devices, the potential of new types of threats also increase.

Protecting Your Computer

Luckily, the advancement of antivirus software has kept pace with current virus threats. Antivirus software is essential to a computer’s ability to fend off viruses and other malicious programs. These products are designed to protect against the ability of a virus to enter a computer through email, web browsers, file servers and desktops. Additionally, these programs offer a centralized control feature that handle deployment, configuration and updating.

A computer user should remain diligent and follow a few simple steps to protect against the threat of a virus:

1. Evaluate your current computer security system.
With the threat of a new generation of viruses able to attack in a multitude of ways, the approach of having just one antivirus software version has become outdated. You need to be confident that you have protected all aspects of your computer system from the desktop to the network, and from the gateway to the server. Consider a more comprehensive security system which includes several features including antivirus, firewall, content filtering, and intrusion detection. This type of system will make it more difficult for the virus to penetrate your system.

2. Only install antivirus software created by a well-known, reputable company.
Because new viruses erupt daily, it is important that you regularly update your antivirus software. Become familiar with the software’s real-time scan feature and configure it to start automatically each time you boot your computer. This will protect your system by automatically checking your computer each time it is powered up.

3. Make it a habit to always scan all new programs or files no matter from where they originate.

4. Exercise caution when opening binary, Word, or Excel documents of unknown sources especially if they were received during an online chat or as an attachment to an email.

5. Perform regular backups in case your system is corrupted. It may be the only way to recover your data if infected.

Recommended Antivirus Software

There are numerous applications available to consumers. With a little research, you can pick the program that is right for you. Many programs provide a trial version which allows you to download the program and test its abilities. However, be aware that some anti-virus programs can be difficult to uninstall. As a precaution make sure to set up a System Restore point before installing.

Here are a few programs which typically receive high marks in terms of cost, effectiveness, ease of use, and customer service.

The Shield Pro 2005™ provides virus protection and hacker security through ongoing support and updates. When a virus breaks out, The Shield Pro 2005™ promises to provide a patch within 2-3 hours and a fix for the virus within 5 hours. You can set your computer to update viruses weekly and run a complete virus scan.

BitDefender 9 Standard provides antivirus protection, as well as Peer-2-Peer Applications protection, full email protection, and heuristics in a virtual environment. This provides a new security layer that keeps the operating system safe from unknown viruses by detecting malicious pieces of code for which signatures have not been released yet.

Kaspersky Anti-Virus Personal 5.0 program is simple to install and use. The user only needs to choose from three levels of protection. It allows updates as frequently as every hour while promising not to disrupt your computer. The program also offers a two-tier email protection feature and round-the-clock technical support.

PC-cillin Internet Security combines antivirus security and a personal firewall—for comprehensive protection against viruses, worms, Trojans, and hackers. It also detects and removes spyware and blocks spam. It even guards against identity theft by blocking phishing and pharming attacks.

AVG Anti-Virus Free Edition is a free downloadable antivirus program that has received high marks for its reliability. In the past, free downloadable antivirus programs have been viewed skeptically because of issues relating to its reliability. However, AVG from Grisoft, remains one of the best-known free anti-virus programs available. While AVG can not be installed on a server operating system and there is no technical support, it still makes a good choice for many home computer users. The best part is that since it is free, you can try it with no further obligation necessary.

VIRUS

Computer Viruses

Every day new computer viruses are created to annoy us and to wreck havoc on our computer systems. Below are ten viruses currently cited as being the most prevalent in terms of being seen the most or in their ability to potentially cause damage. New viruses are created daily. This is by no means an all inclusive list. The best thing you can do is to remain vigilant, keep your anti-virus software updated, and stay aware of the current computer virus threats.

Virus: Trojan.Lodear
A Trojan horse that attempts to download remote files. It will inject a .dll file into the EXPLORER.EXE process causing system instability.

Virus: W32.Beagle.CO@mm
A mass-mailing worm that lowers security settings. It can delete security-related registry sub keys and may block access to security-related websites.

Virus: Backdoor.Zagaban

A Trojan horse that allows the compromised computer to be used as a covert proxy and which may degrade network performance.

Virus: W32/Netsky-P
A mass-mailing worm which spreads by emailing itself to addresses produced from files on the local drives.

Virus: W32/Mytob-GH
A mass-mailing worm and IRC backdoor Trojan for the Windows platform. Messages sent by this worm will have the subject chosen randomly from a list including titles such as: Notice of account limitation, Email Account Suspension, Security measures, Members Support, Important Notification.

Virus: W32/Mytob-EX
A mass-mailing worm and IRC backdoor Trojan similar in nature to W32-Mytob-GH. W32/Mytob-EX runs continuously in the background, providing a backdoor server which allows a remote intruder to gain access and control over the computer via IRC channels. This virus spreads by sending itself to email attachments harvested from your email addresses.

Virus: W32/Mytob-AS, Mytob-BE, Mytob-C, and Mytob-ER

This family of worm variations possesses similar characteristics in terms of what they can do. They are mass-mailing worms with backdoor functionality that can be controlled through the Internet Relay Chat (IRC) network. Additionally, they can spread through email and through various operating system vulnerabilities such as the LSASS (MS04-011).

Virus: Zafi-D
A mass mailing worm and a peer-to-peer worm which copies itself to the Windows system folder with the filename Norton Update.exe. It can then create a number of files in the Windows system folder with filenames consisting of 8 random characters and a DLL extension. W32/Zafi-D copies itself to folders with names containing share, upload, or music as ICQ 2005a new!.exe or winamp 5.7 new!.exe. W32/Zafi-D will also display a fake error message box with the caption "CRC: 04F6Bh" and the text "Error in packed file!".

Virus: W32/Netsky-D
A mass-mailing worm with IRC backdoor functionality which can also infect computers vulnerable to the LSASS (MS04-011) exploit.

Virus: W32/Zafi-B

A peer-to-peer (P2P) and email worm that will copy itself to the Windows system folder as a randomly named EXE file. This worm will test for the presence of an internet connection by attempting to connect to www.google.com or www.microsoft.com. A bilingual, worm with an attached Hungarian political text message box which translates to “We demand that the government accommodates the homeless, tightens up the penal code and VOTES FOR THE DEATH PENALTY to cut down the increasing crime. Jun. 2004, Pécs (SNAF Team)”

Fighting Spam

How prevalent is Spam? According to Scott McAdams, OMA Public Affairs and Communications Department (www.oma.org):

“Studies show unsolicited or “junk” e-mail, known as spam, accounts for roughly half of all e-mail messages received. Although once regarded as little more than a nuisance, the prevalence of spam has increased to the point where many users have begun to express a general lack of confidence in the effectiveness of e-mail transmissions, and increased concern over the spread of computer viruses via unsolicited messages.”

In 2003, President Bush signed the “Can Spam” bill, in December of 2003 which is the first national standards around bulk unsolicited commercial e-mail. The bill, approved by the Senate by a vote of 97 to 0, prohibits senders of unsolicited commercial e-mail from using false return addresses to disguise their identity (spoofing) and the use of dictionaries to generate such mailers. In addition, it prohibits the use of misleading subject lines and requires that emails include and opt-out mechanism. The legislation also prohibits senders from harvesting addresses off Web sites. Violations constitute a misdemeanor crime subject to up to one year in jail.

One major point that needs to be discussed about this: spam is now coming from other countries in ever-greater numbers. These emails are harder to fight, because they come from outside our country’s laws and regulations. Because the Internet opens borders and thinks globally, these laws are fine and good, but do not stop the problem.

So what do you do about this? Her are the top 5 Rules to do to protect from spam.

Number 1: Do what you can to avoid having your email address out on the net.

There are products called “spam spiders” that search the Internet for email addresses to send email to. If you are interested, do a search on “spam spider” and you will be amazed at what you get back. Interestingly, there is a site, WebPoison.org, which is an open source project geared to fight Internet "spambots" and "spam spiders", by giving them bogus HTML web pages, which contain bogus email addresses

A couple suggestions for you: a) use form emails, which can hide addresses or also b) use addresses like sales@company.com instead of your full address to help battle the problem. c) There are also programs that encode your email, like jsGuard, which encodes your email address on web pages so that while spam spiders find it difficult or impossible to read your email address.

Number 2: Get spam blocking software. There are many programs out there for this. (go to www.cloudmark.com or www.mailwasher.net for example). You may also buy a professional version. Whatever you do, get the software. It will save you time. The software is not foolproof, but they really do help. You usually have to do some manual set up to block certain types of email.

Number 3: Use the multiple email address approach.

There are a lot of free email addresses to be had. If you must subscribe to newsletters, then have a “back-up” email address. It would be like giving your sell phone number to your best friends and the business number to everyone else.

Number 4: Attachments from people you don’t know are BAD, BAD, BAD.

A common problem with spam is that they have attachments and attachments can have viruses. Corporations often have filters that don’t let such things pass to you. Personal email is far more “open country” for spamers. General rule of thumb: if you do not know who is sending you something, DO NOT OPEN THE ATTACHMENT. Secondly, look for services that offer filtering. Firewall vendors offer this type of service as well.

Number 5: Email services now have “bulk-mail” baskets. If what you use currently does not support this, think about moving to a new vender. The concept is simple. If you know someone, they can send you emails. If you don’t know them, put them in the bulk email pile and then “choose” to allow them into your circle. Spam Blocking software has this concept as well, but having extra layers seems critical these days, so it is worth looking into.

Get Into the Zone

Malware. An odd sounding word created to lump all malicious software programs, including viruses, worms, trojans, spyware, adware, and other malevolent codes into one cause-your-computer-serious-hurt category.

In 2005, Computer Economics released a report on malware. The good news was that for the first time since 2002, the total worldwide financial losses from malware actually declined to a mere $14.2 billion. The bad news was that the nature of malware was changing from overt threats to more focused, covert attacks. This definitely is not great news for the average computer user just trying to keep up with the hundreds of malware programs that bombard us daily.

It’s not an easy task keeping malware out of your computer system. In order to accomplish this, you need a strong antivirus program. One such program that can deliver the goods is ZoneAlarm Internet Security Suite 6 from Zone Labs. Zone Labs is one of the most trusted brands in Internet Security for good reason. Their product, simply put, kicks serious malware gluteus maximus.

ZoneAlarm has received more review recommendations that any other Internet-security software suite because of its superb firewall and antivirus protection. It blocks pop-up ads, protects against identify theft and provides adequate spam filters that are flexible. It even beats the market leader, Norton Internet Security, which is often criticized for excessive system drag.

Its newest version includes these additional features:

· Triple Defense Firewall to prevent spyware from sending your information across the Internet. It also makes your computer invisible to anyone on the Net.

· Smart Defense Advisor which can automatically adjust your security settings for maximum protection against the latest virus and spyware outbreaks.

· Advanced Identify and Privacy Protection to prevent your personal data from leaving your computer without your approval.

The bonus for the average user who cringes at the idea of setting-up one of these systems is that the interface is easier to understand and use in comparison to most if its competitors. If you choose to venture beyond the out-of-the-box default settings, and install a more elaborate filtering, know that this will require some additional time to set up on your part.

Overall, ZoneAlarm Internet Security Suite is a user-friendly, comprehensive security solution that will have your computer safe from Internet hazards and cyber criminals within minutes of installation.

Heck are Botnets?

"A botnet is comparable to compulsory military service for windows boxes" - Stromberg (http://project.honeynet.org/papers/bots/)

Botnets are networks of computers that hackers have infected and grouped together under their control to propagate viruses, send illegal spam, and carry out attacks that cause web sites to crash.

What makes botnets exceedingly bad is the difficulty in tracing them back to their creators as well as the ever-increasing use of them in extortion schemes. How are they used in extortion schemes? Imagine someone sending you messages to either pay up or see your web site crash. This scenario is starting to replay itself over and over again.

Botnets can consist of thousands of compromised machines. With such a large network, botnets can use Distributed denial-of-service (DDoS) as a method to cause mayhem and chaos. For example a small botnet with only 500 bots can bring corporate web sites to there knees by using the combined bandwidth of all the computers to overwhelm corporate systems and thereby cause the web site to appear offline.

Jeremy Kirk, IDG News Service on January 19, 2006, quotes Kevin Hogan, senior manager for Symantec Security Response, in his article "Botnets shrinking in size, harder to trace", Hogan says "extortion schemes have emerged backed by the muscle of botnets, and hackers are also renting the use of armadas of computers for illegal purposes through advertisements on the Web."

One well-known technique to combat botnets is a honeypot. Honeypots help discover how attackers infiltrate systems. A Honeypot is essentially a set of resources that one intends to be compromised in order to study how the hackers break the system. Unpatched Windows 2000 or XP machines make great honeypots given the ease with which one can take over such systems.

A great site to read up on this topic more is The Honeynet Project (http://project.honeynet.org) which describes its own site's objective as "To learn the tools, tactics and motives involved in computer and network attacks, and share the lessons learned."

Trojan

The Trojan can be tricky. Who hasn’t been online and had an advertisement pop up claiming to be able to rid your computer of some nasty virus? Or, even more frightening, you receive an email that claims to be alerting you to a new virus that can threaten your computer. The sender promises to quickly eradicate, or protect, your computer from viruses if you simply download their “free”, attached software into your computer. You may be skeptical but the software looks legitimate and the company sounds reputable. You proceed to take them up on their offer and download the software. In doing so, you have just potentially exposed yourself to a massive headache and your computer to a laundry list of ailments.

When a Trojan is activated, numerous things can happen. Some Trojans are more annoying than malicious. Some of the less annoying Trojans may choose to change your desktop settings or add silly desktop icons. The more serious Trojans can erase or overwrite data on your computer, corrupt files, spread other malware such as viruses, spy on the user of a computer and secretly report data like browsing habits to other people, log keystrokes to steal information such as passwords and credit card numbers, phish for bank account details (which can be used for criminal activities), and even install a backdoor into your computer system so that they can come and go as they please.

To increase your odds of not encountering a Trojan, follow these guidelines.

1. Remain diligent
Trojans can infect your computer through rogue websites, instant messaging, and emails with attachments. Do not download anything into your computer unless you are 100 percent sure of its sender or source.

2. Ensure that your operating system is always up-to-date. If you are running a Microsoft Windows operating system, this is essential.

3. Install reliable anti-virus software. It is also important that you download any updates frequently to catch all new Trojan Horses, viruses, and worms. Be sure that the anti-virus program that you choose can also scan e-mails and files downloaded through the internet.

4. Consider installing a firewall. A firewall is a system that prevents unauthorized use and access to your computer. A firewall is not going to eliminate your computer virus problems, but when used in conjunction with regular operating system updates and reliable anti-virus software, it can provide additional security and protection for your computer.

Nothing can guarantee the security of your computer 100 percent. However, you can continue to improve your computer's security and decrease the possibility of infection by consistently following these guidelines.

Encryption

About Encryption and Making Your System Secure

What does encryption do for me?

Encryption and cryptographic software has been used in many different ways to make systems more secure. This article discusses only a few ways that such software can make your system more secure, including:

1) Encrypting your email

2) Encrypting your files

To programs are mentioned that will help encrypt information. There are many more programs out there that will help, but these programs are good and a good place to start as any. They have the added benefit of both being free with source code available.

Will encryption stop people from accessing my information?

Encryption simply makes it harder for people to gain access to important information, like passwords or sensitive information in a file. The first thing you should know about encryption is that the algorithm that is used to encrypt can be simple or more complex and that affects how securely what you have encrypted is protected. Encryption systems have been broken when the method of encryption is understood by hackers and is easy to break.

Why bother to encrypt my email?

It should be noted that email is far less secure than paper mail for two very good reasons: first, electronic data can be accessed easily over an Internet and secondly, electronic data is really simple to copy. There is a very good chance that someone has snooped around in your email despite your best intentions to stop it.

How do I go about encrypting my email?

There are many programs out there that can help you encrypt your email. A very popular one is PGP (Pretty Good Privacy) or its Gnu offshoot GPG.

PGP (http://www.pgpi.org/) self-describes itself this way: This "is a program that gives your electronic mail something that it otherwise doesn't have: Privacy. It does this by encrypting your mail so that nobody but the intended person can read it. When encrypted, the message looks like a meaningless jumble of random characters. PGP has proven itself quite capable of resisting even the most sophisticated forms of analysis aimed at reading the encrypted text."

Why bother to encrypt my files?

The answer to this boils down to what you store on your computer. If you have financial data with important information like social security numbers, email addresses, account numbers and passwords, then you open yourself up to losing very valuable information. Most corporate Internet security employees will attest to the widespread theft of very valuable information. As long as you are connected to the Internet you are vulnerable.

How do I go about encrypting my files?

AxCrypt File Encryption Software (http://axcrypt.sourceforge.net/) Self-described as "Free Personal Privacy and Security for Windows 98/ME/NT/2K/XP with AES-128 File Encryption, Compression and transparent Decrypt and Open in the original application."

Soldering

A soldering kit does not have to be very expensive. The tools shown in the above photo are what I could to any soldering job with. You do not need them all and we will discuss the important ones below.

Soldering Iron

For capacitor removal on motherboards it is important to have a reasonably powerful soldering iron. A 40w is the minimum that you would require. Actually a 60w is preferred and an 80w is what many of the professionals use. It must be a grounded soldering iron.


Soldering Stand with Sponge


Soldering Irons are available in corded models or as Soldering Stations which are Analog or Digital. For desoldering capacitors from motherboards it is not absolutely necessary to get a soldering station and definitely not necessary to get a digital one because you will be desoldering capacitors at the maximum temperature (450oC or more).

It is important to get a stand for safety reasons though and most important to have a wet sponge on the stand so that you can clean the iron of solder periodically in order to do the best soldering job.

Soldering Iron Tips

You might think that a very thin tip is required for removing capacitors from motherboards. This is incorrect. A thin tip will not get hot enough and transfer enough heat to the board. The best tip to get is a chisel tip. Around 2mm is the best size.

Solder

Solder is a matter of personal preference. Standard 60/40 solder is fine but preferably around 0.8mm diameter.

Clippers

Large clippers are no use for cutting the capacitor leads. You need small lead clippers.

Solder Sucker

A pneumatic solder sucker is a type of desoldering tool. It is useful for removing excess solder or desoldering damaged connectors. It is not recommended to use it to clear the thru-holes in the board because the pressure is excessive and there is a chance that when it recoils it may hit the board and damage a trace. If you insist on using it to clear thru-holes of solder then use it half-cocked.

Antistatic Wrist Strap

It is not necessary to buy an antistatic soldering station as you can just use an antistatic wrist strap. It is important to have a grounded soldering iron though.

Flux Remover Spray

If you are not using no-clean flux solder then you must clean the flux from the board. This is necessary because some kind of fluxes are slightly corrosive. It also is important for cosmetic reasons if you are fixing someone elses board, when the flux is cleaned the job looks more professional. You can use a flux remover spray for this. I use Flux Off from Cramolin. Alternatively you can use isoprpyl alcohol and a toothbrush.

Canned Air

When you have finished the soldering job there may be some debris on the board like bits of clipped leads or solder flakes. It is a good idea to use some canned air to make sure they dont remain and cause a short.

Bipolar Junction Transistors

The bipolar junction transistor (BJT) was the first solid-state amplifier element and started the solid-state electronics revolution. Bardeen, Brattain and Shockley, while at Bell Laboratories, invented it in 1948 as part of a post-war effort to replace vacuum tubes with solid-state devices. Solid-state rectifiers were already in use at the time and were preferred over vacuum diodes because of their smaller size, lower weight and higher reliability. A solid-state replacement for a vacuum triode was expected to yield similar advantages. The work at Bell Laboratories was highly successful and culminated in Bardeen, Brattain and Shockley receiving the Nobel Prize in 1956.

Their work led them first to the point-contact transistor and then to the bipolar junction transistor. They used germanium as the semiconductor of choice because it was possible to obtain high purity material. The extraordinarily large diffusion length of minority carriers in germanium provided functional structures despite the large dimensions of the early devices.

Since then, the technology has progressed rapidly. The development of a planar process yielded the first circuits on a chip and for a decade, bipolar transistor operational amplifiers, like the 741, and digital TTL circuits were for a long time the workhorses of any circuit designer.

The spectacular rise of the MOSFET market share during the last decade has completely removed the bipolar transistor from center stage. Almost all logic circuits, microprocessor and memory chips contain exclusively MOSFETs.

Nevertheless, bipolar transistors remain important devices for ultra-high-speed discrete logic circuits such as emitter coupled logic (ECL), power-switching applications and in microwave power amplifiers. Heterojunction bipolar transistors (HBTs) have emerged as the device of choice for cell phone amplifiers and other demanding applications.

In this chapter we first present the structure of the bipolar transistor and show how a three-layer structure with alternating n-type and p-type regions can provide current and voltage amplification. We then present the ideal transistor model and derive an expression for the current gain in the forward active mode of operation. Next, we discuss the non-ideal effects, the modulation of the base width and recombination in the depletion region of the base-emitter junction. A discussion of transit time effects, BJT circuit models, HBTs, BJT technology and bipolar power devices completes this chapter.

MOS Field-Effect-Transistors

The n-type Metal-Oxide-Semiconductor Field-Effect-Transistor (nMOSFET) consists of a source and a drain, two highly conducting n-type semiconductor regions, which are isolated from the p-type substrate by reversed-biased p-n diodes. A metal or poly-crystalline gate covers the region between source and drain. The gate is separated from the semiconductor by the gate oxide. The basic structure of an n-type MOSFET and the corresponding circuit symbol are shown in FigureFigure . 1.0.0.1 Cross-section and circuit symbol of an n-type Metal-Oxide-Semiconductor-Field-Effect-Transistor (MOSFET).

As can be seen on the figure the source and drain regions are identical. It is the applied voltages, which determine which n-type region provides the electrons and becomes the source, while the other n-type region receives the electrons and becomes the drain. The voltages applied to the drain and gate electrode as well as to the substrate, by means of a back contact, are referred to the source potential, as also indicated Figure 1.0.0.1


A conceptually similar structure was proposed and patented independently by Lilienfeld and Heil in 1930, but the MOSFET was not successfully demonstrated until 1960. The main technological problem was the control and reduction of the surface states at the oxide-semiconductor interface.

Initially, it was only possible to deplete an existing n-type channel by applying a negative voltage to the gate. Such devices have a conducting channel between source and drain even when no gate voltage is applied. They are called "depletion-mode" devices.

A reduction of the surface states enabled the fabrication of devices, which do not have a conducting channel unless a positive voltage is applied. Such devices are referred to as "enhancement-mode" devices. The electrons at the oxide-semiconductor interface are concentrated in a thin (~10 nm thick) "inversion" layer. By now, most MOSFETs are "enhancement-mode" devices.

While a minimum requirement for amplification of electrical signals is power gain, one finds that a device with both voltage and current gain is a highly desirable circuit element. The MOSFET provides current and voltage gain yielding an output current into an external load, which exceeds the input current, and an output voltage across that external load which exceeds the input voltage.

The current gain capability of a Field-Effect-Transistor (FET) is easily explained by the fact that no gate current is required to maintain the inversion layer and the resulting current between drain and source. The device has therefore an infinite current gain in dc. The current gain is inversely proportional to the signal frequency, reaching unity current gain at the transit frequency.

The voltage gain of the MOSFET is caused by the current saturation at higher drain-source voltages, so that a small drain-current variation can cause a large drain voltage variation.