Introduction let us gets started before we begin with basic electronicsit is a good idea to develop historical perspective of electronics see how things were evolvedand so on and we should mention before starting thatthe images that we are going to see have been taken from the internet all right so let usget started electronics really started with vacuum tubes because in those days they wereknow semiconductor devices and the simplest vacuum tube the diode shown here was inventedby john fleming in nineteen nor four so in this device there is a cathode which is filamentand there is a plate the plate is biased with a positive potential and the cathode is heatedwith this filament supply so that it emits electrons these electrons get attracted tothe plate and that is how the current flows in the external circuit and if this voltageis made negative then there is virtually no current so this device allows current flowin one direction but blogs current flow in the other direction so that is how these diode works in nineteennor seven deforest invented the triode by inserting a third electrode between cathodeand anode here is the schematic diagram and ah this is the third electrode concrete theway this device works is that the cathode emits electrons as i did in the diode andthe plate is positively biased so the electrons emitted by the cathode get attracted to theplate and that is how we have a plate current now the grid electrode is biased with a negativevoltage and therefore it repels these electrons and therefore that causes a reduction in theplate current here is the plot of the plate current in milliamps verses plate potentialin volts and observe how high these voltages are three hundred volts let us take a fixedvoltage lets say two hundred volts for v grid is equal to zero we have that much currentfor v grid equal to minus two the current is smaller for minus four still smaller andso on so this figure clearly shows the control of ah v grid on the plate current here is what the real structure looks likewe have a glass tube under vacuum this one the outer cylinder is the anode after thatthe red structure is the control grid and inside that is the cathode so cathode emitselectrons which make it to the anode and because of the negative potential on the control gridsome of the electrons get repelled thats how the control grid controls the current andthe terminals all of these are brought out like that at the bottom and here is a real photograph showing varioustypes of tubes there are tubes with more than three electrodes and they get quite complicatedbut roughly we can see that there is a glass tube and we also notice that they are largethese dimension could be something like five centimeters for example all right so thissize is one problem with vacuum tubes they tend to be very large compare to the modernsin a semiconductor devises for sure the other problem can be seen in this picture and thesevacuum tubes when they conduct they glow like a light bulb.
we can see that in this pictureand thats the other problem with vacuum tubes they consume way too much power here is acircuit diagram of an audio amplifier made with vacuum tubes these are the tubes herethats the speaker thats the transformer there and what we should tell you notice in thiscircuit diagram is this part here three hundred volts thats a very high voltage and in modernelectronics we simply dont have these kinds of large voltages you probably have ten voltsor fifteen volts but certainly not hundreds of volts here are some pictures of another first computerscalled the eniac computer built with vacuum tubes we are talking about nineteen fortysix so there were no semiconductor devices at that time the vacuum tubes are not seenin this picture but they may be behind these panels here this entire room is one singlecomputer and this is another view of the same computer and these people we see here theyare probably computer engineers or computer programmers of those times let us now lookat some facts and figures about the eniac computer and there is a lot of informationon this topic on the internet and you should really look it up here we will just look atsome other points it was heralded as joint brain by the press and joint brain it wascompare to the electro mechanical computers of those days this computer was a thousandtimes faster it had some seventeen thousand vacuum tubes some six thousand manual switchesand approximately five million and soldered joints this of course was bad news becausewith each hand soldered joint the reliability of the entire computer goes down it consumedone fifty kilo watts thats a huge amount of power input was possible from an ibm cardreader and each card was one statement of a program its clock frequency was one hundredkilohertz and for comparison our modern computers have a clock frequency of two gigahertz orthree gigahertz so you can find out how many times faster todays computers are several tubes burned out almost every dayand also these soldering joints keep problems leaving the computer non functional abouthalf the time so the computer could not be used continuously because of this reason andwhat did the computer do it could be programmed to perform complex sequences of operationsincluding loops branches and subroutines after the program was figured out on paper the processof getting the program into eniac by manipulating its switches and cables could take days todayonce we have a program we can just key it in with our keyboard but in those days thatwas not possible the task of taking a problem and mapping it on to the machine was complexand usually took weeks which is a very long time this point is very interesting the programmersdebugged problems by crawling inside the massive structure to find bad joints and bad tubesthere was really know the choice so they have to do that the first test problem consisted of computationsfor the hydrogen bomb all right so here or some of the facts and figures about this computerbut you should really look up the internet and there is lot of other information overthere another interesting picture about the eniaccomputer these ladies are probably computer operators she is carrying a program listingin her hand she is also carrying some cables to give this other person now this personwho is sitting is actually changing the position of the switches and making connections withcables that she receives from this standing lady all right so let us now talk about thefirst transistor and why was it required in the first place the vacuum tube was a bulkyand fragile device which consumed a significant power so the us government particularly thedepartment of defense was looking for a solid state alternative for the vacuum tube andthis job was given to doctor shocklay at bell laboratories this person sitting here in nineteenforty seven shockley bardeen and brattain this is shockley this is bardeen that is brattainat bell laboratories invented the first transistor this is the picture of the first transistorand this shows its schematic diagram so this triangle that we see over here is actuallya piece of plastic and on both sides we have a gold file one is the emitter and the otherone is the collector and this whole structure is pressed on this semiconductor in thosedays it was germanium and this way a point contact was made between this emitter andthe germanium and also between this collector and this germanium this semiconductor was called the base andthat is how we have this emitter base collector transistor structure the modern transistorlooks very different here is a packaged device and inside there is a single semiconductorpiece which has all three emitter base and collector so the emitter base and base collectorjunctions are all in the same semiconductor .
And that is called monolithic let us now talk about semiconductor technologyas see how it is evolve a time the bipolar transistor which was the first transistorto be invented vacuum nineteen forty seven continues to be an important device both asdiscrete device and as part of integrated circuits that is ics so such (( )) howeverin digital circuits such as processors and memory the mos that is metal oxide semiconductorfield effect transistor has surpassed the bipolar transistor by several miles actuallybecause of the high integration density and low power consumption that the mos technologyoffers it is interesting to note that the idea behind the mos transistors actually itdispatch to nineteen thirty that is before the bipolar transistor was invented in nineteenforty seven as we saw and in nineteen thirty lilienfeld filed a patent for field effecttransistor which forms the basis for mos transistor but the mos technology took several yearsto mature and therefore ics with bipolar technology came first to the market followed soon byics in mos technology the first mos ic was in nineteen sixty four and it was a shiftresistor all right in nineteen fifty eight jack kilby at texas instrument demonstratedthe first integrated circuit consisting of a bipolar transistor and resistors and capacitorsfabricated on a single piece of germanium so this was a big break through and arounda same time robot noise of fair child was also perusing this idea of integrating certainthings on the same semiconductor piece and he also came up with some improvements especiallyrelated to metallization so this work was done more or less in parallel and the rest is history and we will see apart of that in the next few slides we will try to get a glimpse of semiconductor technologythese of course is a vast subject and people have courses one semester long courses onthis topic but we will only take a quick look to get an idea the starting point is a siliconvapor and its diameter is something like thirty centimeters or one foot which is pretty largeand it is relatively thin its thickness is less than one millimeter and after going througha series of processing steps using very sophisticated equipment this is what the vapor looks likethis is the top view it has got some pattern on it and each one of these rectangles forexample this one is one chip now the chip could be as simple as a single diode or asingle transistor or it could be as complex as a microprocessor with ten million transistorsor hundred million transistors the chips are separated from each other andeach chip is mounted in a package like this one this package is called the dip packagebut today we have very sophisticated packages with how much larger number of pins as wellnow this chip has got these little squares here which i called the metal pads and thoseare internally connected to the devices the transistors or whatever on the chip from thismetal pad we have a metal wire bonded to the pins of the ic which looks like this as theend user this package is what we see but you should remember that ah huge amount of efforttime and of course money goes into the design and fabrication of this chip all right thisis what the cross section of this vapor looks like after all the processing steps are overand be see several small reasons here we have n type duping here we have p type duping andso on and we will take a look at part of this how it is done and an important thing to noteis that this entire structure the so called active devices is confined only to a verysmall region on top of the vapor that is only ten microns may be in thickness and the restof the vapor is simply there to support this entire top layer here is the picture of the silicon vapor afterthe processing steps are over and we can see these rectangles here each one of those isgoing to be one chip and notice how shiny it is we can see the reflection of this personshand over here and that is because it requires to be polished to a very smooth finish sothat processing can be accomplished as we mentioned the vapor goes through a seriesof processing steps there is one important process called diffusion in which the vaporis heated to very high temperature such as nine hundred degrees or one thousand degreesand a gas is passed over the surface of the vapor to dope it either p type or n type sohere is a diffusion furnace they are this controls here to control the temperature aswell as the duration of that processing step and the vapors are start over here so thiswhole contraption comes out we start the vapors and push it in and here is a close up so theseare the vapors there be pushed into the furnace the furnace will be locked up for whatevernumber of minutes and that is how this diffusion process takes place .
Let us look at some morepictures of the fabrication process and notice that we dont see any semiconductor vaporsanywhere those are inside this equipment somewhere here is an operator who looks more like asurgeon in an operation theater and this kind of extreme cleanliness is really requiredin the fabrication process because any contamination is absolutely disasters for over devices thatwe are trying to fabricate for example our body goes on shading dead skin cells continuouslywithout our knowledge and if any of those cells lands up on the semiconductor vaporthere is going to kill several transistors and the chip is simply not going to work aswe wanted to another picture this person is taking thesevapors from one equipment to another in each equipment a different processing step takesplace an at the end of all of that we have the final product another picture here iswater silicon vapor looks like and you can see once again that it is very smooth we cansee the reflection of this persons face over here let us go through a very simple fabricationprocess namely fabrication of a p n junction diode here is the cross section of an n typevapor that means its a silicon vapor with n type duping such as phosphorous if firstgrow silicon dioxide on that shown there after that we apply photo resist shown with thispurple layer here now this photo resist is a material which is sensitive to light afterthat we place the mask on top of the photo resist shown here in black color then we exposethis whole thing to ultra violet light now there is a window in the mask over hereso the light goes through and this part gets exposed and this part does not get exposedbecause we have the mask blocking the light over there next though mask is removed andnow we have the photo resist some part of the photo resist has been exposed to uv lightand the rest has not been exposed to uv light all right now we develop the resist that meanswe dip that in a chemical the part that has been exposed to uv light gets dissolved andgoes away living behind the unexposed part of the photo resist that is followed by an etching step in whichthe silicon vapor is immersed in hydrofluoric acid hf now this part of the oxide which isexposed gets dissolved and this part which is protected by the photo resist (( )) soat the end of this step what we do is strip the resist that means we dip the vapor insome other chemical which removes this photo resist and that is followed by a p type diffusionstep that means the vapor is heated to a high temperature like we have seen some time agoin a diffusion furnace and the boron gas bh three passes over the vapor and because ofthis oxide nothing happens if these regions but where there is no oxide the boron atomsget inside and that makes it p type so now we have p type semiconductor here and ouroriginal sample was n type so we have a p n junction after this metallization is carriedon that means one contact is made to this region and another contact is made to thisregion so that is how we have a p n junction diode let us now talk about scaling of mos technologywhich has been happening over the years shrinking of the smallest definable dimension also calledthe feature size on the chip has enabled a huge number of transistors to be integratedon one chip as this size per transistor reduces what it means is we can pack how much largernumber of transistors in the same area and that is how this number of transistors perchip has been going up here have some figures in nineteen seventy the feature size was tenmicron in two thousand ten it was point zero three two micron so much smaller and hereis a plot of the feature size verses year and note that this is a logarithmic scaleso in the seventies we were talking about feature sizes of a few microns and in twothousand ten it is less than point one micron and as a result of the study reduction inthe feature size over the years the number of transistors per chip has been going upcorrespondingly and here is a plot this is the number of transistors per chip and thatis the year and note that this scale is also logarithmic this is thousand transistors perchip this is ten thousand this is hundred thousand one million ten million and so on in the nineteen seventies we were talkingabout a few thousand transistors per chip and now around two thousand ten year we aretalking about a much larger number something like hundred million transistors per chipso this increase is really very dramatic all right now gordon moore of intel predictedthis kind of trend in nineteen sixty five and his prediction was that the number oftransistors will double every two years and this prediction as turned out to be more orless accurate over several decades in fact and it has become known as moores law allright now what is a practical implication of all these developments in the semiconductorindustry in the nineteen seventies we were talking about probably a few hundred gatesper chip and now they have much increased functionality possible and we can talk aboutsystem on a chip which is much more complex than what we use to have in the seventies.
Let us consider ah vacuum tube computer builtwith one million tubes and we should emphasis that such a computer has not been built weonly want to know what it would look like we find that there are several difficultiesand let us see what those are first each vacuum tube would occupy an areaof five centimeters by five centimeters roughly and when you multiply that by one millionthat area of course turns out to be very large its like a football field second each vacuumtube consumes let say something like one watt to ten watts of power and therefore when wemultiply that by one million we are talking about a total power in the range of mega wattsvery large power now this power which is in the mega watts range very large power turnsinto heat and that has to be removed if you dont remove the heat then the computer temperaturewill keep raising and finally things will melt smoke will come out and so on which ofcourse we dont want the next issue is reliability these kind ofcomputer will have very poor reliability because it has a very large number of vacuum tubesand soldering joints and what it means is that this computer is going to require veryfrequent maintenance because every half an hour or one hour some tube or soldering jointis going to go bad and that will need to be fixed and finally even if it was actually builtthe speed would be much lower than a modern cpu because of parasitic capacitances andinductances of the cables that we are going to use for the connections all right so hereis what the computer would look like and we must emphasis once again that it has neverhappened nobody has really built such a computer it will be large and it will below to be housedin a building of this kind one second is going to consume a huge amount of power and thereforewill need to get a separate connection from the state electricity board coming like thatso that is our power supply third such a computer is going to required a massive heat sink andthe only practical way of removing such a large amount of heat is to immerse the wholecomputer in a water fall so that the water keeps taking the heat away on a continuesbases and finally we are going to require a maintenance department to be located onthe same premises because some tube or soldering joint is going to keep failing and we needsomebody to attempt to that so we require a twenty four by seven service for this computerand the best way to do that is to have somebody write on the location all right and now comparethat computer with the mobile phone that you carry in your pocket or your purse so that was the short revive of how electronicshas progressed so far and now it is time to get down to the basics and understand howthings work so see you in the next class