Transistors According to Dictionary.com a transistor is: living with the lab Transistors According to Dictionary.com a transistor is: “a semiconductor device that …switches  the flow of current between two terminals by varying the current or voltage between one of the terminals and a third…” For us, a transistor is a switch that allows us to use a small current to control a larger current. http://learn.mikroe.com/ebooks/componentsofelectronicdevices/chapter/introduction-to-transistors/ http://cen.acs.org/articles/92/web/2014/12/Record-Breaking-Nanowire-Transistors.html

Semiconductors The most common materials for transistors are semiconductors. What is a semiconductor? Material that can be either a conductor or an insulator, depending on the conditions. What is the most common semiconductor? Silicon (Not Silicone) In which periodic table column is Silicon? Column 14 How many valence electrons does it have? 4 Valence Electrons. http://sciencenotes.org/printable-periodic-table/

Silicon Silicon has four valence electrons in its monocrystalline state. e Si When surrounded by other Silicon, each atom has eight valence electrons. Si e In its base state, is Si an insulator or a conductor? Si Insulator How do we make it conduct? e We break up the valence shell How? We add other atoms through a process known as doping.

Doping Doping is the intentional addition of impurities (non-SI atoms). Column 14 elements have four valence electrons. Which column of elements will have five? Column 15 Si Which column will have three? Column 13 e The most common dopants are: Boron, B (3 Valence electrons) Phosphorus, P (5 Valence electrons) http://sciencenotes.org/printable-periodic-table/

n-type doping n-type doping: adding atoms to make structure more negative. e Si Column 15 elements (like P) have 5 valence electrons. What happens when we surround P with Si? P e Si An extra electron is added to an already full valence band. P e This extra electron is available for conduction. e Extra electrons make this Si block negatively (n-type) doped.

p-type doping B e p-type doping: adding atoms to make structure more positive. e Si h Column 13 elements (like B) have 3 valence electrons. What happens when we surround B with Si? B e The structure has one electron fewer than what is required for a full valence band. This vacancy (hole) also allows conduction to occur These holes make this Si block positively (p-type) doped.

Very heavily doped p-type Note on doping Silicon is more than doped or not doped. It is doped in concentrations. Silicon doping is shown by “+” signs. n, n+, and n++ are all increasing n-type doping. Each + represents orders of magnitude change p p+ p++ Lightly doped p-type Heavily doped p-type Very heavily doped p-type

p-n junction B e We can dope a single crystal so that one part is p-type and the other part n-type. e e What happens when we place a voltage across it? h Voltage will push the charges towards each other. p At the pn junction, the electrons cross the border to recombine with holes. This allows current to flow. p n p n + - n + -

B p-n junction What happens if we apply a voltage the other way? Voltage will again push the charges. This time away from the junction. e Electrons cannot recombine with holes. This makes an open circuit. h Open circuits allow no current to flow. p What electrical component do we have that works like this? This is the working model for our diodes. p n p n + - n + -

Recombination B e What happens at the pn junction when electrons and holes meet? e Voltage pushes the electrons across the junction. They recombine with holes. Without this voltage pushing the electrons, the junction stays at thermal equilibrium e Remember that electrons scatter due to thermal effects. h The average distance an electron travels before it combines with a hole is the recombination distance. p The recombination distance is on the order of tens of nanometers. p n n

Transistors B e Sandwiching n-type or p-type semiconductors allows us to make another electronic component. e e These are known as transistors. They come in two types and are named by their dopings: h p n p npn p n pnp n

npn Transistors B e What happens if we attach a voltage source to an npn transistor, as shown? e e Electrons and holes recombine at the left junction. No recombination occurs at the right junction. No current can flow. h What if we attach the voltage source the other way? We have the same problem, reversed. Current remains blocked. p n + - n p + -

How can we make current flow? B e We start by changing the doping. We dope the emitter with orders of magnitude more electrons. e e The higher concentration of electrons wants to move to an area with low electron concentration. h Thermal equilibrium and the base region keep this from occurring. n p Base Collector Emitter Base Collector Emitter n++ p n

How can we make current flow? B e Next we shorten the width of the Base region. e If we want electrons to cross the Base without recombining with holes, how short should it be? e h Shorter than the recombination distance of an electron-hole pair. Tens of nanometers Emitter Base Collector Base Emitter n++ p Collector n n++ p n

How can we make current flow? B e Finally, we apply a second voltage source between the Base and the Emitter. e e This attracts Emitter electrons to the Base. h As the Base is shorter than an electron-hole recombination distance, the electrons continue on into the Collector. Emitter Collector + - Base Emitter n++ p Collector n Base n++ p n + - + -

Transistors as Switches Current is flowing and the transistor is on. VCE can push a large current. Without the second voltage source VBE no current will be allowed to flow VBE can be used as a switch to turn VCE on and off + - Base Emitter n++ p Collector n VBE VCE

Transistors and the Arduino Transistors can be used to have a small current toggle a larger one. Your Arduino can output ~ 40 mA of current. Using transistors with your Arduino allows you to toggle a much larger amperage (such as your 1.5A power supplies). + - Base Emitter n++ p Collector n VBE VCE