Introduction to Transistors

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Implementation of a Transistor Circuit A transistor is a semiconductor device that can be used as an “electrical switch” or as an amplifier Implement the transistor circuit on your Arduino C B E Compare the pin locations of C, B and E to the circuit diagram WARNING: The transistor will get VERY hot if connected incorrectly

Implementation of a Transistor Circuit Write a simple program to make the LED blink on and off void setup() { pinMode(4,OUTPUT); }   void loop() { digitalWrite(4, HIGH); delay(1000); digitalWrite(4, LOW); What is the transistor doing? We will use transistors to activate a relays for switching our fishtank heater on and off and opening and closing solenoid valves.

Transistors Transistors are in most electronic devices A transistor is a semiconductor device that can be used as an “electrical switch” or as an amplifier Transistors are in most electronic devices Advantages of using a transistor over a mechanical switch: Can be very small Works very fast Can be controlled electrically Inexpensive Fishtank applications: Turning heater on and off Opening and closing solenoid valves This chip in your multimeter contains multiple transistors The A8 chip in an iPhone contains 2 billion transistors

Transistors are made from Semiconductors Materials that can either be a conductor or an insulator depending on the conditions Silicon is the most common semiconductor material Given that silicon is in column 14 of the periodic table, how many valence electrons does Silicon have? When surrounded by other silicon atoms, each silicon has 8 valence electrons In this state, is Si an insulator or a conductor? How can we make it conduct? Change the conditions Break up the valence shell Add other atoms through a process called doping e Si 4 Si e Insulator

Doping is the intentional addition of impurities What scenarios would help encourage flow of electrons at the atomic level? An extra electron A missing electron Knowing that a complete valence shell holds 8 electrons and Si has 4 valence electrons, then How many electrons should the doping atom have to provide an extra electron? How many electrons should the doping atom have to provide a hole (or missing electron) electron? Where on the periodic table will we find these elements? Most common dopants are: Boron, B (3 valence electrons) Phosphorous, P (5 valence electrons) 5 3 Column 13 for 3 electrons and column 15 for 5 electrons

N-type doping using an element that has 5 valence electrons Makes the structure more negative by gaining an extra electron Called n-type doping (n for negative) Uses elements from column 15 like Phosphorous, P When P is surrounded by Si, an extra electron is added to an already full valence shell Extra electron is available for conduction e e P e e e

P-type doping using an element that has 3 valence electrons Makes the structure more positive by producing a hole in the valence shell Called p-type doping (p for positive) Uses elements from column 13 like Boron, B Boron has 3 valence electrons When B is surrounded by Si, the structure has one fewer electrons than what is required for a full valence shell This vacancy (hole) allows conduction to occur h B e

P-N Junction We can dope a single crystal so that one part is p-type and the other part is n-type What happens when we place a voltage across it? Voltage will push the charges towards each other At the p-n junction, the electrons will cross the border to recombine with the holes This allows current to flow What happens if we apply voltage the other way? Voltage will push the charges, but this time away from the junction Electrons cannot recombine with the holes resulting in an open circuit and no current flows What electrical component have you used that works like this? p n + - + - p n LED

Transistors Sandwiching n-type or p-type semiconductors allow us to make transistors There are two types of transistors which are named based on their doping: What happens if we attach a voltage source to an npn transistor? Electrons and holes recombine at the left junction and no current can flow What if we reverse the voltage source? We would have the same issue, but this time at the right side pn junction n p p n p n + - npn pnp

How can we make current flow? We can set up the transistor material in a strategic way that will help induce electron flow Add more electrons to the emitter region of the transistor (make it more positive) The higher concentration of electrons will want to move to an area with lower concentration Shorten the base region This will allow the electrons to flow from the emitter region to the collector region without combining with the holes Apply a second voltage source between the base and emitter. Attracts emitter electrons to the base Emitter Base Emitter n++ p Collector n Emitter Base Base Collector Collector n p n++ p n n++ p n + -

Transistors as switches 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 Think about how that relates the transistor/LED circuit you set up at the beginning of class You had 2 voltage sources: 5V power source and voltage supplied by digital pin 4 Which of the two would be VCE and VBE? VBE is the voltage from the digital pin VCE is the 5V power source You were able to switch the LED on by supplying the second voltage (VBE) from the digital pin When you turned the pin LOW you removed the second voltage which turned switched the LED off Transistors can be used to have a small current toggle a larger one Your Arduino can output ~40mA 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

Transistors with Arduino Application Implement the transistor/LED circuit and the thermistor circuit Sketch 1: write a sketch that gives you room temperature as an analog value Room temperature will be your setpoint Sketch 2: write a sketch that uses the transistor to: Blink the LED on for 1 second and off for 1 second when the temperature is above the setpoint Blink the LED on for 300 milliseconds and off for 300 milliseconds when the temperature is below the setpoint Print to your serial monitor when temperature is above and below the setpoint 5V analog input 5 thermistor 10kW C B E Be sure to plug C, B, and E accordingly