1. Introduction to Transistors

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3. 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

4. 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.

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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 + -

13. 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

14. 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