1 Lawndale High School AWIM Program Transistor Theory & Experiment Lecture 5

2 A Little History The Transistor was conceived in the late 1940’s at Bell Laboratories by three scientists; Bardeen, Brattain and Shockley. They were originally working on a Field Effect device to replace the vacuum tube, but unable to make it work, followed another line of research and discovered the transistor. The first transistor used n-type Germanium material and mechanical contacts as shown below in figure 1. This was named a “point-contact transistor. Transistors today use deposited layers of doped materials, and are called ‘Bipolar’ transistors, as shown in figure 2. Figure 1 Figure 2

First Point Contact Transistor

Transistor Applications Transistors as Amplifiers are used in Stereos, TVs, Audio Equipment Transistors as switches are used in Cameras, Cell Phones, Tablet computers, Desktop and Laptop computers.

Transistor Packages

Transistor Characteristics -There are two types of Bipolar transistors; NPN and PNP. - Transistors can be used as Amplifiers or switches, depending on how they are Biased. - Transistors are Current Amplifying devices. - The amplifying factor (gain) of the transistor (Hfe) is the Collector current divided by the base current.

Transistor Symbols

How a Transistor works NPN Transistor at rest showing Junctions and Depletion zones

How a Transistor works -This figure shows a NPN transistor biased for proper operation. Here a small base current Ib controls a larger collector current Ic. -- Note the Base-emitter junction is Forward-Biased, and the Base-Collector junction is Reverse-Biased. -- The Emitter-Base Depletion layer is small, while the Base- Collector Depletion layer is large. -- Current Ie = Ib + Ic Electrons flow from Emitter to Base, called the Base current. This current is small due to the small Base area. Most Electrons will be sucked thru the Base-Collector Depletion layer by the large Vcc field and fill Holes in the collector, which will be replaced from the Vcc battery. This Hole movement causes a Electron flow in the opposite direction, ie Emitter to Collector.

How a Transistor works Current Flows in a active NPN Transistor

How a Transistor works Transistor Characteristic curves

Transistor Amplifier Amplifier

Transistor Amplifier

How a Transistor works -This figure shows a NPN transistor biased for proper operation. Here a small base current Ib controls a larger collector current Ic. -- Note the Base-emitter junction is Forward-Biased, and the Base-Collector junction is Reverse-Biased. Note: This Connection allows us to bias the transistor for operation from a single power source.

Experiment – Transistor Oscillator

17 Transistor Oscillator Experiment The Transistor Oscillator experiment demonstrates the switching abilities of the transistor by cross-coupling to form a oscillator circuit. The two transistor switches are alternately turned On and OFF one at a time. The time between the two actions is caused by the charging and discharging of a capacitor in each of the two switch circuits. The switch cycling frequency is dependent on the size of capacitors and the series resistors. The transistor oscillator circuit is an example of the many basic circuits that are behind most of our modern electronics.

- + Oscillator Circuit Demonstrator Uses Two Capacitors to Alternately Trigger Transistor Switches ON or OFF + _ + - R1 R2 R3R4 C1C2 Q1 6VDC R1, R2= 390 Ω R3, R4 = 11k Ω C1, C2 = 50uf LED1 = Red LED LED2 = Grn LED Q1, Q2 = 2N3904 C1 and C2 are Capacitors LED 1 LED 2 Capacitors in this circuit are used to turn on each transistor as the charge voltage reaches the threshold level (0.6v for Si). When one transistor turns on, the capacitor is discharged, turning the alternate transistor off. The cycle then repeats. 18 NPN Q2 NPN

Transistor Lead Identification Our Experiment Uses this one

Oscillator Construction 1. TOP Long Jumper from Left RED Bus to Right RED Bus 2. Bottom Long Jumper from Left Blue Bus to Right Blue Bus 3. Short Jumper from C12 to Left Blue Bus 4. Short Jumper from C21 to Left Blue Bus 5. R3 (11k ohms) from B11 to Left RED Bus 6. R4 (11k ohms) from B20 to Left RED Bus 7. R1 (390 ohms) from E10 to i10 8. R2 (390 ohms) from E19 to i19

9. RED LED, Long Lead (+) to Right RED Bus, Short (-) Lead to F Green LED, Long Lead (+) to Right RED Bus, Short (-) Lead to F Transistor Q1, Flat side toward RED LED, bottom lead at D12, Center lead to D11, Top lead to D Transistor Q2, Flat side toward Green LED, Bottom lead at D21, Center lead to D20, Top lead to D Capacitor C1, (+) end to C19, (-) end to C Capacitor C2, (+) end to B10, (-) end to C Connect Battery +6v to top Long Jumper (RED Bus) 16. Connect Battery -6v to Bottom Long Jumper (BLUE Bus) Oscillator should start Flashing LEDs at aprox 1 second rate! Oscillator Construction

22 Test Instructions & Data Connect all parts as listed on previous pages Lights should be flashing. If not, check connections and polarities of LED and capacitors. Also check that leads of Capacitors are not shorting to each other or to Transistor Leads. Connect the Voltmeter (-) lead to POWER Supply (-) Probe the junction of R1,C1 to see Q1 Collector voltage. Probe the C1 (-) to see Q2 base voltage. Does the voltage change?, How? What happens when R3 is decreased by adding another 11k Resistor in parallel (A11 to RED Bus). Did the cycling frequency change? How? Name ___________________Date __________

23 What have we learned from this experiment? We have learned that an electronic circuit is made up of electronic parts such as diodes, resistors, capacitors and transistors. We have learned about semiconductors and transistors. ….how a transistor can be used to implement a on/off (switch) function. ….how an oscillator works. ….how to change an oscillator cycling rate.

24 New Terminology Solid State Electronics Deposited layers Integrated Circuits (IC) or Silicon Chip Miniaturization Transistor BASE, EMMITER and COLLECTOR Transistor BASE Voltage – The voltage between the Base and Emitter Transistor Gain (Hfe) – The Collector current / Base current Transistor: A two Junction device that can be used as a solid state switch or an amplifier Bipolar Transistor – A semiconductor consisting of two polar materials joined to form two junctions

25 Semiconductors form the heart of modern electronics. Metals tend to be good conductors of electricity because they usually have "free electrons" that can move easily between atoms, and electricity involves the flow of electrons. While silicon crystals look metallic, they are not, in fact, metals. All of the outer electrons in a silicon crystal are involved in perfect covalent bonds, so they can't move around. A pure silicon crystal is nearly an insulator -- very little electricity will flow through it. A modern semiconductor diode is made of a crystal like silicon that has impurities added to it to create a region on one side that contains negative charge carriers (electrons), called n-type semiconductor, and a region on the other side that contains positive charge carriers (holes), called p-type semiconductor. The diode's terminals are attached to each of these regions. The boundary within the crystal between these two regions, called a PN junction, is where the action of the diode takes place. The crystal conducts a current of electrons in a direction from the N- type side (called the cathode) to the P-type side (called the anode), but not in the opposite direction; that is, a conventional current flows from anode to cathode (opposite to the electron flow, since electrons have negative charge). Appendix - A