1. a) Describe the operation of semiconductor transistors
2. b) Verify the voltage versus current (V–I) characteristic of transistors
3. c) Select an appropriate transistor for a given application
4. d) Construct transistor circuits for given applications
5. e) Appreciate the importance of transistors in electronics
6. f) Identify: transistor operation, V–I characteristics, transistor selection, circuit construction, applications

1. What is a transistor?

A transistor is a small semiconductor device (usually 3 terminals) used to control current. It can act as:

• Ampifier — makes small signals larger.
• Switch — turns current on or off (used in digital circuits).

Two common types for school work:

• BJT (Bipolar Junction Transistor) — has three pins: Base (B), Collector (C), Emitter (E). Example: NPN (BC547, 2N2222) and PNP.
• FET (Field Effect Transistor) — e.g., MOSFET, used for switching higher currents (IRF series).

Basic BJT operation (NPN): a small base current (Ib) allows a larger collector current (Ic) to flow from collector to emitter. The transistor has three regions:

• Cutoff: base current ≈ 0 → transistor off.
• Active: transistor amplifies; Ic ≈ β × Ib (β = current gain, a property of the transistor).
• Saturation: both junctions forward biased → transistor fully on (used for switching).

2. Verify V–I characteristic (practical activity)

Objective

Measure collector current (Ic) versus collector–emitter voltage (Vce) for different base currents and plot the transistor curves.

Materials

• NPN transistor (BC547 or 2N2222)
• Breadboard and jumper wires
• Variable DC supply (0–12 V) or 9 V battery + potentiometer
• Resistors: 1 kΩ, 10 kΩ, 100 kΩ and a 10 kΩ pot
• Two multimeters (or one multimeter switched between current and voltage)
• Switch (optional), notebook and pencil

Procedure (simple school method)

1. Place the transistor on the breadboard and identify B, C, E from the datasheet.
2. Connect collector to the +V through a collector load resistor (Rc). Connect emitter to 0 V (ground).
3. Set base current (Ib) using a base resistor (Rb) from a small supply or from a voltage divider. Example: choose Ib ≈ 10 μA, 50 μA, 100 μA (three steps).
4. For each Ib value:
   • Vary Vce by changing the supply voltage (or use a potentiometer in series with Rc).
   • Measure Vce (voltmeter across collector and emitter) and Ic (ammeter in series with the collector load or by measuring voltage across Rc and using I = V/R).
   • Record pairs (Vce, Ic).
5. Plot graphs Ic (y-axis) against Vce (x-axis) for each Ib. You should see:
   • At small Vce, transistor saturates (Ic limited and Vce low).
   • In the middle region, Ic is roughly constant for a fixed Ib (active region).
   • With Ib = 0, Ic ≈ 0 (cutoff).

Safety

Use low voltages (≤12 V). Do not use mains. Turn off power when changing wiring. Check polarities to avoid damaging the transistor.

3. How to select the right transistor

When choosing a transistor, check the following parameters (from the datasheet):

• Type: NPN or PNP for BJTs; enhancement/depletion for FETs.
• Maximum collector–emitter voltage (Vce max) — must be higher than the supply voltage used.
• Maximum collector current (Ic max) — must exceed the expected current in the circuit.
• Power dissipation (Ptot) — transistor must be able to dissipate heat; use heatsink for power devices.
• Gain (hFE or β) — how much Ic increases per unit Ib; affects resistor choices.
• Switching speed / frequency — important for high-frequency signals.
• Package and availability — small-signal (TO-92) vs power packages (TO-220).

Common, easy-to-find examples in Kenya and classroom use:

• BC547 / 2N2222 — small signal NPN for learning, amplifiers and low current switches.
• BC557 — small-signal PNP.
• TIP122 — Darlington for higher current switching.
• IRFZ44N — MOSFET for power switching (requires suitable gate drive).

4. Example circuits (construct on breadboard)

A. Transistor as a switch — drive an LED

Components: NPN (BC547), LED, resistor Rled ≈ 680 Ω (for 9 V), base resistor ≈ 8.2 kΩ, 9 V battery.

Wiring summary:

• LED + resistor between +9 V and collector.
• Emitter to ground (0 V).
• Base via Rb to a switch or input voltage.

B. Simple common‑emitter amplifier (small signal)

Components: BC547, R1 (100k), R2 (22k), Rc (4.7k), Re (1k), coupling capacitors (10 μF), small signal source (audio player), 9 V supply.

Brief biasing idea:

• Voltage divider R1–R2 sets base voltage.
• Choose Re so that emitter sits at about 1/10 to 1/3 of Vcc (for stability).
• Rc converts collector current changes into an output voltage (gain ≈ Rc/Re for approximate small-signal estimate).

Build, apply small input, observe amplified output on a low-cost oscilloscope or earphones (through coupling capacitor).

Practical tips

• Always check transistor pinout (different transistors have different pin order).
• Use a multimeter's diode test to find base-emitter junction (approx 0.6–0.7 V for silicon).
• If you do not have a variable power supply, use resistor dividers and potentiometers to vary voltages safely.

5. Why transistors matter (appreciation)

Transistors are in almost every electronic device: radios, mobile phones, computers, LED lights, car electronics and more. They allow:

• Miniaturisation: many transistors on one chip (integrated circuits).
• Switching: basis of digital logic (0 and 1).
• Amplification: radios, microphones and sensors.

Understanding transistors helps students design circuits, repair electronics and follow careers in engineering and technology.

6. Classroom activities & assessment

1. Practical: Build the LED switch using a BC547, measure voltages at B, C, E and show transistor in saturation and cutoff. (Outcomes: a, b, d, f)
2. Lab: Perform the V–I measurement activity and submit plotted graphs with explanations of regions. (Outcomes: a, b, f)
3. Selection task: Given three application scenarios (audio amplifier, motor switch, logic input), choose a transistor and justify using datasheet numbers. (Outcome: c, f)
4. Project: Construct a small amplifier or transistor switch circuit and explain its working to the class. (Outcomes: d, e, f)
5. Short quiz: define cutoff, active and saturation; explain hFE. (Outcome: a, f)

7. Safety, tips and resources

• Work with low voltages only (≤12 V) for student experiments.
• Never connect transistor pins without a proper resistor when driving LEDs or bases — this prevents damage.
• Use heat sinks for power transistors; avoid touching hot components.
• Good local sources: electronics shops in Nairobi (e.g., Kimathi Street / electronics markets) or local suppliers. Common parts are inexpensive and available.

Further reading: transistor sections in school physics books, datasheets for BC547/2N2222, and online tutorials (search keywords: "BJT operation", "transistor V-I characteristic", "common emitter amplifier").

• a) Describe operation — covered in "What is a transistor?" and class discussion.
• b) Verify V–I characteristic — lab "Verify V–I characteristic".
• c) Select transistor — "How to select" and classroom selection exercises.
• d) Construct circuits — breadboard examples and project tasks.
• e) Appreciate importance — section "Why transistors matter" and projects.
• f) Identify categories — content headings make the categories explicit.