Diodes and transistors

Why it matters

Diodes and transistors are the building blocks of every power supply, amplifier and digital chip. The most common job you will give a discrete transistor is the simplest: letting a 3.3 V microcontroller pin switch a load (an LED string, a relay coil, a small motor) that needs more current than the pin can supply. Getting the bias right is the difference between a cool, reliable switch and a hot, half-on part.

Diodes

A diode conducts easily in one direction (anode to cathode) and blocks the other. The Shockley diode equationI = I_s·(exp(V / (n·V_T)) − 1): diode current grows exponentially with forward voltage. describes the forward current:

I = I_s * (exp(V / (n * V_T)) - 1)
V_T = k T / q = 25.85 mV at 300 K

I_s is a tiny saturationThe fully-on state of a BJT switch, with only V_CE(sat) of about 0.1 to 0.3 V across it. current (around 1e-14 A for a small silicon diode) and n is between 1 and 2. Because the current is exponential, a silicon diode's voltage stays near 0.6 to 0.7 V over a wide range of currents. With n = 1, every extra 60 mV multiplies the current by about 10: I_s = 1e-14 A gives 0.12 mA at 0.60 V and 1.2 mA at 0.66 V.

Useful diode types:

• Rectifier diodes turn AC into pulsing DC in power supplies.
• LEDs drop more voltage (about 1.8 to 2.2 V for red, about 3 V for blue and white) and need a series resistor to set current.
• Zener diodes are used in reverse at a defined breakdown voltage as simple references.
• Schottky diodes have a lower forward drop (about 0.3 V) and switch fast.
• Flyback diodesDiodes placed across relay coils and motors to absorb the voltage spike when their current is switched off. go across relay coils and motors to absorb the voltage spike when the current is switched off. Leaving one out is a classic way to destroy a transistor.

The bipolar transistor (NPN)

A small base current controls a much larger collector current. In the active region, I_C = beta * I_B, with beta (also called h_FE) somewhere from 50 to 300 and varying widely between parts. That is what amplifiers use.

For a switch we want the opposite: drive the transistor hard into saturation, where it acts like a closed contact with only V_CE(sat) (about 0.1 to 0.3 V) across it. Designers use a forced betaA design ratio of load current to base current (about 10), far below the typical beta, so every part saturates. of about 10, meaning they supply ten times less than the load current as base current, far more than the minimum I_C / beta, so the switch stays saturated for any part from the bin.

Worked example: an LED switched from a 3.3 V pin

A 5 V supply drives an LED (2.0 V drop) through a 150 Ω resistor and an NPN transistor (V_CE(sat) = 0.2 V). The base is driven from a 3.3 V GPIO pin through R_B, and V_BE is about 0.7 V.

I_C = (5.0 - 2.0 - 0.2) / 150 = 18.7 mA
I_B (forced beta 10) = 18.7 mA / 10 = 1.87 mA
R_B = (3.3 - 0.7) / 1.87 mA = 1,393 Ω  ->  use 1.2 kΩ (next standard value down)
P_transistor = 0.2 V * 18.7 mA = 3.7 mW

Choosing the next value down gives a little more base current, which keeps the transistor saturated. Check that the pin can supply about 2.2 mA (most can; the datasheet gives the limit).

MOSFETs

For larger currents, a logic-level N-channel MOSFET is often the better low-side switch. Its gate draws almost no steady current, and when fully on it behaves like a small resistance R_DS(on), so it dissipates P = I^2 * R_DS(on). Check that the datasheet specifies R_DS(on) at your gate voltage (for example 2.5 V or 4.5 V), because "threshold voltage" is only where it barely starts to conduct.

Simulate it

Build the LED switch in Falstad (https://www.falstad.com/circuit/) or LTspice: sweep the base resistor and watch V_CE rise when the transistor leaves saturation. Simulators make it safe to see what too little base drive or a missing flyback diode does. Every hands-on task in this course uses paper, Python or a free simulator (Falstad, LTspice or ngspice). Never build or probe circuits connected to mains (wall outlets, appliances, panels) or utility equipment.

Common mistakes

• Designing a switch with the typical beta instead of a forced beta, so some parts never saturate.
• No base resistor, which overloads both the pin and the base-emitter junction.
• No flyback diode on inductive loads.
• Picking a MOSFET by its threshold voltage rather than R_DS(on) at the actual gate drive.
• Forgetting power: at 1 A, a V_CE(sat) of 0.3 V means 0.3 W to get rid of.

Recap

• Diode current is exponential in voltage; silicon sits near 0.6 to 0.7 V.
• As a switch, saturate the BJT with forced beta about 10: R_B = (V_drive - V_BE) / (I_C / 10).
• Use flyback diodes on coils; consider logic-level MOSFETs for higher currents.

Try it

Open the Do tab to code the diode equation and the transistor-switch design calculation.