Semiconductor Basics

Modern motorcycles replaced most mechanical switches and points with semiconductors — materials that conduct only under certain conditions. Three devices do almost all the work: the diode, the transistor, and the MOSFET. Understanding them here makes the Chapter 2 charging system (MOSFET regulator/rectifier) and PWM control much easier to follow.

Conductor vs insulator vs semiconductor: Copper always conducts, rubber never does. Silicon sits in between — it conducts or blocks depending on the voltage applied. That controllability is the entire foundation of modern electronics.

Diodes

Definition: A one-way valve for current. Allows flow in one direction, blocks the other.

    Anode (+)          Cathode (-)
        ●──────▶|──────●
                ▲
           band marks cathode

Forward biased (+ to anode): conducts, with a small voltage drop
Reverse biased (+ to cathode): blocks, effectively an open circuit
The band on the physical diode marks the cathode (current exits here)

Forward Voltage Drop

A conducting diode always "costs" a small voltage:

Diode Type	Forward Drop	Notes
Silicon (std)	0.6–0.7V	Most rectifier/signal diodes
Schottky	0.3–0.4V	Low loss, fast — used in rectifiers
LED	1.8–3.3V	Varies by color (red low, blue high)
Zener (reverse)	Set value	Conducts backwards at rated voltage

Diode Types on a Motorcycle

Type	Job on the Bike
Rectifier diode	Converts stator AC → DC (six of them in the R/R)
Flyback / freewheel	Clamps back-EMF across relay & injector coils
Zener diode	Voltage reference/regulation, spike clipping
Schottky	Low-loss rectification, reverse-polarity protection
LED	Indicators, tail lights, DRLs (a diode that emits light)

Tie-in: The back-EMF "flyback diode" mentioned in the inductance section (Basic Electricity) is exactly this device — reversed across a coil so the voltage spike has a safe path instead of destroying the transistor driving it.

Testing a Diode

Use the multimeter diode mode (not plain resistance):

Red on anode, black on cathode → reads 0.4–0.7V (forward, conducting)
Reverse the probes → reads OL (blocked)

Result	Meaning
~0.5V one way, OL the other	Good diode
OL both ways	Open diode
~0V (or beeps) both ways	Shorted diode

This is the same test used on each of the six diodes inside a regulator/rectifier (see Chapter 2 — Charging System).

Transistors (BJT) — the Electronic Switch

Definition: A device where a small current controls a much larger one. It can act as a switch (fully on/off) or an amplifier (in between).

A Bipolar Junction Transistor (BJT) has three legs:

            Collector (C)
                │
   Base (B) ───►│   small base current
                │   switches large C→E current
                │
             Emitter (E)

NPN: turns on when the base is pulled high (most common for low-side switching)
PNP: turns on when the base is pulled low

Switch Behavior

Region	Base Drive	Collector–Emitter	Analogy
Cutoff	none	Open (no current)	Switch OFF
Saturation	enough current	Closed (~0.2V drop)	Switch ON

For switching we want the transistor either fully off (cutoff) or fully on (saturation) — never lingering in between, where it dissipates heat.

Why It Matters

The ECU can't directly power an injector or coil — its logic pins output only tiny currents. A transistor lets a weak signal switch a strong load.
Turn-signal flashers, tachometer drivers, and ignition modules are transistor-based.

MOSFETs — the Modern Power Switch

Definition: A voltage-controlled switch. Unlike a BJT (which needs continuous base current), a MOSFET is turned on by a voltage on its gate and draws almost no current to stay on — so it wastes very little energy.

             Drain (D)
                │
   Gate (G) ───┤│   gate VOLTAGE (not current)
    (voltage)   │   switches D→S path
                │
             Source (S)

N-channel: turns on with a positive gate voltage (most common, lowest resistance)
P-channel: turns on with the gate pulled below the source

Why MOSFETs Dominate Modern Bikes

Property	Benefit on a Motorcycle
Very low on-resistance ( $R_{D S (o n)}$ )	Little heat even at high current
Voltage-driven gate	Almost no power wasted holding it on
Very fast switching	Ideal for PWM (thousands of switches per second)
No moving parts	Silent, vibration-proof, long life

MOSFET as a Switch: On or Off

Just like the BJT, a switching MOSFET lives at two extremes:

Fully ON: low $R_{D S (o n)}$ (a few milliohms) → tiny voltage drop, little heat
Fully OFF: open circuit → no current

Because "on" and "off" both waste little power, MOSFETs run cool. This is exactly why the modern MOSFET regulator/rectifier (Chapter 2) runs cooler than the older SCR type, and why PWM works so efficiently — the switch spends no time in the lossy middle region.

Where You'll Find Them

Regulator/rectifier: synchronous rectification — MOSFETs replace the diodes for a smaller voltage drop (regulation may still be shunt or series)
PWM drivers: cooling fans, heated grips, LED dimming, fuel-pump control
ECU output stages: low-side/high-side drivers for injectors, coils, relays, lamps

The Big Picture: Signal → Switch → Load

Almost every actuator on a EURO 5+ bike follows the same chain:

  ECU logic pin  ──►  MOSFET / transistor  ──►  Load (injector, fan, coil, lamp)
   (weak signal)        (the switch)            (needs real current)
        ▲                     ▲
     tiny current      flyback diode protects
     from processor    against coil back-EMF

Diode: directs/blocks current (rectify, protect, reference)
Transistor / MOSFET: lets a small control signal switch a large load
Together they replace every set of points, mechanical regulator, and relay a classic bike once used

Key Takeaways

Diode = one-way valve; conducts forward (~0.7V Si drop), blocks reverse
The band marks the cathode; test with diode mode (0.4–0.7V one way, OL the other)
Flyback diodes protect switches from inductive back-EMF spikes
Transistor (BJT) = small current controls a big one; switch = cutoff or saturation
MOSFET = small voltage controls a big current; very low on-resistance = low heat
MOSFETs switch fast and cool — the basis of modern R/R and PWM control
Actuator pattern: weak ECU signal → semiconductor switch → real load

Semiconductor Basics ​

Diodes ​

Forward Voltage Drop ​

Diode Types on a Motorcycle ​

Testing a Diode ​

Transistors (BJT) — the Electronic Switch ​

Switch Behavior ​

Why It Matters ​

MOSFETs — the Modern Power Switch ​

Why MOSFETs Dominate Modern Bikes ​

MOSFET as a Switch: On or Off ​

Where You'll Find Them ​

The Big Picture: Signal → Switch → Load ​

Key Takeaways ​