Semiconductors: diodes, transistors and MOSFETs as switches

Resistors obey Ohm's law; semiconductors do not, and that is what makes them useful. A diode lets current flow one way only. A transistor lets a small signal control a large current. These two behaviors underlie every logic gate, every regulator, every motor driver and every microcontroller on a robot. This lesson treats them the way a robot builder uses them: as one-way valves and electrically controlled switches. You will learn to read the key datasheet numbers, choose a transistor to let a 3.3 V microcontroller pin switch a 12 V load, calculate the heat it produces and protect it from the inductive spikes that motors, relays and solenoids create.

How semiconductors conduct

Silicon in its pure form is a poor conductor. Adding tiny amounts of other elements, called doping, creates n-type silicon with spare electrons or p-type silicon with spare holes, places where an electron is missing that behave like positive charges. Where p-type and n-type regions meet, a pn junction forms, and the charges near the boundary rearrange into a thin depletion region that blocks current until an external voltage overcomes it. You do not need the solid-state physics to use these parts, but this picture explains why a silicon junction needs roughly 0.6 to 0.7 V before it conducts, and why that voltage falls by about 2 mV for every degree of temperature rise.

Diodes

A diode conducts when its anode is more positive than its cathode by the forward voltage, and blocks current in the other direction until the reverse voltage exceeds its breakdown rating. To a first approximation, a conducting diode drops a fixed voltage: about 0.7 V for a silicon rectifier, 0.2 to 0.45 V for a Schottky diode, and 1.8 to 3.3 V for light-emitting diodes, depending on color. Datasheets give the maximum forward current, the maximum reverse voltage and the forward voltage at a stated current.

Diodes protect circuits in several ways. A diode in series with the battery input blocks current if the battery is connected backwards, at the cost of its forward drop and the heat that drop produces at high current. Clamp diodes from an input pin to the supply and ground rails, built into most microcontrollers, conduct if the input goes outside the rails, so modest overvoltages are absorbed rather than destroying the chip, provided a series resistor limits the current. A TVS diode across a supply or communication line clamps short, high-energy surges, such as those from electrostatic discharge.

Bipolar junction transistors

A bipolar junction transistor (BJT) has three terminals: base, collector and emitter. In an NPN transistor, a small current flowing into the base and out of the emitter allows a much larger current to flow from collector to emitter. The ratio is the current gain, written β or hFE, typically 50 to 300 for small transistors. The base-emitter junction is a diode, so the base sits about 0.7 V above the emitter when the transistor is on.

Used as a switch, a BJT is driven hard into saturation, where the collector-emitter voltage falls to about 0.1 to 0.3 V and the transistor acts like a closed switch. To make sure it saturates, designers supply about ten times the base current the gain alone would require. BJTs are inexpensive and work well for switching up to a few hundred milliamperes: LEDs, small relays, buzzers. For larger currents, the base current a microcontroller would need to supply becomes impractical, and MOSFETs take over.

MOSFETs

A metal-oxide-semiconductor field-effect transistor (MOSFET) is controlled by voltage rather than current. Its gate is insulated from the channel by a thin oxide layer, so in steady state no gate current flows. Applying a voltage between gate and source above the threshold voltage creates a conducting channel between drain and source. When fully on, the channel behaves like a small resistor, RDS(on), which for a good power MOSFET can be a few milliohms. An N-channel MOSFET placed between the load and ground, called a low-side switch, is the workhorse for switching motors, solenoids, LED strips and heaters from a microcontroller.

Three datasheet values matter most. The drain-source voltage rating must exceed the supply plus any spikes, with margin. The continuous drain current rating, which assumes ideal cooling, gives an upper bound but rarely the real limit. And RDS(on) must be read at the gate voltage you will actually apply. A trap catches many beginners here: the threshold voltage, often 1 to 2.5 V, is where the MOSFET only begins to conduct, not where it is fully on. Many power MOSFETs specify RDS(on) only at a 10 V gate drive. Driven from a 3.3 V pin, they may be only partly on, with high resistance and serious heating. Choose a logic-level MOSFET with RDS(on) specified at 2.5 V or 4.5 V, or use a gate driver chip that provides 10 V or more.

Two small resistors complete the circuit. A gate resistor of about 10 to 100 Ω limits the brief current that charges the gate capacitance and damps ringing. A pull-down resistor of about 10 to 100 kΩ from gate to source holds the MOSFET off when the microcontroller pin is floating, as it is during reset and boot. Without it, a motor might twitch every time the robot powers up, which is alarming at best and dangerous at worst.

Power dissipation and heat

A switch that is fully on dissipates power in its on-resistance; a switch that is fully off dissipates almost nothing. In between, during switching, the MOSFET briefly carries current with voltage across it, and that switching loss grows with frequency. For on-off control at low frequency, conduction loss dominates:

The second formula estimates the junction temperature from the ambient temperature and the thermal resistance from junction to ambient, \( R_{\theta JA} \), in degrees per watt, given in the datasheet for a stated mounting. Silicon junctions are usually rated to 150 or 175 °C, but good designs stay well below, at perhaps 100 to 125 °C, because lifetime falls as temperature rises. RDS(on) itself rises with temperature, by roughly 1.5 to 2 times between 25 °C and 150 °C, so a hot MOSFET dissipates more and gets hotter still. Always calculate with the hot value.

def junction_temp(i, rds_on, r_theta_ja, t_ambient):
    p = i**2 * rds_on
    return p, t_ambient + p * r_theta_ja

for label, rds, rth in [("12 mOhm, small pad", 0.018, 50),
                        ("12 mOhm, large copper", 0.018, 30),
                        ("4 mOhm, large copper", 0.006, 30)]:
    p, tj = junction_temp(10, rds, rth, 45)
    print(f"{label:22s} P = {p:4.2f} W  Tj = {tj:5.1f} C")

12 mOhm, small pad     P = 1.80 W  Tj = 135.0 C
12 mOhm, large copper  P = 1.80 W  Tj =  99.0 C
4 mOhm, large copper   P = 0.60 W  Tj =  63.0 C

Inductive loads and flyback

Motors, relays and solenoids contain coils, and a coil resists changes in its current. The voltage across an inductor is proportional to how fast its current changes:

When a switch turns off and tries to stop the current suddenly, dI/dt becomes enormous and the coil produces a voltage spike, often hundreds of volts, that can destroy the transistor in a single event. A flyback diode, also called a freewheeling diode, placed across the coil with its cathode toward the positive supply, gives the current a path to circulate and decay safely. Schottky diodes are preferred for motors because they turn on quickly. The diode slows the release of relays slightly; where fast release matters, a Zener or TVS diode in series with it lets the coil voltage rise to a higher but controlled level. Lesson 11 extends this to H-bridges, where the MOSFETs' own internal body diodes play the same role.

High-side switching

Low-side switches are simple but leave the load connected to the positive supply, which can matter when the load's ground must stay connected, as with a sensor or a module that communicates with the microcontroller. A high-side switch sits between the supply and the load. A P-channel MOSFET can do this: its gate is pulled to the supply to turn it off and pulled lower to turn it on, usually through a small NPN or N-channel transistor because the microcontroller pin cannot reach the supply voltage. P-channel parts have higher RDS(on) than equivalent N-channel ones. For high currents, designers use N-channel MOSFETs on the high side with a gate driver that produces a voltage above the supply, using a bootstrap capacitor or charge pump. Integrated high-side switch chips, common in vehicles, add current limiting and diagnostics in one package.