semiconductors

the building block of everything electronic. switches, amplifiers, logic gates - all semiconductors underneath.

transistors

transistors are voltage-controlled switches. small voltage at base/gate controls big current through collector/drain.

two main types:

• bjt (bipolar junction transistor) - current controlled
• fet (field effect transistor) - voltage controlled

bjt basics

three pins: base, collector, emitter

• npn: current flows collector to emitter when base is high
• pnp: current flows emitter to collector when base is low

common ones i use: 2N2222 (npn), 2N2907 (pnp), BC547 (npn)

fet basics

three pins: gate, drain, source

• n-channel: current flows drain to source when gate is high
• p-channel: current flows source to drain when gate is low

mosfets are the most common fets. i use them for switching loads. common ones: 2N7000 (n-channel), IRF540 (high power n-channel)

mosfet details

metal-oxide-semiconductor field-effect transistor. voltage-controlled device that's great for switching applications.

key advantages

• high input impedance (voltage controlled)
• fast switching speeds
• low on-resistance (rds-on)
• no base current needed

types

• enhancement mode: normally off, turn on with gate voltage
• depletion mode: normally on, turn off with gate voltage
• n-channel vs p-channel determines polarity

practical considerations

• gate threshold voltage (vgs-th) determines when it turns on
• rds-on determines power dissipation when on
• gate charge affects switching speed
• body diode conducts in reverse direction

practical uses

• switching leds, motors, relays
• level shifting between 3.3v and 5v logic
• amplifying audio signals
• building logic gates from scratch
• power management circuits

my design patterns

for switching: mosfet with gate resistor and pulldown for amplification: bjt in common emitter configuration for level shifting: bjt voltage divider or mosfet buffer

gotchas i learned

• always use base/gate resistors to limit current
• body diode in mosfets can cause unexpected behavior
• heat dissipation matters for power transistors
• static electricity can kill mosfets easily
• gate capacitance can cause slow switching without proper drive

understanding semiconductors helps me design better circuits and debug problems. they're everywhere once you know how to spot them.