Back to Courses

Semiconductors & Transistors

Explore the physics of semiconductors and how transistors amplify and switch signals

What are Semiconductors?

Semiconductors are materials with electrical conductivity between conductors and insulators. Silicon and germanium are the most common semiconductor materials.

  • N-type: Doped with atoms that provide extra electrons (negative charge carriers)
  • P-type: Doped with atoms that create "holes" (positive charge carriers)
  • P-N Junction: Where p-type and n-type materials meet, creating a diode
Bipolar Junction Transistors (BJT)

A BJT has three terminals: Base (B), Collector (C), and Emitter (E). Small base current controls large collector current.

Current Gain (β or hFE):

$$I_C = \beta \times I_B$$

Where I_C is collector current and I_B is base current. Typical β ranges from 50 to 300.

Example:

If β = 100 and base current is 0.1mA:

$$I_C = 100 \times 0.0001 = 0.01\text A (10mA)$$

Transistor Operating Regions
  • Cutoff: No base current, transistor is OFF (open switch)
  • Active: Linear relationship between base and collector current (amplifier)
  • Saturation: Maximum collector current, transistor fully ON (closed switch)
MOSFET Transistors

Metal-Oxide-Semiconductor FET (MOSFET) uses voltage on the gate to control current between drain and source.

  • N-channel: Positive gate voltage turns it ON
  • P-channel: Negative gate voltage turns it ON
  • Advantage: Nearly zero gate current (voltage-controlled)
  • Use cases: Power switching, digital logic, amplification
Applications
  • Amplifiers: Increasing signal strength in audio and RF circuits
  • Switches: Digital logic gates and power control
  • Oscillators: Generating clock signals and frequencies
  • Voltage regulators: Maintaining stable output voltage
  • Motor drivers: Controlling high-current loads