13.2.1 Understanding bipolar junction transistors (BJTs)

• There are two main types of BJTs: NPN and PNP.

  True
• The emitter of a BJT provides charge carriers to the base.


  True
• BJTs can switch currents on and off based on a small base current.


  True
• What determines whether a BJT is an NPN or PNP type?

  Material and doping
• What type of applications are NPN BJTs typically used for?

  High-frequency
• Steps in the working principle of an NPN BJT

  1️⃣ The emitter injects electrons into the base region

  2️⃣ A small base current controls electron flow

  3️⃣ Electrons flow from emitter to collector

  4️⃣ The collector collects the electrons
• When a sufficient base current is applied, a BJT turns on, allowing a large collector current to flow.


  True
• In which three modes do BJTs operate?

  Cut-off, active, saturation
• What is the relationship between collector current and base current in active mode?

  I_{C} ≈ βI_{B}</latex>
• Match the BJT type with its primary charge carrier:

  NPN ↔️ Electrons

  PNP ↔️ Holes
• What is the primary charge carrier in an NPN BJT?

  Electrons
• How does a small base current control the collector current in an NPN BJT?

  Increases collector current
• In active mode, the collector current is equal to $βI_{B}$, where $β$ represents the current gain
• What are Bipolar Junction Transistors (BJTs) used for in electronic circuits?

  Amplifiers and switches
• What type of charge carriers do PNP BJTs rely on for conduction?

  Holes
• What is the function of the collector in a BJT?

  Collect charge carriers
• What are the three key regions of a BJT structure?

  Emitter, base, collector
• The base region of a BJT is lightly doped to control charge carrier flow
• BJTs rely on both holes and electrons for charge conduction.


  True
• What type of applications are PNP BJTs typically used for?

  Low-power
• How does a BJT amplify a weak input signal at the base?

  By increasing collector current
• Match the BJT operation mode with its characteristics:

  Cut-off ↔️ $V_{BE} < 0$, no current flow

  Active ↔️ $V_{BE} > 0$, amplification

  Saturation ↔️ $V_{BE} > 0$, maximum current flow
• In cut-off mode, $V_{BE}$ is less than 0, and no current flows.


  True
• A Bipolar Junction Transistor (BJT) is a three-terminal semiconductor device used as both an amplifier and a switch
• Depending on the doping arrangement, BJTs can be either NPN or PNP
• The working principle of BJTs relies on the flow of charge carriers between the emitter, base, and collector
• What type of charge carriers do NPN BJTs rely on for conduction?

  Electrons
• The base of a BJT controls the flow of charge carriers between the emitter and the collector
• BJTs amplify weak input signals into stronger output signals
• The emitter region of a BJT is highly doped to provide charge carriers.


  True
• Match the BJT region with its function:

  Emitter ↔️ Provides charge carriers

  Base ↔️ Controls charge carrier flow

  Collector ↔️ Collects charge carriers
• In an NPN BJT, a small base current controls the flow of electrons from the emitter to the collector
• What are the three operation modes of a BJT?

  Cut-off, active, saturation
• BJTs operate in three modes: cut-off, active, and saturation
• In saturation mode, the collector-emitter voltage $V_{CE}$ is approximately 0.


  True
• Arrange the regions of a BJT based on their doping level from highest to lowest:

  1️⃣ Emitter (Heavily doped)

  2️⃣ Collector (Moderately doped)

  3️⃣ Base (Lightly doped)
• NPN BJTs require opposite voltage polarities compared to PNP BJTs for operation.


  True
• Arrange the BJT operation modes based on the increasing order of current flow:

  1️⃣ Cut-off

  2️⃣ Active

  3️⃣ Saturation
• BJTs are used in switching applications due to their low saturation voltage and fast response time.


  True
• What is the approximate value of $V_{BE}$ in cut-off mode?


  $V_{BE} < 0.7$ V