Bipolar Junction Transistor

Summary

The video script delves into bipolar junction transistors (BJTs), highlighting their PNP or NPN semiconductor sandwich structure and their function as current regulators. It explains the three leads: emitter, base, and collector, with the emitter's arrow indicating the BJT type. BJTs can act as solid-state relays, offering advantages over mechanical switches, such as reduced arcing and shock risk. The script also covers the transistor's operation in cutoff and saturation states and its testing with a multimeter. The importance of understanding BJTs for their widespread use in electronic circuits is emphasized.

Takeaways

• 🌟 Bipolar Junction Transistors (BJTs) are semiconductor devices with either a PNP or NPN structure, named for their dual semiconductor material layers.
• 🔍 BJTs have three leads: emitter, base, and collector, with the emitter's direction indicated by an arrow on the schematic symbol.
• 👉 The direction of the arrow in the schematic symbol helps identify the type of BJT: NPN or PNP.
• 🚦 BJTs function as current regulators, allowing a small emitter-base current to control a larger collector-emitter current.
• ⚡ Proper voltage biasing is essential for BJTs to operate correctly, ensuring the controlling and controlled currents flow in the right direction.
• 🔄 BJTs can be used as switching elements, offering advantages over mechanical switches such as reduced arcing, heating, and shock risk.
• 🛡 A transistor can act as a solid-state relay, turning on or off with a small controlling current and voltage compared to the controlled load.
• 🔒 BJTs have a cutoff state with zero current, where they are non-conducting, and a saturation state with maximum current, where they are fully conducting.
• 🛠 When tested with a multimeter, BJTs behave like two back-to-back PN junctions, requiring proper biasing to function.
• 🔌 The emitter-base PN junction must be forward-biased to allow current flow, while the base-collector junction is normally reverse-biased to block current.
• 🌐 BJTs are versatile components found in almost every electronic circuit, making understanding their operation fundamental to electronics.

Q & A

What is a bipolar junction transistor (BJT)?

A bipolar junction transistor (BJT) is a type of semiconductor device that consists of a PNP or NPN sandwich structure. It is commonly referred to as a BJT because the controlled current must go through two types of semiconductor material, p and n.

How are the schematic symbols for NPN and PNP BJTs different?

The schematic symbols for NPN and PNP BJTs differ by the direction of the arrow. The emitter lead can always be identified by the arrow, and the direction of the arrow identifies the type of BJT. An NPN arrow is not pointed in the same direction as the PNP arrow.

What are the three leads of a BJT called?

The three leads of a BJT are called the emitter, base, and collector.

How does a BJT function as a current regulator?

A BJT functions as a current regulator by allowing a small amount of emitter-base current to control a larger collector-emitter current, provided that the controlling base-emitter current and the controlled collector-emitter current are going in the proper direction.

What are the disadvantages of using a conventional mechanical switch for controlling DC power to a load?

Conventional mechanical switches have several disadvantages, including damage to the mechanical switch due to arcing, heating, and the potential risk of shock to the operator.

How can a transistor be used as an alternative to a mechanical switch?

A transistor can be used as a solid-state relay, offering an alternative to a mechanical switch. With the proper current and voltage biasing, the transistor can be turned on or off, and the controlling current and voltage are typically very small compared to the controlled current and voltage.

What is the state of a transistor when it has zero current through it?

When a transistor has zero current through it, it is said to be in a state of cutoff, meaning it is fully non-conducting or off.

What is the state of a transistor when it has maximum current through it?

When a transistor has maximum current through it, it is said to be in a state of saturation, meaning it is fully conducting or on.

How does a transistor behave when tested with a multimeter in resistance or diode check modes?

When tested with a multimeter in resistance or diode check modes, a transistor behaves like two back-to-back PN junctions, which are better known as a simple diode.

What is necessary for a BJT to be properly biased on or off?

A BJT must have proper voltage applied to bias the transistor on or off. The emitter-base PN junction must be forward biased to allow collector-emitter current to flow, and it is the potential difference between the emitter-base PN junction that turns the transistor on.

Why is it important to understand the emitter-base current flow in a BJT?

It is important to understand the emitter-base current flow in a BJT because it regulates the amount of current between the collector and emitter, even though the base-collector junction is reverse-biased and normally blocks any current from going through the transistor between emitter and collector.