Transistors - Field Effect and Bipolar Transistors: MOSFETS and BJTs

Summary

This video explains the fundamentals of two major types of transistors: Bipolar Junction Transistors (BJTs) and Field Effect Transistors (FETs). It outlines how BJTs use a small current at the middle terminal to control a larger current at the other terminals, while FETs use voltage at the middle terminal to regulate current. The video further compares NPN and PNP BJTs with N-channel and P-channel MOSFETs, highlighting the role of voltage and current at the gate and source terminals. It also addresses the differences in transistor behavior, including the limitations and characteristics of both types in practical applications, such as switching and resistance.

Takeaways

• 😀 The red spheres represent positive current, flowing in the opposite direction of electron current.
• 😀 In a Bipolar Junction Transistor (BJT), a small current at the middle terminal controls a larger current through the other two terminals.
• 😀 Bipolar Transistors are one of two main types of transistors, the other being Field Effect Transistors (FETs).
• 😀 In a Field Effect Transistor (FET), the voltage at the middle terminal (the Gate) controls the current through the other two terminals.
• 😀 Bipolar transistors have two main types: NPN and PNP, distinguished by the direction of the current flow and the arrow's direction on the symbol.
• 😀 In an NPN bipolar transistor, positive current flows in the direction of the arrow, while in a PNP transistor, the current flows out of the middle terminal.
• 😀 MOSFETs are a type of Field Effect Transistor, and there are two main types: N-channel and P-channel MOSFETs.
• 😀 The voltage between the Gate and Source terminals in a MOSFET controls the current through the Drain and Source terminals, with a threshold voltage needed to start the process.
• 😀 In an N-channel MOSFET, increasing the Gate-Source voltage increases the current, whereas in a P-channel MOSFET, the relationship is reversed.
• 😀 MOSFETs are unable to block current in the reverse direction because of an inherent diode built into their structure.
• 😀 MOSFETs also have capacitors between their terminals, which can cause momentary current through the Gate terminal when the Gate voltage changes.

Q & A

What is the role of the middle terminal in a Bipolar Junction Transistor (BJT)?

In a Bipolar Junction Transistor, a small current flowing into the middle terminal (called the base) controls a much larger current flowing between the two other terminals (the collector and emitter).

What is the primary difference between a Bipolar Junction Transistor and a Field Effect Transistor?

The key difference is that in a Bipolar Junction Transistor, the current through the transistor is controlled by the current flowing into the base terminal, while in a Field Effect Transistor, the current is controlled by the voltage between the gate and source terminals.

How can Bipolar Transistors be identified based on their arrow direction?

Bipolar Transistors can be identified by the direction of the arrow on the base terminal. In an NPN transistor, the arrow points away from the base, while in a PNP transistor, the arrow points toward the base.

What does the voltage between the gate and source terminals control in a Field Effect Transistor?

In a Field Effect Transistor, the voltage difference between the gate and source terminals controls the flow of current through the transistor.

What are the two main types of Metal Oxide Semiconductor Field Effect Transistors (MOSFETs)?

The two main types of MOSFETs are the N Channel MOSFET and the P Channel MOSFET, where the current flow direction and the behavior in response to voltage changes differ.

How does a decrease in gate voltage affect an N Channel MOSFET?

In an N Channel MOSFET, a decrease in the voltage on the gate relative to the source will cause a decrease in current, while an increase in gate voltage will increase the current.

Why is it important to exceed the threshold voltage in MOSFETs?

Exceeding the threshold voltage between the gate and source terminals is necessary for the MOSFET to start conducting current, as the transistor remains off below this voltage.

What happens to the current in a MOSFET when the drain voltage is reduced?

Reducing the drain voltage in a MOSFET causes a significant decrease in current, but increasing the drain voltage does not have a large effect on current.

How does the behavior of a Bipolar Transistor differ when it operates as a switch compared to a MOSFET?

When a Bipolar Transistor operates as a switch, it experiences a small voltage drop (around 0.3 volts) between the collector and emitter. In contrast, a MOSFET behaves like a resistor in the on position, with a small resistance, but lacks the ability to block current in the reverse direction.

What is an inherent characteristic of MOSFETs that affects their behavior in a circuit?

MOSFETs inherently include capacitors between their terminals, which means that charging and discharging these capacitors causes a momentary current through the gate terminal when the gate voltage changes.