Transistors vs MOSFETs Explained: How They Work and Why MOSFETs Dominate Modern Electronics

Overview

An engineer explains the core differences between transistors and MOSFETs, how each device controls electrical current, and how MOSFETs achieve faster switching with minimal energy, enabling modern electronics. The video uses water flow analogies and circuit examples to illustrate transistor gain, switching, and real-world applications like amplifier stages and motor control. It also covers packaging quirks and common pitfalls when driving MOSFET gates.

Key insights

• Transistors regulate current with base drive, giving current gain and enabling amplification, while MOSFETs use a gate voltage to form a conduction channel with minimal static power.
• BJT and MOSFET operation can be understood through PN junctions and field effect, respectively, including the role of forward bias and threshold voltage.
• MOSFETs enable fast switching and high currents with low gate power, but gate charge storage requires proper pull-downs and resistors to avoid staying on.
• Relays are mechanically isolated switches, but transistors and MOSFETs are the fast, compact alternatives used in most modern circuits.

Introduction and context

The video begins by contrasting relays, BJTs, and MOSFETs as devices used to switch circuits, highlighting why engineers moved from mechanical to semiconductor switches for speed and reliability. The host emphasizes that although BJTs and MOSFETs can both act as switches, they operate on different physical principles and have different strengths and weaknesses.

The bipolar junction transistor (BJT) fundamentals

A transistor, in practice a BJT, has three terminals: emitter, base, and collector. Current into the base controls a much larger current from collector to emitter, providing current gain. The base-emitter junction must be forward biased, which creates a conduction path for electrons across the junction. The internal structure relies on PN junctions, where the base region modulates the flow of carriers. However, maintaining conduction requires continuous base current, which leads to energy waste and heating as loads increase.

How BJTs amplify and switch

Transistors can amplify tiny signals, such as microphone outputs, into larger voltages to drive a speaker. In amplifier configurations, two transistors can share the load and improve efficiency. The video uses intuitive water-flow analogies to illustrate the concept of current gain where a small base current yields a larger collector current and how this underpins analog amplification and switching with a BJT.

Understanding MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors)

MOSFETs have three pins named gate, drain, and source. Like a transistor, they can block current until a voltage is applied to the gate. The gate forms a capacitor with the underlying silicon dioxide insulating layer. Increasing gate voltage creates an electric field that attracts carriers in a P-type channel region, forming a conductive path between drain and source. This makes MOSFETs excellent for switching with very little current required to hold the gate voltage.

Inrush, gate charge, and how to control MOSFETs

The gate behaves as a capacitor, so charging it during turn-on causes a brief surge of current that can damage microcontroller pins if not limited. A gate resistor helps limit this inrush. Conversely, when the controller turns off, the stored charge can keep the MOSFET on unless the gate is discharged. A pull-down resistor to ground ensures the gate returns to a defined low state when not driven.

Practical considerations and packaging

MOSFETs can share the same physical packages with other components such as voltage regulators and SCRs, enabling highly compact designs. The video also reinforces that MOSFETs are favored for fast switching, high efficiency, and reduced heat, which is why they dominate modern electronics. The water-pipe analogy is revisited to reinforce the conceptual distinction between blocking and enabling current flow.

Conclusion

Relays offer galvanic isolation and are suitable for high voltage AC loads but are slow and mechanical. BJTs provide straightforward analog amplification but require continuous base current. MOSFETs solve the power and speed problems, enabling efficient, fast switching across a wide range of applications from motor control to power supplies.

Sponsorship note: the video is sponsored by Brilliant, which is highlighted as a platform for visual, interactive math and electronics learning, including a new personal tutor feature.