Understanding MOSFETs and Their Types
MOSFETs, based on their conductive channel types, are categorized into two types: NMOS and PMOS. Figure 1 displays a schematic of an NMOS where you can see a diode connected in parallel between the drain (D) and source (S) terminals. This diode is often referred to as either a parasitic diode or a body diode, so many people are curious: Why is this diode connected in parallel? Can it be removed? What purpose does it serve?
This inclusion of a diode is inherent to the structure and function of MOSFETs, crucial for certain operational features and protection mechanisms within electronic circuits.
The Structure and Process of MOSFETs
To understand this, we need to discuss the manufacturing and structure of MOSFETs and diodes, both which are made from semiconductor materials. We know that a diode is formed from a pair of PN junctions. As shown in figure 2, the P-type region corresponds to the anode of the diode, while the N-type region corresponds to the cathode, with the PN junction situated between them. This fundamental semiconductor structure is pivotal in understanding how these components function.
Revisiting the structure of MOSFETs, as depicted in figure 3(1), the oxide ‘O’ refers to silicon dioxide (SiO2), which does not conduct electricity. This results in negligible current flow through the Gate (G) terminal, making the MOSFET a voltage-driven device and thus contributing to its low power consumption.
Additionally, apart from the Drain (D), Gate (G), and Source (S) terminals, there is a body terminal in the MOSFET, which is linked to the Source via a dashed line. Consequently, the internal arrow in the MOSFET symbol, which points towards the channel, is connected to the Source.
Figure 3(3) shows the configuration from the Drain’s N-type region through the body’s P-type region to the Source, which effectively forms a diode structure. This structural formation is the reason why a diode is depicted in parallel in the MOSFET symbol, illustrating how the intrinsic properties of the MOSFET’s design contribute to its overall functionality.
The Inherent Diode Within MOSFETs
The internal structure from the Drain’s N-type region through the body’s P-type region to the Source forms a diode structure. This is why the symbol for a MOSFET includes a parallel diode.
This diode is a result of the MOSFET’s structure and manufacturing process. While calling it a parasitic diode might not be entirely appropriate, the term “body diode” is more accurate. The body diode can sometimes be undesirable as it allows for the potential of leakage current between the Source and Drain.
Role of the Body Diode in Practical Applications
In some scenarios, the properties of the body diode are utilized to ensure the system operates correctly. For example, in battery protection circuits, if a lithium battery is overcharged, the protection mechanism might shut off the discharge MOSFET. When a charger is connected, the body diode of the MOSFET allows the circuit to conduct, enabling the system to function normally.
Conclusion
The inclusion of a diode parallel to the MOSFET in the symbol and practical use serves specific functions, and understanding its role can be crucial for designing effective electronic systems. This article aims to assist in grasping the reasons behind the MOSFET’s design and its practical implications.
