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# Understanding the curves of a MOSFET

I am trying to understand the curves of a MOSFET. Sorry if the question is very basic.

Where the red point is is the saturation zone of the MOSFET, therefore the source drain voltage must be 0V because at this point the MOSFET is saturated conduction at maximum current, because on the X axis of the graph called Vds marks 10V for the red point.

• 1therefore the source drain voltage must be 0v No, the red dot is at the point where VDSVDS = 10 V, see the X-axis of the graph. The source drain voltage is VDSVDS. Look up when a MOSFET is in saturation, there is an equation which tells you that VDSVDS needs to be larger than a certain value. – Bimpelrekkie 7 hours ago
• Ah, let me see. (1) The green region is the “saturation” region. (2) The yellow region is the “linear”, or “ohmic”, or “triode” region. (3) In the saturation region, the thick horizontal (well, slightly tilting upwards) straight lines (well, OK, curves) represent the points in the region of a particular Vgs value. (4) So for example, the curve that the red dot sits represents the points of Vgs = 2.5V. (5) The vertical lines 0, 5, 10, 15, 20 mean the voltage across Drain and Source, Vds. (6) Now the red dot operating point says this: (a) Vds = 10V, (b) Vgs = 2.5V, Id = approx 16A . – tlfong01 6 hours ago
• You may be confusing “saturation” in a bipolar transistor with “saturation” in a MOSFET. Unfortunately they mean practically the opposite phenomenon but have the same name. – Brian Drummond 1 hour ago

therefore the source drain voltage must be 0v because at this point the mosfet is saturated conduction at maximum current

No, you have this wrong. Maybe you were perhaps thinking of the BJT saturation region (when the collector-emitter voltage is close to 0 volts)? If so, then you’d be correct but, it’s the other way round for a MOSFET – the channel is saturated rather than the base/collector on a BJT.

From Wiki on MOSFETs: –

Question

What do the curves and the red dot represent in the following MOSFET Id vs Vds and Vgs characteristic graph?

1. The green region is the “saturation” region.
2. The yellow region is the “linear”, or “ohmic”, or “triode” region.
3. In the saturation region, the thick horizontal (well, slightly tilting upwards) straight lines (well, OK, curves) represent the (connected) points in the region of a particular Vgs value.
4. So for example, the curve that the red dot sits represents the points of Vgs = 2.5V.
5. The vertical lines 0, 5, 10, 15, 20 mean the voltage across Drain and Source, Vds.
6. Now the red dot operating point says this: If (a) Vds = 10V, and (b) Vgs = 2.5V, then (c) Ids = approx 16A.

References

Appendices

Appendix A – Recommended reading list of the Jaeger book

Part 1 Solid State Electronics and Devices

Chapters

1. Chapter 4 Field-Effect Transistors page 145,
2. Chapter 5 Bipolr Junction Transistors page 217

Sections

1. Saturation of the I-V characteristics, Section 4.2.4, Page 154, Fig 4.8
2. Mathematical Model in the Saturation (Pinch-off) Region, Section 4.2.5, Page 155, Fig 4.10
3. NMOS Transistor Mathematical ModelSummary (Cutoff region, Triode region, Saturation region, Threshold voltage) Chapter 4, page 160.

Appendix B – Clarifying concepts and terms in MOSFET characteristics graph

Linear Mode Operation and Safe Operating Digram of Power-MOSFETs – J Schoiswohl, Infenion, App Note V1.1 2017may

Appendix C – Comparing and Constrasting between MOSFET and BJT

Introduction

MOSFET and BJT, by their structure and operation mode, cannot be easily compared, though can be more easily constrasted. The following discussion is limited to NPN BJT and N-channel MOSFET, and are over simplified and therefore potentially misleading.

1.1 BJT is basically a “current device”. So we talk about (a) current amplification gain Ic/Ib and (b) current switching.

1.2 MOSFET is basically a “voltage device”. We change Vgs which causes a change in Rds and therefore Ids and Vload. So the amplification is more indirect.