What is the Depletion Region in NMOSFET Transistors?

[Bassalisk]

Hello,

I am a bit confused about depletion region in NMOSFET transistor. It says here in my book, that when you increase Uds (drain source) above Ugs(gate source) that the current get higher. Ok, makes sense if nothing, from Ohms law. Simultaneously, the depletion region get smaller in length due to that voltage, that is bigger than drain source one(I can grasp with that concept too) but when I put that together, it doesn't make sense...

How can current get higher when the path through it can travel gets smaller(on the other end though).

I attached an relevant image.

 

[Bassalisk]

UPDATE: Relevant to this question: parasitic capacitance, I understand somewhat how it occurs. But why are they so bad? And why would one try to get rid of that capacitance?

 

[uart]

UPDATE: Relevant to this question: parasitic capacitance, I understand somewhat how it occurs. But why are they so bad? And why would one try to get rid of that capacitance?

Parasitic capacitances reduce the high frequency performance of the transistor. They shunt away both input current and output current and hence reduce both input impedance and gain at high frequencies. The worst of all is the feedback capacitance, for example C_{gd} (aka the reverse transfer capacitance C_{rss}) in the common source configuration. The voltage variation across this capacitance gets multiplied by the voltage gain of the circuit, and so in turn does it's determent on the high frequency performance.

 

[uart]

RE your original question :

How can current get higher when the path through it can travel gets smaller(on the other end though).

You have to understand that this increase in current is a second order effect. Meaning that to a first order approximation the current actually remains constant (The very mode of which your question refers to is call either "saturation" or "constant current" mode.)

So sorry to give you the "run around" here, but to understand the more esoteric second order effect you must first understand the basic "first order" approximation. To understand why the current increases you must first understand why it remains (approx) constant.

So you probably should ask that question first.

 

[Bassalisk]

Took me 2 weeks to swallow the transistor effect... countless hours of searching. I guess I will have to do with FETs the same...

Thanks for your help, I have a good start though.