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GIDL & Subthreshold Leakage in MOSFETs: A Complete Guide for VLSI Interviews

GIDL & Subthreshold Leakage in MOSFETs: A Complete Guide for VLSI Interviews

DIBL & SUB THRESHOLD LEAKAGE – SHORT CHANNEL PROBLEM

Hi Everyone,

The next topic today we will learn about is sub threshold leakage.

Now, as for short channels, we’ve brought the drain and source closer to each other so much, and the channel is so narrow, that there is a problem of subthreshold leakage.

In simple terms, even if the transistor is OFF, there will be leakage, and it’ll consume power. It’s disastrous for hand-held devices like mobiles where power saving is very crucial, so we need to take care of it to be as low as possible.

SUB THRESHOLD LEAKAGE – is the MOSFET really turned OFF?

We might think that as soon as the Vgs < Vth, the device turns off, but that’s not the case in advanced nodes, there will be leakage or we can say still power consumption is there even though the Vgs < Vth as well as Vth = 0.  (Similar to early effect in BJT — if you wanna learn that, comment in my Telegram channel)

Subthreshold leakage on Log Scale – what really happens below Vth ?

As nature doesn’t allow discontinuity, so does in mosfets its not the case. Graph 1 looks like it, but when we map it to log scale, it will look like graph 2.

From Subthreshold Leakage to GIDL — What happens at negative Vgs?

Now we’ll reduce the voltage further and infact go for negative Vgs to turn it off fully, but after some -ve value of Vgs , the Id (drain current) will again shoot up. This phenomenon is called as GIDL (Gate Induced Drain Leakage) which occurs due to tunneling efect. Tunneling effect we’ll learn in separate blog. So we measure the gates on basis of how quickly the voltage drops in FinFets to call them as good gates/std cells. The GIDL is also shown above in the image in graph 2

Interview questions for GIDL & SUB THRESHOLD LEAKAGE

  1. What is subthreshold leakage, and why does current flow even when VGS < Vth?
  2. Why is subthreshold leakage a major concern in advanced technology nodes?
  3. Why do we use a log-scale ID–VGS graph to understand subthreshold leakage?
  4. What is GIDL, and what causes it in a MOSFET?
  5. What is GIDL?

Thanks for reading.

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