2 Flashcards
Width of p channel twice width of nchannel to match.
Always some resistance in mosfets
On Resistance (R_on) has to be matched between n channel and p channel, although practically they differ very slightly.
(a few microohms for example)
Why is it important to match R_on for both PUN and PDN?
If load is modeled as capacitor and resistor in parallel, resistor from mosfet acts as a low pass filter and no longer a digital circuit. Rise time is slower due to this filter. Instead a square wave, a distorted wave it happening.
Mosfets have some capacitance that is not idea
Leaky capacitance.
Reason our processors aren’t getting much faster.
Ron cant get much smaller due to capacitance. Instead we make parallel processors.
How can you reduce R_on?
Increase width of transistors. There is a tradeoff between space effectiveness and speed.
Although Decreasing Ron also increases capacitance.
Via contact
Metal vertical connector on mosfets.
New technologies have up to 40 via layers. Most have about 4 layers.
VDD is only connected to one via ontop and to the other to the side to avoid short circuiting.
Visual cortex in brain takes lots of volume such as graphics sl chips take up lots of semiconductor chips.
Stick diagram ignores substrate connection on mosfet.
Threshold voltage variation occurs if substrate of B parallel mosfet gets shorted if not connected to VDD like in A.
Why do we need nwell
Acts as substrate for p channel transistor. If nwell wasn’t there p+ will short circuit with other p in other transistor.
Depletion region acts as
Insulator.
Why is there an overlap in the gate metal connection in stick diagrams?
Misalignment issue while manufacturing. (Mask) sometimes there might not be a total connection if it’s flush.
Manufactures will give spacing rules or guidelines for their manufacturing of your circuits that as designers we need to consider named this.
Lamba rules
What is lamba in lamda rules?
Lamda = length (L) / 2.
Why do we need lamba rules
Accuracy of manufacturing process, but also two metals form a capacitance and need some distance to be sufficiently small.
What is Tristate not gate?
A not gate with an extra pmos and extra nmos in series with enable inputs.
IIIE1164 Logic levels consists of how many levels?
9, although we only consider a few:
Strong 1
Strong 0
H (weak 1)
L (weak 0)
Z high impedence.
Don’t care
What makes a strong logic signal
Typically Resistance around 0 in thevenin equivalent (or less than equal 10 ohms)
What consists of a weak logic gate
Higher Ron resistance from about 10-100 ohms.
What consists of high impedence logic circuit.
Resistance is so high that it it basically an open circuit.
What are parasitic capacitors?
A mosfter (4 terminal device Gate, drain source, substrate) with a capacitor in between all nodes.
