Lab X: NMOS/FET & Digital Logic

The purpose of this lab was to understand digital logic. We started out by forking the lab from github . Then we built a NOT gate and branched off of it by playing with it by transforming it to a NOT NOT gate and incorporating LEDs to observe how it worked.
Then we built an NAND gate and again incorporated an LED to observe how it worked.
There was also an extra part that asked us to show how a NAND gate can be used to build any other gate.
At the end I added a little side note about how this lab reminded me of stuff I learned from a   program I went to last summer about nanotechnologies.

2N7000 transistor
270ohm resistor

Logic Gates
Before this lab, I had no idea what logic gates were or how they worked. Luckily, wonderful things like youtube exist. So I watched some videos from a really awesome series of videos I found on logic gates:
Introduction to Logic Gates
NOT gates
Truth Tables, AND, OR
Combining Logic Gates
Universal Gates:NAND
CMOS MOSFET (starts talking about CMOS at 6:15)
Also, just to clarify where the gate, drain and source are:

                        Image 1: Transistor  (from this website)

NOT gate
After watching the video on NOT gates, I learned that its purpose is to output the inverse of its input. The NOT gate we were asked to construct consisted of the resistor, the transistor, while using +5 for the drain/ source and use 0 or +5 V for FALSE/TRUE. We were then asked to measure the current through the resistor(drain/source) and measure the voltage after the resistor(at drain).

GATE T (+5V) GATE F (0V)
Current (mA) 17.79 0
Voltage (V) 0 4.96

We were then asked to make sense of the voltage data and if the gate=input and drain=output, how the circuit demonstrated a NOT logic function. Since the NOT function acts as an inverter, the voltage readings make sense; when the gate(input) is true, the drain(output) is false and visa versa.

NOT NOT gate
By putting two NOT gates in series, you end up inverting the inversion, so the input=output. I had some trouble wiring the two in series, since this involved two resistors and two transistors, so I gave up and followed PhD. Koch’s picture of his circuit. We were then asked to verify that input=output, which it did.

One more element was added to the NOT circuit, an LED. This means there were two in total for the NOT NOT circuit.  I was pretty confused on how the LEDs were supposed to switch, which mine were not doing anyway. After noticing that my transistor leads were shorting since they were touching and that my LEDs were backwards, he explained that there’s one LED that’s connected to the output (drain) of ONE of the transistors. This means that the output=output of a NOT function, since it’s only going through one transistor.
The input that goes into the second LED has passed through TWO transistors, aka it has passed through a NOT NOT function. So input=output.
Hence, when the input is FALSE, LED 1 will turn on (TRUE) and LED 2 will be off (FALSE). And visa versa.

                Image 2: NOTNOT with LEDs in series. The input is FALSE, hence the LED after the first transistor will turn on. (Sorry for the quality. I forgot my camera, so I took pictures with my ghetto phone; didn’t know how to download pictures from my ghetto phone, so I took pictures of the pictures on my phone.)

Image 3: NOT NOT with LEDs in series. TRUE input.

NAND gate
We were first asked to write down a logic table for the NAND function. Since the NAND=NOT AND, it’s like inverting the AND function. So I wrote down what the output would be for the AND function then inverted it. (The video on truth tables also helped on this, since I had no clue what a logic table was to begin with.)

Input 1 Input 2 Output (AND) Output (NAND)
1 1 1 0
1 0 0 1
0 1 0 1
0 0 0 1

Then we had to arm a circuit from this  wikipedia article , which represents a NAND function. We also had to measure the current through the resistor-drain-source-drain-source and the output voltage.

                                    Image 4: NAND circuit


Input 1(V) Input 2 (V) Current(mA) Votage( V)
5 5 17.83 0.105
5 0 0 4.96
0 5 0 4.96
0 0 0 4.96
I wondered why the current was zero, and PhD. Koch explained to me it was because of high resistance due to the transistors. Something else he noticed was the 0.105 voltage I was getting which was supposed to be zero. After checking the transistors, and replacing one of them the voltage was still unchanged.  :/  . So, the circuit was working like a NAND gate for the most part.
Next, we incorporated an LED into the circuit, to demonstrate that it worked like a NAND gate.

Image 5: NAND with LED.  This is for any of the three TRUE outputs.

Image 6: NAND with LED. This was for the one FALSE output.

We were then asked if NMOS would be ideal to string a bunch of NAND gates. The answer would be no, since a notable amount of current is drawn. This would be reeeeeeally impractical and expensive when you consider wiring up a bunch of them for any practical purposes. After learning about CMOS (again, the video up there helped me understand it), which is composed of NMOS +PMOS and acts like a pair of switches. Also, Koch told me it only drew current when it was switching– which is waaaay more practical!

This reminded me of a program I participated in last summer on introduction to nanotechnologies. After looking back at the notes that I brought back with me, I went over what I learned about data rate limitations and switching was mentioned! One of the ways of improving the efficiency of electronics, nanostructures for CMOS are exploited by stacking nanowires and 3D multichannels. The nanowires increase the hole and electron mobility extraction but limit the available surface for conduction, and the multichannels increase the available surface– at least that’s what’s indicated on my notes.
I don’t know how they work together or how it is that they improve the hole and electron mobility, but I thought it was an interesting thing to note.