ece124: add transistors

docs/1b/ece124.md

@@ -195,6 +195,22 @@ A **canonical product of sums (POS)** is a function expressed as a product of ma
 
 $$f(x_1,x_2,\dots)=\prod M(a,b,\dots)$$
 
+## Transistors
+
+Binary is represented in hardware via switches called **transistors**. Above a certain voltage threshold, its output is $1$, whlie it is $0$ if below a threshold instead.
+
+A transistor has three inputs/outputs:
+
+- A ground
+- An input **source**, which has voltage that determines whether the circuit is connected to the ground
+- An output **drain**, which will either be grounded or have a voltage depending on whether the switch is closed.
+
+<img src="https://upload.wikimedia.org/wikipedia/commons/6/61/IGFET_N-Ch_Enh_Labelled_simplified.svg" width=200>(Source: Wikimedia Commons)</img>
+
+A **negative logic** transistor uses a NOT bubble to represent that it is closed while the voltage is **below** a threshold.
+
+<imgg src="https://upload.wikimedia.org/wikipedia/commons/c/c4/IGFET_P-Ch_Enh_Labelled_simplified.svg" width=200>(Source: Wikimedia Commons)</img>
+
 ## VHDL
 
 VHDL is a hardware schematic language.