# EXPERIMENT 4

It's time to make your first transistor. Construct the circuit shown at right. Be sure to use LIGHTLY doped p-type silicon (light red) for the middle section of the transistor. The two end pieces can be either N or N+, but N+ is better because it has a lower resistance. Make sure you connect the N+ and P together by dragging from one to the other. Also, make sure you DON'T put a via on the metal track forming the gate -- it needs to sit over the P region but must not connect to it.
Now turn on the Q-Goggles and watch what happens to the output when you turn the switch on and off. If you've done everything correctly, when you flip the switch up the output should go low, and when you flip it down the output should go high.
This circuit is called an inverter and it performs a logical "not" function: the output is low when the input is high, and vice versa.

## Amplification

The transistor provides a solution to one of the problems that our diode or-gate had. Because the gate of the transistor is insulated, it draws very little power from whatever is connected to it. So one output can drive many transistors without any difficulty. Put another way, the transistor provides amplification.
However, this circuit is still not a very good logic gate, because it uses power continuously as long as the transistor is turned on, and it takes a long time for the output to go high after the transistor turns off.
People used to build computers from circuits like this some time back in the pleistocene era, but they got really hot and took forever to load a web page. In the next couple of lessons we'll see how to do better.