簡單的說- 2N3904 NPN晶體管

　　2N3904 NPN晶體管是一個非常受歡迎的，是作為一個簡單的電子開關或放大器，可處理200馬（絕對值）和高達100 MHz的頻率作為一個放大器。

　　Figure 1： An illustration of the pin out of a standard TO-92 package for Digi-Key part number 2N3904FS-ND： E = Emitter， B = Base， C = Collector.

　　Figure 2 shows a circuit example which turns on the collector emitter junction by energizing the base or biasing the transistor to turn it on by bringing 5 volts to the base via a slide switch. This example lights an LED which is considered to be the load in this case. The proper use of resistors to prevent over current draw is required when biasing the base. I used leaded parts in a bread board to test my example circuit. Most engineers will use surface-mount components （much smaller size then a TO-92 package） when it comes to using transistors in a new product design going to market. Here is a link that shows various package sizes for 3904 transistors.