Vce (Breakdown voltage between collector and emitter)
This is the voltage that causes current to flow (to crash through is more like it) from emitter to collector without the minimum threshold voltage applied at the base to keep the transistor turned on. It is a very good idea to keep the collector voltage well below the indicated Vce.
40V

Ic (Maximum collector current allowed)
This is the maximum current that is allowed to flow from emitter to collector. However, this does not mean that you can pass this much current through your transistor at any voltage. You have to stay within the transistor's power dissipation limits. For instance, if you want to pass 1 amp through the PN2222A, then your collector voltage cannot exceed 625 millivolts.

1.0 A

Hfe (Current gain)
Minimum and maximum values will be shown at a specified collector current. This is the transistor's amplification factor, also referred to as the transistor's beta. A small current flowing between emitter and collector causes a much greater to flow from emitter to collector. For instance, if your transistor has a beta of 100 at 1ma of collector current, then a current of 10 microamps flowing between emitter and base will cause 1 milliamp of current to flow between emitter and collector. The beta is dependent on current flow from the emitter to the collector. Typically, as the collector current goes up, the amplification factor goes up along with it, but only to a point before it starts to fall as the current is increased further. It is important to note that the Hfe can vary wildly among transistors even if they have the same part number and is produced by the same manufacturer. Even though the data sheet for the PN2222A indicates an Hfe of 50 at 1 ma of collector current, my transistor measured at 200 using a multimeter with a built-in Hfe checker. If you want your amplifier to operate with the highest current gain possible, it is best to measure the Hfe yourself.
50 @ 1.0 ma

Ft (Frequency Transition)
This marks the frequency in Mhz where gain falls to one. As the operating frequency goes up, the amplification factor tends to go down.

300 Mhz (gain falls to 1 at 300 Mhz)


Pd (Power dissipation in millwatts) 
This is the maximum amount of heat energy that the transistor is allowed to dissipate in an ambient temperature of 25 degrees Celsius. The collector voltage and current cannot exceed values that will produce more heat energy than the maximumamount of power dissipation indicated. For instance, if the collector current is 150 ma, then the collector voltage cannot exceed 4 volts.
625 mW

Case (Case style)
This designates the outer physical construction of the transistor. Transistors are manufactured in a wide variety of cases. The most common being TO-18, TO-92, TO-220, and the TO-3.

TO-92
When working with transistors it is always a good idea to look at their absolute maximum ratings indicated. You can find specifications for any transistor on the manufacturer's data sheet (ususally published on the web). Specifications for the PN2222A transistor are indicated at the bottom of each definition.
2007 Dave Cline. All Rights Reserved.