A transistor is made up of three doped regions: the emitter, base, and collector regions.
The base is a very thin and lightly doped region which is sandwiched between the emitter and collector regions.
The emitter region is the most heavily doped region in a transistor. Its function is to emit or inject current carriers into the base region.
The collector region is moderately doped and is the largest of all three transistor regions. Most of the current carriers injected into the base are attracted into the collector region rather than flowing out from the base lead.
In a transistor, IE = IB + IC.
The dc alpha (αdc) is the ratio of dc collector current to dc emitter current: αdc = IC/IE.
The dc beta (βdc)is the ratio of dc collector current to dc base current: βdc = IC/IB.
A transistor has four operating regions: the breakdown region, active region, saturation region, and the cutoff region.
When a transistor is operating in the active region, the collector acts like a current source whose value is IC = IB x βdc.
The power dissipation rating of a transistor decreases for temperatures above 25°. Manufacturers specify a derating factor in W/°C so that the transistor's power rating can be determined for any temperature.
Transistors must be properly biased to obtain the desired circuit voltages and currents. When properly biased, the transistor can amplify ac signals.
The most commonly used biasing techniques for transistors include base bias, voltage-divider bias, and emitter bias.
Base bias is seldom used because it has a very unstable Q point.
Voltage-divider bias is the most common way to bias a transistor. It provides a very stable Q point when designed properly.
Emitter bias using two power supplies provides a very stable Q point.
A dc load line is a graph that shows all possible operating points for IC and VCE in a transistor amplifier.
The end points for the dc load line are labeled IC(sat) and VCE(off). All points between cutoff and saturation are in the active region.
