Hobbies And Interests
External Description
The MPSA18 consists of a piece of silicon with three metal contacts. This piece of silicon is housed within a cylindrical plastic package. The plastic package has a notch on one side to indicate the internal alignment of the transistor. The plastic package is between 0.175 and 0.205 inches in width and between 0.170 and 0.210 inches in length. The metal contacts are attached to one end of the plastic package.
Internal Description
The silicon has been doped so that it consists of three consecutive sections. The first section is called an N-type semiconductor because it contains an excess of negative charge carriers (electrons). The central section is called a P-type semiconductor because it contains an excess of positive charge carriers (holes). The final section is another N-type semiconductor. For this reason, the MPSA18 is referred to as an NPN transistor. This is in contrast to a PNP transistor. Each of the three metal contacts are attached to one of the semiconductor sections.
Contact Names
The contact attached to the central P-type silicon is called the "base" of the transistor. The two other contacts are called the "collector" and the "emitter." In normal operation, a supply voltage is applied to the collector terminal. This supply voltage is usually greater than the voltage at the emitter, which is usually kept at zero volts.
Current Gain
The MPSA18 acts as a current amplifier. The current that flows between the collector and the emitter is proportional to the current applied to the base. In normal operation, the maximum current flowing between the collector and the emitter is many times larger than the associated base current. The current gain of the MPSA18 is determined by dividing the collector-emitter current by the base current. The current gain varies with temperature and with the size of the supply voltage. The current gain is usually between 580 and 1150. Note that the gain is the ratio between the collector-emitter current and the base current. As such it is a dimensionless number and does not have units.
Breakdown Voltage
In theory, when the base current is zero the collector-emitter current should be zero as well. However, if the supply voltage applied to the collector terminal exceeds a certain amount, the transistor will undergo breakdown and current will flow between the collector and the emitter. This voltage is called the breakdown voltage. For an MPSA18, the breakdown voltage is 45 volts. Exceeding the breakdown voltage permanently damages the MPSA18, and so should be avoided.