Transistor - Basic Properties, Terminology and Theory
What is a Transistor?
Transistors are a semiconductor device commonly used to amplify or switch electronic signals. There are a variety of transistors available including Bipolar, CIM, Darlington, Insulated Gate Bipolar Transistors (IGBT), MOS Bipolar, and MOSFET. A transistor is made of a solid piece of a semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals. Because the controlled current can be much larger than the controlling current, the transistor provides amplification of a signal.
The most basic element of a transistor power module is the silicon chip. Because of the high gain of Darlington configurations, most bipolar transistor modules contain Darlington transistor chips. Some of these chips are planar structures, as illustrated in Figure 1.1. The surface of a planar chip can be easily treated, simplifying mass production. Various manufacturers employ state-of-the-art fine line emitter patterns, resulting in excellent gain and safe operating area performance. High blocking voltages are achieved by using a triple diffusion process and guard rings.
Figure 1.2 illustrates the internal construction of a transistor module. The transistor chip is soldered to a molybdenum base. The molybdenum base alleviates thermal stress on the chip due to the nearly equivalent thermal expansion coefficients of silicon and molybdenum. This assembly is next soldered to a copper collector electrode along with a freewheeling diode chip. The copper electrode is in turn soldered to a ceramic substrate. The ceramic substrate can withstand 2000 to 2500 volts without adding significantly to the device's thermal resistance. The chips are bonded with aluminum wire and then encapsulated with silicon gel to guard the chip surfaces. Finally, the package is back-filled with epoxy resin to increase mechanical and environmental strength.
The proper application of power semiconductors requires an understanding of their maximum ratings and electrical characteristics, information that is presented within the device data sheet. Good design practice employs data sheet limits and not information obtained from small sample lots.
Rating
A rating is a maximum or minimum value that sets a limit on device capability. Operation in excess of a rating can result in irreversible degradation or device failure. Maximum ratings represents extreme capabilities of a device. They are not to be used as design conditions.
Characteristic
A characteristicis a measure of device performance under specified operating conditions expressed by minimum, typical, and/or maximum values, or shown graphically.
Transistors are available with operating voltages from 2V to 1200V. Current handling is available in 2A to 300A. Package styles include the single transistor, dual, triple, six, seven and 16 transistor versions.