SEMICONDUCTORS

1. SEMICONDUCTORS Bipolar transistors basics

2. SEMICONDUCTORS • A transistor is a semiconductor device used to amplify and switch electronic signals and electrical power, these components are referred to as BJT (bipolar junction transistor). • It is composed of 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.

3. SEMICONDUCTORS • When a third layer is added to a semi-conductor diode, a device is produced that can amplify power, current or voltage. • This device is called a bipolar junction transistor or just transistor.

4. SEMICONDUCTORS • A bipolar transistor is constructed from germanium or silicon just like a PN junction diode. • Transistors are constructed of three alternatively doped semiconductor regions. • These three regions are arranged in two different ways.

5. SEMICONDUCTORS • Here is a PNP transistor with the leads labeled. • The middle section is usually smaller and is lightly doped compared to the outer sections.

6. SEMICONDUCTORS • Here is the NPN transistor with the leads labeled. • The middle section is usually smaller and is lightly doped compared to the outer sections.

7. SEMICONDUCTORS Here is the side by side comparison of the two types of bipolar transistors

8. SEMICONDUCTORS • Transistors are constructed using the alloyed method or the diffusion method. • These processes involve the use of indium pellets or another type of trivalent or pentavalent impurities on each side of a thin N-type or P-type semiconductor crystal. • These processes are similar to the construction of PN junction diodes.

9. SEMICONDUCTORS Some of the many transistor packages

10. SEMICONDUCTORS • Transistor packaging uses a reference designation that indicates it’s size and configuration, the letters TO (transistor outline) followed by a number is commonly used. • As an example a TO-92 is a common plastic transistor however this number does not indicate NPN or PNP type, that is specified separately EXAMPLE OF A TO-92 PACKAGE

11. SEMICONDUCTORS Some more transistor packages

12. SEMICONDUCTORS • It is important to know how a transistor is packaged so that you can identify the three leads. • These leads will be connected in a circuit in specific order to insure proper biasing just like a PN Junction diode. E=emitter B=base C=collector

13. SEMICONDUCTORS • The schematic symbol for a transistor may not always show the three letters indicating the emitter, base and collector. • The base is almost always the middle lead and the emitter always has the arrow on its lead

14. SEMICONDUCTORS • Because of the huge assortment of different transistor packages it is difficult to form an absolute rules for identifying the various leads or terminals on each component. • It is necessary to refer to the manufactures specifications for accurate terminal identification and type (NPN or PNP)

15. SEMICONDUCTORS • Notice that for a NPN BJT the arrow never points in. • This type of transistor acts like a normally closed switch

16. SEMICONDUCTORS • Notice that for a PNP BJT the arrow points in. • This type of transistor acts like a normally open switch

17. SEMICONDUCTORS • The most basic and important function of a transistor is to provide power amplification. • These components are used to boost the strength or amplitude of electronic signals. • The signals applied to the transistor and the amplified signals obtained from the transistor can be expressed in terms of voltage, current or power

18. SEMICONDUCTORS • The BJT transistor is controlled by the current flowing through its terminals and this current is controlled by varying the input voltage. • The transistor must be properly biased by external voltages so that its emitter, base and collector regions interact in the desired manner.