Operation | Construction of Transistor

In 1948, Bell Laboratories developed the first working junction transistor. A transistor is a three-element, two-junction device used to control electron flow. By varying the amount of voltage applied to the three elements, the amount of current can be controlled for purposes of amplification, oscillation, and switching.

Construction of Transistor

When a third layer is added to a semiconductor diode, a device is produced that can amplify power, current, or voltage. The device is called a bipolar transistor, also referred to as a junction transistor or transistor. The term transistor will be used here.

A transistor, like a junction diode, can be constructed of germanium or silicon, but silicon is more popular. A transistor consists of three alternately doped regions (as compared to two in a diode). The three regions are arranged in one of two ways.

In the first method, the P-type material is sandwiched between two N-type materials, forming an NPN transistor (Figure 1).

construction of transistor
Fig. 1.

In the second method, a layer of N-type material is sandwiched between two layers of P-type material, forming a PNP transistor (Figure 2).

construction of transistor
Fig. 2

In both types of transistor, the middle region is called the base and the outer regions are called the emitter and collector. The emitter, base, and collector are identified by the letters E, B, and C, respectively.

Transistor Types and Packaging

Transistors are classified by the following methods:

  • According to type (either NPN or PNP)
  • According to the material used (germanium or silicon)
  • According to major use (high or low power, switching, or high frequency)

Most transistors are identified by a number. This number begins with a 2 and the letter N and has up to four more digits. These symbols identify the device as a transistor and indicate that it has two junctions.

A package serves as protection for the transistor and provides a means of making electrical connections to the emitter, base, and collector regions. The package also serves as a heat sink, or an area from which heat can be extracted, removing excess heat from the transistor and preventing heat damage.

Transistor packages are designated by size and configuration. The most common package identifier consists of the letters TO (transistor outline) followed by a number.

Because of the large assortment of transistor packages available, it is difficult to develop rules for identifying the emitter, base, and collector leads of each device. It is best to refer to the manufacturer’s specification sheet to identify the leads of each device.

Operation of Transistor

A diode is a rectifier and a transistor is an amplifier. A transistor may be used in a variety of ways, but its basic functions are to provide current amplification of a signal or to switch the signal.

A transistor must be properly biased by external voltages so that the emitter, base, and collector regions interact in the desired manner.

In a properly biased transistor, the emitter junction is forward biased and the collector junction is reverse biased. A properly biased NPN transistor is shown in Figure 3.

operation of transistor
Fig. 3(A) BLOCK DIAGRAM OF A BIASED NPN TRANSISTOR
operation of transistor
Fig. 3(b) B) SCHEMATIC DIAGRAM OF A BIASED NPN TRANSISTOR

Electrons are caused to flow from an NPN transistor emitter by a forward bias. Forward bias is a positive voltage on the base terminal with respect to the emitter terminal. A positive potential attracts electrons, creating an electron flow from the emitter.

The electrons that are attracted into the base are now influenced by the positive potential applied to the collector. The majority of electrons are attracted to the collector and into the positive side of the reverse-biased voltage source.

A few electrons are absorbed into the base region and support a small electron flow from it. For this action to occur, the base region must be extremely thin. In a properly biased PNP transistor, the batteries must be reversed (Figure 4).

operation of transistor
Fig. 4(A) BLOCK DIAGRAM OF A BIASED PNP TRANSISTOR
operation of transistor
Fig. 4(B) SCHEMATIC DIAGRAM OF A BIASED PNP TRANSISTOR

The difference between the NPN and PNP transistors is twofold: The batteries have opposite polarities, and the direction of the electron flow is reversed. As with the diode, a barrier voltage exists within the transistor.

In a transistor, the barrier voltage is produced across the emitter-base junction. This voltage must be exceeded before electrons can flow through the junction. The internal barrier voltage is determined by the type of semiconductor material used. As in diodes, the internal barrier voltage is 0.3 V for germanium transistors and 0.7 V for silicon transistors.

The collector-base junction of a transistor must also be subjected to a positive potential that is high enough to attract most of the electrons supplied by the emitter.

The reverse-bias voltage applied to the collector-base junction is usually much higher than the forward-bias voltage across the emitter-base junction, supplying this higher voltage.

Testing of Transistor

Transistors are semiconductor devices that usually operate for long periods of time without failure. If a transistor does fail, the failure is generally caused by excessively high temperature, current, or voltage. Failure can also be caused by extreme mechanical stress.

As a result of this electrical or mechanical abuse, a transistor may open or short internally, or its characteristics may alter enough to affect its operation.

There are two methods for checking a transistor to determine if it is functioning properly: with an ohmmeter and with a transistor tester.

A conventional ohmmeter can help detect a defective transistor in an out-of-circuit test.

Resistance tests are made between the two junctions of a transistor in the following way: emitter to base, collector to base, and collector to emitter.

In testing the transistor, the resistance is measured between any two terminals with the meter leads connected one way. The meter leads are then reversed. In one meter connection, the resistance should be high, 10,000 ohms or more. In the other meter connection, the resistance should be lower, less than 10,000 ohms.

Each junction of a transistor exhibits a low resistance when it is forward biased and a high resistance when reverse biased. The battery in the ohmmeter is the source of the forward- and reverse-bias voltage.

The exact resistance measured varies with different types of transistors, but there is always a change when the ohmmeter leads are reversed. This method of checking works for either NPN or PNP transistors (Figure 5). If a transistor fails this test, it is defective. If it passes, it may still be defective.

testing of transistor
Fig. 5. Resistance measurements of transistor junctions.

A more reliable means of testing a transistor is by using a transistor tester. Transistor testers are designed specifically for testing transistors and diodes. There are two types: an in-circuit tester and an out-of-circuit tester. Both may be housed in the same package.

The transistor’s ability to amplify is taken as a rough measure of its performance. There is an advantage with an in-circuit tester because the transistor does not have to be removed from the circuit.

An out-of-circuit transistor tester can not only determine whether the transistor is good or defective, but also determine the leakage current. Leakage tests cannot be made in-circuit.

Transistor testers contain controls for adjusting voltage, current, and signal. Refer to the manufacturer’s instruction manual for the proper settings.

Transistor Substitution

Numerous guides have been prepared by manufacturers to provide cross references for transistor substitution. Most substitutions can be made with confidence. If the transistor is unlisted or the number of the transistor is missing, the following procedure can be used to make an accurate replacement selection.

1. NPN or PNP? The first source of information is the symbol on the schematic diagram. If a schematic is not available, the polarity of the voltage source between the emitter and collector must be determined.

If the collector voltage is positive with respect to the emitter voltage, then it is an NPN device. If the collector voltage is negative with respect to the emitter voltage, then it is a PNP device. An easy way to remember the polarity of the collector voltage for each type of transistor is shown in Figure 6.

transistor substitution
Fig. 6. How to remember the polarity of the collector voltage.

2. Germanium or silicon? Measure the voltage from the emitter to the base. If the voltage is approximately 0.3 V, the transistor is germanium. If the voltage is approximately 0.7 V, the transistor is silicon.

3. Operating frequency range? Identify the type of circuit and determine whether it is working in the audio range, the kilohertz range, or the megahertz range.

4. Operating voltage? Voltages from collector to emitter, collector to base, and emitter to base should be noted either from the schematic diagram or by actual voltage measurement. The transistor selected for replacement should have voltage ratings that are at least three to four times the actual operating voltage. This helps to protect against voltage spikes, transients, and surges that are inherent in most circuits.

5. Collector current requirements? The easiest way to determine the actual current is to measure the current in the collector circuit with an ammeter. This measurement should be taken under maximum power conditions. Again, a safety factor of three to four times the measured current should be allowed.

6. Maximum power dissipation? Use maximum voltage and collector current requirements to determine maximum power requirements (P = IE). The transistor is a major factor in determining power dissipation in the following types of circuits:

  • Input stages, AF, or RF (50 to 200 mW)
  • IF stages and driver stages (200 mW to 1 W)
  • High-power output stages (1 W and higher)

7. Current gain? The common emitter small signal DC current gain referred to as or Beta should be considered. Some typical gain categories are:

  • RF mixers, IF and AF (80 to 150)
  • RF and AF drivers (25 to 80)
  • RF and AF output (4 to 40)
  • High-gain preamps and sync separators (150 to 500)

8. Case style? Frequently, there is no difference between the case styles of original parts and recommended replacements. Case types and sizes need only be considered where an exact mechanical fit is required. Silicon grease should always be used with power devices to promote heat transfer.

9. Lead configuration? This is not a prime consideration for replacement transistors, although it may be desirable for ease of insertion and appearance.

Summary

A transistor is a three-layer device used to amplify and switch power and voltage.

A bipolar transistor is also called a junction transistor or simply a transistor. Transistors can be configured as NPN or PNP.  The middle region of the transistor is called the base, and the two outer regions are called the emitter and collector.

A transistor is classified according to whether it is NPN or PNP, silicon or germanium, high or low power, and switching or high frequency. Transistors are identified with a prefix of 2N followed by up to four digits.

The transistor package provides protection, a heat sink, and a support for the leads. Transistor packages are identified with the letters TO (transistor outline).

In a properly biased transistor, the emitter-base junction is forward biased and the collector-base junction is reverse biased. PNP transistor bias sources are the reverse of NPN bias sources.

The internal barrier voltage for germanium transistors is 0.3 V and for silicon transistors is 0.7 V.

The reverse-bias voltage applied to the collector-base junction is higher than the forward-bias voltage applied to the emitter-base junction.

When a transistor is tested with an ohmmeter, each junction exhibits a low resistance when it is forward biased and a high resistance when it is reverse biased. Transistor testers are available for testing transistors in and out of circuit.

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