Theory 30 :- Types of transistors and applications. Leads of transistors and their identification.

 

Transistors are the semiconductor devices having three or four leads terminals.

Fig 1 a shows some typical transistors.

Fig shows the symbols used for different types

Transistors are mainly used for enlarging or amplifying small electrical /electronic signals   The circuit which uses transistors for amplifying is known as a transistor amplifier. One other important application of transistors is it use as a solid state switch. A solid state switch is nothing but a switch which does not involve any physical ON/OFF contacts for switching. Transistors can be thought of as two PN junction diodes connected back to back.

Compared with the present day transistors the vacuum tubes were big in size, consumed more power, generated lot of unwanted heat and were fragile. Hence vacuum tubes became absolute as soon as transistors came to market. Transistors were invented by Walter H. Brazil and John Barlow of Bell Telephone Laboratories on 23rd Dec. 1947. Compared to vacuum tubes (also known as valves),transistors have several advantages.  

Some important advantages are:

•  Very small in size  
•  Light in weight
•  Minimum or no power loss in the form of heat
•  Low operating voltage
•  Rugged in construction 

To satisfy the requirements of different applications, several types of transistors in different types of packaging are available , transistors are given a type number such as BC107,2N 6004 etc., The characteristics data corresponding to these type numbers are given in Transistor data

CLASSIFICATION OF TRANSISTORS

1 Based on the semiconductor used.

•  Germanium transistors
•  Silicon transistors 

Like in diodes, transistors can be made, using any one of the above two important semiconductors. However, most of the transistors are made using silicon. This is because, silicon transistors work better over a wide temperature range (higher thermal stability) compared to germanium transistors. Transistor data books give information about the semiconductor used in any particular transistor

2 Based on the way the P and N junctions are organized  as shown

•  NPN transistors 
•  PNP transistors 

Both NPN and PNP transistors are equally useful in electronic circuits. However, NPN transistors are preferred for the reason that NPN has higher switching speed compared to PNP.

3 Based on the power handling capacity of transistors

Low power transistors, also known as small signal amplifiers, are generally used at the first stage of amplification in which the strength of the signal to be amplified is low. For example, to amplify signals from a microphone, tape head, transducers etc., 267 Medium power and high power transistors, also known as large signal amplifiers are used for achieving medium to high power amplification. For example, signals to be given to loudspeakers etc. High power transistors are usually mounted on metal chassis or on a physically large piece of metal known as heat sink. The function of heat sink is to, take away the heat from the transistor and pass It  to air. 

4 Based on the frequency of application High power transistors

•  Low freq. transistors (Audio frequency or AF transistors)
•  High freq. transistor (Radio frequency or R/F transistors

Amplification required for signals of low or audio range of frequencies in Tape recorders, PA systems etc., make use of A/F transistors. Amplifications required for signals of high and very high frequencies as, in radio receivers, television receivers etc., use R/F transistors

5 Based on the manufacturing method 

• Grown junction
• Alloy junction
• Planar type
• Point contact
• Epitaxial 
• Mesa

 6 Based on the type of final packaging

•   Metal
•   Plastic ‐ Ceramic

Metal packaged transistors are generally used in medium and high power amplifications. Plastic packaging is generally used for low power amplification. Some plastic packages come with a metal heat sink. Such transistors are used for medium power amplification. Ceramic packaging is used for special purpose very high frequency applications, for higher temperature stability etc., some examples of packaging type codes used with transistors are, TO‐3, TO‐92, SOT‐25 and so on.

Transistor data books give information about the type of packaging and its case outline. Chart 1 at the end of this lesson illustrates some popular transistor case outlines and pin/lead information.

INSIDE A TRANSISTOR

Inside a transistor there are two PN junctions connected to each other   Outside a transistor, one can see only three leads. These leads are known as base, emitter and collector as shown  in Fig, the three leads/pins/pigtails called base, emitter and collector are taken from each of doped semiconductor material.

In simple terms, as shown in Fig , the function of the base, emitter and collector regions of a transistor are, Emitter ‐ emits current carriers(electrons/holes) 

• Collector ‐ collects current carriers 
• Base ‐ controls flow of current carriers from emitter to collector. 

While connecting a transistor to a circuit, it is necessary  to identify the base, the emitter and the collector pins. A Transistor data book gives information on pin identification of transistors. However, it is convenient to put sleeve wires over the transistor pins for the following reasons; 

• for easy identification while wiring 
• sleeves act as spacers while mounting and soldering 
• they ensure the required minimum lead distance from the solder joint to the transistor body. 

Following colour scheme is suggested for putting sleeves to transistor pins although, any convenient colour scheme may be adopted.

• Base pin ‐ Blue colour sleeve 
• Emitter pin ‐ Red colour sleeve 
• Collector pin ‐ Yellow colour sleeve 
• Shield pin  ‐ Black colour sleeve