Transistor (NPN & PNP)

Transistor Report

        1. Introduction   

        Transistor is a semiconductor device which is used to amplify the signals as well as in switching circuits. Generally transistor is made of solid material which contains three terminals such as emitter (E), Base (B) and Collector (C) for connections with other components in the circuit. Some transistors contains fourth terminal also substrate (S). Transistor is one of the active components. Transistor is the proper arrangement of different semiconductor materials. General semiconductor materials used for transistor are silicon, germanium, and gallium-arsenic. Basically the transistors are classified depending on their structure.

2. Types of Transistor

Transistor Tree Diagram

.The transistors classification can be understood by observing the above tree diagram. Transistors are basically classified into two types: They are Bipolar Junction Transistors (BJT) and Field Effect Transistors (FET).

. The BJTs are again classified into NPN and PNP transistors.

. The FET transistors are classified into JFET and MOSFET.

. Junction FET transistors are classified into N-channel JFET and P-channel JFET depending on their function.

. MOSFET transistors are classified into Depletion mode and Enhancement mode. Again depletion and enhancement mode transistors are classified into N-channel JFET and P-channel.

 

Ø (BJT) Bipolar Junction Transistors

 

o   Junction transistor is generally called as Bipolar Junction Transistor (BJT). The BJT transistors have three terminals named emitter (E), Base (B), Collector (C). The name itself indicates that it has two junctions between p-type and n-type semiconductors. The BJT transistors are classified in to NPN and PNP transistors depending on the construction.

o  Unlike FET transistors, the BJT transistors are current-controlled devices. If small amount of current flows through the base of a BJT transistor then it causes to flow large current from emitter to collector. The BJT transistors have low input impedance and it causes to flow large current through the transistor.

o   The BJT transistors are only the transistors which are turned ON by the input current which is given to the base. Bipolar junction transistors can operate in three regions, they are

     - Cut-off Region: Here the transistor is in ‘OFF’ state the current flowing                                through the transistor is zero.

- Active Region: Here the transistor acts as an amplifier.

- Saturation Region: transistor is in fully ‘ON’ state and also works as a                              closed switch.

 

·        NPN Transistor

-       NPN is one of the two types of Bipolar Junction Transistors (BJT). The NPN transistor consists of two n-type semiconductor materials and they are separated by a thin layer of p-type semiconductor. Here the majority charge carriers are electrons and holes are the minority charge carriers. The flowing of electrons from emitter to collector forms the current flow in the transistor through the base terminal.

-        A small amount of current at base terminal causes to flow large amount current from emitter to collector. Nowadays the generally used bipolar transistor is NPN transistor, because the mobility of electrons is greater than mobility of holes. The standard equation for the currents flowing in the transistor is:  I_E = I_B + I_C

·       PNP Transistor

-      PNP is another type of Bipolar Junction Transistors (BJT). The PNP transistors contain two p-type semiconductor materials and are separated by a thin layer of n-type semiconductor. The majority charge carriers in the PNP transistors are holes and electrons are minority charge carriers. The arrow in the emitter terminal of transistor indicates the flow of conventional current. In PNP transistor the current flows from Emitter to Collector.

-   The PNP transistor is ON when the base terminal is pulled to LOW with respect to emitter. The symbol and structure for PNP transistor is shown below.

Ø (FET) Field Effect Transistors

 

    The Field-Effect-Transistor (FET) is another transistors type. Basically the FET transistors have three terminals they are gate (G), Drain (D) and Source (S). FET transistors are classified into Junction Field Effect transistors (JFET) and Insulated Gate FET (IG-FET) or MOSFET transistors. For the connections in the circuit we also consider fourth terminal called base or substrate. The FET transistors have control on the size and shape of a channel between source and drain which is created by applied voltage. The FET transistors are unit-polar transistors because they perform single channel operation whereas BJT transistors are bipolar junction transistors. The FET transistors have high current gain than BJT transistors.

 

 

·       JFET (Junction-Field Effect Transistor)

                   The Junction-Field-Effect transistor (JFET) is an earliest and simple type of FET transistors. These JFETs are used as switches, amplifiers and resistors. This transistor is a voltage controlled device. It doesn’t need any biasing current. The voltage applied between gate and source controls the flow of electric current between source and drain of a transistor. The JFET transistors are available in both N-channel and P-channel types.

§  N-Channel JFET

In N-channel JFET the current flow is due to the electrons. When voltage is applied between gate and source, a channel is formed between source and drain for current flow. This channel is called N-channel. Nowadays N-channel JFET transistor is most preferable type than P-channel JFET. The symbols for N-channel JFET transistor are given below.

§  P-Channel JFET

       In this JFET transistor the current flow is because of holes. The channel between source and drain is called P-channel. The symbols for P-channel JFET transistors are given below. Here arrow marks indicates the direction of current flow.

·       MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor)

-     Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) is most useful type of among all transistors. The name itself indicates that it contains metal gate terminal. The MOSFET has four terminals drain, source, gate and body or substrate (B). MOSFET has many advantages over BJT and JFET, mainly it offer high input impedance and low output impedance. It is used in low power circuits mainly in chip designing technologies.

-  The MOSFET transistors are available in depletion and enhancement types. Further the depletion and enhancement types are classified into N-channel and P-channel types.

      

§  N-Channel MOSFET

The MOSFET having N-channel region between source and drain is called N-channel MOSFET. Here the source and gate terminals are heavily doped with n-type materials and substrate is doped with p-type semiconductor material. Here the current flow between source and drain is because of electrons. The gate voltage controls the current flow in the circuit. N-channel MOSFET is most preferable than P-channel MOSFET because the mobility of electrons is high than mobility of holes. The symbols for N-channel MOSFET transistors are given below.

§  P-Channel MOSFET

The MOSFET having P-channel region between source and drain is called as P-channel MOSFET. Here the source and drain terminals are heavily doped with P-type material and the substrate is doped with N-type material. The current flow between source and drain is because of holes concentration. The applied voltage at gate will controls the flow of current through channel region. The symbols for P-channel MOSFET transistors in depletion and enhancement types are given below.

    3. Measurement terminals (E, B, C)

v How to check the transistor emitter, base & collector with multi meter:

1. Using an ohmmeter, measure the resistance between each pair of transistor terminals The two lowest values are from base to emitter and from base to collector. This identifies the base terminal.

2. Whether the transistor is PNP or NPN is shown by the polarity of the meter test leads in measurement of forward resistance.

3. Apply the ohmmeter test leads to the collector and emitter terminals of the transistor (which is C and which is E is unknown at this time).

4. Pinch the base lead and one of the other leads between the thumb and forefinger to provide "bleeder resistance." Note the resistance reading.

5. Pinch the base lead and the remaining other lead between the thumb and forefinger to provide "bleeder resistance." Note the resistance reading.

6. Reverse the ohmmeter leads and repeat steps 4 and 5.
7. The collector is the terminal that provides the lowest resistance reading when its test voltage is "bled" into the base terminal.

 

    4. Measurement transistor to find Error

                        Testing transistors in circuit.

Most resistance tests on semiconductors assume that the transistor, diode etc. has first been undersold and removed from the circuit. However this is only one way to test a transistor, and is usually used to confirm earlier tests done with the circuit in "working" (though faulty) condition. These tests involve measuring the voltages on the suspect transistor with the circuit switched on and are part of a full fault finding process. There are however some simple voltage indications that can indicate with a good degree of certainty, whether a suspect transistor is faulty.

1. More than 0.7V difference between base and emitter voltages indicates an open circuit b-e junction.

2. The same voltage on two or more terminals MAY indicate one or more short circuit  junctions.

3. A LOWER than expected collector voltage generally means that the transistor is conducting heavily (turned on).

4. A HIGHER than expected collector voltage generally means that the transistor is not conducting (turned off).

    5. Build test circuit NPN/PNP (Schematic)

NPN (BC547)

Pin Number	Pin Name	Description
1	Collector	Current flows in through collector
2	Base	Controls the biasing of transistor
3	Emitter	Current Drains out through emitter

BC547 Transistor

- Bi-Polar NPN Transistor

- DC Current Gain (h_FE) is 800 maximum

- Continuous Collector current (I_C) is 100mA

- Emitter Base Voltage (V_BE) is 6V

- Base Current(I_B) is 5mA maximum

- Available in To-92 Package

Schematic of NPN and PNP

PNP (2N2907)

Pin No.	Pin Name	Description
1	Emitter	Current Drains out through emitter
2	Base	Controls the biasing of transistor
3	Collector	Current flows in through collector

2N2907 PNP Transistor

     

            - Having a high value of current (max. 600 mA)

-  Low voltage value (max. 40 V)

-  Comes in different type of packages – TO-92, TO-18

-  These are Lead (PB) free devices

-  Collector to Emitter voltage (VCEO) is 40v (max.)

-  Collector to Base voltage (VCBO) is 60v (max.)

-  Emitter to Base voltage (VEBO) is 5v (normally)

-  Maximum value of Collector current is 600mA

-  Power dissipation at ambient temperature is about 400mW

-  Having DC current gain (HFE) of 100 to 300 (max.)

-  Temperature of operation and storage is -65 to +150 °C

Schematic of PNP and NPN 

    6. Experiment (simulation)

v Switch (Relay)

v Microphone

v Flip flop

v Light Switch

    7. Experiment (Real Test)

v Switch (Relay)

v Flip flop