PNP and NPN transistors

PNP and NPN transistor, transistor input, output and transfer characteristics in different configurations:

Bipolar Junction Transistor (BJT):

The transistor is a three terminals semiconductor device in which two P-N junction diode is sandwich in a single semiconductor crystal. Thus, transistor is semiconductor device from by back to back connection of two P-N junction diode. The transistor are basically, use for amplification of current signal. A bipolar junction transistor (BJT) is a type of transistor that uses both electron and hole charge carriers, such .In contrast, unipolar transistors, only use one kind of charge carrier. For their operation, BJTs use two junctions between two semiconductor types N-types and P-types.

BJTs are manufactured in two types, NPN and PNP, and are available as individual components, or fabricated in integrated circuits, often in large numbers. The basic function of BJT is to amplify current. This allows BJTs to be used as amplifier or switches, giving them wide applicability in electronic equipment, including computers, television, mobile phones, audio amplifier, industrial control and radio transmitters.

Each transistor has three region called emitter(E),base(B) and collector(C) and two P-N junction namely emitter base junction (E/B junction) and collector base junction(C/B junction ).Depending upon central region of transistor these are two types ;

(1)NPN transistor

(2)PNP transistor

NPN transistor:

The transistor whose central region is made by P-types impurities which separate the two side region made by N-types impurities.

Figure

PNP transistor:

The transistor whose central region is made by N-types impurities which separates two side region made by P-types impurities.

Figure

Each transistor have following three different region regardless its types PNP and NPN.

(1)Emitter (E):

It is of side region of transistor which is heavily doped. It is called emitter as it has to emit the large number of majority charge carrier into base region. Due to this flow of majority charge carrier large current flows through emitter is called emitter current.

(2)Base (B):

It is the central region of transistor is very thin and lightly doped. Due to recombination of majority charge carriers with that of charge carriers with that of charge carrier in base region. Small current flows through base terminal called as base current\(\left(I_B\right)\).The size of base current is made thin to reduce the loss of current due to recombination. The base region is narrow region than other two regions.

(3)Collector (C):

It is one of the side regions of transistor which is large in size and made totally doped. This means it is lightly doped than emitter region but heavily doped than base region. The main function of collector is accommodating the majority charge carrier that are injected from the inter conversion between emitter and collector region isn’t possible. Due to recombination of charge carrier through the collector base junction, collector current \(I_c\) follows through the collector.

Biasing of transistor:

For the normal operation of transistors, it is essential to supply the voltage of current polarity to transistor circuit which is called as biasing of transistor. During biasing of transistor there should be;

(1)The emitter base junction is always forward biased.

(2)The collector base junction is always reverse biased.

(3)The proper selection of point.

Transistor action and current:

Figure

Let us take PNP transistor which is connected to external supply i.e. \(V_{EE}\) across collector base junction with proper biasing i.e. emitter base junction is forward biased and collector base junction is reversed biased.

In such biasing positive terminal of \(V_{EE}\) is connected to p-region if emitter so that the positive charge carriers are injected from emitter to base producing emitter current through the circuit. However, base region receives the holes from the emitter so that the electron hole recombination occurs in best regions producing small base current \(I_B\).The negative terminal of \(V_{cc}\) affects the holes from collector region so that majority charge carrier that present in N-region injected into collector region producing the collector current \(I_C\).

From Kirchhoff’s law the total current entering into transmitter should always be equal to that leaving through transmitter that is ,$$ I_E=I_B+I_C$$This condition is valid for both PNP and NPN transistors.

Applications of BJTs:

The BJT remains a device that excels in some applications, such as discrete circuit design, due to the very wide selection of BJT types available, and because of its high transconductance and output resistance compared to MOSFETs

The BJT is also the choice for demanding analog circuits, especially for very-high-frequency applications, such as radio-frequency circuits for wireless systems. Emitter-coupled logic (ECL) use BJTs. Bipolar junction transistor can combined with MOSFETs in an integrated circuit by using a BiCMOS process of water fabrication to create circuits that take advantage of the application strengths of both types of transistor.

Vulnerabilities for resistor:

Exposure of the transistor to ionizing radiation causes radiation damage .Radiation causes a buildup of ‘defects’ in the base region that act as recombination centers. The resulting reduction in minority carrier lifetime causes gradual loss of gain of transistor.

Power BJTs are subjected to a failure mode called secondary breakdown, in which excessive current and normal imperfections in the silicon die cause portions of the silicon inside the device to become disproportionately hotter than the others. The electrical resistivity of doped silicon, like other semiconductors, has a negative temperature coefficient, meanings that it conducts more current at higher temperatures. Thus, the hottest part of the die conducts the most current, causing its conductivity to increase, which then causes it to become progressively hotter again, until the device fails internally. The thermal runway process associate with secondary breakdown, once trigged, occurs almost instantly and may catastrophically damage the transistor package.

If the emitter-base junction is reverse biased into avalanche or Zener mode and current flows for a short period of time, the current gain of the BJTs will be the permanently degraded.

References:

(1)Theraja, B.L. Basic Electronics. N.p.: S.Chand, n.d. Print.

(2)C.L.Arora. Refresher Course in Physics. Vol. II and III. N.p.: S.Chand, 2006. Print.

(3)Malvino. Electronic Principles. N.p.: Tata McGraw-Hill, n.d. Print.

(4)N.Nelkon and P.Parker. Advanced Level Physics. 5th ed. N.p.: Arnold Heinemann, n.d. Print.

(5)Priti Bhakta Adhikari,Diya Nidhi Chaatkuli, Ishowr Prasad Koirala. A Textbook of Physics (2nd Year). N.p.: Sukunda Pustak Bhawan, 2070. Print.