Tranzistora Bipolara Junction

Dîrok BJT

Bipolar Junction Transistor (BJT an jî BJT Transistor) yek dêvice semiconductorka sê terminalî ye ku ji du p-n junctions ve hatîye çêkirin. Ev dêvice dikevin taybetmendiyên signalan biguhezine. Ên dêvice ya kontrolkirina rojan ye. Sê terminalên BJT hene: base, collector û emitter. BJT cûrî transistora e ku hem elektron û hole yên bi guhertina rojê bikar dike.

Yek signal ku amplitûdê ya bicîk dikê, heta base serê, li ser collector ên transistorek da ku amplitûdê ya zêde hatiye girtin. Ev û ewa bêtguhertina ku BJT pêşkêş dike. Li her navberî, ev xwesteke şevirina DC power supply-ê din ku bêtguhertina bike.

Hejmar div bistolarî bipolar junction transistors ne – NPN transistors û PNP transistors. Wêne dîroka bistolarî bipolar junction transistors de heye:

Li ser wêneya jor, dibe ku her BJT hase sê parçeyên emmitter, base û collector. JE û JC wekheviyên emmitter û collector nîşan dide. Li gorînav ber, ya basa bêtgotin û ji bo collector-base junctions bêtgotina werdigire. Mijara dawer dikare bistolarî transistorean dike.

NPN Bipolar Junction Transistor

Di n-p-n bipolar transistor (an npn transistor) de, yek p-type semiconductor di du n-type semiconductors de ne. Wêne dîroka n-p-n transistor de heye. Ji bo emitter, IC û IE hene current an emitter û collector. VEB û VCB voltage an emitter-base û collector-base nîşan dide. Li gorî navberî, îma ku ji bo emitter, base û collector current IE, IB û IC current di transistore de derbas bike, nîşanîya current positive hatiye girêdan û ger current ji transistore derbas bike, nîşanîya negative hatiye girêdan. Dibe ku me tabulate different currents û voltages inside the n-p-n transistor bikin.

PNP Bipolar Junction Transistor

Lambar bîyî, ji bo p-n-p bipolar junction transistor (an pnp transistor), yek n-type semiconductor di du p-type semiconductors de ne. Wêne dîroka p-n-p transistor de heye.

Ji bo p-n-p transistors, current di transistore de derbas bike ji bo terminala emitter. Li gorî navberî bistolarî bipolar junction transistor, emitter-base junction forward biased û collector-base junction reverse biased e. Me dibe ku tabulate emitter, base û collector current, as well as the emitter-base, collector base û collector-emitter voltage for p-n-p transistors bikin.

Çalakî BJT

Wêne dîroka n-p-n transistor de heye ku active region de bias kirin (transistor biasing binihêrin), BE junction forward biased e û CB junction reverse biased e. Width of the depletion region of the BE junction piqnik e ku têra CB junction.

Forward bias at the BE junction barrier potential lower kirin, electrons flow from the emitter to the base. Ji bo base thin û lightly doped, holes yekêm dikin. About 2% of the electrons from the emitter recombine with holes in the base û flow out through the base terminal.

Ev base current e, it flows due to recombination of electrons and holes (Note that the direction of conventional current flow is opposite to that of the flow of electrons). The remaining large number of electrons will cross the reverse-biased collector junction to constitute the collector current. Thus by KCL,

Base current very small as compared to emitter and collector current.

Here, the majority of charge carriers are electrons. The operation of a p-n-p transistor is same as of the n-p-n, the only difference is that the majority charge carriers are holes instead of electrons. Only a small part current flows due to majority carriers and most of the current flows due to minority charge carriers in a BJT. Hence, they are called as minority carrier devices.

Cihaneke Equivalent BJT

A p-n junction is represented by a diode. As a transistor has two p-n junctions, it is equivalent to two diodes connected back to back. This is called as the two diode analogy of the BJT.

Characteristics of Bipolar Junction Transistors

The three parts of a BJT are collector, emitter and base. Before knowing about the bipolar junction transistor characteristics, we have to know about the modes of operation for this type of transistors. The modes are

• Common Base (CB) mode

• Common Emitter (CE) mode

• Common Collector (CC) mode

All three types of modes are shown below

Now coming to the characteristics of BJT there are different characteristics for different modes of operation. Characteristics is nothing but the graphical forms of relationships among different current and voltage variables of the transistor. The characteristics for p-n-p transistors are given for different modes and different parameters.

Common Base Characteristics

Input Characteristics

For p-n-p transistor, the input current is the emitter current (IE) and the input voltage is the collector base voltage (VCB).

As the emitter-base junction is forward biased, therefore the graph of IE Vs VEB is similar to the forward characteristics of a p-n diode. IE increases for fixed VEB when VCB increases.

Output Characteristics

The output characteristics show the relation between the output voltage and output current IC is the output current and collector-base voltage and the emitter current IE is the input current and works as the parameters. The figure below shows the output characteristics for a p-n-p transistor in CB mode.

As we know for p-n-p transistors I E and VEB are positive and IC, IB, VCB are negative. These are three regions in the curve, active region saturation region and the cut off region. The active region is the region where the transistor operates normally.

Here the emitter junction is reverse biased. Now the saturation region is the region where both the emitter-collector junctions are forward biased. And finally, the cut off region is the region where both emitter and the collector junctions are reverse biased.

Common Emitter Characteristics

Input characteristics

IB (Base Current) is the input current, VBE (Base – Emitter Voltage) is the input voltage for CE (Common Emitter) mode. So, the input characteristics for CE mode will be the relation between IB and VBE with VCE as a parameter. The characteristics are shown below

The typical CE input characteristics are similar to that of a forward-biased of p-n diode. But as V CB increases the base width decreases.

Output Characteristics

Output characteristics for CE mode is the curve or graph between collector current (IC) and collector-emitter voltage (VCE) when the base current IB is the parameter. The characteristics is shown below in the figure.

Like the output characteristics of common – base transistor CE mode has also three regions named (i) Active region, (ii) cut-off regions, (iii) saturation region. The active region has collector region reverse biased and the emitter junction forward biased.

For cut-off region, the emitter junction is slightly reverse biased and the collector current is not totally cut-off. And finally for the saturation region both the collector and the emitter junction are forward biased.