Npn Transistor Forward Bias

In the npn transistor in what is called active mode the baseemitter voltage and collectorbase voltage are positive forward biasing the emitterbase junction and reverse biasing the collectorbase junction. In the previous tutorial we saw that the standard bipolar transistor or bjt comes in two basic forms.

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Transistor biasing can be defined as the proper flow of zero signal collector current and the maintenance of proper collector emitter voltage during the passage of signal.

Npn transistor forward bias. Working of npn transistor the emitter base junction of a transistor is forward biased whereas collector base junction is reverse biased. This causes the emitter current i e. The base emitter junction behaves like any other pn junction when viewed alone.

In the active mode of operation electrons are injected from the forward biased n type emitter region into the p type base where they diffuse as minority carriers to the reverse biased n type collector and are swept away by the electric field in the reverse biased collector. Due to the forward applied voltage at the emitter base junction the width of the depletion region gets narrowed. If you forward bias a diode the diode conducts.

This is just like a diode. The collector is the thickest region and the base is the thinnest region of the npn transistor. Transistor bias voltages are largely dependent on transistor beta b so the biasing set up for one transistor may not necessarily be the same for another transistor as their beta values may be different.

The figure shows the biased condition of npn transistor. If for a moment we ignore the presence of emitter base junction then practically note 1 no current would flow in the collector circuit because of the reverse bias. If the base emitter junction is forward biased the transistor is on.

A pnp transistor behaves exactly the same as an npn device with the exception that the majority charge carriers are holes. An npn negative positive negative type and a pnp positive negative positive type. If you reverse bias the diode the diode cuts off.

The emitter of the npn transistor is heavily doped. The npn transistor consists two n type semiconductor material which is separated by a thin layer of p type material. When the forward bias is applied across the emitter the majority charge carriers move towards the base.

Transistor biasing can be achieved either by using a single feed back resistor or by using a simple voltage divider network to provide the required biasing voltage. The basic purpose of transistor biasing is to keep the base emitter junction forward biased and collector base junction reverse biased at any instant of the applied signal. Similarly the reverse applied voltage broadens the width of the collector base junction.

As illustrated in fig. Holes are emitted from the p type emitter across the forward biased be junction into the base. 4 9 the be junction is forward biased by an external voltage source and the cb junction is reverse biased.

Npn transistors are three terminal three layer devices that can function as either amplifiers or electronic switches. The base of the npn transistor is lightly doped. The emitter base region of the transistor is in forward biasing and the collector base region is connected in reverse bias.

If it is reverse biased the transistor is off. The electrons enter into the p type material and combine with the holes.

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