How does a bjt operation

And, as mentioned earlier, the emitter region is heavily doped. So in an npn transistor, the n-type emitter region has a very high density of free electrons while in a pnp transistor the p-type emitter region has a very high density of holes.

Since the base-emitter junction is forward-biased, free electrons from the emitter region easily cross the base-emitter junction and go into the very thin and lightly doped p-type base region.

In this case, only a small percentage of the free electrons from the emitter region can recombine with the holes in the base region. The small number of free electrons from the emitter region that recombined with the holes in the base region move through the base region as valence electrons.

But when they leave the base region and move through the metallic base lead, they become free electrons and produce the external base current, which then goes out through the metallic lead, into the external circuit, and then, eventually, return to the emitter region. Since the collector region is connected to the positive side of the external bias voltage, the free electrons are attracted to the positive side and are swept across into the collector region.

They exit the collector region and and also move through the metallic collector lead, into the circuit, and return into the emitter region. So in this case, we know that the emitter current is the sum of the base and collector currents. Therefore, the emitter current is slightly greater than the collector current.

The operation inside a pnp transistor is very similar to the npn type. But the roles of the electrons and holes are swapped.

The external bias voltages and the current directions are all reversed. In this way very small changes in base current cause very large changes in the current flowing from emitter to collector, so current amplification is taking place. Hons All rights reserved. Revision Semiconductors 2. Diodes 3. Bipolar Junction Transistors 4. Field Effect Transistors 5. Opto-Coupled Devices 6. Equation 4 expresses this rule mathematically. Alpha describes the ratio of the DC collector current to the DC emitter current.

Refer to the figure below. We know that V BE is about 0. Once we know this, finding the base current is a cinch. Take a gander at the circuit below. Beta is Determine all node voltages and branch currents. Some may ask why bother using a transistor as a switch when we can just use a mechanical switch.

There are a few good reasons for this. First, transistors can switch on and off extremely fast, typically a fraction of a micro-second or less. Second, they can be driven from other circuits such as a microcontroller whereas a mechanical switch cannot.

Mechanical switches wear out and the contacts bounce while transistors do not. And there are other reasons. Cutoff mode is self-explanatory — the device is off and is not conducting any current except for a negligible leakage current. When the BE junction is forward biased and there is enough base current to produce a maximum collector current the transistor is saturated.

Equation 8 gives the formula for collector current when the device saturates. The minimum base current the BJT needs for saturation is:. Finally, some inquisitive readers may be asking if there is a formula for V CE sat. The LED in the figure below needs 30 mA to emit an aesthetically pleasing level of light. So, the collector current should be about 30 mA. Use double the minimum base current as a safety margin because ruining the mood lighting by having the LED turn off would make her want to leave.

This gives us about 1. We know that the beta of any random BJT varies widely. Because beta is somewhat unpredictable, you should avoid designing and building circuits that depend specifically on beta values to work correctly.

In other words, a circuit that depends on a particular value of beta is a bad circuit. Also, all transistors have a maximum collector current rating as well maximum collector to base, emitter to base, and collector to emitter breakdown voltages. You also need to be aware of the maximum power dissipation rating.

Ignoring these specs can kill your BJT. Bipolar junction transistors come in a variety of packaging types. Some come in plastic, others in metal can-like packages. Look at the package to see if it has a pinout diagram on it.

A good diode shows high resistance with reverse bias and a low resistance with forward bias. An open diode will have a high resistance both ways. A shorted diode will have low resistance both ways. That wraps up our intro to BJTs.

Meanwhile, leave a comment and tell me about your latest endeavor using discrete BJTs. What are you driving? An LED?