Bipolar Junction Transistors - Part 1

In this series of articles I will be covering the bipolar junction transistor, how it works and how it can be used in a huge variety of circuits.

History

John Bardeen, William Shockley and Walter Brattain.

The Bipolar Junction Transistor (BJT) was invented in December 1947 by John Bardeen and Walter Brattain at Bell Telephone Laboratories in the United States of America, this point contact transistor was unreliable, expensive and by today's standards performed terribly, as such commercial use was extremely limited.

The far superior grown-junction transistor was invented on June 23 1948 by William Shockley and was announced by Bell Labs in 1951, improvements came rapidly with the modern diffusion transistor being released in 1954, low cost planar transistors which could be used in integrated circuits were invented in 1959 by Jean Hoerni at Fairchild.

By the late 50’s Bipolar Junction Transistor were rapidly replacing vacuum tubes offering lower cost, smaller size and immunity to vibration, by the late 60’s vacuum tubes were essentially obsolete except in specialist applications.

The BJT remained the only transistor of choice until the invention of the Junction Field Effect Transistor (JFET) and Metal Oxide Semiconductor Field Effect Transistor (MOSFET), these have their own advantages and disadvantages and all three remain in usage.

Device Overview

The BJT is a three terminal semiconductor device, the terminals are called the Collector, Base and Emitter, two different types of BJT are available, the NPN and PNP, these are very similar with the main difference being how they are connected to the circuit.

The most basic view of a BJT is as a current controlled device, the base-emitter current programs the collector-emitter current, this is useful as the base current is often orders of magnitude smaller than the collector current, in other words a tiny base current results in a much larger collector current, in essence power gain which is what makes transistors so useful.

In a manner the amplification of a BJT is similar to a relay where the small coil current controls the typically larger current though the contacts, except unlike a relay which can only be on or off, a BJT can be at any state between on and off allowing for the accurate reproduction of analog signals.

Junctions

The BJT is constructed from a combination of N and P type semiconductors (usually silicon), NPN and PNP, this results in two junctions similar to a pair of back to back diodes, this is important for correctly biasing the transistor however you’ll note you can’t simply make a BJT from diodes, the transistor action is mainly a result of the very thin base region.

In typical operation for a NPN the Base-Emitter junction is forward biased and the Base-Collector junction in reverse biased, with a PNP the inverse is true, Emitter-Base junction is forward biased and Collector-Base is reverse biased.

Current Gain

One very important parameter is the forward current gain called Beta (β) or hFE, this is a unit-less number defined as:

`beta = I_C / I_B`

Where
`I_C` = DC Collector Current
`I_B` = DC Base Current

For your typical transistor this may be around 100, so if you put 1mA in to the base the collector current will be 100 times greater, or 100mA.

It’s important to keep in mind that current gain is not a stable parameter, it varies with collector current, temperature and other factors so it’s wise to design circuits that do not depend on a specific value for beta, nevertheless this simple view of the BJT is enough to make some useful circuits.

Transistor Switch

Transistors are ideal for switching, unlike mechanical switches these can operate much faster and do not generally wear out, the BJT however cannot switch AC since the reverse breakdown voltage of the base-emitter junction is approximately 6V.

The main concern when designing a BJT switch is to ensure there is sufficient base current to turn the transistor on fully, to minimize power dissipation you want the voltage drop across the collector-emitter junction to be as small as possible, the minimum drop is called the VCE saturation voltage which is around 0.2V to 0.4V, as such it's always a good idea to supply more base current than is technically required.

As a practical example let's assume we want to switch 100mA 10V, using the very common 2N3904 NPN transistor, looking at the datasheet a minimum current gain at 100mA is given as 30, from that we can calculate the needed base current.

`I_C / beta = I_B`

`0.1 / 30 = 3.33mA`

As a precaution we will increase this to 5mA which is a nice round figure, to calculate the base resistor to provide this we will make an assumption that the base-emitter voltage drop is 0.7V, this is usually close enough, assuming we are connecting the base to the 10V supply the resistance needed is:

`(10-0.7)/0.005 = 1860 Omega`

1.8k is a standard E12 series resistor so we will go with that, we can then calculate the power dissipation, assuming VCE(SAT) is 0.3V the collector dissipation is:

`0.3V * 0.1A = 30mW`

The base current is a small contribution but it's still worth including.

0.7V * 5mA = 3.5mW

The result is a total dissipation of 33.5mW which is well within the 625mW limit of the 2N3904.

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In the next part more useful amplifier circuits will be covered.


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