Semiconductors and Transistors 101 – The BJT
Image Credit: Wikipedia
In the previous post we saw what a semiconductor, depletion layer and p-n junction are. Let’s now see how they are they used to make transistors?
The above image is a replica of the first working point contact single transistor! Huge isn’t it? These days hundreds can fit onto a pin head,
Transistors
There are several types of transistor:
- Bipolar junction transistor – BJT (the original active semiconductor device)
- Junction field effect transistor – JFET
- Metal oxide semiconductor FET – MOSFET – Sometimes referred to as an insulated gate FET – IGFET
The first working point contact transistor was invented in 1947, the first JFET in 1953 and the first MOSFET in 1959.
Let’s take a look at the construction, operation and uses of each of these devices.
The Bipolar Junction Transistor – BJT
Like the triode valve, in order to make the PN junction really useful we need to be able to control the flow of electrons across it. This is achieved by making either a PNP or NPN sandwich of semiconductor material. PNP transistors were the earliest type of devices due to the available materials technology. Now NPN devices are much more widely used, and will only be considered here.
Simplified NPN BJT Construction
If we dope a piece of intrinsic material as shown above we end up with three distinct extrinsic connection areas:
- Collector
- Base
- Emitter
Bipolar Transistor Symbols
BJT NPN Operation
If we connect batteries as shown above then with Vbe=0 volts no collector current (Ic) will flow due to the collector/base junction being reverse biased and the base/emitter P-N junction depletion layer voltage. If we now gradually increase Vbe, the depletion layer voltage is eventually overcome and base current (Ib) flows from the emitter to the base. As the P type base region is very narrow and lightly doped, only a few (<5%) electrons combine with holes to create the base/emitter current (Ib), the remaining electrons (>95%) are able to flow across it into the collector region where they create an external collector current (Ic). Adjusting the base current by feeding it a varying voltage signal, therefore changes the collectors current. The device therefore acts as a current amplifier with current gains of over 500. This changing collector current is converted to a voltage by inserting a load into the collector circuit. The NPN devices resulting characteristic is as shown below:
Typical NPN BJT Characteristic
The basic differences for a silicon PNP BJT is that both battery voltages are reversed as is the current flow.
The range of BJT NPN and PNP devices is vast being specifically designed for high/low power, AF, RF, low noise, switching, high/low voltage and all combinations in between. Very common audio devices would include: voltage amplifiers BC107/8/9, ZTX107/108/109 and power devices like the BFY51, BFX88, 2N3055 and BDY90. There is a vast range of circuit designs that use these devices in many configurations and it is found in many items of HT equipment from amplifiers, processors to projectors and TV’s. They support all sorts of signal processing functions from audio to RF as voltage and current amplifiers and will be found in phono and line level voltage amplifiers, equalizers, speaker current power amplifiers, power supplies and even tape machine motor control. BJT’s can be used as light sensitive devices, photo-transistors, like the early OCP71 and many BJT devices are available as PNP versions and can be manufactured to create matched and complementary NPN/PNP BJT pairs for class AB push-pull audio power amplifiers.
Typical individual BJT devices – low power to high power
The next post in this series will review the Field Effect family of transistors (FET).
Here are the other two parts to this series of posts:
- Semiconductors and Transistors 101
- Semiconductors and Transistors 101 – The FET (Field Effect Transistor)
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