A bipolar junction transistor (BJT) is a semiconductor device constructed by layering three doped semiconductor materials to form two P-N junctions. An electrode is attached to each layer of the the BJT.
The middle layer of the bipolar junction transistor, called the base, consists of either n-type or p-type material, and the outside layers, called the collector and emitter, are the opposite of the base type. As such, there are are two types of bipolar junction transistors - PNP transistors and NPN transistors.
Figure 1. NPN bipolar junction transistor (left) and PNP bipolar junction transistor (right)
The amount of current allowed to flow through a bipolar junction transistor (the collector-emitter current) is controlled by the base-emitter (control) current. The potential applied to the base of the bipolar junction transistor can thus be used to control the current through the device.
Figure 2. Collector current IC versus Collector-emitter voltage VCE for different values of Base current IB
Bipolar junction transistors are commonly used in this way as amplifiers and switches and are employed in an enormous variety of applications either as individual components or as parts of other devices. The simplicity and low production cost of bipolar junction transistors makes them one of the most important devices in the field of electronics.
Basic Bipolar Junction Transistor Circuits
The various possibilities of making connections to the three electrodes of the bipolar junction transistor result in three basic BJT circuits:
- Common Emitter BJT: in the common emitter circuit, the emitter terminal connection is common to both the base and collector. The input current is the emitter current and the output current is the collector current.
- Common Base BJT: in the common base circuit, the base terminal connection is common to both the emitter and collector. The input current is the emitter current and the output current is the collector current.
- Common Collector BJT: in the common collector circuit, the collector terminal connection is common to both the emitter and base. The input current is the base current and the output current is the emitter current.
These three basic configurations for bipolar junction transistors are shown in figure 3.
Figure 3. Basic bipolar junction transistor circuits (Ue = Input Voltage, Ua = Output Voltage, Ucc = Battery Voltage)
Current Amplification with a Bipolar Junction Transistor
The collector current IC in the bipolar junction transistor is controlled by a much smaller base current IB. When the base current is zero, it is difficult for current to pass through the depletion zone of the collector-base P-N junction. When the base current is greater than zero, the depletion zone is lessened, and the transistor allows a large number of the electrons that come from the emitter through the the thin base. These electrons are accelerated into the collector. The collector current is proportional to the base current (current amplification). The thinner the base, the stronger the current amplification.
Figure 4. NPN bipolar junction transistor with base current = 0 (left) and base current >0 (right)
The degree of current amplification is different for each of the bipolar junction transistor configurations mentioned above and is also dependent on the characteristics of the individual transistor. The current gain of the transistor is defined as the ration of the output current to the input current, .
Common base bipolar junction transistor
The current gain for the common base configuration of the bipolar junction transistor is given by , where IC is the collector current and IE is the emitter current. The value of α is always less than 1.
Common base bipolar junction transistors are characterized by their low imput impedance and sensitive response to high frequencies. Common base bipolar junction transistors are frequently used as pre-amplifiers for microphones and as high frequency amplifiers.
Common emitter bipolar junction transistor
The current gain for the common emitter configuration of the bipolar junction transistor is given by , where IC is the collector current and IB is the base current.
For the common emitter bipolar junction transistor, the emitter current may be written as .
Combining this with the equations for α and β gives and .
Common emitter bipolar junction transistors are generally characterized by their high current and power gain. Common emitter bipolar junction transistors are the most commonly used transistor configuration, often being employed for the amplification of faint signals.
Common collector bipolar junction transistor
The current gain for the common base configuration of the bipolar junction transistor is given by , where IE is the collector current and IB is the base current.
Using the results above, this may be written as . Given that β is generally large, often one simply writes for the common collector configuration.
Common collector bipolar junction transistors are characterized by their low output impedance and high current and power gain. For this reason, the common collector configuration is often used as an impedance matching amplifier.
Combination of Bipolar Junction Transistors
For some applications it is beneficial to use multiple transistors in combination in order to meet design requirements. Two examples of such instances are given below.
Single stage and Darlington transistor
Current amplification is extremely low in high-blocking transistors since the base zone must be thickened in order to absorb reverse voltage. To rectify this, two transistors are used to form a Darlington circuit in order to increase the current amplification in the emitter circuit by .
Figure 5. Single stage and Darlington bipolar junction transistors
Driver and final stage transistors can be integrated into a single piece of silicon, as shown on the right in figure 5.
Three level bipolar junction transistor
Figure 8. Three level bipolar junction transistor
Semikron's three level transistor, also known as the Trilington, is made up of three transistors. The switching behaviour of a transistor worsens as transistor levels increase. This switching behavior between the transistors can be improved significantly using the so called "speed-up" diodes. These speed-up diodes can be integrated into the transistor monolithically.
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