Bipolar Junction Transistor

A Bipolar Junction Transistor is a semiconductor device consisting of two P-N Junctions connecting three terminals called the Base, Emitter and Collector terminals. The arrangement of the three terminals affects the current and the amplification of the transistor. The behavior of Bipolar junction transistors is also very different for each circuit configuration. The three different circuit configurations produce different circuit characteristics with regards to input impedance, output impedance and gain. These characteristics affect whether the transistor exhibits voltage gain, current gain or power gain. One of the primary operations of a bipolar junction transistor is to amplify the signal of the current. Bipolar junction Transistors are able to regulate the current so that the current magnitude is proportional to the biased voltage applied at the base terminal of the transistor. The application of Bipolar Junction Transistors can be found in devices that utilize analog circuits such as computers, mobile phones and radio transmitters.

INTRODUCTION Bipolar Junction Transistors have three semiconductor regions. The three regions are the emitter region (E), base region (B), and the collector region (c) and these regions are differently doped depending on the type of bipolar transistor it is.
The two types of bipolar transistors are the PNP Transistor, whose three regions are p type, n type, and p type respectfully, and NPN Transistor, whose regions are n type, p type, and n type respectfully. Both types of transistors have one P-N junction between the collector region and base region and another P-N junction between the base region and emitter region. The base region is always the structure's center connection with the emitter and collector regions connected on either side. Both types of transistors also have the same principle of operation, with the single difference being in the polarity of power and biasing for each type. Bipolar Junction Transistors ability to amplify a signal, through the regulation of current, allows for the transfer of an input signal from one circuit to another, regardless of the different level of resistance in each circuit. The amount of current flowing through the transistor is proportional to the magnitude of the biasing voltage applied to the base terminal. This allows the transistor to act like a current-controlled switch. Depending on whether the bipolar transistor is PNP or NPN, the controlled current will flow from the collector to the emitter or from the emitter to the collector while the smaller controlling current will flow from base to emitter or from emitter to base respectively.
The transistor contains a maximum allowed current that is able to restrict the amount of current as it passes from terminal to terminal. Depending on the order of the terminals in the transistor, the transistor will act as either a conductor or an insulator when in the presence of a controlled current. This ability to change between these two states, insulator or conductor, enables the transistor to act like a switch or as an amplifier of small amplitude signals applied to the base depending on the structure and order of the three semiconductor regions.

NPN Bipolar Junction Transistor
A NPN Bipolar Junction Transistor has a P-doped semiconductor base in between an N-doped emitter and N-doped collector region. NPN bipolar transistors are the highest used bipolar transistors due to the ease of electron mobility over electron hole mobility.
For this type of transistor, large magnitude collector and emitter currents get produced through the amplification of a small current which enters through the base. This small current only gets amplified when the transistor becomes active.
In this active state, a positive potential difference is found between both the base region to the collector region and the emitter region to the base region which results in current that gets carried by electrons, between the collector and emitter regions. The construction and terminal voltages for a NPN Transistor are shown in Figure 1   The current flowing out of the transistor must be equal to the currents flowing into the transistor as the emitter current is given as Note: "Ic" is the current flowing into the collector terminal, "Ib" is the current flowing into the base terminal and "Ie" is the current flowing out of the emitter terminal.
Since the physical construction of the transistor determines the electrical relationship between these three currents, (Ib), (Ic) and (Ie), any small change in the base current ( Ib ), will result in a much larger change in the collector current ( Ic ).
The ratio of the collector current to the emitter current is called Alpha (α).
Alpha (α) = Ic/Ie (2) The current gain of the transistor from the Collector terminal to the Emitter terminal, Ic/Ie, is a function of the electrons By combining the expressions for both Alpha, α and Beta, β the current gain of the transistor can be given as: As seen from the equations above, electron mobility between the Collector and Emitter circuits is the only link between these two circuits. This link is the main feature of transistor action. Since transistor action is constituted by initial electron movement through the base region, the amplifying properties of the transistor comes from the consequent control the Base exerts on the current between the Collector and Emitter. As long as the flow of the biasing current into the base terminal is steady, the base region can be treated as a current control input.  1. Cutoff: The Cutoff region is when the transistor is inactive due to minimal current being passed through the transistor, which makes the transistor appear as an open circuit. Both VBE and VBC are reverse biased so all depletion region edges exhibit small minority carrier densities. This region has biasing conditions opposite of saturation.

PNP Bipolar Junction Transistor
2. Forward-active: The Forward-active region occurs when the transistor is in its active state which allows the transistor to amplify the voltage variations present on the base. With the base-emitter junction is forward biased and the base-collector junction is reversed biased, the transistor can amplify voltage because the collector to emitter voltage is greater than the base to emitter voltage and is also in between the cutoff and saturation states. The output current is proportional to the base current and can be extracted at the collector.
3. Reverse-active: The Reverse-active region occurs when the transistor is in its active state but the maximum current gain in the reverse active mode is much smaller than the forward active mode. The biasing conditions are reversed so that the base collector junction is forward biased and the base emitter junctions is reverse biased, which switches the roles of the collector and emitter regions. The base contains a much lower reverse bias voltage than in the forward-active region.

Saturation:
The saturation region allows the transistor to conduct current from the emitter to the collector. With both the base collector junction and the base emitter junction forward-biased, the base current is so strong it exceeds the magnitude at which it can increase the collector current flow. As a result, the circuit between the collector and emitter terminals appears to have short circuited due to the over saturation of current.

CONFIGURATIONS
There are three methods of connection for a Bipolar Junction Transistor within an electronic circuit. The Common Base configuration, Common Emitter configuration and Common Collector configuration all respond differently to the circuit's input signal, thus varying the characteristics of each configuration.

The Common Base Configuration
The common base configuration has a strong high frequency response which is good for single stage amplifier circuits.
However, is not very common due to its low current gain characteristics and low input impedance. The input signal gets applied between the base and emitter terminals while the output signal is taken from between the base and collector terminal. For this to occur, the base terminal has to be grounded so the reference voltage is a fixed amount. The Common Base Configuration is shown below.

Figure 5 The Common Base Transistor Circuit
This type of amplifier configuration is a non-inverting voltage amplifier circuit. The configuration has a resistance gain due to the ratio between the load resistance (Rload) in series with the collector and the Rin resistor. The input current flowing into the emitter is the sum of both the base current and collector current respectively therefore, the collector current output is less than the emitter current input resulting in a current gain. Its input characteristics represent that of a The Common Emitter Configuration The common emitter amplifier configuration produces the highest current and power gain of all the three bipolar transistor configurations which is why this type of configuration is the most commonly used circuit for transistor based amplifiers. The input signal applied between the base and emitter is small due to the forward biasing of the PN junction and the output taken from between the collector and emitter is large due to the reverse biased PN junction.
This is mainly because the input impedance is small as it is connected to a forward biased PN-junction, while the output impedance is large as it is taken from a reverse biased PN-junction. However, its voltage gain is much lower. The Common Emitter configuration is shown below. Figure 6 The Common Emitter Amplifier Circuit The common emitter configuration is an inverting amplifier circuit. Therefore the output signal is out-of-phase with the input voltage signal.

The Common Collector Configuration
The common collector configuration is very useful for impedance matching applications because of the very large ratio of input impedance to output impedance. The configuration has the input signal directly connected to the base. With the emitter region in series with the load resistor, the current flowing through the load resistance is the same value as the emitter current. This is why the output is taken from the emitter load and the current gain of the configuration is approximately equal to the β value of the transistor.

Figure 7 The Common Collector Transistor Circuit
This type of bipolar transistor configuration is a non-inverting circuit in that the signal voltages of Vin and Vout are "inphase". The load resistance receives both the base and collector currents which results in a large current gain as well as providing good current amplification with very little voltage gain.