BJT (Bipolar Junction Transistor) in power electronics is a transistor and its operation is purely based on the contact that two semiconductors make. BJT acts as a switch, amplifier or oscillator and it is called as a bipolar transistor and it needs two types of charge carriers (holes and electrons) for an operation. Holes comprise of the leading charge carriers in P-type semiconductors where as electrons will be the key charge bearers in N-type semiconductors.
BJT will have two P-N junctions that are connected back to back and also share a common region B (base). This connection will make sure that the contacts are made in all the regions which are base, collector and emitter. Below diagram shows the structure of a PNP bipolar transistor.
BJT shown above includes two diodes that are connected back to back, leading to the depletion of the regions called quasi-neutral. Width of quasi-neutral of the emitter, base and collector are indicated above as WE’, WB’ and WC’. They are obtained as shown below
Conventional signs of the currents for the emitter, base and collector will be denoted by IE, IB and IC correspondingly. Thus, the collector and base current will be positive when a positive current is meeting the collector or base contact. Additionally, emitter current will be positive when the current is leaving the emitter contact. So,
While applying a positive voltage to the base contact comparative to the collector and emitter, base-collector voltage and base-emitter voltage will also become positive.
For ease, VCE is assumed to be zero.
Diffusion of electrons happens from the emitter to the base when diffusion of holes is originated from the base to the emitter. When the electrons have reached the base-collector depleted region, they will be cleaned up by an electric field using the region and these electrons will be forming the collector current.
While BJT is inclined in the forward active mode, total emitter current will be achieved by adding the electron diffusion current (IE,n), hole diffusion current (IE, p) and base emitter current.
Total collector current will be given by the electron diffusion current (IE,n) less than the base recombination current(Ir,B).
Sum of the base current IB can be achieved by adding the hole diffusion current (IE, p), base recombination current (Ir,B) and the base-emitter recombination current of the depletion layer (Ir,d).
Transporter factor can be achieved by the ratio of the collector current and the emitter current.
By applying the Kirchhoff’s current law, we can observe that the base current is given by the difference between the emitter current and the collector current.
Current gain can be acheived by the ratio of the collector current to the base current.
Above formula describes how a BJT will produce current amplification and the transport factor (α) will approach one if the collector current is equal to the emitter current. Therefore, current gain (β) will be greater than one.
For more analysis, transport factor (α) will be rewritten as a product of the emitter efficiency (γE) the base transport factor (αT) and the recombination factor of the depletion layer (δr). It can be rewritten as shown below
below is the summary of the emitter efficiency, base transport factor and depletion layer recombination factor.
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