311 TRANSISTOR B SIDES FREE
The problem now is that a free charge requires some extra energy to overcome the barrier that now exists for it to be able to cross the depletion region junction.Then the presence of impurity ions on both sides of the junction causes an electric field to be established across this region with the N-side at a positive voltage relative to the P-side.As the N-type material has lost electrons and the P-type has lost holes, the N-type material has become positive with respect to the P-type.Punch-through is undesirable and hence it is avoided in circuit design.In this condition holes are swept directly from the emitter region to the collector and the normal transistor action is lost.The punch-through breakdown occurs if the collector-base reverse bias voltage is increased far enough to the extent that the depletion region fills the entire base.This is explained with the help of the following V CE (Reverse voltage) vs I C (Collector current) curve: The slope introduced by the Early effect is almost linear with I C and the common-emitter characteristics extrapolate to an intersection with the voltage axis V A, called the Early voltage.The problem now is that a free charge requires some extra energy to overcome the barrier that now exists for it to be able to cross the depletion region. The decrease in base width causes ß to increase and hence collector current increases with collector voltage rather than staying constant. Then the presence of impurity ions on both sides of the junction causes an electric field to be established across this region with the N-side at a positive voltage relative to the P-side.This reduction in the effective base width causes less recombination of carriers in the base region which results in an increase in collector current.This reduces the effective base width and hence the concentration gradient in the base increases.As reverse biasing of the collector to base junction increases, the depletion region penetrates more into the base, as the base is lightly doped.A large collector base reverse bias is the reason behind the early effect manifested by BJTs.Voltage-controlled device: voltage between gate and source control the current through the device.Ĭurrent-controlled device: Base current control the amount of collector current Unipolar device: Uses only one type of charge carrierīipolar device: Uses both electron and hole
The difference between FET and BJT is explained in the following table: majority and minority electrons and holes) It is a bipolar device (current conduction by both types of carriers, i.e.The current through the two terminals is controlled by a current at the third terminal (base).J = Junction refers to the two PN junctions between emitter and base, and collector and base.Basically, in this type of transistor construction the two diodes are reversed with respect to the NPN type giving a P ositive- N egative- P ositive type of configuration, with the arrow which also defines the Emitter terminal this time. B = Bipolar (because conduction is due to two opposite type of carriers Holes and electrons) The PNP Transistor is the exact opposite to the NPN Transistor device we looked at in the previous tutorial.BJT stands for Bipolar junction transistor
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