165.2Base CurrentBase current consists of two components.DiffusioncurrentRecombinationcurrentRecombination current is dominant.The value of base current is very small.DVRE1WN.WiβD, N, LpB2 D,Tb2025/11/27
5.2 Base Current 2 1 1 2 BE BE T T v v p A V V C B s s n D P n b D N W W i i I e I e D N L D = + = = 2025/11/27 16 ⚫ Base current consists of two components. ➢ Diffusion current ➢ Recombination current ⚫ Recombination current is dominant. ⚫ The value of base current is very small
175.3Common-Emitter Current GainExpressionforcommon-emittercurrentgain:N1WV2NDD1β ishighlyinfluenced bytwo factors> thewidth of the base region>the relative dopings of the baseand theemitter regions2025/11/27
5.3 Common-Emitter Current Gain ⚫ Expression for common –emitter current gain: 2025/11/27 17 2 p A n D P n b i D N W 1 W = 1 i D N L 2 D = + C B c b e iE iC iB ⚫ is highly influenced by two factors ➢ the width of the base region ➢ the relative dopings of the base and the emitter regions
185.3Common-Emitter Current GainAhighβishighlydesirablesinceitrepresentsa gainparameterInordertoobtainahighβ,thebaseregionshouldbethin and lightly dopedand theemitter regionheavily dopedβisintherange50to200forgeneraltransistors2025/11/27
5.3 Common-Emitter Current Gain ⚫ A high is highly desirable since it represents a gain parameter ⚫ In order to obtain a high , the base region should be thin and lightly doped and the emitter region heavily doped. ⚫ is in the range 50 to 200 for general transistors 2025/11/27 18
195.3Explanation of the Physical Structureohighlydopingintheemitterregion andforwardbiasedEBJ:>Moreelectronscan beemitted fromthisregiontothebaseregion1017EnnnoBnoDpO1010CBnMnoEPODno1032025/11/27
⚫highly doping in the emitter region and forward biased EBJ: 2025/11/27 19 5.3 Explanation of the Physical Structure 1017 1010 103 ➢ More electrons can be emitted from this region to the base region
205.3ExplanationofthePhysicalStructurevery thin base region:Much sharpergradientMuchlessrecombinationlargeareathecollectorregionandreversebiasedCBJ:Muchmoreelectronsemittedfromtheemitterregioncanbe“captured"by the strong electricfieldinthereverseCBJintothecollectorregion2025/11/27
⚫ very thin base region: ➢Much sharper gradient ➢Much less recombination ⚫ large area the collector region and reverse biased CBJ: ➢Much more electrons emitted from the emitter region can be “captured” by the strong electric field in the reverse CBJ into the collector region 2025/11/27 20 5.3 Explanation of the Physical Structure n +