TF EIM Chapt5

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Bipolar Transistor

The Bipolar transistor concept

From the last chapter we saw a p-n junction diode which had similar characteristic current-vs- voltage responses when biased either in the forward direction or backward.


TF EIM Diode V-vs-I curve.jpg


TF EIM ForwardBiased bottomhalfBipolar.jpg
The n-p junction. The depletion region is represented as the shaded square The potential
TF EIM ReverseBiased tophalfBipolar.jpg
The n-p junction. The depletion region is represented as the shaded square. Notice the depletion region (and E-field in that region) is larger than the forward biased n-p juntion the potential
TF EIM BipolarJunction.pngTF EIM pnpBipTran.png TF EIM BipolarJunctionCircuit.png
The n-p junction. The depletion region is represented as the shaded square. Notice the depletion region (and E-field in that region) is larger than the forward biased n-p juntion the potential

The bottom p-n junction, when forward biased, provides a large current with a small voltage bias.

The top half of the p-n-p transistor is reversed biased, providing a "throttle" for the large current from the lower half.

The electric field points to the "collector" in the depletion region enlarged by the reverse bias. This moves the majority charge carriers away from the collector-base junction. Once outside the depletion zone, the charge carriers rely on diffusion to escape the material.

BIpolar transistor currents

Conservation of Current
[math]\Rightarrow I_E = I_B + I_C[/math]

Here it is the case that the positive current is representing the flow of positively charged holes.


You might think that the base current is larger than the collector current because of the reverse bias on the collector which leads to the base having lower resistance than the collector.

The above is not the case.

Transistor Physical features

The three layers

A transistor has three semi-conductor payers. The three layers occur in the order of

p-n-p= holes-electron-hole

n-p-n= electron-hole-electron


The three layers are formed from a single crystal such that the doping of the single crystal defines the boundaries. The three layers are not "glued" together.

Base-Emmiter-Collector

Forest_Electronic_Instrumentation_and_Measurement