All bipolar devices including LEDS and solar cells have emitter layers which injects minority carriers into adjacent base region. According to the Shockley theory , the emitter is characterized by an injection efficiency which is the ratio of the current injected from the emitter into the base to the sum of this current and the back injected current from the base into the emitter which makes out the total emitter current. For good operation of the bipolar devices the emitter injection efficiency must be made as high as possible.he
This is achieved basically by making the doping of the emitter layer is much greater than that of the base. Practically, it is observed there there is a limit with such increase. Increasing the doping of the emitter beyond certain value reduces the emitter injection efficiency again.
Within the scope of this question, it is required to discuss the factors limiting the emitter injection efficiency and the methods of further enhancing it.
I observed that no one tell now tried to answer this question. Therefore, i divide it here into parts to be more specific. So it is required to discuss the limitations on the emitter injection efficiency of the the homojunction bipolar transistor. Then we discuss how the injection efficiency is enhanced in the advanced bipolar transistor structures like the hetero junction bipolar transistor and the use of strained silicon as a base material.
One other device in which the emitter efficiency is very important is the light emitting diode since as the emitter injection efficiency increase the light emitting efficiency of the diode rises up.
So, let us confine ourselves on these two important devices.
Thank you
The injection efficiency of the transistor emitter is controlled by the ratio of emitter doping concentration to the base doping concentration. As this ratio increases, the injection efficiency increases. Since the base doping is determined by the required base resistance one has to increase the doping of the emitter to the highest possible level, the solid solubility of the dopant atoms in the semiconductor material. This doping limit is about 10^20/cm^3. Unfortunately it is observed that the injection efficiency increase with the emitter doping concentration reaches a maximum value at doping concentration of 10^19/cm^3 and then decreases again with further increasing the doping. The investigations showed that the high doping effects are responsible of this phenomenon.
The high doping effects are the smaller minority carrier lifetime and the effective increase in the intrinsic concentration because of the band gap narrowing. The researcher then discovered that there is a fundamental limit to the emitter injection efficiency due to high doping effects.
In order to increase further the injection efficiency one has to mitigate this high doping effects. This accomplished this by:
-Hetero junction emitter with wider bandgap than the base
-lowering the bandgap in the base region by straining its material.As an example for straining material. is doping silicon with germanium. The resulting silicon geremanide has lower band gap.
Here is a paper demonstrating how using the SiGe layer has considerably boots the gain of the transistor.
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