Question

1) Briefly explain the difference between the I-V relationships of a silicon and a gallium arsenide PN diode. 2) A certain semiconductor has energy gap of 2.48eV. Determine the range of wavelengths that are absorbed by the semiconductor 21 Why is the collector current in a PNP transistor in the inverse active region much smaller than in the active region?

Answer 1

1.) I-V characteristics curves of both Si and GaAs based P-N diodes have nearly same nature. The difference between their I-V characteristics are-

1. In Forward region, cut-in potential or barrier height for both p-n junctions are different. It is V_b= 0.7 V for Si based p-n junction while for GaAs based p-n junction V_b=1.2 V.
2. In reverse region, GaAs based p-n diode has less reverse saturation current (I_s= 1pA Approx.) than Si based p-n diode (i_s= 10pA Approx.). And in case of Si based p-n diode reverse breakdown earlier than that of GaAs based p-n diode as shown in figure.

CneAs

2.) Energy band gap E_g=2.48 eV.

photons with energy less than the band gap energy will pass though the semiconductor while photons with energy greater than band gap energy are absorbed.

The wavelength of photons that can be absorbed by semiconductor can be determined as

ic ic

6.626 × 10-34 × 3 × 108 2.48 × 1.6 × 10-19

λ= 5.009 × 10-m 5009.4

Thus photons with wavelength less than   are absorbed within the semiconductor.

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3.)In active region of PNP transistor the emitter-base region is forward biased, while the collector-base junction is reverse biased so holes are injected into the base. The base is very thin, and most of the holes cross the reverse-biased base–collector junction and to be collected at the collector.So collector current is mainly due to the injection of holes from heavily doped emitter.

$I_{c}=\beta I_{E}$

where $\beta$ is current amplification factor.

But in case of inverse active region E-B junction is reverse biased, so minority charge carriers i.e. electrons will be injected into base ,since base is very thin so electrons are now collected at collectors. Here collector current is due to minority charge carriers of heavily doped emitter so this current must bs less than the current in cace of active region.