Semiconductor Electronics: Materials, Devices And Simple Circuits

- What you are getting from a statement might not be necessarily true. Let's burst some of the common misconceptions
1
Where exactly do hole current and electron current flow inside a semiconductor?
  At room temperature, some electrons in the valence band gain enough energy to jump in conduction band, thus becoming free electrons. The generation of free electrons leaves behind vacancies in the valence band, called the holes. The current constituted by free electrons within the conduction band is called electron current.
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Electrons that remain in the valence band can move into a nearby vacancy (hole) left due to free electron. This movement of valence electrons within the valence band constitutes the hole current.
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NOTE: Electron current and hole current both are caused due to flow of electrons. Under an external electric field, both the free electrons responsible for electron current and valence electrons responsible for hole current flow towards the positive terminal. Conventional direction of both the currents is taken to be opposite to the flow of electrons.
2
At the breakdown voltage, can the reverse current through the zener diode increase upto any value?   From the I-V characteristics of a Zener diode, it may seem that the reverse current can increase upto any any value.
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However, for safe operation, following parameters are defined for the zener diode in reverse bias:
  • Power rating : The maximum power the Zener diode can dissipate; given by the product of voltage across the diode and the current flowing through.
  • Current : Maximum current at the rated Zener voltage
  • Current : Minimum current required for the diode to break down.
The curent shouldn't exceed the prescribed value given by or else the diode will be permanently damaged due to excessive heating caused by the large reverse current.
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3
Can we use an LED as a light source directly by connecting it across a battery?
No, LEDs are very delicate devices. We must use a resistor in series with the LED to limit the current flowing through the LED. If we connect LED to the battery without any resistor, then the LED will burn immediately.
But what should be the value of resistor to be used with LED? Generally, an LED consumes 20mA of current (Forward Current ) and has voltage drop called Forward Voltage () of 2-3 V.
So here we can calculate the resistor value by using the basic Ohm's law which states:

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4
Is sunlight always required for a solar cell to generate EMF?
The photo-excitation in a solar cell takes place as long as the energy of incident photons is greater than the band gap of the semiconductor used, i.e. . So, we can use any light whose photon energies satisfy this condition. We can even use the light from an incandescent bulb!
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Sunlight is required to generate EMF only in solar cells which use semiconductors having band gap in the range 1.0 to 1.8 eV. It is because the solar radiation (sunlight) contains a majority of photons having energy concentrated at E=1.5 eV. Thus, it is economically more efficient to exploit these abundant photons by using semiconductors like Si () and GaAs ().
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