Test: Electronic Devices - 1

• The material which is not a good conductor, or a good insulator is called as a semiconductor.For example, silicon, germanium, etc.
• The charge carriers which are present in more quantity in a semiconductor compared to other particles are called the majority charge carrier.
• The impurity atoms added are called dopants and semiconductors doped with the impurity atoms are called extrinsic or doped semiconductors.
• An ideal, perfectly pure semiconductor (with no impurities) is called an intrinsic semiconductor.
• At absolute zero temperature valence band is full of electrons and the conduction band is empty, hence there are no free electrons in the conduction band and holes in the valence band.
• The charge carrier concentration is zero at zero absolute temperature.
• Hence, at zero absolute temperature the intrinsic semiconductor behaves like insulator.

There are two types of semiconductors:

P-type semiconductor:

• The semiconductor having holes as majority charge carriers and electrons as a minority charge carrier is called a P-type semiconductor.

N-type semiconductor:

• The semiconductor having electrons as majority charge carriers and holes as a minority charge carrier are called as N-type semiconductors.

Under equilibrium:

• The fermi level across the entire material will be sum and does not vary with distance.
  
• If there is any disturbance in the material, like junction contact, injection of impurities at any point, the charge carriers redistribute themselves such that the fermi-potential is same in entire material.

Under non-equilibrium:

• The fermi level is uneven with gradient of charge distribution across distance, in material.
• It can be studied using quasi-fermi states, related to charge distribution.

Property of Semiconductors:

• Semiconductors are the materials that have a conductivity between conductors (generally metals) and non-conductors or insulators (such as ceramics).
• Semiconductors can be compounds such as gallium arsenide or pure elements, such as germanium or silicon.
• Semiconductors have an almost empty conduction band and an almost filled valence band.
• In a semiconductor, the mobility of electrons is higher than that of the holes.
• Its Resistivity lies between 10^-5 to 10⁶ Ωm
• Conductivity lies between 10⁵ to 10^-6 mho/m
• The temperature coefficient of resistance for semiconductors is Negative.
• The current Flow in the semiconductor is mainly due to both electrons and holes.

Using the relation, D = 0.025 × 1900 cm²/sec
= 47.5 cm²/sec

As 25% of donor atoms are ionised, the occupation probability of donor level is 0.75.

E_D - E_F = -0.028 eV

The intrinsic concentration is related to bandgap by:

E_g = 0.0518 ln (10⁸) eV

= 0.954 eV

Diffusion constant: 

Diffusion length: 

• Hole concentration at any distance x is sum of equillibrium and excess hole concentration excess hole concentration varies with the distance x as Δp e^-x/Lp
• p = p_o­ + Δp e^-x/Lp
  Where p_o = Doping concentration
  Δp = extra hole concentration
  = (1+.379) x 10¹⁷
  1.379 × 10¹⁷ cm^-3

• In an intrinsic semi-conductor the Fermi energy:Where N_C and N_V are density of state in valence and conduction bond respectively.
• Since N_C ∝ m_n, effective mass of electron and N_v ∝ m_p, effective mass of hole. Since m_n and m_p are not equal: 

Given ND = 2.25 × 1015 cm^-3

Partial ionization of 55%, number of electrons is:

⇒ n = 0.55 × 2.25 × 1015 cm-3

= 1.2375 × 1015 cm-3

Electron mobility μn = 1000 cm2/v-s

Resistivity of n-type silicon = 

= 5.05 Ω – cm

= 1.6 × 10¹³/cm³

= 0.16 × 10¹⁴/cm³