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Electronic Devices

(ECE F214)

Dr. Manish Gupta

Department of Electrical and Electronics Engineering BITS-Pilani, K K Birla Goa Campus

Lecture – 19

PN-Junction Diode

Content

❑ Distribution of Carriers during Forward Bias and Reverse

Bias

❑ Quasi Fermi Level in PN-Junction

❑ Capacitance of PN-Junction

Carrier Concentrations

Reverse Bias Condition (assuming NA> ND)

• Under the reverse bias condition, the depletion region act like a ‘sink’ for minority carriers.
• The minority carrier concentration at the depletion edge reduces to zero or very minimal.
• Deep in to the semiconductor, the minority carrier concentration is equals to the equilibrium minority carrier concentration.

Quasi Fermi Level in PN Diode

Forward Bias Condition

• The above figure shows that the equilibrium fermi-level (EF) is split into the quasi fermi-levels i.e. F n and F p

• The quasi fermi level within depletion region i.e. ‘W’ is separated by an energy qV, where V is the applied bias.
• The recombination of the carriers on the either side of the junction i.e. electrons on p-side and holes on n-side depends on the diffusion length ( L n and L p

Quasi Fermi Level in PN Diode

Reverse Bias Condition

The minority carrier concentration on either side of the junction is given by (4) (5)

Quasi Fermi Level in PN Diode

Reverse Bias Condition

If a sufficiently high reverse bias is applied i.e. V = - Vr, eq. 4 and eq. 5 can be written as If Vr >> KT/q, Similarly,

Capacitance of PN-Junction

The two types of capacitance associated with the junction diode

• Junction Capacitance:
  • It arises due to the dipole (positive and negative ions) in the transition region
  • Dominant when the diode is reverse biased
• Charge Storage Capacitance:
  • Due to the storage of the charges
  • Dominant when the diode is forward biased

Capacitance of PN-Junction

Also, the positive charge due to ions on the n-side exactly balances the negative charge due to ions on the p-side

Capacitance of PN-Junction

Also,

On substituting eq. 5 in eq. 4 (6)

Differentiating eq. 6 with respect to (V 0 - V)

Capacitance of PN-Junction

Junction Capacitance or (Using eq. 3)

Problem

A silicon sample is maintained at 300 K and it is characterized by the energy band diagrams as shown below in Fig. a and Fig. b i) Do the equilibrium condition prevails? ii) Sketch the electrostatic potential (V) and electric field as a function x. Also, sketch n and p (on log scale) as a function x.

Thank you