Semiconductor Physics: p-n Junctions

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Questions and Answers

What is the primary function of the native oxide on silicon in the context of p-n junctions?

It lowers the resistance of the silicon surface.

It facilitates the bonding of dopants.

It acts as a good insulator preventing current flow. (correct)

It enhances current flow through the junction.

Which of the following processes is NOT typically used to form a p-n junction?

Dopant diffusion

Ionic implantation

Thermal oxidation (correct)

Epitaxy

What is crucial about the placement of a p-n junction in relation to the semiconductor surface?

It should be as close to the surface as possible.

It must be formed in the bulk of the semiconductor. (correct)

It needs to be on the surface for effective operation.

It can be formed anywhere with no constraints.

What role does a concentration gradient play in carrier diffusion?

It drives the movement of carriers from high to low concentration. Signup and view all the answers

What must be true for a n-well in relation to a p-type substrate?

It needs to be reverse biased from the substrate. Signup and view all the answers

What causes parasitic resistance in a p-n junction?

The well resistance in the junction. Signup and view all the answers

What is the result of removing the divider in the gas diffusion analogy?

Gas will fill the entire volume of the new chamber. Signup and view all the answers

What is indicated by a discontinuity in the Fermi level across a p-n junction?

A barrier to carrier movement. Signup and view all the answers

What components contribute to the total current in a p-n junction at equilibrium?

Both drift and diffusion Signup and view all the answers

In a resistor, how does the carrier density behave?

It is approximately uniform Signup and view all the answers

What must be constant in a solid under thermal equilibrium?

Fermi level (Ef) Signup and view all the answers

What is referred to as the built-in potential in a p-n junction?

The potential barrier height across the junction Signup and view all the answers

Which derivatives are used to express the equilibrium conditions in a p-n junction?

First derivatives of charge density and potential Signup and view all the answers

What happens to the total current in a PN junction when the system is at thermal equilibrium?

It is zero Signup and view all the answers

Which equation represents the drift current in a p-n junction?

$J = q\mu nE$ Signup and view all the answers

What characterizes the depletion region in a p-n junction?

Absence of majority carriers Signup and view all the answers

What happens when a small voltage greater than 0 is applied to an MS(n-type) contact with m < s?

There is a large current flow from S to M. Signup and view all the answers

Under equilibrium, what condition is met at an MS(n-type) contact with m > s?

The net flow of carriers is zero, and the Fermi-level remains constant. Signup and view all the answers

What is the term used for the energy barrier encountered by electrons in a semiconductor flowing from M to S?

Barrier height ($B$) Signup and view all the answers

What occurs shortly after the formation of the contact when m > s in an MS(n-type) contact?

A depletion layer is formed, resulting in a built-in electric field. Signup and view all the answers

When V < 0 is applied to an MS(n-type) contact with m < s, what is the expected behavior?

Electrons can flow easily from S to M. Signup and view all the answers

What happens to the diffusion current without external bias?

It remains small since few carriers can penetrate the barrier. Signup and view all the answers

How is the drift current affected in a p-n junction when reverse bias is applied?

It remains unchanged. Signup and view all the answers

What is the primary effect of applying forward bias to a p-n junction?

Diffusion current increases exponentially. Signup and view all the answers

Which statement about the diffusion current in a reverse-biased p-n junction is correct?

It decreases exponentially. Signup and view all the answers

What defines the net result of a p-n junction under large forward bias?

A large forward current is generated. Signup and view all the answers

Which of the following statements is true regarding the effects of minority carriers in drift and diffusion currents?

Diffusion current depends exponentially on the barrier. Signup and view all the answers

What factor contributes to the small current observed in a reverse-biased p-n junction?

The combination of increased barrier and constant drift current. Signup and view all the answers

What condition leads to a strong diffusion current in a p-n junction?

Forward bias applied. Signup and view all the answers

What happens when the voltage is changed to its negative value in a p-n junction?

The diode conducts current in the reverse direction. Signup and view all the answers

What characterizes the reverse saturation current of an ideal diode?

It increases exponentially with temperature. Signup and view all the answers

Which of the following factors is NOT related to the current flow in a p-n junction?

Fermi level stability Signup and view all the answers

What is the significance of the breakdown voltage (VB) in a p-n junction?

Large currents begin to flow once this voltage is applied. Signup and view all the answers

Which of the following correctly describes the diode IV relation?

It is an exponential function related to carrier energy. Signup and view all the answers

What occurs to the current through a p-n junction during reverse bias?

The current initially increases slightly then becomes steady. Signup and view all the answers

In a p-n junction, which effect leads to a current flow in the forward bias condition?

Enhanced thermal energy affecting majority carriers. Signup and view all the answers

Which statement is true regarding the current flow in an ideal diode?

Current is negligible in reverse bias until breakdown voltage is reached. Signup and view all the answers

Study Notes

Forming p-n Junction

Challenges:
- Limited points for contact
- Native oxide layer is a good insulator
- Dangling bonds interrupt bonding
Solutions:
- Forming in the bulk of the semiconductor as far from the surface as possible
- Processes:
  - Dopant diffusion
  - Ionic implantation
  - Epitaxy
  - Low damage deposition (CVD, PVD, etc.)

p-n Junction

Discontinuity in the Fermi level across the junction
The Fermi level must be the same everywhere in a solid under thermal equilibrium, even at a junction of different materials
There are two types of current:
- Drift current: due to the movement of carriers by electric field
- Diffusion current: due to the concentration gradient of carriers
The ideal junction at equilibrium is when both drift and diffusion currents are present, and the total current is given by their sum.
The built-in potential across a p-n junction is the potential barrier height due to the depletion region.
The built-in potential is also known as the junction potential.

p-n Junction without External Bias

Limited Diffusion current due to few carriers with sufficient energy to overcome the barrier
Small Drift current due to low minority carrier concentration
Important point: minority drift current is independent of the barrier.

p-n Junction with Reverse Bias

Applied voltage increases the potential barrier, resulting in a reduced diffusion current.
The drift current remains constant.
Net result: Small reverse current.
The depletion region width increases with reverse bias.

p-n Junction with Forward Bias

Forward bias decreases the potential barrier.
Diffusion current increases exponentially.
Drift current remains constant, but is reduced due to the forward bias.
Net result: Large forward current.

Ideal Diode

The ideal diode equation is valid for both forward and reverse bias, which is an exponential function.
The exponential function is due to the Boltzmann distribution of carriers versus energy.
The current due to the drift current of reverse bias is called as the reverse saturation current.

Reverse Breakdown

A reverse bias applied across the junction will result in a small current.
At a specific high voltage, known as the breakdown voltage, a large current flows through the junction.

Schottky Junction

MS (n-type) contact with m < s
- No barrier for electron flow from the semiconductor (S) to the metal (M)
- Large current flows when the voltage is applied
MS (n-type) contact with m > s
- A barrier forms for electron flow from M to S.
- The barrier height is B = m –  where  is the electron affinity of the semiconductor.
- Electrons in the semiconductor encounter an energy barrier of m – s while flowing from S to M.
The Schottky junction is a metal-semiconductor junction where a depletion region exists due to the difference in work functions.
This depletion region creates a potential barrier for electron flow from the metal to the semiconductor.
The barrier height is determined by the work function of the metal and the electron affinity of the semiconductor.
The Schottky junction can be used to create rectifiers, diodes, and other electronic components.

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Description

This quiz covers the formation and characteristics of p-n junctions in semiconductor physics. Explore the challenges, solutions, and electrical properties associated with these critical components. Test your understanding of drift and diffusion currents and their significance in achieving thermal equilibrium.