Doping, P-N Junction & Diodes

Doping

Doping means the introduction of impurities into a semiconductor crystal to the defined modification of conductivity. Two of the most important materials silicon can be doped with, are boron (3 valence electrons = 3-valent) and phosphorus (5 valence electrons = 5-valent). Other materials are aluminum, indium (3-valent) and arsenic, antimony (5-valent).

The dopant is integrated into the lattice structure of the semiconductor crystal, the number of outer electrons define the type of doping. Elements with 3 valence electrons are used for p-type doping, 5-valued elements for n-doping. The conductivity of a deliberately contaminated silicon crystal can be increased by a factor of 10⁶.

n-doping

The 5-valent dopant has an outer electron more than the silicon atoms. Four outer electrons combine with ever one silicon atom, while the fifth electron is free to move and serves as charge carrier. This free electron requires much less energy to be lifted from the valence band into the conduction band, than the electrons which cause the intrinsic conductivity of silicon. The dopant, which emits an electron, is known as an electron donor (donare, lat. = to give).

The dopants are positively charged by the loss of negative charge carriers and are built into the lattice, only the negative electrons can move. Doped semimetals whose conductivity is based on free (negative) electrons are n-type or n-doped. Due to the higher number of free electrons those are also named as majority charge carriers, while free mobile holes are named as the minority charge carriers.

n-doping with phosphorus

Arsenic is used as an alternative to phosphorus, because its diffusion coefficient is lower. This means that the dopant diffusion during subsequent processes is less than that of phosphorus and thus the arsenic remains at the position where it was introduced into the lattice originally.

p-doping

In contrast to the free electron due to doping with phosphorus, the 3-valent dopant effect is exactly the opposite. The 3-valent dopants can catch an additional outer electron, thus leaving a hole in the valence band of silicon atoms. Therefore the electrons in the valence band become mobile. The holes move in the opposite direction to the movement of the electrons. The necessary energy to lift an electron into the energy level of indium as a dopant, is only 1 % of the energy which is needed to raise a valence electron of silicon into the conduction band.

With the inclusion of an electron, the dopant is negatively charged, such dopants are called acceptors (acceptare, lat. = to add). Again, the dopant is fixed in the crystal lattice, only the positive charges can move. Due to positive holes these semiconductors are called p-conductive or p-doped. Analog to n-doped semiconductors, the holes are the majority charge carriers, free electrons are the minority charge carriers.

p-doping with boron

Doped semiconductors are electrically neutral. The terms n- and p-type doped do only refer to the majority charge carriers. Each positive or negative charge carrier belongs to a fixed negative or positive charged dopant.

N- and p-doped semiconductors behave approximately equal in relation to the current flow. With increasing amount of dopants, the number of charge carriers increases in the semiconductor crystal. Here it requires only a very small amount of dopants. Weakly doped silicon crystals contain only 1 impurity per 1,000,000,000 silicon atoms, high doped semiconductors for example contain 1 foreign atom per 1,000 silicon atoms.

Electronic band structure in doped semiconductors

Through the introduction of a dopant with five outer electrons, in n-doped semiconductors there is an electron in the crystal which is not bound and therefore can be moved with relatively little energy into the conduction band. Thus in n-doped semiconductors one finds a donator energy level near the conduction band edge, the band gap to overcome is very small.

Analog, through introduction of a 3-valent dopant in a semiconductor, a hole is available, which may be already occupied at low-energy by an electron from the valence band of the silicon. For p-doped semiconductors one finds an acceptor energy level near the valence band.

Band model of doped semiconductors

Diodes: 

Diodes is a two terminal electronic device which allows the flow of current in only one direction and high resistance in opposite direction. The most common diode used currently are p-n junction diode. The diodes are of different types as illustrated below:

1. p-n junction Diodes
2. Avalanche Diodes
3. Zener Diodes
4. LED(light emitting diode)
5. Photo Diodes
6. Sckottky Diodes
7. Tunnel Diodes
8. Gunn Diodes
9. Laser Diodes
10. Pin Diodes

p-n junction diode:

 A pn junction diode is made up ofsilicon or germanium doped at certain level. Any pentavalent or trivalent impurities can be added to form pn junction diode. It can be either a donor if pentavalent impurities are added or acceptor when trivalent impurities are added. p-n junction diodes are the electronic components that can allow the flow of current in only forward direction and gives 100 percent resistance in reverse direction.

Avalanche Diode:

 These are the diodes which conducts in the reverse direction when the reverse bias voltage exceeds the breakdown voltage. These are electrically similar to the zener diodes but having a different breakdown mechanism (avalanche effect).

Zener Diode:

 These diodes are termed as reverse breakdown diodes because these are mainly used in reverse direction. breakdown occurs at below the breakdown voltage of 5 volts. The zener diodes conduct some minute amount of current (micro amperes) in reverse direction.

LED: 

Light emitting diodes are directly made up of gallium arsenide that cross the junction and emit photons when they combine with the majority carrier on the other side. The different voltages corresponds to different colors like 2.1 volts correspond to red and 4 volts corresponds to violet.

Photo Diode: 

All semiconductors are subject to optical charge carrier generation. A photo diode is used generally to sense light coming from various outlets. So a photo diode is packaged in a material that allows light to pass through. Photo diodes can be used in line following robot, etc.

Sckottky Diode: 

These diodes have very less forward voltage drop, even lesser than p-n junction diodes. Their voltage drop at forward current of 1mA is at 0.15V to 0.45V, which makes them usefull in voltage clamping applications. They are also used as low loss rectifier.

Tunnel Diode: 

Tunnel diode or Esaki diode have a region of operation showing negative resistance showing quantum tunneling allowing amplification of signals. These diodes are very fast and can be used in spacecrafts because of their high magnetic field and high radiation environment susceptibility.

Gunn Diode:

 Gunn diodes are similar to tunnel diodes but these offer negative differential resistance. These diodes are used in high frequency microwave oscillators.

Laser Diode: 

If a LED is contained in a resonant cavity formed by polishing the parallel end faces a laser can be formed. Laser diodes are used mainly for optical communication.

Pin Diodes:

A pin diodes has a central undoped or intrinsic layer forming a p-type or n-type structure. They are used as radio frequency switches and attenuators.These are also used in power electronics.