Question

1a. What does doping do to silicon?

1b. When no current is applied to a diode, what forms at the junction between the N-type and P-type layers?

1c. What is the biggest problem to account for while manufacuring silicon chips?

Answer 1

Hi,

solution of 1a:

Doping in silicon increases the conductivity of the silicon.

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.

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.

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.

solution of 1 b:

When no current is going through to a diode, it means a thick depletion width forms at the junction between the N-type and P-type layers making the diode reverse biased.

Solution of 1c

Some problem to account for while manufacuring silicon chips are

• Silicon foundries cost ten times more every few years , e.g. $3 billion today
• Development spend growing very rapidly
• Interconnects means failures and expense
• Large area or small orders mean expense
• The lowest cost silicon chips have been 5 - 10 cents for 20 years
• Traditionally silicon is thick and brittle but thinner, smaller chips are emerging
• Difficult to integrate with other components quickly and cheaply