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(ELEC101)tutorial11.pdf
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Fall 2006
ELEC101 BASIC ELECTRONICS



Outline

.Semiconductor

CSemiconductor

CIntrinsic Silicon

CExtrinsic Silicon

.Diode (or PN junction)

CDiode

CDiode clamping circuit

CDiode voltage regulator

CDiode rectifier

CCapacitor filter

CVoltage doubler




Semiconductor

Semiconductor lies on the middle of the table. They have electrical conductivities between metal and non-metal. They are Group IV elements because they have 4 outermost shell electrons



Sillicon

Silicon is the most important semiconductor today. Silicon atoms are arranged in a tetrahedral structure.



Intrinsic (pure) Semiconductor

In intrinsic semiconductor, electron density nand hole density pareequal. As the electron-hole pair is created by the thermal generation and at the same time, there is a recombination of the electron-hole pair. So, at a given temperature,

n= p= ni


Extrinsic (impure/doped) Semiconductor

Extrinsic semiconductor is created by doping the intrinsic semiconductor.Doping is a process of adding very precise amounts of a very small concentration of certain impurities named dopant. After dopingprocess, the doped semiconductorhas higher conductivityand we can control the conductivity. Because for both intrinsic and extrinsic semiconductor, the following equation should hold at a given temperature.

np= ni2 = constant

These dopantsare elements from the columns in the Periodic Table adjacent to silicon. The 2 types of dopantsare donorand acceptor


N-type Si

For example, we can replace some silicon atoms witharsenic atoms which has5 valence (penta-valent) electrons.Sinceonly 4 electrons are needed to form covalent bonds with silicon, theextra electron of the arsenic (dopant) atom is unbonded.

The electron is free to move when the temperature (thermal energy) is high enough.
When an electric field (voltage) is applied, the electron can contribute to the current.
We call atoms from column 5 of the periodic table donors, and the doped Si as N-type Si.



P-type Si

On the other hand, if anelement from column 3 is added (e.g. boron B) , an unfilled bond (called a hole) is formed. When the temperature is high enough, electron in othercovalent bond can break the bond (hence form a holein the bond), move around, and fill this hole.

Hence holes can also move (indirectly).
When an electric field is applied, the hole can also contribute to the current.
We call atoms from column 3 of the periodic table acceptors, and the doped Si as P-type Si.



Drift Current and Conductivity

In a conductor, the atoms are arranged in a crystal lattice where the electrons are free to move about (sea of electrons). Due to the thermal energy (kT), the electrons move randomlybut on average there is no net motion (current).

If an electric field E is applied, the electrons will tend to drift (accelerate by E and decelerate by collisions with lattice and ion) from regions of