Conduction in Metals

•Atoms form a crystal•Atoms are in close proximity to each other•Outer, loosely-bound valence electron are not associated with any one nucleus

Conduction in Metals

•If a uniform electric field is applied, the electrons are accelerated

Pure (Intrinsic) Semiconductors

(Silicon and Germanium)

•Belong to Group IV of the periodic table (Tetravalent)•Tetrahedral structure: each atom has 4 neighbors held by covalent bonds

Periodic Table

Pure (Intrinsic) Semiconductors

Symbolic 2-D Representation

Note: Inner shells do not influence electrical properties

At 0oK•bonds are complete•Electrons are held tightly to parent atom•Insulator

Pure (Intrinsic) Semiconductors at Room Temperature

At Room Temperature

•A few covalent bonds are broken•Free electrons will be generated•Holes will be generated

Hole Conduction

At Room Temperature

•Application of an electric field will cause a drift of electrons and holes; holes toward the negative, electrons towards the positive

•Pure semiconductors have few direct applications•High (negative) temperature coefficient•Can be used in temperature sensitive devices; e.g., thermistor.

Doped (Extrinsic) Semiconductor

•Conductivity of a semiconductor can be increased greatly and precisely controlled by adding small amount of impurities (Doping)•Two types of impurities:

N-Type•Antimony•Phosphorous•Arsenic

N-Type Properties•Pentavalent•Comparable in size to Germanium and Silicon•Donor

P-Type•Boron•Gallium•Indium

N-Type Properties•Trivalent•Comparable in size to Germanium and Silicon•Acceptor

Semiconductor Elements

Doped (Extrinsic) Semiconductors

Definition of Charge Carriers:

Majority Carriers:•Holes in the P-Region and electrons in the N-Region

Minority Carriers:•Holes in the N-Region and electrons in the P-Region

Drift and Diffusion in a P-N Junction

Two Different Conduction Mechanisms:

Drift and Diffusion

Diffusion:•The migration of majority charge carriers from a region of high concentration to one of low concentration•This results in the generation of a depletion region (i.e., an area where all charge carriers have been depleted) and the creation of an electric field.•The electric field is the result of the potential difference across the junction due to the oppositely charged sides of the junction.•Electric field acts as a barrier to diffusion current.

Drift:•Under the action of the electric field (thermally generated) minority carriers drift across the junction.

•Under open circuit conditions the net current is zero; diffusion of majority carriers is balanced by drift of minority carriers.

Diffusion

Reverse Biasing a P-N Junction

•The polarity of the external voltage source is reversed.•This increases the potential barrier.•A small current (the reverse saturation current), IS, is observed.•The reverse saturation current, which can often be neglected, is due to thermally generated minority carriers in the depletion region.

Forward Biasing a P-N Junction

•An external voltage source is connected such that the positive terminal is connected to the P side (anode) and the negative terminal to the N side (cathode).•This reduces the potential barrier resulting in a significant forward current, IF.

An expression for the current through a semiconductor (P-N) diode can be derived based on the probability of a carrier possessing sufficient energy to diffuse across the junction:

Current-Voltage Characteristic for a P-N Diode

Where:

e = charge on an electron = 1.602 X 10-19 Ck = Boltzmann Constant: average kinetic energy corresponding to a

temperature T (oK) (1.38 X 10-23 J/ oK)η = 1 for Germanium and Silicon at high currents (V>0.5) and 2 for

silicon at low currents (V approaching zero).V = bias voltage (positive for forward, negative for reverse)IS = Reverse saturation current

•Independent of junction potential•Varies exponentially with temperature•At room temperature: IS = 2μA for Germanium

IS = 2nA for Silicon