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 0 o K 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), I S, 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, I F.

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 = X C k = Boltzmann Constant: average kinetic energy corresponding to a temperature T ( o K) (1.38 X J/ o K) 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) I S = Reverse saturation current Independent of junction potential Varies exponentially with temperature At room temperature:I S = 2μA for Germanium I S = 2nA for Silicon