AELE237Semiconductor Materials1 Semiconductor Materials and pn Junctions T. Floyd, “Electronic Devices”, Maxwell Macmillan International Editions, Chapter 2

AELE237Semiconductor Materials2 Atoms An atom is the smallest particle of an element that retains the characteristics of that element. It is made out of a nucleus and a number negatively charged electrons. Electrons are orbiting in one or more shells around the nucleus. The nucleus consists of positively charged particles, called protons, and uncharged particles called neutrons.

AELE237Semiconductor Materials3 Examples of Atoms Some typical elements are the Hydrogen that has only 1 electron and one proton, the Helium that has 2 electrons, two protons and two neutrons, and the Silicon that has 14 electrons and 14 protons.

The Bohr Model AELE237Semiconductor Materials4

AELE237Semiconductor Materials5 Valance Electrons, Free Electrons and Conductivity Orbiting electrons in an atom are attracted by the positively charged protons. Electrons in orbits farther from the nucleus are less tightly bound to the atom than those closer to the nucleus. Electrons in the outermost shell of an atom are relatively loosely bound to the atom. These electrons are called the valance electrons and contribute to chemical reactions and bonding within the structure of a material. When an atom absorbs energy from a heat source or from light, its electrons gain energy and move to an orbit farther from the nucleus.If a valance electron gains sufficient amount of energy, it can be completely removed from the atom, and become a free electron. The number of free electrons in a material determines its resistance to electric current. This number increases as the temperature increases. Materials with relatively few free electrons are called insulators, and posses a high resistance to electric current. Materials with relatively large number of free electrons are called conductors, and posses a low resistance to electric current.

AELE237Semiconductor Materials6

AELE237Semiconductor Materials7 Semiconductor Crystals Two widely used types of semiconductor materials are Silicon and Germanium. Their atoms have 4 valance electrons. When their atoms combine to form a solid material, they arrange themselves in a fixed pattern called a crystal. The atoms within a crystal are held together by covalent bonds, which are created by the valance electrons of each atom. Covalent bonds the atoms together, because the valance electrons of adjacent atoms are attracted equally by the protons in the nucleus of the atoms. A small number of valance electrons can escape and become free electron, leaving a positively charged Hole in the atom.

AELE237Semiconductor Materials8

AELE237Semiconductor Materials9

AELE237Semiconductor Materials10 n-Type Semiconductors Intrinsic (pure) semiconductor materials have a low conductivity, due to the small number of free electrons. Their conductivity can be increased and controlled by the addition of impurities. This process is called doping. Atoms such as the Antimony (Sb) have 5 valance electrons. When these atoms are added in pure silicon, then each antimony atom forms a covalent bond with the four adjacent silicone atoms, leaving one free electron that is not attracted by any atom. The number of free electrons can be controlled by the amount of impurity added to the silicon. The semiconductor produced by adding pentavalent (impurities with 5 valance electrons) impurities to pure semiconductors is called the n-type semiconductor.

AELE237Semiconductor Materials11 p-Type Semiconductors Atoms such as the Boron (B) have 3 valance electrons. When these atoms are added in pure silicon, then each boron atom forms a covalent bond with the four adjacent silicone atoms. An electron is though missing since boron has only 3 valance electrons, leaving a positively charged hole. The number of holes in the semiconductor material can be controlled by the amount of impurity added to the silicon. The semiconductor produced by adding trivalent (impurities with 3 valance electrons) impurities to pure semiconductors is called the p-type semiconductor.

AELE237Semiconductor Materials12 pn- junction A pn junction is formed by connecting a p- type semiconductor with an n-type semiconductor. Initially, at the point of contact, the free electrons of the n-type region recombine with the holes of the p-type region. Due to the force of attraction between the electrons and holes, some electrons move to the p-region while some holes move to the n-region. At some point the force on the electrons that moved in the p-region is the same as the force due to the holes that moved in the n-region, thus these electrons are trapped in the p-region. The same applies to the holes that moved in the n-region. This creates a barrier between the two regions, called the Depletion Layer.

AELE237Semiconductor Materials13

AELE237Semiconductor Materials14 Biasing the pn-junction:- Forward Bias (V < VD) If a voltage source is connected to a pn-junction with the positive terminal connected on the p-region and the negative on the n-region then the junction is forward biased. In this case the positive terminal of the source push the holes of the p-region towards the n-region, while the negative terminal push the electrons towards the p-region. This reduces the width of the depletion layer. If the source voltage is less than the depletion voltage, then the width of the depletion layer is only reduced, not eliminated. Thus, only a very small current flows through the circuit.

AELE237Semiconductor Materials15 Biasing the pn-junction:- Forward Bias (V >= VD) If the source voltage is greater than the depletion voltage, then the depletion layer is eliminated, and a large number of electrons gain enough energy to become valance electrons. Thus a large current flows through the pn-junction. In this case the pn-junction behaves like a resistance (Rp + Rn) with a voltage source (V D ). The bulk resistance (Rp + Rn) is very low (few ohms), thus to avoid damaging the pn-junction, a liming resistor is usually connected in series. The depletion voltage for silicon pn-junctions is 0.7V and for germanium 0.3V.

AELE237Semiconductor Materials16 Biasing the pn-junction:- Reverse Bias If the source voltage is connected in such a way so that the negative terminal is connected on the p-region and the positive on the n-region, then the pn-junction is reversed biased. In this case the source voltage widens the depletion layer. Only a very small currents flows the circuit due to the small number of minority curriers in the pn-junction. If the source voltage increases further, then the depletion layer widens more, creating a gap between the p-region and the n-region, that behaves as a capacitance. Further increase in the source voltage will lead to a state known as the avalanche breakdown, where a large reverse current flows through the circuit.

AELE237Semiconductor Materials17 The PN-Junction Diode A pn junction is formed by connecting a p-type semiconductor with an n-type semiconductor. Initially, at the point of contact, the free electrons of the n-type region recombine with the holes of the p-type region. Due to the force of attraction between the electrons and holes, some electrons move to the p-region while some holes move to the n-region. At some point the force on the electrons that moved in the p-region is the same as the force due to the holes that moved in the n-region, thus these electrons are trapped in the p-region. The same applies to the holes that moved in the n-region. This creates a barrier between the two regions, called the Depletion Layer.

AELE237Semiconductor Materials18 Biasing the Junction Diode:- Forward Bias (V < VD) If a voltage source is connected to a pn- junction with the positive terminal connected on the p-region and the negative on the n-region then the junction is forward biased. In this case the positive terminal of the source pushes the holes of the p-region towards the n-region, while the negative terminal pushes the electrons towards the p-region. This reduces the width of the depletion layer. If the source voltage is less than the depletion voltage, then the width of the depletion layer is only reduced, not eliminated. Thus, only a very small current flows through the circuit.

AELE237Semiconductor Materials19 Biasing the Junction Diode:- Forward Bias (V >= VD) If the source voltage is greater than the depletion voltage, then the depletion layer is eliminated, and a large number of electrons gain enough energy to become valance electrons. Thus a large current flows through the pn-junction. In this case the pn-junction behaves like a resistance (Rp + Rn) with a voltage source (V D ). The bulk resistance (Rp + Rn) is very low (few ohms), thus to avoid damaging the pn-junction, a liming resistor is usually connected in series. The depletion voltage for silicon pn-junctions is 0.7V and for germanium 0.3V.

AELE237Semiconductor Materials20 Junction Diode Forward Characteristics Ideal Junction Diode: (Assume that the internal resistance of the diode is zero) –Silicon diode: if V 0.7V then I = ∞. For germanium V=0.3V. Non-ideal Junction Diode: –Need to consider the internal resistance of the pn-junction

AELE237Semiconductor Materials21 Biasing the pn-junction:- Reverse Bias If the source voltage is connected in such a way so that the negative terminal is connected on the p-region and the positive on the n-region, then the pn-junction is reversed biased. In this case the source voltage widens the depletion layer. Only a very small currents flow in the circuit due to the small number of minority curriers in the pn-junction. If the source voltage increases further, then the depletion layer widens more, creating a gap between the p-region and the n-region, that behaves as a capacitance. Further increase in the source voltage will lead to a state known as the avalanche breakdown, where a large reverse current flows through the circuit.