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pn-diode (1).pdf

  1. 1. PN Junction Diode pn-juntion-Diode
  2. 2. A p-n junction consists of two semiconductor regions with opposite doping type as shown in Figure. The region on the left is p-type with an acceptor density Na, while the region on the right is n-type with a donor density Nd. The dopants are assumed to be shallow, so that the electron (hole) density in the n-type (p-type) region is approximately equal to the donor (acceptor) density. Cross-section of a p-n junction pn-juntion-Diode
  3. 3. Flatband diagram The principle of operation will be explained using a gedanken experiment, an experiment, which is in principle possible but not necessarily executable in practice. We imagine that one can bring both semiconductor regions together, aligning both the conduction and valence band energies of each region. This yields the so-called flatband diagram shown in Figure. Energy band diagram of a p-n junction (a) before and (b) after merging the n-type and p-type regions
  4. 4. Note that this does not automatically align the Fermi energies, EF,n and EF,p. Also, note that this flatband diagram is not an equilibrium diagram since both electrons and holes can lower their energy by crossing the junction. A motion of electrons and holes is therefore expected before thermal equilibrium is obtained. The diagram shown in Figure (b) is called a flatband diagram. This name refers to the horizontal band edges. It also implies that there is no field and no net charge in the semiconductor. pn-juntion-Diode
  5. 5. At Thermal Equilibrium A short time after the junction is established and thermal equilibrium is achieved, charge carriers in the vicinity of the junction will neutralize each other (electrons combining with holes), leaving the unneutralized negatively ionized acceptors, Na - , in the p-region and unneutralized positively ionized donors, Nd + , in the n-region. This region of ionized donors and acceptors creates a space charge and its region is called the depletion region. The edge of the depletion region given by -xp on the p-side and +xn on the n-side. the ionized donors and acceptors are located in substitutional lattice sites and Cannot move in the electric field. The concentration of these donors and acceptors are selected to give the p-n junction desired device properties pn-juntion-Diode
  6. 6. i.e. the Fermi level in the p- and n- type semiconductors must be equal. This requirement for constant Fermi level pushes the n-type semiconductor Fermi level down to be constant with the p-type semiconductor Fermi level, as shown in the diagram. The amount the bands are bent is the difference In work function. The depletion width xd, where xd = xp + xn may be calculated from Drift Diffusio n Drift Diffusio n bi a d d V N N q x ÷ ÷ ø ö ç ç è æ + = - + 1 1 2e 0 = dx dEf Energy Band Diagram at Thermal Equilibrium At thermal equilibrium Energy band diagram of a p-n junction in thermal equilibrium While in thermal equilibrium no external voltage is applied between the n-type and p-type material, there is an internal potential, f, which is caused by the workfunction difference between the n-type and p-type pn-juntion-Diode
  7. 7. Impurity distribution illustrating the space charge region Electric field variation with distance, x Potential variation with distance, x The build-in potential may be expressed as: 2 ln i d a bi n N N q kT V + - = Where, mV V q kT T 26 = = K – Boltzman constant VT = Thermal voltage At T=300K Junction Potential pn-juntion-Diode
  8. 8. The built-in potential in a semiconductor equals the potential across the depletion region in thermal equilibrium. Since thermal equilibrium implies that the Fermi energy is constant throughout the p-n diode, the built-in potential equals the difference between the Fermi energies, EFn and EFp, divided by the electronic charge. It also equals the sum of the bulk potentials of each region, fn and fp, since the bulk potential quantifies the distance between the Fermi energy and the intrinsic energy. This yields the following expression for the built- in potential. The built-in potential pn-juntion-Diode
  9. 9. No Applied Voltage A semiconductor diode is created by joining the n-type semiconductor to a p-type semiconductor. In the absence of a bias voltage across the diode, the net flow of charge is one direction is zero. Bias is the term used when an external DC voltage is applied Semiconductor Diode pn-juntion-Diode
  10. 10. When an external voltage VD is applied as shown, with - terminal to n-side and +terminal to p-side, it forms a forward bias configuration. In this setup, electrons and holes will be pressured to recombined with the ions near the boundary, effectively reducing the width and causing a heavy majority carrier flow across the junction. As Vd increases, the depletion width decrease until a flood of majority carriers start passing through. Is remains unchanged. Forward Bias n ~ 1 When an external voltage VD is applied as shown, with + terminal to n-side and – terminal to p-side, the free charge carriers will be attracted away by the voltage source. This will effectively increase the depletion region within the diode. This widening of the depletion region will create too great a barrier for the majority carriers to overcome, effectively reducing the carrier flow to zero. The number of minority carriers will not be affected. This configuration is called reverse Bias. This small current flow during reverse bias is called the reverse saturation current, Is. Reverse Bias ÷ ÷ ø ö ç ç è æ - = 1 T D nV V s D e I I Biasing the Junction Diode pn-juntion-Diode
  11. 11. We now consider a p-n diode with an applied bias voltage, Va. A forward bias corresponds to applying a positive voltage to the anode (the p-type region) relative to the cathode (the n-type region). A reverse bias corresponds to a negative voltage applied to the cathode. Both bias modes are illustrated with Figure. The applied voltage is proportional to the difference between the Fermi energy in the n-type and p-type quasi-neutral regions. As a negative voltage is applied, the potential across the semiconductor increases and so does the depletion layer width. As a positive voltage is applied, the potential across the semiconductor decreases and with it the depletion layer width. The total potential across the semiconductor equals the built-in potential minus the applied voltage, or: Energy band diagram of a p-n junction under reverse and forward bias pn-juntion-Diode
  12. 12. What Are Diodes Made Out Of? • Silicon (Si) and Germanium (Ge) are the two most common single elements that are used to make Diodes. A compound that is commonly used is Gallium Arsenide (GaAs), especially in the case of LEDs because of it’s large bandgap. • Silicon and Germanium are both group 4 elements, meaning they have 4 valence electrons. Their structure allows them to grow in a shape called the diamond lattice. • Gallium is a group 3 element while Arsenide is a group 5 element. When put together as a compound, GaAs creates a zincblend lattice structure. • In both the diamond lattice and zincblend lattice, each atom shares its valence electrons with its four closest neighbors. This sharing of electrons is what ultimately allows diodes to be build. When dopants from groups 3 or 5 (in most cases) are added to Si, Ge or GaAs it changes the properties of the material so we are able to make the P- and N-type materials that become the diode. Si +4 Si +4 Si +4 Si +4 Si +4 Si +4 Si +4 Si +4 Si +4 The diagram above shows the 2D structure of the Si crystal. The light green lines represent the electronic bonds made when the valence electrons are shared. Each Si atom shares one electron with each of its four closest neighbors so that its valence band will have a full 8 electrons. pn-juntion-Diode
  13. 13. N-Type Material: When extra valence electrons are introduced into a material such as silicon an n-type material is produced. The extra valence electrons are introduced by putting impurities or dopants into the silicon. The dopants used to create an n-type material are Group V elements. The most commonly used dopants from Group V are arsenic, antimony and phosphorus. The 2D diagram to the left shows the extra electron that will be present when a Group V dopant is introduced to a material such as silicon. This extra electron is very mobile. +4 +4 +5 +4 +4 +4 +4 +4 +4 pn-juntion-Diode
  14. 14. P-Type Material: P-type material is produced when the dopant that is introduced is from Group III. Group III elements have only 3 valence electrons and therefore there is an electron missing. This creates a hole (h+), or a positive charge that can move around in the material. Commonly used Group III dopants are aluminum, boron, and gallium. The 2D diagram to the left shows the hole that will be present when a Group III dopant is introduced to a material such as silicon. This hole is quite mobile in the same way the extra electron is mobile in a n-type material. +4 +4 +3 +4 +4 +4 +4 +4 +4 pn-juntion-Diode
  15. 15. The Biased PN Junction P n + _ Applied Electric Field Metal Contact “Ohmic Contact” (Rs~0) + _ Vapplied I The pn junction is considered biased when an external voltage is applied. There are two types of biasing: Forward bias and Reverse bias. These are described on then next slide. pn-juntion-Diode
  16. 16. The Biased PN Junction Forward Bias: In forward bias the depletion region shrinks slightly in width. With this shrinking the energy required for charge carriers to cross the depletion region decreases exponentially. Therefore, as the applied voltage increases, current starts to flow across the junction. The barrier potential of the diode is the voltage at which appreciable current starts to flow through the diode. The barrier potential varies for different materials. Reverse Bias: Under reverse bias the depletion region widens. This causes the electric field produced by the ions to cancel out the applied reverse bias voltage. A small leakage current, Is (saturation current) flows under reverse bias conditions. This saturation current is made up of electron-hole pairs being produced in the depletion region. Saturation current is sometimes referred to as scale current because of it’s relationship to junction temperature. Vapplied > 0 Vapplied < 0 pn-juntion-Diode

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