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P-n Junction Diode

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Published in: Physics
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This note explains the concept and working of P-n Junction diode. The p-n junction is a versatile element, which can be used as a rectifier, as an isolation structure and as a voltage-dependent capacitor.

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  1. PN Junction Diode pn-juntion-Diode
  2. Basics of p-n junction? A p-n junction is the metallurgical boundary between the n and p-regions of a semiconductor crystal. P-n junctions consist of two semiconductor regions of opposite type. Such junctions show a pronounced rectifying behavior. They are also called p-n diodes in analogy with vacuum diodes. The p-n junction is a versatile element, which can be used as a rectifier, as an isolation structure and as a voltage-dependent capacitor. In addition, they can be used as solar cells, photodiodes, light emitting diodes and even laser diodes. They are also an essential part of Metal-Oxide-Silicon Field-Effects- Transistors (MOSFETs) and Bipolar Junction Transistors (BJTs).
  3. 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. < > n-type anode cathode Cross-section of a p-n junction pn-juntion-Diode
  4. We will assume, unless stated otherwise, that the doped regions are uniformly doped and that the transition between the two regions is abrupt. We will refer to this structure as an abrupt p-n junction. Frequently we will deal with p-n junctions in which one side is distinctly higher-doped than the other. We will find that in such a case only the low-doped region needs to be considered, since it primarily determines the device characteristics. We will refer to such a structure as a one- sided abrupt p-n junction.
  5. The junction is biased with a voltage Va as shown in Figure. We will call the junction forward-biased if a positive voltage if a is applied to the p-doped region and reversed-biased negative voltage is applied to the p-doped region. The contact to the p-type region is also called the anode, while the contact to the n-type region is called the cathode, in reference to the anions or positive carriers and cations or negative carriers in each of these regions.
  6. 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. qX p-type n-type p -type n-type (b) Energy band diagram of a p-n junction (a) before and (b) after merging the n-type and p-type regions
  7. Note that this does not automatically align the Fermi energies, EFn and EFp. 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
  8. 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, in the n-region. This region of ionized donors and acceptors creates a space charge and its region is called the depletion region. ——8 Space Charge I 0+0+1 e e e 10-0-0- e+å+je e e to-c-o- +++ete ee 0 01-0-0-0 e 01 + e + e 10 e Distance x 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
  9. Energy Band Diagram at Thermal Equilibrium At thermal equilibrium i.e. dx the Fermi level in the p- semiconductors must be requirement for constant pushes s sqVbi = F and n- equal. Fermi the n-type semiconductor Fermi type This level 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 x d, where x d = XP + xn may be calculated from p-type Distance x (b) n-type 28 1 1 hi Energy band diagram of a p-n junction in thermal equilibrium hile in thermal equilibrium no external voltage is applied between the n-type and p-type material, there is an internal potential, (b, which is caused by the workfunction difference between the n-type and p-type pn-juntion-Diode
  10. Junction Potential Impurity distribution illustrating the space charge region p-side n -side The build-in potential may be expressed as: (a) (b) (c) hi Electric field variation with distance, x Potential variation with distance, x pn-juntion-Diode — In bi q Where, 2 n = VT = 26mV At T=300K K— Boltzman constant VT = Thermal voltage
  11. The built-in potential 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, and P' 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. pn-juntion-Diode
  12. Semiconductor Diode 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 = O mA vn=ov (no bias) = O mA Depletion region e:ee.e VD=OV (no bias) (a) = O rnA Metal contact Minority carrier flow (b) pn-juntion-Diode Majority carrier flow (c)
  13. Biasing the Junction Forward Bias ID majority 1 majority e+e-e -+ 0+ -o Depletion region Diode (Similar) (b) Reverse Bias Minority-carrier now ete+$e+e+e Depletion region (a) l) (Opposite) (b) 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. unchanged. 1 1 e NT-I remains 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. pn-juntion-Diode
  14. 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. Reverse bias As a negative voltage is applied, the potential the across semiconductor increases and so E does the depletion layer width. As a positive voltage is applied, the Forward bias potential across qva the semiconductor decreases and with it the depletion layer width. The total potential across the semiconductor equals the built-in the applied E qKI voltage, or:' Energy band diagram of a p-n junction under reverse and forward bias pn-juntion-Diode
  15. Electrostatic analysis of a p-n diode The electrostatic analysis of a p-n diode is of interest since it provides knowledge about the charge density and the electric field in the depletion region. It is also required to obtain the capacitance-voltage characteristics of the diode. The analysis is very similar to that of a metal-semiconductor junction. A key difference is that a p-n diode contains two depletion regions of opposite type. pn-juntion-Diode
  16. 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. pn-juntion-Diode +4 +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 lines green 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.
  17. N-Type Material: 0+00 o 000 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. pn-juntion-Diode
  18. P-Type Material: 0+00 O o 000 P-type material is produced when the dopant that is introduced is from Group Ill. Group Ill 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 Ill dopants are aluminum, boron, and gallium. The 2D diagram to the left shows the hole that will be present when a Group Ill 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. pn-juntion-Diode
  19. ionized acceptors h+ drift The PN Junction Metallurgical Junction Space Charge Region E-Field h+ diffusion e- diffusion n ionized donors e- drift pn-juntion-Diode
  20. p ionized acceptors h+ drift The Metallurgical Junction Space Charge Region E-Field PN Junction n ionized donors e- drift Steady State When no external source is connected to the pn junction, diffusion and drift balance each other out for both the holes and electrons h+ diffusion e- diffusion Space Charqe Reqion: Also called the depletion region. This region includes the net positively and negatively charged regions. The space charge region does not have any free carriers. The width of the space charge region is denoted by W in pn junction formula's. Metallurqical Junction: The interface where the p- and n-type materials meet. Na & Nd: Represent the amount of negative and positive doping in number of carriers per centimeter cubed. Usually in the range of 1015 to 1020. pn-juntion-Diode
  21. The Biased PN Junction Metal Contact 'Ohmic Contact" (Rs-O) App ied Electric Field 1 applied 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.
  22. Forward Bias: applied Reverse Bias: applied The Biased PN Junction 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. 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. pn-juntion-Diode
  23. Properties of Diodes Figure 1.10 — The Diode Transconductance Curve2 = Bias Voltage ID = Current through Diode. ID is Negative for Reverse Bias and Positive for Forward Bias Is = Saturation Current VBR = Breakdown Voltage V = Barrier Potential Voltage
  24. Properties of Diodes The transconductance curve on the previous slide is characterized by the following equation: ID = Is(eVDh1VT — 1) As described in the last slide, ID is the current through the diode, Is is the saturation current and VD is the applied biasing voltage. VT is the thermal equivalent voltage and is approximately 26 mV at room temperature. The equation to find VT at various temperatures is: q = 1.6 x 10-19 C k = 1.38 x 10-23 J/K T = temperature in Kelvin is the emission coefficient for the diode. It is determined by the way the diode is constructed. It somewhat varies with diode current. For a silicon diode is around 2 for low currents and goes down to about 1 at higher currents pn-juntion-Diode
  25. Diode Circuit Models The Ideal Diode The diode is designed to allow current to flow in Model only one direction. The perfect diode would be a perfect conductor in one direction (forward bias) and a perfect insulator in the other direction (reverse bias). In many situations, using the ideal diode approximation is acceptable. Example: Assume the diode in the circuit below is ideal. Determine the value of ID if a) VA = 5 volts (forward bias) and b) VA = -5 volts (reverse bias) Rs = 50 Q a) With VA > O the diode is in forward bias and is acting like a perfect conductor so: ID = = 5 V 1 50 Q = 100 mA b) With VA < O the diode is in reverse bias and is acting like a perfect insulator, therefore no current can flow and ID = O
  26. Diode Circuit Models The Ideal Diode with This model is more accurate than the simple Barrier Potential ideal diode model because it includes the approximate barrier potential voltage. Remember the barrier potential voltage is the voltage at which appreciable current starts to flow. Example: To be more accurate than just using the ideal diode model include the barrier potential. Assume = 0.3 volts (typical for a germanium diode) Determine the value of ID if VA = 5 volts (forward bias). Rs = 50 Q With VA > O the diode is in forward bias and is acting like a perfect conductor so write a KVL equation to find ID: 4.7 v = 94 mA 50 Q
  27. The Ideal Diode with Barrier Potential and Linear Forward Resistance AID Diode Circuit Models This model is the most accurate of the three. It includes a linear forward resistance that is calculated from the slope of the linear portion of the transconductance curve. However, this is usually not necessary since the RF (forward resistance) value is pretty constant. For low-power germanium and silicon diodes the RF value is usually in the 2 to 5 ohms range, while higher power diodes have a RF value closer to 1 ohm. AID AVD Linear Portion of transconductance curve
  28. The Ideal Diode with Barrier Potential and Linear Forward Resistance Rs = 50 Q Diode Circuit Models Example: Assume the diode is a low-power diode with a forward resistance value of 5 ohms. The barrier potential voltage is still: V = 0.3 volts (typical for a germanium diode) Determine the value of ID if VA = 5 volts. Once again, write a KVL equation for the circuit: v A = 5-0.3 50 +5 = 85.5 mA
  29. Diode Circuit Models Values of ID for the Three Different Diode Circuit Models Ideal Diode Model 100 mA Ideal Diode Model with Barrier Potential Voltage 94 mA Ideal Diode Model with Barrier Potential and Linear Forward Resistance 85.5 mA These are the values found in the examples on previous slides where the applied voltage was 5 volts, the barrier potential was 0.3 volts and the linear forward resistance value was assumed to be 5 ohms. pn-juntion-Diode
  30. The Q Point The operating point or Q point of the diode is the quiescent or no- signal condition. The Q point is obtained graphically and is really only needed when the applied voltage is very close to the diode's barrier potential voltage. The example 3 below that is continued on the next slide, shows how the Q point is determined using the transconductance curve and the load line. First the load line is found by substituting in different values of into the equation for ID using the ideal diode with barrier potential model for the diode. With Rs at 1000 ohms the value of RF wouldn't have much impact on the results. Using V (b values of O volts and 1.4 volts we obtain ID values of 6 mA and 4.6 mA respectively. Next we will draw the line connecting these two points on the graph with the transconductance curve. This line is the load line.
  31. The Q Point 8 6 4.6 4 2 12 10 1.0 1.2 The transconductance curve below is for a Silicon diode. The Q point in this example is located at 0.7 V and 5.3 mA. The intersection of the load line and the transconductance curve. 0.2 0.4 0.6 Q Point: 0.8 VD (Volts) 1.4
  32. The dynamic resistance of the diode is mathematically determined as the inverse of the slope of the transconductance curve. Therefore, the equation for dynamic resistance is: The dynamic resistance is used in determining the voltage drop across the diode in the situation where a voltage source is supplying a sinusoidal signal with a dc offset. The ac component of the diode voltage is found using the following equation: ac The voltage drop through the diode is a combination of the ac and dc components and is equal to: F
  33. Dynamic Resistance Example: Use the same circuit used for the Q point example but change the voltage source so it is an ac source with a dc offset. The source voltage is now, = 6 + sin(wt) Volts. It is a silicon diode so the barrier potential voltage is still 0.7 volts. The DC component of the circuit is the same as the previous example and therefore ID = 6V - 0.7 V = 5.2 mA 1000 Q = DY..I= 26 mV = 4.9 Q 5.3 mA = 1 is a good approximation if the dc current is greater than 1 mA as it is in this = sin(wt)V 4.9 Q example. = 4.88 sin(wt) mV ac rF + Rs Therefore, V 4.9 Q + 1000 Q D = 700 + 4.9 sin (wt) mV (the voltage drop across the diode)
  34. Types of Diodes and Their Uses PN Junction Diodes: Are used to allow current to flow in one direction while blocking current flow in the opposite direction. The pn junction diode is the typical diode that has been used in the previous circuits. Schematic Symbol for a PN Junction Diode p n Representative Structure for a PN Junction Diode Zener Diodes: Are specifically designed to operate under reverse breakdown conditions. These diodes have a very accurate and specific reverse breakdown voltage. Schematic Symbol for a Zener Diode
  35. Types of Diodes and Their Uses Schottky Diodes: Schematic Symbol for a Schottky Diode Shockley Diodes: Schematic Symbol for a four-layer Shockley Diode These diodes are designed to have a very fast switching time which makes them a great diode for digital circuit applications. They are very common in computers because of their ability to be switched on and off so quickly. The Shockley diode is a four-layer diode while other diodes are normally made with only two layers. These types of diodes are generally used to control the average power delivered to a load.
  36. Types of Diodes and Their Uses Light-Emittinq Diodes: Light-emitting diodes are designed with a very large bandgap so movement of carriers across their depletion region emits photons of light energy. Lower bandgap LEDs (Light-Emitting Diodes) emit infrared radiation, while LEDs with higher bandgap energy emit visible light. Many stop lights are now starting to use LEDs because they are extremely bright and last longer than regular bulbs for a relatively low cost. K The arrows in the LED representation indicate emitted light. Schematic Symbol for a Light-Emitting Diode
  37. Types of Diodes and Their Uses While LEDs emit light, Photodiodes are sensitive to Photodiodes: received light. They are constructed so their pn junction can be exposed to the outside through a clear window or lens. K In Photoconductive mode the saturation current increases in proportion to the intensity of the received light. This type of diode is used in CD players. In Photovoltaic mode, when the pn junction is exposed to a certain wavelength of light, the diode generates voltage and can be used as an energy Schematic Symbols for source. This type of diode is used in the Photodiodes production of solar power.
  38. re n ces Dailey, Denton. Electronic Devices and Circuits, Discrete and Inteqrated. Prentice Hall, New Jersey: 2001. (pp 2-37, 752-753) 2 Figure 1.10. The diode transconductance curve, pg. 7 Figure 1.15. Determination of the average forward resistance of a diode, pg 11 3 Example from pages 13-14 Liou, J.J. and Yuan, J.S. Semiconductor Device Physics and Simulation. Plenum Press, New York: 1998. Neamen, Donald. Semiconductor Physics & Devices. Basic Principles. McGraw-Hill, Boston: 1997. (pp 1-15, 211-234) 1 Figure 6.2. The space charge region, the electric field, and the forces acting on the charged carriers, pg 213.

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