PN Junction Diode Table of Contents. What Are Diodes Made Out Of?

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1 PN Junction iode Table of Contents What are diodes made out of?slide 3 N-type materialslide 4 P-type materialslide 5 The pn junctionslides 6-7 The biased pn junctionslides 8-9 Properties of diodesslides iode Circuit Models slides The Q Pointslides ynamic Resistanceslides Types of diodes and their uses slides Sourcesslide 25 What Are iodes Made Out Of? licon () and Germanium (Ge) are the two most common single elements that are used to make iodes. A compound that is commonly used is Gallium Arsenide (GaAs), especially in the case of LEs because of it s large bandgap licon 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, 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. The diagram above shows the 2 structure of the crystal. The light green lines represent the electronic bonds made when the valence electrons are shared. Each atom shares one electron with each of its four closest neighbors so that its valence band will have a full 8 electrons. 1

2 N-Type Material 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 2 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. P-Type Material 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 2 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. The PN Junction Steady State 1 Na Metallurgical Junction Nd P ionized acceptors Space Charge Region E-Field h drift = h diffusion e- diffusion = e- drift n ionized donors 2

3 P ionized acceptors Na Metallurgical Junction Space Charge Region E-Field The PN Junction Nd = = h drift h diffusion e- diffusion e- drift n ionized donors 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 Space Charge Region: 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. Metallurgical 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 to The Biased PN Junction Metal Contact Ohmic Contact (Rs~0) P Applied Electric Field n I V 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. Forward Bias: V applied > 0 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. Reverse Bias: V applied < 0 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. 3

4 Properties of iodes Figure The iode Transconductance Curve I (ma) V = Bias Voltage I S I = Current through iode. I is Negative for Reverse Bias and Positive for Forward Bias V BR ~V V I S = Saturation Current V BR = Breakdown Voltage V = Barrier Potential Voltage (na) Properties of iodes The Shockley Equation The transconductance curve on the previous slide is characterized by the following equation: I = I S (e V /V T 1) As described in the last slide, I is the current through the diode, I S is the saturation current and V is the applied biasing voltage. V T is the thermal equivalent voltage and is approximately 26 mv at room temperature. The equation to find V T at various temperatures is: V T = kt q k = 1.38 x J/K T = temperature in Kelvin q = 1.6 x C 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 The Ideal iode Model iode Circuit Models The diode is designed to allow current to flow in 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. etermine the value of I if a) V A = 5 volts (forward bias) and b) V A = -5 volts (reverse bias) V A R S = 50 I a) With V A > 0 the diode is in forward bias and is acting like a perfect conductor so: I = V A /R S = 5 V / 50 = 100 ma b) With V A < 0 the diode is in reverse bias and is acting like a perfect insulator, therefore no current can flow and I = 0. 4

5 iode Circuit Models The Ideal iode with This model is more accurate than the simple ideal Barrier Potential diode model because it includes the approximate barrier potential voltage. V 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 V = 0.3 volts (typical for a germanium diode) etermine the value of I if V A = 5 volts (forward bias). V A R S = 50 I V With V A > 0 the diode is in forward bias and is acting like a perfect conductor so write a KVL equation to find I : 0 = V A I R S - V I = V A - V = 4.7 V = 94 ma R S 50 The Ideal iode with Barrier Potential and Linear Forward Resistance iode 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 R F (forward resistance) value is pretty constant. For low-power germanium and silicon diodes the R F value is usually in the 2 to 5 ohms range, while higher power diodes have a R F value closer to 1 ohm. V R F I Linear Portion of transconductance curve R F = ΔV Δ I Δ I V Δ V The Ideal iode with Barrier Potential and Linear Forward Resistance iode 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) etermine the value of I if V A = 5 volts. R S = 50 V A I V Once again, write a KVL equation for the circuit: 0 = V A I R S - V - I R F I = V A - V = = 85.5 ma R S R F 50 5 R F 5

6 iode Circuit Models Values of I for the Three ifferent iode Circuit Models Ideal iode Model Ideal iode Model with Barrier Potential Voltage Ideal iode Model with Barrier Potential and Linear Forward Resistance I 100 ma 94 ma 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. 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 below that is continued on the next slide, shows how the Q point is determined using the transconductance curve and the load line. V A = 6V R S = 1000 I V First the load line is found by substituting in different values of V into the equation for I using the ideal diode with barrier potential model for the diode. With R S at 1000 ohms the value of R F wouldn t have much impact on the results. I = V A V Using V values of 0 volts and 1.4 volts we obtain I 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. R S I (ma) The Q Point The transconductance curve below is for a licon diode. The Q point in this example is located at 0.7 V and 5.3 ma Q Point: The intersection of the load line and the transconductance curve V (Volts) 6

7 ynamic Resistance 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: r F = V T I 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 (when a waveform has unequal amounts of signal in the positive and negative domains). The ac component of the diode voltage is found using the following equation: v F = v ac r F r F R S The voltage drop through the diode is a combination of the ac and dc components and is equal to: V = V v F ynamic 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, v in = 6 sin(wt) Volts. It is a silicon diode so the barrier potential voltage is still 0.7 volts. v in R S = 1000 I V The C component of the circuit is the same as the previous example and therefore I = 6V 0.7 V = 5.2 ma 1000 r F = V T = 1 * 26 mv = 4.9 I 5.3 ma = 1 is a good approximation if the dc current is greater than 1 ma as it is in this example. v F = v ac r F = sin(wt) V 4.9 = 4.88 sin(wt) mv r F R S Therefore, V = sin (wt) mv (the voltage drop across the diode) Types of iodes and Their Uses PN Junction iodes: 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. A K P n Schematic Symbol for a PN Junction iode Representative Structure for a PN Junction iode Zener iodes: A Are specifically designed to operate under reverse breakdown conditions. These diodes have a very accurate and specific reverse breakdown voltage. K Schematic Symbol for a Zener iode 7

8 Types of iodes and Their Uses Schottky iodes: A K 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. Schematic Symbol for a Schottky iode Shockley iodes: A 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. K Schematic Symbol for a four-layer Shockley iode Types of iodes and Their Uses Light-Emitting iodes: 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 LEs (Light-Emitting iodes) emit infrared radiation, while LEs with higher bandgap energy emit visible light. Many stop lights are now starting to use LEs because they are extremely bright and last longer than regular bulbs for a relatively low cost. A K The arrows in the LE representation indicate emitted light. Schematic Symbol for a Light-Emitting iode Types of iodes and Their Uses Photodiodes: A A Schematic Symbols for Photodiodes K K While LEs emit light, Photodiodes are sensitive to received light. They are constructed so their pn junction can be exposed to the outside through a clear window or lens. In Photoconductive mode the saturation current increases in proportion to the intensity of the received light. This type of diode is used in C 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 source. This type of diode is used in the production of solar power. 8

9 iodes Circuits Key Words: iode Limiter multi diode Circuits Rectifier Circuits v i v i - R v o vo - Von vo t t When v i > V on, on v o v i ; v i < V on, off v o = 0 5V multiple diodes Circuits V 1 1 V 2 2 R V o V 1 (V) V 2 (V) V o (V) Logic output

Rectifier Circuits One of the most important applications of diodes is in the design of rectifier circuits. Used to convert an AC signal into a C voltage used by most electronics.

10 Rectifier Circuits One of the most important applications of diodes is in the design of rectifier circuits. Used to convert an AC signal into a C voltage used by most electronics. Rectifier Circuits mple Half-Wave Rectifier What would the waveform look like if not an ideal diode? Rectifier Circuits Bridge Rectifier Looks like a Wheatstone bridge. oes not require a center tapped transformer. Requires 2 additional diodes and voltage drop is double. 10

11 iode Circuits: Applications Applications Rectifier Circuits Half-Wave Rectifier Circuits Applications Rectifier Circuits Battery-Charging Circuit 11

12 Half-Wave Rectifier with Smoothing Capacitor Large Capacitance i=dq/dt or Q = I L T Q = V r C then C ~ (I L T) / V r Half-Wave Rectifier with Smoothing Capacitor Large Capacitance V r Peak-to-peak riple voltage Start i=dq/dt or Q = I L T then Q = V r C Forward bias Reverse bias charge cycle discharge cycle C ~ (I L T) / V r Full-Wave rectifier Circuits The sources are out of phase 12

13 Wave Shaping Circuits Clipper Circuits Batteries replaced by Zener diodes Half-Wave Limiter Circuits I flow below 600 mv I flow Above 600 mv 600 mv Current flows thru the resistor until 600 mv is reached, then flows thru the iode. The plateau is representative of the voltage drop of the diode while it is conducting. Voltage divider mv Linear Small gnal Equivalent Circuits (1) When considering electronic circuits in which dc supply voltages are used to bias a nonlinear devices at their operating points and a small ac signal is injected into the circuit to find circuit response: Split the analysis of the circuit into two parts: (a)analyze the dc circuit to find the operating point (b)consider the small ac signal 13

14 Linear Small gnal Equivalent Circuits (1) nce virtually any nonlinear ch-tic is approximately linear (straight) if we consider a sufficiently small segment THEN The small signal diode circuit can be substituted by a single equivalent resistor. We can find a linear small-signal equivalent circuit for the nonlinear device to use in the ac analysis Linear Small gnal Equivalent Circuits (2) dc supply voltage results in operation at Q An ac signal is injected into the circuit and swings the instantaneous point of operation slightly above and below the Q point For small changes di i v dv Q i the small change in diode current from the Q-point v the small change in diode voltage from the Q-point (di /dv ) the slope of the diode ch-tic evaluated at the point Q Linear Small gnal Equivalent Circuits (2) dc supply voltage results in operation at Q An ac signal is injected into the circuit and swings the instantaneous point of operation slightly above and below the Q point For small changes ynamic resistance of the diode 1 di i v dv di r Q dv Q i i the small change in diode current from the Q-point v the small change in diode voltage from the Q-point (di /dv ) the slope of the diode ch-tic evaluated at the point Q v r 14

15 Linear Small gnal Equivalent Circuits (3) From small signal diode analysis vd i I s exp nv 1 T kt VT q ifferentiating the Shockley eq. di dv I S 1 v exp nvt nvt and following the math on p.452 we can write that dynamic resistance of the diode is nvt r I Q where I Q vq ~ I s exp nvt Example - Voltage-Controlled Attenuator Find the operating point and perform the small signal analysis to obtain the small signal voltage gain C control signal 1 Z C j C C 1, C 2 small or large? C in dc circuit open circuit C in ac circuit short Example - Voltage-Controlled Attenuator C control signal c circuit for Q point (I Q, V Q ) nv T r Compute at the Q I point (I Q Q, V Q ) 15

16 Example - Voltage-Controlled Attenuator The dc voltage source is equivalent to a short circuit for ac signals. v R p Av 0 v Voltage R Rgain in p Sources ailey, enton. Electronic evices and Circuits, iscrete and Integrated. Prentice Hall, New Jersey: (pp 2-37, ) 2 Figure The diode transconductance curve, pg. 7 Figure etermination of the average forward resistance of a diode, pg 11 3 Example from pages Liou, J.J. and Yuan, J.S. Semiconductor evice Physics and mulation. Plenum Press, New York: Neamen, onald. Semiconductor Physics & evices. Basic Principles. McGraw-Hill, Boston: (pp 1-15, ) 1 Figure 6.2. The space charge region, the electric field, and the forces acting on the charged carriers, pg

semiconductor p-n junction Potential difference across the depletion region is called the built-in potential barrier, or built-in voltage:

semiconductor p-n junction Potential difference across the depletion region is called the built-in potential barrier, or built-in voltage: Chapter four The Equilibrium pn Junction The Electric field will create a force that will stop the diffusion of carriers reaches thermal equilibrium condition Potential difference across the depletion