Electronics The Seventh and Eighth and Lectures Eighth week 28 / 12/ 1436 هـ - 1 / 1/ 1437 هـ أ / سمر السلمي

Chapter Two: Junction Diode Physical Electronics Forward-biased and Reverse- biased Junction depletion region and applying bias, and Diffusion and Drift currents Drive excess of minority carrier in Forward-biased and Reverse- biased Junction Mathematical description of excess of minority carrier in Forward-biased and Reverse- biased Junction Drive diffusion current in forward-biased and reverse- biased Junction I – V characteristic of forward-biased and reverse-biased Junction Diode Models Pn Junction’s mission(half and full wave rectifiers, filters, regulator) Zener diode Ohmic Contact Metal–Oxide–Semiconductor Contact (MOS) Structure, Effect of voltage Energy levels forms in MOS in different bias Outline for today

Time of Periodic Exams The first periodic exam in / 1 / 1437 هـ , Please everyone attend In her group The second homework I put the second homework in my website in the university homework Due Thursday 2 / 1/ 1437 H in my mailbox in Faculty of Physics Department, I will not accept any homework after that, but if you could not come to university you should sent it to me by in the same day

 Forward-biased and Reverse- biased Junction Our discussion until now is about equilibrium condition (The absence of power supply (battery) between the parties of diode).  But what about the condition (the existence of power supply between the parties of diode?  How the power supply connect between the parties of the diode ?? What happens to contact potential increased or decreased?  What will happened to diffusion and drift currents in the junction? In the condition of power supply existence, so that positive terminal of it connect with p-type side and negative terminal of it connect with n-type side; this condition called forward bias. however, If the two terminals reverse, the condition called reverse bias as in the figure blow. =

 Forward-biased Junction (depletion region and applying bias) In forward bias, we notice that the outer field (forward voltage) opposes built-in field created by the space charges in the depletion region. Therefore, the number of donor and acceptor ions reduces; thus depletion region width decreases. The potential barrier is lower in this condition than in equilibrium condition; in addition voltage difference across the diode decreases (( V 0 –V f )) as in the figure. Also we notice that Fermi level will not be at the same energy level in the two types as in equilibrium condition.

 Forward-biased Junction (Diffusion and Drift) This decreasing of voltage difference across the diode in forward bias affect the diffusion current ( due to injection of holes in p-type and electrons in n-type ) so, we obtain a huge flow of diffusion currents of the electrons and holes compared with it in equilibrium condition ; also due to climbing of electron easily to new voltage difference level. However, drift current is not effected by decreasing of voltage difference or length of potential barrier due to not effecting of minority carrier which remain pulled inside the diffusion current at the edge of the depletion region. Therefore, drift current in this condition remains as in equilibrium condition

 Reverse-biased Junction (depletion region and applying bias) In reverse bias, we notice that the outer field (reverse voltage ) in the same direction of built-in field created by the space charges in the depletion region. Therefore, the number of donor and acceptor ions rise; thus depletion region width increases. The potential barrier is higher in this condition than in equilibrium condition; in addition voltage difference across the diode increases ((V 0 + V r )) as in the figure. Also we notice that Fermi level will not be at the same energy level in the two types as in equilibrium condition =

 Reverse-biased Junction (Diffusion and Drift) In reverse bias which opposite of forward bias, the increasing of voltage difference across in the diode affect negatively to diffusion current. There shall be a few flow of diffusion currents of the electrons and holes transported to the other end compared with it in equilibrium condition ; also due to climbing of electron difficulty to new voltage difference level. Similar to forward bias, drift current is not effected by increasing of voltage difference or length of potential barrier. Therefore, drift current in this condition remains as in equilibrium condition =

 Forward-biased and Reverse- biased Junction Look at increases of direction of electron and hole diffusion in forward bias and decreases in reverse bias and drift as the same as in equilibrium condition.

 Drive excess of minority carrier in Forward-biased and Reverse- biased Junction In thermal equilibrium, we represent relations However, forward and reverse bias represent relations as follows The forward- biased voltage and reverse -biased voltage In the case of biased, the low-level injection or carriers minority concentration is very weak so will not affect the equilibrium majority carriers; thus we can consider

 Drive excess of minority carrier in Forward-biased and Reverse- biased Junction We start by holes in non equilibrium condition From relation By substitute We obtain By subtracting p n0 from both sides, we get a excess of minority -carriers of holes concentration in n-type

 Drive excess of minority carrier in Forward-biased and Reverse- biased Junction Similarly, electrons in non equilibrium condition From relation By substitute We obtain By subtracting n p0 from both sides, we get a excess of minority -carriers of electrons concentration in p-type

 Mathematical description of excess of minority carrier in Forward-biased and Reverse- biased Junction From previous lectures, we discussed about the equation for diffusion of minority carriers The solution for it is where A and B are constants which can be found from boundary conditions (from the following figure a semiconductor p-type variable concentration in one dimension) (In the case of bias {non- equilibrium} we will deal of low-level injection) Therefore, we assume that And Therefore, we obtain

 Mathematical description of excess of minority carrier in Forward- biased and Reverse- biased Junction Similar, in case of bias in junction =

 Mathematical description of excess of minority carrier in Forward- biased and Reverse- biased Junction to Drive diffusion current Similar, in case of bias in junction Thus, the equation for diffusion of minority carriers in n –type & p- type is By substituting to excess minority -carriers concentration from previous derivation In focusing of borders depletion region, we obtain

 Drive diffusion current in Forward-biased and Reverse- biased Junction We notice that diffusion current variables in equilibrium condition (or the absence of bias) while drift current does not change its status in equilibrium condition. To get the current density in the junction : first from p-type side (minority electrons current density) To focus in one dimension and to solving equations of diffusion of the minority carriers At borders of depletion region =

 Drive diffusion current in Forward-biased and Reverse- biased Junction from n-type side diffusion current density (minority holes current density) To focus in one dimension and to solving equations of diffusion of the minority carriers At borders of depletion region To get the total current density of junction must collect two current densities for n-type and p-type, however, we should notice the direction of them

 Drive diffusion current in Forward-biased and Reverse- biased Junction From the figure To calculate the current Where the reverse saturation current is

 Drive diffusion current in Forward-biased and Reverse- biased Junction Figure illustrates the distribution of the electron and hole currents in the junction in the forward bias

 I – V Characteristic of Forward-biased and Reverse-biased Junction You studied at electronic lab the relation between voltage and current of diode in forward-biased and reverse-biased depending on electrical conductivity to terminals of battery and diode as in the figure We conduct first in forward bias then reverse bias. In addition, a resistor is used to limit the forward current to a value that will not overheat the diode and cause damage. This resistor called dynamic resistance. To calculate resistance value is inverted slope dI/dV tangent to the curve. The value of resistance in forward bias less than the value in reverse bias R f < R b

 I – V Characteristic of Forward-biased and Reverse-biased Junction At forward bias circuit, we notice when increasing forward voltage, there is a flow in the current (diffusion current here). After the voltage value reaches to (barrier potential), the forward current increase rapidly (barrier potential for Si is 0.7v and Ge is 0.3 v ). From figure and explanation, we notice that diode is not linear which not depend on Ohm’s law due to first quarter of graph. =

 I – V Characteristic of Forward-biased and Reverse-biased Junction At reverse bias circuit as fourth quarter of graph, there is small value of voltage and constant in relation between reverse voltage and diffusion current (which called Reverse Saturation Current). Keeping in mind, reverse voltage has large value compared with forward voltage to reached a particular voltage called Avalanche Voltage. This is shown clearly when we study a special type of diodes : Zener diode. The current increase very rapidly after Zener Breakdown or Breakdown voltage V BR

 I – V Characteristic of Forward-biased and Reverse-biased Junction the relation between diffusion current and applied voltage Forward-biased Reverse-biased is an exponential relation Where I 0 is reverse saturation current

 I – V Characteristic of Forward-biased and Reverse-biased Junction Temperature Effect : for forward-biased diode as temperature is increased, the forward current increases for a given value of forward voltage. For a given value of forward current, the forward voltage decrease, as shown in figure. The blue curve is at room temperature and red curve is elevated temperature (300K + ΔT) notice that barrier potential decreases as temperature increases. For reverse-biased diode as temperature is increased, the reverse current increases. However, there different between two curves. Keep in mind that the reverse current breakdown remains extremely small and can usually be neglected

 Diode Models There are three models for diode 1-The Ideal Model: This model is a simple switch. When the diode is forward-biased, it acts like a closed (on) switch. When it is reverse –biased, it acts like a closed (off) switch. the barrier potential, the forward dynamic resistance, and the reverse current are all neglected in this model.

 Diode Models 2- The practical Model The practical model adds the barrier potential to ideal model. When the diode is forward-biased, it is equivalent to a closed switch in series with a small equivalent voltage source equal to the barrier potential. When the diode is reverse-biased, it is equivalent to an open switch just as ideal model because barrier potential does not affect reverse bias.

 Diode Models 3- The Complete Model: This model consists of the barrier potential, the small forward dynamic resistance, and the large internal reverse resistance.

 Pn Junction’s mission Of the most important uses and benefits of the pn junction is the rectifier current from alternating current to direct current and you will study in detail that in Electronics Lab. where junction connect to AC source (which current shape wave or sine with time) to take advantage of the forward bias (passing current) reverse bias (not passage current) are rectifier the alternating current. There are two types of rectifier: half – wave rectifier and full wave rectifier. In addition to rectifier process, the filter process came next. By putting capacitor that will improve the current form of half wave or full wave into a form close to a straight line by putting a number of capacitors After rectifier and filter processes, regulator process came third (which we can put Zener diode) that give straight current. Three processes (rectifier, filter, regulator) to transformation from alternating current to direct current

 Half-Wave Rectifiers =

To calculate average value of half-wave rectified output voltage However, this calculation for ideal diode. So in practical diode, we take in the account the barrier potential

 Full-Wave Rectifiers There two circle for full –wave rectifiers 1)A center- tapped full –wave rectifiers 2) A bridge full –wave rectifiers

 center- tapped Full-Wave Rectifiers =

 The Bridge Full-Wave Rectifiers =

 Filters By using capacitor half or fall – wave is filtered. By putting number of capacitors, the wave is filtered more than before

 Regulator The final step to transformation from alternating current to direct current is regulator. There are numbers of regulator devices. One of them is Zener diode

 Zener diode Zener diode differs from normal pn junction in its designed to work in reverse breakdown area without the occurrence of any problems. Difference in Zener diode differential has more impurities in one of side of diode than the other as (n + p ) or ( p + n). In equilibrium condition Fermi level at the same energy level in n-type and p-type, but in non equilibrium condition opposite as we knew. However, in the case of reverse bias aligns the conduction band in the n-type and valence band in the p-type as if they are at the same energy level and thus the electron from the conduction band in the n-type moves (or tunneling) to the valence band in p-type as shown in the figure. Among the benefits of this diode is used in regulate voltage.

Previously, we discussed about pn junction (a semiconductor type of n-type next to the other type p-type) But what about another junction as semiconductor next to insulator or metal  Ohmic Contact This contact consists metal and semiconductor (which follows Ohm's Law and does not have the ability to rectifier current and (current and voltage) characteristic is linear in both forward and reverse bias ) The scientific method to create This contact 1- by increasing impurities in a semiconductor at the contact area so that the charge carriers crosses barrier 2- by selecting a work function of metal (eΦ) close to work function of semiconductor work function : the required energy to remove an electron from Fermi level or required energy for ionization metal =

 Ohmic Contact The left figure illustrates Fermi level and work function in the metal and semiconductor (n-type) before contact. The right figure illustrates them when configure junction of metal and semiconductor (n-type) in equilibrium condition and how Fermi level aligning between metal and semiconductor (when there are negative charges close to surface of metal, they attract positive charges in metal ) =

 Ohmic Contact The left figure illustrates Fermi level and work function in the metal and semiconductor (p-type) before contact. The right figure illustrates them when configure junction of metal and semiconductor (p-type) in equilibrium condition and how Fermi level aligning between metal and semiconductor (when there are positive charges close to surface of metal, they attract negative charges in metal ) =

 Metal–Oxide–Semiconductor Contact (MOS)  structure In this contact, a thin layer of oxide is put on the surface of a semiconductor n- type or p-type. Then, pole metallic (metal) is put above the surface of the oxide layer. We should choose a good electrical insulation of oxide which has a large energy gap and isolates the metal from the semiconductor which no passing electrical current between them. In thermal equilibrium condition In the absence of application of the electric field or the voltage, the Fermi and connection and valence levels are horizontal and flat. When applying an electric field, there is a bending in energy levels

 Metal–Oxide–Semiconductor Contact (MOS)  Effect of voltage bias According to the applied voltage on this contact, it will consist three different situations such as what is shown in figure. 1- depletion 2- inversion 3- accumulation =

 Metal–Oxide–Semiconductor Contact (MOS)  Effect of voltage bias (metal and n-type contact) 1 - Depletion : When applying negative bias voltage at the surface of metal, a small amount of negative charges is made. Then, the oxide layer prevent electric current from passage to semiconductor. However, the electrons in substrate of semiconductor n-type will be affected by these negative charges and moved away from the area located under oxide and created the depletion region in semiconductor similar to those that created in pn Junction =

 Metal–Oxide–Semiconductor Contact (MOS)  Effect of voltage bias (metal and n-type contact) 2 - Inversion When increasing a negative bias voltage on surface of metal, Instead of expanding more of depletion region within the semiconductor, inversion status is formed which holes gather next to the surface of the oxide. Those holes is the minority carriers in the semiconductor n-type. 3 - Accumulation When applying positive bias voltage at the surface of metal, negative majority carriers attract and accumulate at the surface of the oxide in semiconductor n- type. =

 Metal–Oxide–Semiconductor Contact (MOS)  Energy levels forms in MOS in different bias (metal and p-type contact) 1- Depletion : When applying positive bias voltage, Fermi level move down from its first location in thermal equilibrium condition. Also, straight bend at the energy level in oxide and energy levels of the semiconductor p-type move down near the interface of oxide. In addition, electrons drop down in potential well. We notice that the distribution of carriers density of per unit area in semiconductor p-type equal in the metal =

 Metal–Oxide–Semiconductor Contact (MOS)  Energy levels forms in MOS in different bias (metal and p-type contact) 2 - Inversion When increasing positive bias voltage more than threshold voltage V T ; the semiconductor inverse and electrons occupy inversion layer. Fermi level move more down from its first location in thermal equilibrium condition. Also, straight bend at the energy level in oxide and energy levels of semiconductor p-type move more down near interface of oxide. In addition, electrons drop more down in potential well. We notice that the distribution of carriers density of per unit area in semiconductor p-type for maximum depletion region W max in addition to carrier of inversion layer Q n equal in the metal =

 Metal–Oxide–Semiconductor Contact (MOS)  Energy levels forms in MOS in different bias (metal and p-type contact) 3- Accumulation When applying negative bias voltage, Fermi level move up from its first location in thermal equilibrium condition. Also, straight bend at the energy level in oxide and energy levels of the semiconductor p-type move up near the interface of oxide. In addition, holes climb up in potential well. We notice that the distribution of carriers density of per unit area in semiconductor p-type equal in the metal =