Semiconductor Doping and Diode Characteristics - Prof. Aurangzeb Khan, Study notes of Electrical and Electronics Engineering

Download Semiconductor Doping and Diode Characteristics - Prof. Aurangzeb Khan and more Study notes Electrical and Electronics Engineering in PDF only on Docsity! Review of Basic Concepts Extrinsic Semiconductors • Extrinsic semiconductor can be formed from an intrinsic semiconductor by added impurity atoms to the crystal in a process known as doping. • An n-type semiconductor is formed by doping the silicon crystal with elements of group V of the periodic table (antimony, arsenic, and phosphorus). • A p-type semiconductor is formed by doping the silicon crystal with elements of group III of the periodic table (aluminum, boron, gallium, and indium). Si Si Si SiPSi Si Si Si - q+ q Figure 2.5 - An extra electron is available from a phosphorus donor atom Si Si Si SiBSi Si Si Si Vacancy Figure 2.6(a) - Covalent bond vacancy from boron acceptor atom E C Electrons ED ND E V Figure 2.10 - Donor level with activation energy (EC - ED). This figure corresponds to the @ Holes and free electrons related by @ Room temperature (300 K) itil) ‘ @lf @ Then @ Therefore where Semiconductor in “thermal equilibrium” Seema elec melt 4 Seis | Labi @ Then lis) a | @ ptype material: Seared ee ee) El DRIFT & DIFFUSION CURRENTS @ Drift current —> Carrier movement is caused by an electric field. . © Diffusion current —> Carriers move from Two more points about high to low concentrations. doped semiconductors Pe ee aT ee ee ae ll E EL sO — Si, li oe PTE ees Me fet Leeman Mar od — G e — J, EXCESS CARRIERS @ Valence electrons acquiring more energy to break additional covalent bonds (i.e., at high temperature). LB eee M meets [etl Te em mm @® Excess electrons & excess holes: n=n +4, P = p, +4, Chapter-2: Diode circuits Diode: Why we need to understand diode? • The base emitter junction of the BJT behaves as a forward bias diode in amplifying applications. • The behavior of the diode when reverse bias is the key to the fabrication of the integrated circuits. • The diode is used in many important nonamplifer applications. p-n Junction Diode p-n Junction p-Type Materia l n-Type Material p-Type Materia l n-Type Material p-n Junction A p-n junction diode is made by forming a p-type region of material directly next to a n-type region. — a separation of charge anda = Il, P type N type raqlon reglon Erietd Forward Bias Nery large Anode + mee titel =] as : ; | tear ee edt fo='a (lott Maa =a fey meme t-laia || barrier. Reducing barrier height increases the NetNames] ada est de Holes flow from the p to the n region (minority carrier injection). Electrons flow from the n to the p region (majority carrier injection). Large current densities produced by small voltages. Forward bias: Minority-carrier distribution in a forward-biased pn junction. It is assumed that the p region is more heavily doped than the n region; NA >> ND. Departure from ideal behavior The four major reason why the actual diode do not correspond exactly to the ideal. 1. Ohmic resistance and contact resistance in series with the diode cause the VI curve to become linear at high forward current. 2. Avalanche or Zener breakdown take place at high reverse voltage, causing an abrupt increase in reverse current. 3. Surface contaminants cause an ohmic layer to form across the junction, which is Increasing the reverse current as reverse voltage is increased. 4. Recombination of current carrier in the depletion region take place due to traps. Nonlinear Modeling • Nonlinear problems are difficult to solve • The diode is a nonlinear device • Picewise linear models can simplifying the solution of non linear circuits problems. Demonstration of the difficulty in solving nonlinear problems (cont.) In order to determine Vout we have to solve another equation which can be written as by Kirchhoff’s law, V1 = IR1 + Vout ⇒V1= 200 X 10-10(eVout/0.026-1) + Vout Again, there is no close form solution of the above equation. Perhaps the quickest method for solving this problems is a trial and error iterative method. If we guess many time, finally we will be able to show that, when Vout = 0.505215 ~ 0.5V, the right side of the above equation is 5.99V, which is essentially equal to the value of the left side of the equation. Finally ,I =0.02747≅0.027A. Possible model of the problem (constant voltage drop model) • One possible model for the forward bias diode is a simple 0.6V voltage source. • When this model replaces the diode, the circuit appear as shown in the figure and is very easy to analyze. • For this circuit the current is calculated to be • I=(V1-0.6)/200 = 0.027A • And the Vout = 0.6V • These values compare well to the results calculated from the exact equations, but much easier to obtained. • The above example demonstrate that how model simplifies the solution. A load line approach • An alternate and more traditional graphical method to analyze a circuit containing a nonlinear element is that of using a load line. • The load line can yield accurate results and used extensively in the evaluation of the electronic circuits. Piecewise Linear Diode Model Slope = 1/Rf V VD ID Approximate the diode characteristics with straight line Forward Bias Reverse Bias Conductor-to-Semiconductor Contact @® Must have some way of making electrical connections to semiconductors. @ If metal (Al, Pt, Au, Ag, Cu, etc.) deposited on clean semiconductor surface (Ge, Si, GaAs, etc.), the resulting contact: # Rectifying # Ohmic  Metal-to-Semiconductor Contacts @® With contact between metal and semiconductor (let's assume Si), @ Some path exists for electron flow from one material to the other.  Rectifying Contacts ® Result, depletion region (space charge region) exists in silicon next to metal contact. stains Space change reglon ® Width of depletion region inversely dependent on doping density  Rectifying Contacts @ Contact region exhibits diode or rectifying properties In = Islexp A) - 1]  Resulting Device @ Schottky diode. @ Symbol: C @ Lower turn-on voltage than pn junction (other benefit of one junction covered later). @ Note: p-type semiconductor can also be used. EX1.7 fy \ | in = exp| > —| a I ‘ so 10° =(10°*)| ex -1 ( ) a (0.026, | a . | lo | Solving for the diode voltage, we find v, =(0.026)In = +1] or Vp = (0.026) In(10"") which yields Vy = 0.599 V Test Your Knowledge 1.13 VV I PS − = γ Given Vγ (pn) = 0.7V Vγ (SB) = 0.3V rf = 0Ω for both diodes Calculate ID in each diode. R Diode Piecewise Equivalent Circuit The diode is replaced by a battery with voltage, Vγ, with a a resistor, rf, in series when in the ‘on’ condition (a) and is replaced by an open when in the ‘off’ condition, VD < Vγ. If rf = 0, VD = Vγ when the diode is conducting. Other Diode Types ® Varactors ® Zener Diodes ® Photodiodes and Solar Cells @ Light-Emitting Diodes  Charge Storage and Diode Capacitance ® Amount of charge stored in depletion region varies with the applied bias voltage. @ In other words, the junction exhibits capacitive behavior. # Not linear, where @ = CV # Non linear, Reverse bias The pn junction excited by a constant-current source I in the reverse direction. To avoid breakdown, I is kept smaller than Is. Note that the depletion layer widens and the barrier voltage increases by Vr volts, which appears between the terminals as a reverse voltage. The charge stored on either side of the depletion layer as a function of the reverse bias. I-V Characteristic Curve Risted voltage: Intercept of slope f Wz Wio Geode voltage . = | 4 Rated currert *Z_ slope = Lire Reverse bios conditions Breakdown Voltage The magnitude of the breakdown voltage (BV) is smaller for heavily doped diodes as compared to more lightly doped diodes. Current through a diode increases rapidly once breakdown has occurred. Reverse Bias External reverse bias adds to the built-in potential of the pn junction. The shaded regions below illustrate the increase in the characteristics of the space charge region due to an externally applied reverse bias, vD. Reverse Breakdown Increased reverse bias eventually results in the diode entering the breakdown region, resulting in a sharp increase in the diode current. The voltage at which this occurs is the breakdown voltage, VZ. 2 V < VZ < 2000 V Reverse Breakdown Mechanisms • Avalanche Breakdown As the electric field increases, accelerated carriers begin to collide with fixed atoms. As the reverse bias increases, the energy of the accelerated carriers increases, eventually leading to ionization of the impacted ions. The new carriers also accelerate and ionize other atoms. This process feeds on itself and leads to avalanche breakdown. Reverse Breakdown Mechanisms (cont.) • Zener Breakdown Zener breakdown occurs in heavily doped diodes. The heavy doping results in a very narrow depletion region at the diode junction. Reverse bias leads to carriers with sufficient energy to tunnel directly between conduction and valence bands moving across the junction. Once the tunneling threshold is reached, additional reverse bias leads to a rapidly increasing reverse current. • Breakdown Voltage Temperature Coefficient Temperature coefficient is a quick way to distinguish breakdown mechanisms. Avalanche breakdown voltage increases with temperature, whereas Zener breakdown decreases with temperature. For silicon diodes, zero temperature coefficient is achieved at approximately 5.6 V. Chap 3 -62 Other Diode Types ® Conductor-to-Semiconductor Contacts # Schottky Diodes @ Junction Capacitance # Varactors ® Breakdown Region # Zener Diodes ® Photodiodes and Solar Cells ®@ Light-Emitting Diodes  Photodiodes and Solar Cells ® Use phenomenon that light photons can be absorbed by semiconductor diodes. @ If light of enough energy at depletion zone, electron-hole pairs (EHP) can be formed. Photogenerated Current When the energy of the photons is greater than Eg, the photon’s energy can be used to break covalent bonds and generate an equal number of electrons and holes to the number of photons absorbed. Photodiodes and Solar Cells @ Photodiode symbol: 4 @ Solar-cell or photodiode symbol: 2y Y)  Light-Emitting Diodes (LED) @ If semiconductor is a direct bandgap material, such as GoAs, # Electrons can drop their energy level from conductance band to valence band and directly recombine with holes #* No change in momentum, ® Energy liberated can be transformed to photos: (light). @® Cannot happen in indirect bandgap materials, such as Si and Ge.  Light-Emitting Diodes (LED) ® Light-emitting diodes are made of direct bandgap material, e.g., GaAs. ® Symbol: A @® Laser diodes — LED integrated into an “optical cavity.”