المملكة العربية السعودية وزارة التعليم العالي - جامعة أم القرى كلية الهندسة و العمارة الإسلامية قسم الهندسة الكهربائية ELECTRONIC DEVICES K INGDOM OF S AUDI A RABIA Ministry of Higher Education Umm Al-Qura University College of Engineering and Islamic Architecture Electrical Engineering Department Lecture 5 By: Dr Tarek Abdolkader

27/11/1433Electronic devices (802311) Lecture 5 Dr Tarek Abdolkader Pn junction at equilibrium Biasing pn-junction Diode equation Diode resistances Diode models 2

27/11/1433Electronic devices (802311) Lecture 5 Dr Tarek Abdolkader The p-n junction is constructed by forming p-type and n-type regions in intimate contact. Holes will tend to diffuse right leaving fixed negative ions (N a - ) near the junction. Electrons will tend to diffuse left leaving fixed positive ions (N d + ) near the junction. An electric field (  ) will be established from N d + to N a - across certain width (w) and tries to retard holes back into the p-region and electrons back into the n-type. A built-in potential ( V bi ) will appear across the junction region. - h - h - h - h - h - h - h - h p-TYPEn-TYPE + e + e + e + e + e + e + e + e + e + e + e + e + e + e + e + e h - h - h -- - h - h -- + e + e + e + + e + e + e + + e + e + e + + e + e + e + V bi e + -  The junction region is depleted from free carriers (electrons and holes) and thus called Depletion region (Depletion layer). 3

27/11/1433Electronic devices (802311) Lecture 5 Dr Tarek Abdolkader When an external voltage is applied across the pn junction, the NET VOLTAGE ( V j ) on the junction will be the sum of the external voltage and the built in voltage. There are TWO different modes of biasing (Forward bias and Reverse bias) compared to the zero bias mode: FORWARD BIASNO BIASREVERSE BIAS p-side is biased positively relative to the n-side. no bias p-side is biased negatively relative to the n-side. external voltage is opposite to the built-in voltage No external voltage external voltage is added to the built-in voltage. V j = V bi - V f V j = V bi V j = V bi + V R pn VfVf pn V bi +- pn VRVR 4

27/11/1433Electronic devices (802311) Lecture 5 Dr Tarek Abdolkader FORWARD BIASNO BIASREVERSE BIAS Width of the depletion layer decreases Width of the depletion layer increases V j = V bi - V f V j = V bi V j = V bi + V R pn VfVf pn V bi +- pn VRVR n p -x p xnxn w n p xnxn w n p w xnxn x V V bi - V f -x po x no x V V bi -x p xnxn x V V bi + V R 5

27/11/1433Electronic devices (802311) Lecture 5 Dr Tarek Abdolkader Reverse bias: Fermi level in the two sides will be different. The barrier is larger than V bi The electric field in the depletion region is larger than zero bias field. majority carriers (electrons in the n-side and holes in the p-side) cannot diffuse to the other side. minority carriers (electrons in the p-side and holes in the n-side) is pulled strongly by the electric field causing small current ( called reverse saturation current ). Current is carried by drift in the depletion region and by diffusion in the neutral regions Current is carried by minority carriers ( very small current ) pn VRVR E cn E in E vn E cp E ip E vp E fp E fn Reverse Bias J R (  ) = J n (  ) + J p (  ) Very small current 6

27/11/1433Electronic devices (802311) Lecture 5 Dr Tarek Abdolkader Forward bias: Fermi level in the two sides will be different. The barrier is smaller than V bi The electric field in the depletion region is smaller than zero bias field. majority carriers (electrons in the n-side and holes in the p-side) can diffuse to the other side. Current is carried mainly by diffusion Current is carried by majority carriers ( Large current ) pn VfVf E cn E in E vn E cp E ip E vp E fp E fn J f (  ) = J n (  ) + J p (  ) Large current Forward Bias 7

27/11/1433Electronic devices (802311) Lecture 5 Dr Tarek Abdolkader Diode characteristics: Ideal diode equation I o is the reverse saturation current -I s I ~ I s exp(eV /kT). I ~- I s VDVD IDID Slope = e/KT Increasing doping concentration  I s decreases Increasing temperature or decreasing band gap  n i increases  I s increases For bias voltages more than several KT/e volts: 8

27/11/1433Electronic devices (802311) Lecture 5 Dr Tarek Abdolkader Determine the ideal reverse saturation current density in a silicon pn junction at T = 300 K. Consider the following parameters: See Example 8.2 page 288 in the textbook N a = N d = 1×10 16 cm -3 n i =1.5×10 10 cm -3  n0 =  p0 = 5×10 -7 s D n = 25 cm 2 /s D p = 10 cm 2 /s J s = 4.15× A/cm 2 If A = cm 2 I s = 4.15× A 9

27/11/1433Electronic devices (802311) Lecture 5 Dr Tarek Abdolkader (a) Consider an ideal pn junction diode at T = 100 K operating in the forward-bias region. Calculate the change in diode voltage that will cause a factor of 10 increase in current. (b) Repeat part (a) for a factor of 100 increase in current. See Problem 8.1 page 323 in the textbook 10

27/11/1433Electronic devices (802311) Lecture 5 Dr Tarek Abdolkader In a pn-junction at equilibrium, There is a depletion region (depleted from carriers) between n- and p-sides There is a built in electric field directed from n-side to p-side bi There is a built in potential V bi with n-side potential higher than p-side Outside the depletion region, there are two neutral regions which have no charges, no electric field and no change in potential E c is flat in neutral regions and changing in depletion region. E c in the p-side is higher than in the p-side Diffusion current due to carrier density difference between n-side and p-side and drift current due to the electric field in the depletion region cancel each other and the current is zero. 11

27/11/1433Electronic devices (802311) Lecture 5 Dr Tarek Abdolkader 12 In a biased pn-junction, The electric field in the depletion layer increases for reverse bias and decreases for forward bias. The potential across the junction increases for reverse bias and decreases for forward bias. The width of the depletion layer increases for reverse bias and decreases for forward bias. Outside the depletion region, there are still two neutral regions which have no charges, no electric field and no change in potential In reverse biasing, current is a drift current of minorities ( very small ) In forward biasing, current is a diffusion current of majorities ( very large ) (reverse)(forward)

27/11/1433Electronic devices (802311) Lecture 5 Dr Tarek Abdolkader DC resistance: AC or dynamic resistance: r D formula is used only for forward bias (it is called forward resistance) Both resistances are bias dependent Both resistances decreases with increasing bias DC resistance is the inverse of the slope of the line joining the origin to the operating point AC resistance is the inverse of the slope of the tangent of the curve at the operating point 13

27/11/1433Electronic devices (802311) Lecture 5 Dr Tarek Abdolkader Determine the dc and ac resistance of a diode operating at ( i ) 2 mA, ( ii ) 25 mA. 14

27/11/1433Electronic devices (802311) Lecture 5 Dr Tarek Abdolkader Diode as a circuit element: The operating point (called the Q-point ) of the diode can be determined from the intersection of the diode characteristics with the load line ( Load line analysis ) Diode symbol 15

27/11/1433Electronic devices (802311) Lecture 5 Dr Tarek Abdolkader Determine the operating point of the diode shown with R ( i ) 1 kΩ, ( ii ) 2 kΩ. 16

27/11/1433Electronic devices (802311) Lecture 5 Dr Tarek Abdolkader Ideal diode model: Practical model: 17

27/11/1433Electronic devices (802311) Lecture 5 Dr Tarek Abdolkader Piecewise linear model: 18

27/11/1433Electronic devices (802311) Lecture 5 Dr Tarek Abdolkader Determine the operating point of the diode shown with R = 1 kΩ. Use (i)ideal diode model, (ii)practical model, (iii)Piecewise linear model ( r D = 10 Ω). 19