Semiconductor Device Physics Lecture 6 Dr Ing Erwin

Semiconductor Device Physics Lecture 6 Dr. -Ing. Erwin Sitompul President University http: //zitompul. wordpress. com President University Erwin Sitompul SDP 6/1

Chapter 6 pn Junction Diodes: I-V Characteristics Qualitative Derivation Majority carriers President University Erwin Sitompul SDP 6/2

Chapter 6 pn Junction Diodes: I-V Characteristics Current Flow in a pn Junction Diode n When a forward bias (VA > 0) is applied, the potential barrier to diffusion across the junction is reduced. n Minority carriers are “injected” into the quasi-neutral regions Δnp > 0, Δpn > 0. n Minority carriers diffuse in the quasi-neutral regions, recombining with majority carriers. President University Erwin Sitompul SDP 6/3

Chapter 6 pn Junction Diodes: I-V Characteristics Ideal Diode: Assumptions n Steady-state conditions. n Non-degenerately doped step junction. n One-dimensional diode. n Low-level injection conditions prevail in the quasi-neutral regions. n No processes other than drift, diffusion, and thermal R–G take place inside the diode. President University Erwin Sitompul SDP 6/4

Chapter 6 pn Junction Diodes: I-V Characteristics Current Flow in a pn Junction Diode n Current density J = JN(x) + JP(x) n JN(x) and JP(x) may vary with position, but J is constant throughout the diode. n Yet an additional assumption is now made, that thermal recombination-generation is negligible throughout the depletion region JN and JP are therefore determined to be constants independent of position inside the depletion region. President University Erwin Sitompul SDP 6/5

Chapter 6 pn Junction Diodes: I-V Characteristics Carrier Concentrations at –xp, +xn n Consider the equilibrium carrier concentrations at VA = 0: p-side n If low-level injection conditions prevail in the quasi-neutral regions when VA 0, then: President University Erwin Sitompul SDP 6/6

Chapter 6 pn Junction Diodes: I-V Characteristics “Law of the Junction” n The voltage VA applied to a pn junction falls mostly across the depletion region (assuming that low-level injection conditions prevail in the quasi-neutral regions). n Two quasi-Fermi levels is drawn in the depletion region: President University Erwin Sitompul SDP 6/7

Chapter 6 pn Junction Diodes: I-V Characteristics Excess Carrier Concentrations at –xp, xn p-side President University n-side Erwin Sitompul SDP 6/8

Chapter 6 pn Junction Diodes: I-V Characteristics Example: Carrier Injection n A pn junction has NA=1018 cm– 3 and ND=1016 cm– 3. The applied voltage is 0. 6 V. a) What are the minority carrier concentrations at the depletion -region edges? b) What are the excess minority carrier concentrations? President University Erwin Sitompul SDP 6/9

Chapter 6 pn Junction Diodes: I-V Characteristics Excess Carrier Distribution n From the minority carrier diffusion equation, n We have the following boundary conditions: n For simplicity, we develop a new coordinate system: n Then, the solution is given by: • LP : hole minority carrier diffusion length President University Erwin Sitompul SDP 6/10

Chapter 6 pn Junction Diodes: I-V Characteristics Excess Carrier Distribution n New boundary conditions n From the x’ → ∞, From the x’ → 0, n Therefore n Similarly, President University Erwin Sitompul SDP 6/11

Chapter 6 pn Junction Diodes: I-V Characteristics pn Diode I–V Characteristic n-side p-side President University Erwin Sitompul SDP 6/12

Chapter 6 pn Junction Diodes: I-V Characteristics pn Diode I–V Characteristic • Shockley Equation, • President University for ideal diode I 0 can be viewed as the drift current due to minority carriers generated within the diffusion lengths of the depletion region Erwin Sitompul SDP 6/13

Chapter 6 pn Junction Diodes: I-V Characteristics Diode Saturation Current I 0 n I 0 can vary by orders of magnitude, depending on the semiconductor material, due to ni 2 factor. n In an asymmetrically doped pn junction, the term associated with the more heavily doped side is negligible. n If the p side is much more heavily doped, n If the n side is much more heavily doped, President University Erwin Sitompul SDP 6/14

Chapter 6 pn Junction Diodes: I-V Characteristics Diode Carrier Currents • Total current density is constant inside the diode • Negligible thermal R-G throughout depletion region d. JN/dx = d. JP/dx = 0 President University Erwin Sitompul SDP 6/15

Chapter 6 pn Junction Diodes: I-V Characteristics Carrier Concentration: Forward Bias n Law of the Junction n Low level injection conditions Excess minority carriers President University Erwin Sitompul Excess minority carriers SDP 6/16

Chapter 6 pn Junction Diodes: I-V Characteristics Carrier Concentration: Reverse Bias n Deficit of minority carriers near the depletion region. n Depletion region acts like a “sink”, draining carriers from the adjacent quasineutral regions President University Erwin Sitompul SDP 6/17

Chapter 6 pn Junction Diodes: I-V Characteristics Deviations from the Ideal I-V Behavior n Si pn-junction Diode, 300 K. Forward-bias current Reverse-bias current “Slope over” “Breakdown” Smaller slope President University No saturation Erwin Sitompul SDP 6/18

Chapter 6 pn Junction Diodes: I-V Characteristics Homework n This time no homework. n Prepare well for midterm examination. President University Erwin Sitompul SDP 6/19