1 / 22

Chapter 6 pn Junction Diodes: I - V Characteristics

Semiconductor Device Physics. Chapter 6 pn Junction Diodes: I - V Characteristics. Chapter 6. pn Junction Diodes: I - V Characteristics. M ajority carriers. M ajority carriers. Qualitative Derivation. Chapter 6. pn Junction Diodes: I - V Characteristics.

sadah
Télécharger la présentation

Chapter 6 pn Junction Diodes: I - V Characteristics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Semiconductor Device Physics Chapter 6 pn Junction Diodes: I-V Characteristics

  2. Chapter 6 pn Junction Diodes: I-V Characteristics Majority carriers Majority carriers Qualitative Derivation

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

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

  5. Chapter 6 pn Junction Diodes: I-V Characteristics Current Flow in a pn Junction Diode • Current density J = JN(x) + JP(x) • JN(x) and JP(x) may vary with position, but J is constant throughout the diode. • 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.

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

  7. Chapter 6 pn Junction Diodes: I-V Characteristics “Law of the Junction” • 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). • Two quasi-Fermi levels is drawn in the depletion region:

  8. Chapter 6 pn Junction Diodes: I-V Characteristics Excess Carrier Concentrations at –xp, xn p-side n-side

  9. Chapter 6 pn Junction Diodes: I-V Characteristics Example: Carrier Injection • 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?

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

  11. Chapter 6 pn Junction Diodes: I-V Characteristics Excess Carrier Distribution • New boundary conditions • From the x’ → ∞, • From the x’ → 0, • Therefore • Similarly,

  12. Chapter 6 pn Junction Diodes: I-V Characteristics pn Diode I–V Characteristic n-side p-side

  13. Chapter 6 pn Junction Diodes: I-V Characteristics pn Diode I–V Characteristic • Shockley Equation,for ideal diode • I0 can be viewed as the drift current due to minority carriers generated within the diffusion lengths of the depletion region

  14. Chapter 6 pn Junction Diodes: I-V Characteristics Diode Saturation Current I0 • I0 can vary by orders of magnitude, depending on the semiconductor material, due to ni2 factor. • In an asymmetrically doped pn junction, the term associated with the more heavily doped side is negligible. • If the p side is much more heavily doped, • If the n side is much more heavily doped,

  15. 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  dJN/dx = dJP/dx = 0

  16. Chapter 6 pn Junction Diodes: I-V Characteristics Excess minority carriers Excess minority carriers Carrier Concentration: Forward Bias • Law of the Junction • Low level injection conditions

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

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

  19. Chapter 6 pn Junction Diodes: I-V Characteristics Homework • This time no homework. • Prepare well for midterm examination. • Midterm examination covers material of Lecture 1 until Lecture 6. • Two A4- formula sheets may be used during exam. Formula sheets may not contain problems and solutions. • The use of calculator is allowed. • No notebooks, tablets, smart phones, nor any other electronics may be used during exam.

  20. Chapter 6 pn Junction Diodes: I-V Characteristics Exercise Problems A certain Silicon sample is doped differently in two regions. Region A is doped with Boron (7×1016 cm–3) and Arsenic (3×1016 cm–3) while Region B only with Arsenic (1018 cm–3). At 300 K, calculate the resistance of the block. An Si sample is given with ND = 1018 cm–3. The minority carrier lifetimes are given by 10–7 s. Holes are injected at x=0 to generate a certain hole diffusion current density of 10μA/cm2. Low level injection condition is assumed to be prevailed. If the hole concentration decays exponentially as x increases with a certain diffusion length constant, determine the excess concentration required to be supplied at x=0.

  21. Chapter 6 pn Junction Diodes: I-V Characteristics Exercise Problems A certain Si pn-junction diode is doped with NA=2×1018 cm–3 and ND= 4×1018 cm–3. The constants are given by DN = 18 cm2/s, DP = 12 cm2/s, and τp = τn = 10–7 s. Calculate the built-in voltage Vbi and the depletion width W at T = 300 K. Which side of the depletion layer will be wider, on the p-side or on the n-side? Determine Jdiff if VA=0.5 V? What is the change if VA=–0.2 V?

More Related