Kristin Ackerson, Virginia Tech EE Spring 2002

3 The diode is the simplest and most fundamental nonlinear circuit element. Just like resistor, it has two terminals. Unlike resistor, it has a nonlinear current-voltage characteristics. Its use in rectifiers is the most common application. Introduction

SJTU Zhou Lingling4 The most important region, which is called pn junction, is the boundary between n-type and p-type semiconductor. Physical Structure

SJTU Zhou Lingling5 (a) diode circuit symbol; (b) i– v characteristic; (c) equivalent circuit in the reverse direction; (d) equivalent circuit in the forward direction. Symbol and Characteristic for the Ideal Diode

What Are Diodes Made Out Of? Silicon (Si) and Germanium (Ge) are the two most common single elements that are used to make Diodes. A compound that is commonly used is Gallium Arsenide (GaAs), especially in the case of LEDs because of it’s large bandgap. Silicon and Germanium are both group 4 elements, meaning they have 4 valence electrons. Their structure allows them to grow in a shape called the diamond lattice. Gallium is a group 3 element while Arsenide is a group 5 element. When put together as a compound, GaAs creates a zincblend lattice structure. The concept of majority carrier and minority carrier is important in semiconductor devices. Majority carrier is the carrier type in a doped semiconductor with the highest concentration. Minority carrier is the carrier type with the lowest concentration.

7 Conducting in one direction and not in the other is the I-V characteristic of the diode. The arrowlike circuit symbol shows the direction of conducting current. Forward biasing voltage makes it turn on. Reverse biasing voltage makes it turn off. Characteristics

Forward Bias hole diffusion electron diffusion pn hole drift electron drift +- Applied potential lowers the potential barrier, I diffusion > I drift Mobile carriers drift through the dep. region into neutral regions become excess minority carriers and diffuse towards terminals

Forward Bias p n0 n p0 -W 1 W 2 0 p n ( W 2 ) n-regionp-region L p diffusion Typically avoided in Digital ICs x W n Metal contact to n-region -W p Metal contact to p-region p (x) n n (x) p minority carrier concentration

Reverse Bias hole diffusion electron diffusion pn hole drift electron drift -+ Applied potential increases the potential barrier Diffusion current is reduced Diode works in the reverse bias with a very small drift current

Reverse Bias x n p0 -W 1 W 2 0 n-region p-region p n0 diffusion The Dominant Operation Mode -W p Metal contact to p-region W n Metal contact to n-region n p0

12 The pn junction under open-circuit condition I-V characteristic of pn junction  Terminal characteristic of junction diode.  Physical operation of diode. Junction capacitance P-N junction

13 Usually the pn junction is asymmetric, there are p + n and pn +. The superscript “+” denotes the region is more heavily doped than the other region. pn Junction Under Open-Circuit Condition

14 Fig (a) shows the pn junction with no applied voltage (open- circuited terminals). Fig.(b) shows the potential distribution along an axis perpendicular to the junction. pn Junction Under Open-Circuit Condition

15 The procedure of forming pn the dynamic equilibrium of drift and diffusion movements for carriers in the silicon. In detail, there are 4 steps: a) Diffusion b) Space charge region c) Drift d) Equilibrium Procedure of Forming pn Junction

16 diffusion  Both the majority carriers diffuse across the boundary between p-type and n-type semiconductor.  The direction of diffusion current is from p side to n side. Procedure of Forming pn Junction

17 Space charge region  Majority carriers recombining with minority carriers results in the disappearance of majority carriers.  Bound charges, which will no longer be neutralized by majority carriers are uncovered.  There is a region close to the junction that is depleted of majority carriers and contains uncovered bound charges.  This region is called carrier-depletion region or space charge region. Procedure of Forming pn Junction

18 Drift  Electric field is established across the space charge region.  Direction of electronic field is from n side to p side.  It helps minority carriers drift through the junction. The direction of drift current is from n side to p side.  It acts as a barrier for majority carriers to diffusion. Procedure of Forming pn Junction

19 Equilibrium  Two opposite currents across the junction is equal in magnitude.  No net current flows across the pn junction.  Equilibrium conduction is maintained by the barrier voltage. Procedure of Forming pn Junction

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