ECE 313 Electronic Devices & Circuits

SEMICONDUCTOR DIODE

1. Function-A semiconductor diode is a discrete electronic device or component that allows current to pass in only one direction in a circuit.2. Construction-A semiconductor diode is formed by joining N-type and P-type silicon. This basic diode is called a PN-junction diode or Junction Diode -In actual manufacturing, as the N-type silicon crystal is being grown, the process is halted to grow the P-type silicon crystal. The joined (PN) crystal, is then coated by either glass or plastic

-It has two electrodes (terminals). The one connected to the P-type is called the Anode, while the one connected to the N-type called the Cathode.

3.1. SYMBOL AND ACTUAL APPEARANCE (Junction Diode)

4. Operating Conditions :-For semiconductor diodes the term Bias refers to the application of voltage source for the diode to operate.

4.1 NO BIAS / UNBIASED -The diode is not connected to a voltage source or anything.

- Near the junction of the P and N type material, there is a diffusion of free electrons from the N-type to fill-in the vacant holes of nearby P-type.This is called recombination movement of free electrons to a hole.

- The pairing of these dipoles (charged ions) produces an electric field that in turn produces a potential or voltage barrier (VB). This is also known as the depletion layer where there are no carriers (No free electrons or holes). - This VB must first be overcome in order for a diode to conduct current. VB for Silicon is 0.7 V while for Ge is 0.3 V 4.2 FORWARD BIAS: CLOSED SWITCH

4.3 REVERSE BIAS: OPEN SWITCH

4.4 BREAKDOWN

2. Zener Effect due to increase Magnetic Field strength. Strong enough to rupture the covalent bond, generating free electron-hole pairs. So even a small increase in reverse voltage can produce large number of current carriers.A special diode operates in this breakdown or Zener region. Aptly called the Zener Diode.

5. DIODE CHARACTERISTIC CURVE

DIODE APPLICATIONS6. Negative Temperature Coefficient An increase in temperature of a semiconductor can result in a substantial increase in the number of free electrons in the material. Semiconductor materials such as Ge and Si that show a reduction in resistance with increase in temperature are said to have a negative temperature coefficient.

7. ZENER DIODES - AZener diodeis adiodewhich allows current to flow in the forward direction in the same manner as normaldiode, but also permits it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage or "Zenerknee voltage",

8. LIGHT EMITTING DIODES (LED) - A diode that will give off visible light when it is energized. - The process of giving off light by applying an electrical source of energy is called electroluminescence.

9. IDEAL DIODE

10. TEMPERATURE EFFECTS

11. DIODE TESTING

12. RESISTANCE LEVELS

- As the operating point of a diode moves from one region to another the resistance of the diode will also change due to the nonlinear shape of the characteristic curve. 1. DC or Static ResistanceThe application of a dc voltage to a circuit containing a semiconductor diode will result in an operating point on the characteristic curve that will not change with time.2. AC or Dynamic ResistanceThe application of a varying input will move the instantaneous operating point up and down a region of the characteristics and thus defines a specific change in current and voltage .With no applied varying signal, the point of operation would be the Q-point (Q-point is derived from the word quiescent, which means still or unvarying.) 3. Average AC ResistanceIf the input signal is sufficiently large to produce a broad swing such as indicated in the resistance associated with the device for this region is called the average ac resistance. The average ac resistance is, by definition, the resistance determined by a straight line drawn between the two intersections established by the maximum and minimum values of input voltage.

13. DIODE EQUIVALENT CIRCUIT - An equivalent circuit is a combination of elements properly chosen to best represent the actual terminal characteristics of a device, system, or such in a particular operating region. In other words, once the equivalent circuit is defined, the device symbol can be removed from a schematic and the equivalent circuit inserted in its place without severely affecting the actual behavior of the system.