CHAPTER 1 Introduction To Diodes

OBJECTIVES Describe and Analyze: Function of Diodes Some Physics of Diodes Diode Models

Introduction Diodes let current flow one way, but not the other Conventional current flows from anode to cathode Electrons flow from cathode to anode

Diodes are Important The humble silicon diode is the simplest of all the semiconductor devices. It is also one of the most important. Without diodes, you could not build electronic equipment Applications for diodes range from power supplies to cell phones and everything in between.

Diodes are Important It’s important to understand what a diode is and what makes it work. Diodes use a PN junction. Later, we will see how PN junctions play a key role in transistors.

Bias: Forward & Reverse A forward-biased diode conducts. A diode is forward-biased when the voltage on the anode is positive with respect to the cathode. A reverse-biased diode does not conduct. A diode is reverse-biased when the voltage on the anode is negative with respect to the cathode. A diode acts like a voltage-controlled switch.

Valence Electrons The outer band of electrons in an atom is called the valence band. Atoms in a conductor (e.g. copper) have valence electrons that can move freely through the material. The valence electrons of insulators are bound to the atoms and can not move freely.

Covalent Bonds A covalent bond is formed when atoms can share valence electrons with adjacent atoms. The result is crystalline material such as silicon. Covalent bonds are very strong. They are what make diamonds hard.

Doping Adding different atoms to a crystal is called doping. Donor atoms (e.g. arsenic) add movable electrons to the crystal’s valence band. Acceptor atoms (e.g. gallium) add movable “holes” : open spaces in the valence band to accept electrons. Holes act like positive charge carriers.

Semiconductors: N & P Doped silicon becomes a semiconductor. Current can flow through a semiconductor, but not as easily as through metal conductors. N-Material is silicon that has been been doped with donor atoms. P-Material is silicon that has been been doped with acceptor atoms. When a single crystal has N-material on one side and P-material on the other side, things get interesting.

The PN Junction Anode is P-material. Cathode is N-material. The interface is the PN junction, which is a diode.

Reverse-Biased PN Junction The electrons and holes are drawn away from the junction, leaving a depletion region devoid of charge carriers. No current can flow across the junction.

Forward-Biased PN Junction Electron are forced to move across the junction and fall into the holes on the other side. Current is flowing. The energy required to make the electrons and holes combine shows up as a 0.7 Volt drop across the junction.

Other Semiconductors Silicon is not the only material used to make semiconductors. Germanium was used originally back in the 1950s, and is used today in high frequency applications. Germanium has a 0.3 Volt drop (approximately) across a PN junction. Other kinds of semiconductor materials can have PN drops up to 1.5 Volts or even up to 3 Volts.

Ideal Diode Model A perfect diode would behave as shown.

Practical Diode Model Closer to how real diodes behave.

Detailed Diode Model Very close to how real diodes behave.

Ideal vs. Practical vs. Detailed The only difference between the ideal model and the practical model is the 0.7 Volt battery. That may be important when working with circuits that use, for example, a 1.5 Volt battery. The detailed model includes the reverse leakage current and the diode’s internal resistance. Leakage current is not usually a problem with silicon. Usually, the diode’s resistance is only important when the applied forward-bias is a low voltage.

Choosing a Model The ideal model shows the key feature of a diode: one-way conduction of current. For most purposes, the practical model is sufficient. The detailed model may be needed when low voltages and small currents need to be analyzed.

Summary Doping changes silicon from an insulator to a semiconductor. Donor atoms make silicon into N-material. Acceptor atoms make silicon into P-material. A PN junction makes a diode.

Summary Diodes let current flow when anode to cathode is positive about 0.7 Volts. Diodes block current when anode to cathode voltage is negative. Diodes can be modeled by combining basic circuit elements: switch, battery, resistor.