Transcript Ch 1
CHAPTER 1 Introduction To Diodes OBJECTIVES Describe and Analyze: • Function of Diodes • Some Physics of Diodes • Diode Models Introduction <insert figure 1-2 here> •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.