Introduction to Semiconductor Devices Louis E. Frenzel A presentation of eSyst.org

Prerequisites To understand this presentation, you should be able to: Draw the structure of an atom, including electrons, protons, and neutrons. Define resistance and conductance. Label an electronic schematic, indicating current flow. Define Ohm’s and Kirchhoff’s laws. Describe the characteristics of DC and AC (sine wave) voltages. A presentation of eSyst.org

Student Learning Outcomes Upon completion of viewing this presentation, you should be able to: Define active and passive components. Name three main categories of active semiconductor devices. Name the two elements of a diode and state the primary operational characteristics of a diode. Define bias and name the two types of bias and their effects on diode operation. Explain the concept of a transistor. Explain how a transistor can switch or amplify. Define integrated circuit. A presentation of eSyst.org

Passive Components Semiconductor materials can be used to form almost any kind of electronic component, including passive components. Passive components like resistors, capacitors or inductors are very common. A resistor is made with a piece of N or P-type semiconductor material doped to the appropriate resistance level. A capacitor is made with two plates of a highly conductive semiconductor material separated by a pure semiconductor material or another insulator like glass silicon dioxide (SiO2). An inductor is made by making a spiral of highly conductive semiconductor material. A presentation of eSyst.org

Active Electronic Components An active electronic component either amplifies or switches. The most common active components are diodes and transistors. Both diodes and transistors are easy to make with semiconductors With semiconductor materials it is possible to create complete circuits of active and/or passive components wired together. These circuits are formed on a single chip of silicon and are called integrated circuits (ICs). A presentation of eSyst.org

Diodes A semiconductor diode is formed with pieces of N and P-type material are joined. The P material is called the anode. The N material is called the cathode. The resulting structure is called a PN junction. A PN junction (or diode) is a switch or component through which electrons will flow easily in one direction but not in the opposite direction. A presentation of eSyst.org

Biasing a PN Junction To get current to flow in PN junction or diode, you have to apply an external voltage called bias. With this connection, current only flows freely from cathode across the junction to the anode. You’d say the switch is closed when electrons can flow through the diode. Note: The current flow may be so high that an external resistance R is usually needed to minimize the current flow to a level a diode can withstand. This arrangement is called forward bias. A presentation of eSyst.org

Reverse Bias A diode with the external voltage polarity reversed is called reverse bias. With this connection no current will flow. You’d say the switch is open when electrons can’t flow through the diode. A presentation of eSyst.org

Diode Symbol The PN junction forms a diode. To represent the diode in schematic diagrams, we use the symbol shown on the right. Note the designations for the anode and cathode. A presentation of eSyst.org

Diode Current Flow The direction of current flow (electrons) is shown by the arrow. A presentation of eSyst.org

A practical diode has two wire leads. Note the end with the band circling the body is the cathode end. Wire Leads A presentation of eSyst.org

Forward and Reverse Bias Examples of both forward and reverse biased diodes: A presentation of eSyst.org

Practical Diodes Diodes, like other electronic components, are not perfect. Diodes have an upper current limit that if exceeded will destroy the diode. You can get diodes with current limits of a few milliamperes up to hundreds or thousands of amperes. Diodes also have an upper voltage limit when reverse biased. This is the reverse break down voltage which may be only a few volts or hundreds or thousands of volts. A presentation of eSyst.org

Diode Voltage Drop Diodes also have a voltage drop while they are conducting. This is called forward voltage drop. It is in the 0.5 to 0.9 volt range for silicon diodes. A typical value is 0.7 volts. The forward drop in a germanium diode is in the 0.2 to 0.4 volt range. Diodes also have a threshold voltage approximately equal to the forward voltage drop. This is the minimum amount of forward voltage that must be applied to get the diode to conduct. For example if you apply a forward bias of less than abut 0.7 volts to a silicon diode, it will not conduct. As soon as the bias voltage rises about 0.7 volts the diode will conduct. A presentation of eSyst.org

Transistors A transistor is a 3-terminal semiconductor device that is used to amplify or switch. By applying an external DC voltage, current will flow from terminal 1 through the device to terminal 3. A resistor is used to set the current level. A voltage or current applied to terminal 2 is used to control how much current flows from terminals 1 to 3. A very small voltage or current variation at terminal 2 can produce a very large current variation between terminals 1 and 3. A presentation of eSyst.org

How Transistors Switch The transistor is connected in series with a resistor. With no (zero) voltage on terminal 2, no current will flow in the transistor. The transistor acts like an open switch. The output voltage is 3 volts as seen through the resistor R. If a large enough voltage is applied to terminal 2, the transistor will conduct heavily and act like a very low resistance between terminals 1 and 3. It then acts like a closed switch. The output voltage is near zero. A presentation of eSyst.org

How a Transistor Amplifies The circuit is the same as the switch. With a small voltage on terminal 2, a large variation in current from terminals 1 to 3 occurs. For example, if a sine wave is applied to the input, the current through the transistor will be a sine wave and it will produce a sine wave voltage across the resistor and the transistor. Since the current variation in the transistor is very large, the out put voltage is larger than the smaller input voltage. The transistor amplifier is said to have gain. A presentation of eSyst.org

How a Transistor Amplifies (continued) A key point to note in the amplifier is that the transistor does not actually make the input voltage bigger. Instead, the small input controls the larger current through the device produced by the external DC voltage. The transistor just generates a larger separate version of the input voltage in the output. A presentation of eSyst.org

Types of Transistors There are two commonly used types of transistors, metal oxide semiconductor field effect transistors (MOSFETs) and bipolar junction transistors (BJTs) called bipolars. MOSFETs are the most widely used although BJTs are still used in selected applications. There are sub categories of each type and you will learn how each works in a later course. A presentation of eSyst.org

Integrated Circuits Integrated circuits (ICs) are semiconductor devices that are complete circuits made up of transistors, diodes, capacitors, resistors and inductors. The complete circuit is made on a single piece of silicon called a chip. See Figure. Any circuit from a simple amplifier to a quad core microprocessor used in a PC can be made. A presentation of eSyst.org

Figure shows a typical board. Most electronic equipment is made up of a collection of ICs and a small selection of discrete passive components like resistors and capacitors connected together on a printed circuit board (PCB). Figure shows a typical board. A presentation of eSyst.org

In Summary The simplest semiconductor device is a diode made by forming a junction between a P and N-type semiconductor material. The diode will pass current from cathode to anode but not in the other direction. If the anode is made positive and the cathode negative, the diode is said to be forward biased and current will flow. The reverse condition is called reverse bias and no current flows. A three terminal (element) semiconductor device is called a transistor. The voltage on one element controls the current between the other two elements. A transistor is used to switch voltages or currents or to amplify small signals into larger ones. MOSFETs and BJTs are the two major types of transistors A presentation of eSyst.org