Lecture 5 Biasing a Diode The Diode As

Lecture 5: Biasing a Diode The Diode As mentioned, a diode is made from a small piece of semiconductor material, usually silicon, in which half is doped as a p region and half is doped as an n region with a pn junction and depletion region in between. The p region is called the anode and is connected to a conductive terminal. The n region is called the cathode and is connected to a seconductive terminal. The basic diode structure and schematic symbol are shown in Figure 13: The Diode. 1

Biasing a Diode: No electrons move through the pn junction at equilibrium. Generally the term "bias" refers to the use a dc voltage to establish certain operating conditions for an electronic device. In relation to a diode, there are 2 bias conditions: forward and reverse. Either of these bias conditions is established by connecting a sufficient dc voltage of the proper polarity across the pn junction. Forward Bias To bias a diode, you apply a dc voltage across it. Forward bias is the condition that allows current through the pn junction. Figure 14 shows a dc voltage source connected by conductive material (contacts and wire) across a diode in the direction to produce forward bias. 2

This external bias voltage is designated as VBIAS. The resistor limits the forward current to a value that will not damage the diode. Figure 14: a diode connected for a forward bias. Forward Bias Requirements: 1) The negative side of VBIAS is connected to the n region of the diode and the positive side is connected to the p region. This is one requirement forward bias. 2) The bias voltage, VBIAS, must be greater than the barrier potential, 0. 7 Si, 0. 3 Ge. 3

Reverse Bias Reverse bias is the condition that prevents current through the diode. Figure 15 shows a dc voltage source connected across a diode in the direction to produce reverse bias. This external bias voltage is designated as VBIAS just as it was forward bias. Notice that the positive side of VBIAS is connected to the n region of the diode and the negative side is connected to the p region. Also note that the depletion region is shown much wider than in forward bias, because the positive side of VBIAS (Battery) withdraw the electrons in the n region at its boundary and the something in the p region. 4

Figure 15: a diode connected for a reverse bias. Review Questions 1. What is the diode? 2. Describe forward bias of a diode. 3. Explain how to forward-bias a diode. 4. What are the conditions to forward-bias the diode? 5. Describe reverse bias of a diode. 6. Explain how to reverse-bias a diode. 5