DIODE CHARACTERISTICS

by

Ima Student

June 6, 2000

ETR 148-91B

Instructor Al Koon, Associate Professor

PURPOSE The purpose of this lab is to verify the effects of forward and reverse biasing a silicon, germanium and zener diodes. Additionally, various means of obtaining characteristics curves for each of these diodes will be demonstrated. THEORY Diodes are active devices constructed to allow current to flow in one direction. The diode consists of N-type and P-type materials (see diagram shown below). Each of these materials originally consisted of pure silicon doped to obtain the type of characteristic desired. Doping is the process of adding impurities to the pure semiconductor material. N-type material is formed when the impurities with five electrons in the outer most shell (pentavalent) are added to the pure semiconductor material. Pentavalent materials are elements such as antimony, arsenic, and phosphorus. The same procedure is then performed for the P-type material using an atom containing only three electrons in its outer shell (trivalent). Trivalent materials are elements such as boron, gallium, and indium. A diode is formed when a piece of pure material is doped half as N-type and half as P-type material. It is not constructed by fusing the N and P type materials together. The N-type material is called the cathode and the P-type material is called the anode. A junction, called the depletion region, is formed where the two materials meet. Some of the free electrons begin diffusing across the junction and fill the holes in the P-type material. When this occurs the atom that the electron joined becomes a negative ion. When an electron leaves N-type material, it leaves a hole creating a positive ion. Eventually the diffusion of the free electrons and holes in the junction of the two materials will decrease to a point where it will stop. The area of positive and negative ions created around the junction is called the depletion layer. The free electrons in the N-type material are blocked from diffusing to the P-type material by the negative ions in the P material.

Holes from the P material are blocked by the positive ions in the N material. For charge carriers to flow through the layer, a small voltage potential of approximately 0.7V (silicon) or .3V (germanium)is required to break it down. This is called the barrier potential. There are two types of biasing that can be applied to a diode. For a diode to be forward biased, a power supply is connected with the positive terminal to the P-type material (anode) and the negative terminal to the N-type material (cathode). As the potential across the diode approaches the barrier, the diode begins conducting. For a diode to be reversed biased, the power supply leads are set up with the negative terminal attached to the P-type material and the positive terminal attached to the N-type material. With this configuration, the electrons in the N-type material and the holes in the P-type material are drawn away from the depletion layer increasing the width of the layer. Even though the majority current has stopped flowing, minority current is still flowing due to thermal energy. Minority current is also called saturation current and can only be increased by an increase in temperature. There is also a second minute current that flows through the resistive paths created by surface impurities. This type of current is called surface-leakage current. The sum of saturation current and the surface-leakage current is called the reverse current. A diode, such as the silicon diode, will conduct in the reverse bias direction with a significant amount of voltage applied. At the point of conduction, the diode breaks down resulting in the diode being damaged. The value of voltage at which the diode breaks down is called the breakdown voltage. Other types of diodes, such as zener diodes, function in the reverse bias region. The effect of a zener diode breaking down in the reverse region allows it to be used as a voltage regulator. The graph of a silicon diode shows the response curve including the forward and reverse bias regions (See Figure 1 next page). The graph of the silicon diode shows the point in the forward bias region where the barrier potential is exceeded and the diode begins conduction. The voltage at which the diode begins conducting is called the knee voltage. In the reverse bias region, the point at which the diode begins breaking down is also shown. The voltage level at the moment of breakdown is called the breakdown voltage as mentioned previously.

ID(mA)

10

20

30

.5 1 1.5VD(Volts)

Knee Voltage

ForwardBias Region

ID(uA)

VD(Volts)204060

10

20

30

BreakdownVoltage

Reverse BiasRegion

FIGURE 1 PROCEDURE See Appendix A. No errors were encountered in following the procedure.

RESULTS

RESISTANCE DIODE FORWARD SCALE REVERSE SCALE SILICON 833Ω 2KΩ Open 20MΩ GERMANIUM 303Ω 2KΩ 310KΩ 2000KΩ 5.1V ZENER 994Ω 2KΩ Open 20MΩ

TABLE 1 FORWARD BIASED DIODES

SILICON GERMANIUM 5.1V ZENER VD ID ID ID .1 0A 18.5uA 0A .2 0.2uA 245uA 0A .3 1.4uA 1.9mA 0.1uA .4 10.4Ua 10.6mA 0.2uA .5 85Ua 39.4mA 1.6uA .6 708uA 111.3mA 35uA .7 6.9mA 1.13mA .8 30.5mA 41.7mA

TABLE 2 REVERSE BIASED DIODES

SILICON GERMANIUM VD ID ID 2V 0.2uA 3.3uA 4V 0.4uA 5.4uA 6V 0.6uA 8.1uA 8V 0.8uA 11.3uA

10V 1uA 12.8uA 12V 1.2uA 16.0uA 14V 1.5uA 19.2uA 16V 1.6uA 22.3uA 18V 1.9uA 26.2uA

TABLE 3 REVERSE BIASED ZENER

5.1V ZENER VD ID 1V 0.2uA 2V 0.4uA 3V 0.6uA 4V 0.8uA

4.9V 1.4uA 5.0V 560uA 5.1V 2.8mA 5.2V 30mA

20V 1V

30mA

60uA

.5V10V

20mA

10mA

40uA

20uA

SiliconGermanium

Zener

Diode Curves From TablesID

VD

Germanium

Silicon

Zener

20V 1V

30mA

60uA

.5V10V

20mA

10mA

40uA

20uA

SiliconGermanium

Zener

Diode Curves From Curve TracerID

VD

Germanium

Silicon

Zener

CONCLUSIONS The forward and reverse bias characteristics of a silicon, germanium and 5.1V zener diode were verified. The extremely high resistance in the reverse direction, open or nearly open, and the low resistance in the forward direction, less than 1KΩ, proves that current under normal operating conditions will only flow in the forward bias direction.

For the silicon and zener diode, the barrier potential was reached at .65V and .67V, respectively. For the germanium diode, the barrier potential was reached at .27V. The value of voltage at which the barrier is overcome is called the knee voltage.

The voltage necessary to cause breakdown of the diode was not possible to reach with the lab power supply and was not practical due to probable damage to the diode. If the power supply was capable of causing breakdown, the diode would begin conducting heavily in the negative region until the diode burned up or disintegrated. The readings that were taken proved that current is restricted from flowing in the reverse bias region. The reverse bias dynamic characteristic of the zener diode was verified. The significant difference between the zener diode and the silicon or germanium diodes is that the zener diode functions in the reverse bias region. As the voltage applied to the zener reaches the rated value of the zener, in this lab 5.1V, the voltage levels off at the rated value and current begins flowing at a higher rate. This result makes the zener diode an effective voltage regulator.

Each diode was then tested in the forward and reverse bias directions using the curve tracer. The results of these tests verified the results of the measured values of forward and reverse bias using the voltmeter and ammeter.

PSpice

Diode Characteristic Curve V1 1 0 0 ;diode voltage VX 2 3 0 ;Dummy source, used to measure current D1 1 2 D1N4148 ;Diode anode first cathode second R1 3 0 330 ;resistor used to limit current .LIB ;get diode from library .DC V1 -20 .8 .1;Step diode voltage from -20 to .8 volts in .1 volt ;increments .PROBE ;run probe when through .END

Appendix A