Semiconductor - Diode ABE 3413 – Bioinstrumentation I Dr. Filip To Ag and Bio Engineering, Mississippi State University

Semiconductor

  A conductor is a material that has a lot of free electrons so it readily conducts current. A semiconductor is a material that does not have much free electron, its atomic structure can be disturbed that electrons on its valence layer can be made to jump from one atom to another under some influences. Material like silicon and germanium have relatively few free electrons, so they are not a very good conductor. But they can be doped with impurities so that they can become a P-type or and N-type semiconductor. We are not interested in knowing how it is made, we just need to know that there is a P-type and an N-type semiconductor.

Diode   A P-type and an N-type joint together to form a diode.

The P side of a diode is the Anode, the N side of a diode is the Cathode.

  There are several kinds of diodes: Rectifier Diode, Zener Diode, Tuning Diode, Photo Diode, Light Emitting Diode (LED). The schematic diagram symbols are as follows:

Anode

Cathode

N P

How Diodes Work   A diode is like a one-way valve of electric current, it conducts

current one way, but not in reverse.

  If the Anode has a voltage is higher than the Cathode it is forward biased, it conducts (the valve opens) current. That positive voltage difference between anode and cathode that makes it conduct is the Forward Voltage (VF) of the diode. For a Silicon Diode, VF ≈ 0.6 volts VF is the voltage drop across a diode when it is forward biased. Forward Bias is the condition when anode is more positive than the cathode. VF is a “factory made” voltage it is not derived from Ohm’s law.

  If a diode is biased in such away that the cathode voltage is higher than the anode voltage then it is reverse biased. When reverse biased, a diode does not conduct current (0 current) until the reverse bias voltage gets beyond the Break Down voltage beyond which the diode conducts/breaks down.

Simple Example

  The above LED circuit: the one on the right is reverse biased. The Anode voltage is lower than the cathode voltage. There is no current flowing thru R1

  The following LED (light emitting diode) circuit: the one on the left is forward biased. The Anode voltage is higher than the cathode voltage. There is a current flowing thru R1 and there is VF drop across the LED

Simple Example   Given the following circuit, the LED has a forward

voltage (VF) of 1.5 volts, and the maximum current = 50 mA. Design this circuit so that the LED can only get a maximum of 50 mA.

  First, the circuit must be forward biased. Then we limit the amount of current flowing by adding a series resistor. We need to calculate the value of R1 that will limit the current to 50 mA.

The VF = 1.5 volts, so R1 must drop 12 - 1.5 = 10.5 volts, and also limit the current to 50 mA (IR1 = 50 mA)

R1 = VR1/IR1 = 10.5 / 50 = 0.21 K = 210 ohms

Zener Diode Zener is special diode that is specially made to operate in a reverse bias condition. It has a relatively low breakdown voltage (VB) (usually the breakdown voltage of a non-zener diode is greater than 100 volts and when it is exceeded it becomes permanently damaged).

Zener diode is not destroyed when its breakdown voltage is reached as long as its current capacity is not exceeded.

  So, if a Zener diode is reversed biased and connected to fluctuating voltage source, the voltage across the zener diode will only be up to its breakdown voltage, because when its reverse bias voltage is less than VB it does not conduct, and when the voltage is > VB then it conducts with a voltage drop of VB. When forward biased a zener diode behaves just like normal diode, it drops VF volts.

  Only zener diode can do this. Other diode when its breakdown voltage is surpassed, it conducts and the voltage drop is 0 (it becomes short circuit), and it does not recover.

Example   The following is a schematic diagram of a zener diode

connected in a reverse bias fashion. The breakdown voltage is VZ = VD1. What is VR1 if VZ = 6.2 volts, what must be the valur of R1 and how much power is dissipated by the zener diode if maximum current is 0.5 A??

The Voltage drop of R1, VR1 = V1 – VZ = 5.8 volts. Since R1 is to limit the current to 0.5A then R1 = VR1/IR1 = 11.6 homs.

The power consumption is VZ * IR1 = 6.2 * 0.5 = 3.06 W. So, R1 cannot be just any resistor, it needs to be > 3.06W

Rectifier Diode

  Because of the one-way valve characteristics of a diode, it can be used as a rectifier, a circuit that converts AC into DC voltage

  Two versions of rectifier circuits: Half Wave, and Full Wave.

  Think of a sine wave voltage, a portion of the wave is positive, and the other portion is negative. Since a diode conducts only when it is forward biased, and not when it is reverse biased, if the AC wave is connected to a diode, only one portion of the voltage is conducted and the other portion is cut off.

Half Wave Rectifier Circuit Channel B (Yellow), Half Wave Rectified

voltage of R1. VR1 = V2 - VF

Full Wave Rectifier Circuit

Which diodes conduct during the positive going portion of a sine wave and which diodes conduct during the negative going portion of the sine wave?

Homework Due Next Class   Google the following part number for a blue LED and look at the

spec sheet (data sheet): C4SMF-BJS-CR0U0452. Jot down the typical forward voltage (VF) value and the typical forward current (IF). [We do not want to use Absolute Maximum values because they are the "do not exceed" values.] We want to make a Christmas lighting which will operate with 24 volts (door bell voltage). In order for this LED to work under this situation, a resistor must be put in series with it for limiting the total current flow to IF. 1. Draw the schematic diagram of your circuit with proper labeling. 2. What value of resistor must be used in order to accomplish this? 3. how many watts of a resistor is needed at the minimum? 4. If the Christmas light is to have a string of five LEDs and only one resistor is to be used for cost effectiveness the LEDs must be connected in series, What value of resistor is needed to do this? 5. Does the resistor's wattage need to be increased or not? explain. Show your derivation and steps in all problems.