1 SEMICONDUCTORS Diode ratings and construction

2 SEMICONDUCTORS Diodes have two active electrodes between which the signal of interest can flow The word diode comes from the Greek words meaning “two paths” They are usually used for unidirectional current properties This property is called the rectifying property Diodes have two active electrodes between which the signal of interest can flow The word diode comes from the Greek words meaning “two paths” They are usually used for unidirectional current properties This property is called the rectifying property

3 SEMICONDUCTORS When we combine these two types of semi-conductors, we create a PN Junction (the space between the two). Recall that positive charges develop in the N-type and Negative charges develop in the P-type. The result is a “barrier” voltage being developed across the PN junction When we combine these two types of semi-conductors, we create a PN Junction (the space between the two). Recall that positive charges develop in the N-type and Negative charges develop in the P-type. The result is a “barrier” voltage being developed across the PN junction

4 SEMICONDUCTORS Germanium diodes have a typical barrier voltage of 0.3 Volts and Silicon diode barrier is typically about 0.7 volts. The main characteristic of diodes is their ability to either pass or stop current flow. Note that a diode will eventually reach a voltage level where it will begin to breakdown (the breakdown voltage). Germanium diodes have a typical barrier voltage of 0.3 Volts and Silicon diode barrier is typically about 0.7 volts. The main characteristic of diodes is their ability to either pass or stop current flow. Note that a diode will eventually reach a voltage level where it will begin to breakdown (the breakdown voltage).

5 SEMICONDUCTORS Biasing is a function of diodes. Forward biasing involves allowing current to flow in one direction where reverse biasing blocks current in the opposite direction. Diodes are like check valves in a water system. Note that diodes do no behave in a LINEAR fashion when it comes to current and voltage. Biasing is a function of diodes. Forward biasing involves allowing current to flow in one direction where reverse biasing blocks current in the opposite direction. Diodes are like check valves in a water system. Note that diodes do no behave in a LINEAR fashion when it comes to current and voltage.

6 SEMICONDUCTORS Diode characteristics can be plotted on a graph, which is referred to as the V-I curve The diodes forward and reverse bias voltages Vc & Vr are plotted to the right and left on the horizontal axis of the graph Diode characteristics can be plotted on a graph, which is referred to as the V-I curve The diodes forward and reverse bias voltages Vc & Vr are plotted to the right and left on the horizontal axis of the graph

7 SEMICONDUCTORS The diodes forward and reverse bias Currents If & Ir are plotted to the top and bottom on the vertical axis of the graph. The vertical and horizontal crossing point is the zero reference point. The diodes forward and reverse bias Currents If & Ir are plotted to the top and bottom on the vertical axis of the graph. The vertical and horizontal crossing point is the zero reference point.

8 SEMICONDUCTORS Germanium/Silicon diode V-I characteristics The point which current will start to flow after the knee

9 SEMICONDUCTORS When reverse-biased, an ideal diode would block all current, a real diode lets perhaps 10 micro amps through -- not a lot, but still not perfect. If you apply enough reverse voltage (V), the junction breaks down and lets current through. Usually, the breakdown voltage is a lot more voltage than the circuit will ever see, so it is irrelevant. When reverse-biased, an ideal diode would block all current, a real diode lets perhaps 10 micro amps through -- not a lot, but still not perfect. If you apply enough reverse voltage (V), the junction breaks down and lets current through. Usually, the breakdown voltage is a lot more voltage than the circuit will ever see, so it is irrelevant.

10 SEMICONDUCTORS Germanium/Silicon diode break down voltage

11 SEMICONDUCTORS Diode ratings are specified by manufactures to insure that they do not become damaged or allow unsafe reverse voltages to flow through a circuit. Temperature is also a factor that has to be taken into consideration, the diode characteristic that is most adversely affected by temperature is the reverse current. Extremely high temperature allows more reverse current flow than low temperatures. Diode ratings are specified by manufactures to insure that they do not become damaged or allow unsafe reverse voltages to flow through a circuit. Temperature is also a factor that has to be taken into consideration, the diode characteristic that is most adversely affected by temperature is the reverse current. Extremely high temperature allows more reverse current flow than low temperatures.

12 SEMICONDUCTORS Diodes can become damaged by excessive forward current so manufactures usually specify the maximum forward current (I f max ) that each type of diode can safely handle. Diodes can also be damaged by excessive reverse voltages that cause it to break down and allow dangerously high reverse voltages, this reverse breakdown voltage is specified as PIV (peak inverse voltage). Diodes can become damaged by excessive forward current so manufactures usually specify the maximum forward current (I f max ) that each type of diode can safely handle. Diodes can also be damaged by excessive reverse voltages that cause it to break down and allow dangerously high reverse voltages, this reverse breakdown voltage is specified as PIV (peak inverse voltage).

13 SEMICONDUCTORS The breakdown voltage tends to increase as temperature increases and the forward voltage drop decreases allowing to pass current sooner, this is true for both germanium and silicon

14 SEMICONDUCTORS Diode symbol showing current flow (I f )

15 SEMICONDUCTORS How forward and reverse biased diodes are represented in schematic form.

16 SEMICONDUCTORS Diodes are made by a technique known as the growth method. PN junctions are grown by placing an intrinsic semiconductor a P type impurity into a quartz container and then heated until the two materials melt. A small semiconductor crystal (seed) is then lowered into the molten mixture and the seed is rotated withdrawn from the mixture, this forms a P type semiconductor. Diodes are made by a technique known as the growth method. PN junctions are grown by placing an intrinsic semiconductor a P type impurity into a quartz container and then heated until the two materials melt. A small semiconductor crystal (seed) is then lowered into the molten mixture and the seed is rotated withdrawn from the mixture, this forms a P type semiconductor.

17 SEMICONDUCTORS The growth technique under an electron microscope

18 SEMICONDUCTORS As the seed is withdrawn the molten mixture can be doped with N or P type impurities to create either type of layer within the crystal as it grows. The resulting crystal can then be cut into many PN junctions. As the seed is withdrawn the molten mixture can be doped with N or P type impurities to create either type of layer within the crystal as it grows. The resulting crystal can then be cut into many PN junctions.

19 SEMICONDUCTORS PN junctions can also be constructed using the alloyed method. This is done by placing a small pellet of indium on N type semiconductor crystal and then it is heated until the pellet fuses with the crystal. Since indium is trivalent impurity it produces a P type semiconductor. PN junctions can also be constructed using the alloyed method. This is done by placing a small pellet of indium on N type semiconductor crystal and then it is heated until the pellet fuses with the crystal. Since indium is trivalent impurity it produces a P type semiconductor.

20 SEMICONDUCTORS The preferred method is the diffusion technique. A thin section of N or P type material (wafer) is exposed to an impurity element which is in a gaseous state. The impurity atoms penetrate or diffuse through the exposed surfaces of the wafer. The preferred method is the diffusion technique. A thin section of N or P type material (wafer) is exposed to an impurity element which is in a gaseous state. The impurity atoms penetrate or diffuse through the exposed surfaces of the wafer.