Semiconductors have properties that are in between an insulator and a conductor. An insulator is a substance or a device, which has a high amount of resistance. It does not conduct electricity, and they are mostly non – metals. On the other side, a conductor is a material with a low amount of resistance. It can conduct electricity and are mostly metal. A semiconductor lies in between an insulator and conductor. It operates in either condition depending on whether it is activated or not.
A semiconductor has lower conductivity and often metalloids like silicon and germanium. How do you increase the conductivity of a semiconductor? There are two ways to raise its conductivity by exposing it to high or low temperature or add impurities. Increasing the heat across the semiconductor lowers the activation threshold, which enables electrical flow.
The intrinsic semiconductor has an empty conduction band but a filled valence band. When we apply heat to it, some of the electrons move to the conduction band. It leaves holes in the valence band that make it conductive. The intrinsic semiconductor has lower resistance as its temperature rises. It is not practical to subject semiconductors to a high temperature to increase their conductivity.
The best option is to add impurities and increase the conductivity through doping. An intrinsic semiconductor has low conductivity, which is why it undergoes doping to raise it. Doping is the process of adding impurities to a semiconductor. As a result, we formed an extrinsic semiconductor, which is a p-type or n-type material. P-types are semiconductors that are a byproduct of adding a trivalent dopant to intrinsic materials. Boron, Gallium, Indium, and Aluminum are some examples of trivalent impurities. In contrast, N-types are negatively-charged material, which results from adding a pentavalent dopant.
When we join these materials together, we can form a PN junction or a semiconductor diode. A semiconductor diode is a two-terminal device that enables current to flow in one direction. It only conducts electricity when our biasing reached the breakdown voltage. It heavily restricts current flow when it is not activated and in a reverse flow. Our electronic appliances and gadgets utilize a semiconductor diode via a rectifier that juice these gadgets.
A rectifier is a device that transforms the alternating currents in our outlet to direct current in our gadgets. We will talk about that on a separate blog. What we highlight here is the biasing conditions of a diode. There are three biasing conditions for our diode: no bias, forward bias, and reverse bias. Biasing a diode means applying a voltage to it so that the diode activates and conducts electricity. There are two commonly used diodes in electronics: silicon and germanium. Silicon diode has a breakdown voltage of 0.7V, while germanium diodes take 0.3V.
When we formed the PN junction, we create a depletion region due to the attraction of holes and electrons from each material. The depletion region creates a barrier in the diode so that electricity can not flow across it. We can overcome it when we apply enough energy to break it. The depletion region dictates the amount of voltage needed to activate a diode. We can say that the germanium diode has a slimmer depletion region as to silicon diodes.
PN Junction at no bias. wikimedia
Once the diode is activated, it is in a forward bias state, and electricity enables it to pass through it. The no bias state happens when the applied voltage is zero or lower than the breakdown voltage. At no bias, the diode is off, and the current can not pass across it. The voltage applied to the diode should be greater than the breakdown voltage and a positive voltage across the cathode to activate the forward bias state. On the other hand, a reverse bias state happens once we connect a negative voltage at the cathode.
The diode acts as a close switch when it is at a forward bias condition. In contrast, the diode is an open switch when it is at reverse bias. At forward bias, we repel the majority carriers of the p-type material to force the depletion region to open up. When we apply higher potential energy to it, there is a higher chance to break the barrier. The barrier opens up once we reached the breakdown voltage. A forward bias voltage will allow an easy current flow by pushing the free electrons in the n-type material and the holes (protons) in the p-type material to recombine with the ions and reduce the depletion region. The higher voltage applied to the diode results in a thinner depletion region. The electrons can pull the protons intensely close to themselves, which makes the depletion region thinner.
PN Junction at forward bias state. wikimedia
On the other hand, the depletion region thickened when we apply a negative potential across the cathode. We attracted the majority carriers of the p-type into the terminal that stretched out the depletion region in reverse bias. A reverse bias condition is just the exact opposite of a forward bias. The depletion region thickness increases as the negative voltage applied to it increase. What happens inside the diode is the majority carrier of each material gets stuck in the terminals.
PN Junction in Reverse Bias. wikimedia
We can identify the terminal of an unmarked diode by understanding when a diode is in a forward and reverse bias state. We can test using the diode testing function on our digital multimeter. We start with connecting the positive probe of a multimeter to one terminal of the diode while the other probe to the other diode terminal. We say that the unmarked diode is a silicon diode. When we measure a 0.7V across it, we can conclude that the cathode is the terminal we connect the positive probe. If we read OL (open line), we can have the diode terminal as the anode.
In semiconductor diodes, There are three biasing conditions for our diode: no bias, forward bias, and reverse bias. Biasing a diode means applying a voltage to it so that the diode activates and conducts electricity. No bias condition occurs when the voltage supplied to it is zero. It is a thermal equilibrium state where we have a balanced number on both ends. At a forward bias, there is a sufficient voltage to enables current to pass through the junction. In contrast, a reverse bias state occurs when a negative voltage is applied that causes the depletion region to widen.
Experiment
An electronic training board
I am sharing with you this experiment to better understood what happened to a diode when at no bias, forward bias, and reverse bias. We start by testing the equipment if it is good or not. If not, we can change it directly. Here are the tabulated values for the didoes and resistors used in the circuit.
Measured values for the diode and resistance.
After testing the components, we configure the circuit shown below in the electronic trainer. The electronic circuit simulates the forward bias condition of a diode. Unfortunately, I do not have pictures of the actual setup because I was busy monitoring the experiment. The electronic circuit comprises a voltage source, a 330 ohms resistor, and a general-purpose diode (1N4001).
Circuit for the experiment
Next, we change the supply voltage gradually until the diode achieves activation. Every time we increased the voltage, we record the voltage and current changes in the resistor. Here is a tabulation of the changes in voltage and current.
Plot of voltage vs current in the resistor
We can observe that the voltage and current across the resistor increase as we increase the biasing voltage across the circuit. When we apply a higher biasing voltage to a forward bias diode, we observed that the resistor experiences an increase in current intake.
Circuit for the reverse bias experiment
Then, we reversed the diode in the circuit to form the circuit shown above. Using the electronic circuit, we examine the reverse bias state of a reverse bias condition. As mentioned earlier, the diode doesn't allow electricity to flow across it at reverse bias conditions. We can observe it with the result presented in the table. When we increase the biasing voltage, the resistor still experiences zero current across it.
Experiment results for reverse bias state.
In this activity, we have verified that the diode when forward bias acts as a close switch. Whether we place the diode before or after the resistor and parallel it, it still works the same. Also, we can't expect a zero current across the diode when it is at reverse bias.
References
- Boylestad and Nachelsky, Electronic Devices and Circuit Theory
- Floyd, Electronics Fundamentals. Circuits, Devices, and Applications
- Schultz, Grob’s Basic Electronics
Note:
All images are from the author except with citation.