Category Archives: Guides

Gate-Drive and Isolation Transformers

Controlling the current flow between the drain and the source of a MOSFET requires the application of a drive voltage to the gate of the MOSFET. Switching power supplies operate the MOSFET as a current switch by applying a pulsed voltage drive to the gate for turning the drain-source current on and off. Delivering the controlling pulse requires a gate drive transformer to provide isolation between the controlling drive circuit and the MOSFET. Companies like Coilcraft offer off-the-shelf gate drive transformers for the purpose.

Gate drive circuits must provide an isolated or floating bias supply for maintaining the necessary turn-on bias when the MOSFET source rises to the input voltage. While driving the MOSFET gate, not only does the gate drive transformer help in isolating the controlling gate drive circuit from the switch node, it may also scale the output voltage via a suitable turns-ratio between its primary and secondary.

Some applications use optocouplers or digital isolators for driving the MOSFET directly. However, the use of a gate drive transformer is preferable, as it can provide a higher voltage requirement, much lower turn-on and turn-off delay times, and it can scale voltages by the ratio of its turns. These advantages make the simple gate driver transformer the best-performing solution for high-frequency and high-voltage applications that require maintaining accurate and fast signal timing.

Typical low-power applications use a simple single-output, transformer-coupled, high-side gate driver circuit. Additional components like capacitors, resistors, and diodes may be necessary depending on the duty cycle and other circuit conditions. These include preventing the development of a DC voltage across the transformer, as this may cause it to saturate. The additional components also help in the coupling capacitance and magnetizing inductance from resonating with specific duty cycle ratios. For single-ended circuits, the highest duty cycle is preferably 0.5.

Higher power applications may require half-bridge and full-bridge configurations coupled with transformers. Double-ended or DC-coupled bridge configurations may use a theoretical maximum duty cycle of 1.0. Designers use isolation transformers for isolation and voltage scaling in power supply applications. These serve three main purposes.

First, the transformer helps to connect circuits with grounds at different potentials—this prevents ground loop formation. Second, the transformer provides galvanic isolation, thereby preventing any flow of direct current. Lastly, the transformer provides voltage transformation—stepping up or stepping down from one voltage to another.

Isolation transformers may be available as signal transformers, power-supply transformers, communication transformers, data-line transformers, and many others. These are versatile and aptly suited for several industrial and commercial data communications and power supply applications.

Using off-the-shelf gate drive and isolation transformers can simplify the design of the gate drive circuit and significantly reduce the design cycle time. Coilcraft transformers typically use high-permeability ferrite cores for maximizing the inductance and minimizing the magnetizing current.

The designer can determine the required transformer size by the volt-time product of the application. This forms the first selection criterion for a gate drive transformer, as the designer can select the appropriate volt-time or V-µsec rating from the datasheet of the transformer. The rating must be equal to or greater than the highest applicable voltage-time product 

MCUs Working Sans Batteries

Nature is exceptionally efficient. It maximizes available and additional resources by using as much of it as possible. Humans are now beginning to follow in nature’s footsteps. Doing this allows us to improve performance, thereby reducing waste and minimizing cost. One of the methods in use today is energy harvesting. We can power electrical devices using ambient energy. For devices operating on batteries, it is possible to use energy harvesting for extending the useful life of the battery, or even replace the energy contribution of the battery entirely.

We have ultra-low-power microcontroller units or ULP MCUs as the logical choice for demonstrating energy harvesting. Many devices like wireless sensors, wearable technology, and edge applications use ULP MCUs because it is essential for these devices to extend their battery lives. Reviewing the working practice of energy harvesting is important to understand its value to ULP MCUs.

The principles of energy harvesting are simple. It must overcome the finite nature of the primary source of energy, here, the battery. However, as no process can be one hundred percent efficient, there will be losses when converting the source power to usable energy, even when there is boundless ambient energy available for capture. This is evident in wind turbines, a renewable large-scale energy source. The wind provides the turbines with potential energy, making the blades rotate. This movement turns a generator, producing electrical power. Other similar large-scale ambient energy sources also exist—geothermal heat, oceanic waves, and solar.

Wearables and other similar small-scale devices harvest thermal, kinetic, or environmental electromagnetic radiation energy. However, each of these makes use of different mechanisms for converting the source power to useful usable energy. It is necessary to consider the utility and practicality of each conversion mechanism, as the application defines the size and mass of the energy conversion technology.

For instance, making use of thermal radiation is more suitable for wireless sensor applications, as the sensor placement and design can take advantage of both forms of energy. Likewise, vehicles can use sensors that make use of radiant heat emanating from the road surface. As engine components like wheels are high-vibration locations, it is possible to harvest motion energy from near them. For wearables using ULP MCUs, harvesting the kinetic energy from the human user’s motion provides the most practical means of conversion to usable energy.

In wearable technology, the primary application of the ULP MCU is to process the edge data gathered by the sensor. And, it is critical to process this data with the minimum power consumption. Energy harvesting supplements the power from the battery, which has a finite amount of energy, and requires periodic replenishment in the form of recharging or replacement as its power depletes. There are three ways of capturing energy for ULP MCUs—using piezoelectric, electromagnetic, or triboelectric generators.

Kinetic forces compressing piezoelectric materials can make it generate an electric field, which can add as much as 10 mW to the battery. Harvesting energy from electromagnetic radiation like infrared, radio, UV, and microwaves can contribute about 0.3 mW of harvested power. Triboelectric generators use friction on dissimilar material surfaces rubbing together from mechanical movements like oscillation, vibration, and rotary motions to generate 1-1.5 mW of electricity.

Improving Power Management Efficiency

Design engineering teams face considerable challenges handling conflicting requirements for portable types of medical devices. Most of these devices are always-on types and must be capable of managing battery life with maximum efficiency and effectiveness. They also must have suitable dimensions tailored to the patient’s comfort, especially as most of these are meant to be worn 24 hours a day. Therefore, not only must their construction be robust, but they must also deliver the highest levels of performance. Designers use PMICs or power management integrated circuits for optimizing the power utilized by the ultra-low architecture, improving the sensitivity of measurements, and keeping the SNR or signal-to-noise figures on the high side.

Wearable technology is benefitting from the growing popularity of mobile networks from the perspective of both, the healthcare and the consumers. Although their design was initially meant for sports and wellness, the medical market is now finding increasing use for wearables. As a consequence, newer generations of medical wearable devices are available using MEMS or micro-electromechanical system sensors, including heart-rate monitors, gyroscopes, and accelerometers. Other sensors are also in use, such as for determining skin conduction and pulse variability. However, the more sensitive the sensor, the more it faces SNR issues so designers need to use better noise reduction techniques along with more efficient energy-saving solutions.

For instance, the accuracy of optical instruments depends on many biological factors. Therefore, design engineers maximize the sensitivity of optical instruments by improving their SNR over a wide range. They use voltage regulator ICs with low quiescent currents along with elements that improve the SNR by reducing ripple and settling times.

Maxim offers a complete SCODAS or single-channel optical data acquisition system, the MAXM86161. They have designed its sensor module for use in in-ear and mobile applications. They have optimized it for SPO2 or oxygen saturation in the blood, HR or reflective heart rate, and continuous monitoring of HRV or heart rate variability. There are three high-current programmable LED drivers on the transmitter part of the MAXM86161. While the receiver part has a highly efficient PIN photo-diode along with an optical readout channel. It features a low-noise signal conditioning ALE or analog front end. It includes a 19-bit ADC or analog to digital converter, a high-performance ALC or ambient light cancellation circuit, and a picket fence type detect-and-replace algorithm.

Optimizing the energy efficiency of an optical measuring instrument is a constraint on its design. Rather than use regular LDO or low-drop-out regulators, designers now use novel switching configurations to improve the efficiency further. The requirement is that the voltage regulation element provides a low level of ripples at high frequencies so that there is no interference when measuring heart rates. To operate LEDs at voltages different from what the Li-ion batteries can supply, designers use new buck-boost converter technologies, thereby curbing energy consumption and saving board space. For instance, they use the SIMO or single-inductor multiple-output buck-boost architecture for reducing the number of inductors and ICs the circuit requires.

The MAXM86161 from Maxim Integrated is a PMIC or power-management integrated circuit and is meant for applications that are space-constrained and battery-powered, where the efficiency must be high within a small space.

Silicon-Based MEMS Micro Speakers

For the past 100 years or so, the audio industry has been using coil-based driver technology for its loudspeakers. Although the technology has several disadvantages, it has dominated the landscape for so long simply because of the absence of a suitable alternative cost-effective technology. Now, this is likely to change, at least for the next generation of earbuds using micro speakers. A new startup, the CA-based firm xMEMS, has been perfecting its MEMS driver.

The company has created three MEMS or micro-electro-mechanical-systems micro speakers, suitable for use in hearing aids, wired and wireless earbuds, smart glasses, loudspeaker tweeter arrays, and virtual reality headsets.

xMEMS is promising a long list of advantages for its solid-state micro speakers. For starters, the driver is only about 1 mm (1/25th of an inch) thick. That leaves more room for sensors, batteries, and other components. The entire speaker is made of silicon, including the actuator and membrane. This eliminates the need for matching the driver and calibrating it. Being entirely solid-state, the MEMS technology allows mass production of the high-resolution capable micro speaker in more precise configurations than is possible with traditional designs. It does not involve the tedious manual assembly of balanced-armature drivers, as in regular coil-based speakers.

The solid-state micro speakers boast a flat frequency response for the full audio spectrum ranging from 20 Hz to 20 kHz. While there are no in-band resonances, these speakers exhibit an astonishing ±1° phase consistency for spatial performance. As the MEMS speakers show a superior high-frequency response in comparison to coil speakers, their clarity and presence are outstanding. The high-speed mechanical response results in a group delay of less than 50µs, while their total harmonic distortion is only about 0.5% or 94dB at 1kHz. The near-zero phase delay results in improved noise suppression.

In addition to the superior performance characteristics above, the new MEMS speakers can withstand mechanical shock to a greater extent than their coil-based counterparts can. This is due to their monolithic design, eliminating the spring and suspense structure of coil-based speakers. Being totally solid-state, the new speakers consume far less power for the same output, thereby improving battery life. No added membrane is necessary for resistance to dust and moisture up to IP58.

In a blog, xMEMS claimed their MEMS speakers are suitable for high-resolution audio. Although for high-resolution audio, the focus is more on the codec’s ability to achieve suitable bit depth and sampling rates, requirements from the speaker are just as stringent.

Typically, the digital signal chain and the codec are responsible for the highest quality of data stream. Since the speaker is the ultimate transducer for the sound that people hear, it must accurately render and reproduce the sound as the artist intended.

In this respect, the performance of solid-state MEMS micro speakers suits the standard requirements significantly better than coil-based speakers can. The MEMS speaker’s extended bandwidth and its mechanical and ultrasonic near-flat response above 20 kHz are responsible for that.

The MEMS driver works on the principles of inverse piezoelectric effect. The application of voltage causes the actuator to contract and expand, converting electrical energy into mechanical sound energy.

The Function of Ferrites in Electronics

Engineers often use ferrite components in electronic circuits. These ferrite components are nonconductive, ceramic compound materials made with numerous combinations of iron oxides. Electronic components typically use them because of their high electrical resistivity and low eddy current losses. Ferrites can have various properties depending on their condition of synthesis, sintering temperature, composition, and grain size.

Manufacturers classify ferrites based on their crystal structure and magnetic properties. In general, they are of two types—soft and hard. Soft ferrites, made from magnesium, manganese, nickel, cobalt, and zinc, have low coercivity, such that their magnetism changes easily, and they act as conductors of magnetic fields. On the other hand, hard ferrites make very good permanent magnets, owing to their high coercivity.

It is also possible to classify ferrites based on their crystal structure. Typically, there are four groups— spinel, garnet, ortho, and hexagonal. Manufacturers distinguish them based on the molar ratio of ferric oxide to other oxide compounds present in the ferrite ceramic.

Crystallizing spinel ferrite results in a cubic structure with oxygen anions in a closely packed arrangement. Here, a unit cell comprises 32 oxygen ions. The anions form an FCC or face-centered cubic array.

Ferrites typically exhibit a permanent type of magnetism that physicists refer to as ferrimagnetism. This is a phenomenon that aligns the magnetic moments of atoms in both antiparallel and parallel directions. This alignment partially cancels the magnetic field, making the overall magnetic field of a ferrite material weaker than that of ferromagnetic materials.

Various types of ferrites are available. In electronic circuits, engineers typically use them as beads. For a ferrite bead, the resistivity is the strongest in a thin frequency band. This feature makes ferrite beads very useful as frequency-dependant resistors. Above the frequency band, the impedance of the bead begins to appear capacitative.

Other types of ferrites structures are also available for use in electronics. For instance, there are flat ferrites, typically rectangular or disc-shaped. Engineers use them in applications where they need a flat shape, such as power inductors, planar transformers, filters, and power inductors. Flat ferrites are very useful for suppressing radio frequency interference and electromagnetic emissions.

Ferrite rings and sleeves are also available. These are cylindrical-shaped components, suitable for placing around a wire or cable. It acts like a filter that can block high-frequency noise, allowing only low-frequency signals to pass through the wire or cable. Manufacturers choose the inner diameter of the ferrite to closely match the outer diameter of the cable, as this maximizes the benefits of interference suppression. Ferrite rings and sleeves are very useful in applications like data communications, consumer electronics, and power supplies to improve signal integrity and reduce interference effects on circuit performance.

Multi-hole ferrite beads are cylindrical cores with typically 6 through-holes running along the axis of the cylinder. When a trace or wire in a circuit is wound through its holes, the multi-hole ferrite bead behaves as a low-pass filter. It blocks unwanted high-frequency interference signals and allows only low-frequency signals to pass through the wire.

DAWSense Turns Any Surface into an Input Device

Although we are used to traditional interfaces like touchscreens and keyboards, interfacing with computers has traversed a long distance over the years. Now, it is possible to turn any surface into an input device. DAWSense can do this by utilizing machine learning and taking advantage of surface acoustic wave technology. With different situations requiring varying methods of input, researchers are now exploring newer methods of human-computer interfacing. One of them is to embed the interface within everyday objects, thereby enhancing user experiences.

Human-machine interfaces may take many forms. For instance, the industry often uses microphones or cameras to control devices using methods like speech or gesture recognition. Although such systems may be of immense help in certain applications, they may not be practical for others. In a camera-based system, it is easy to obscure the arrangement by introducing objects in front of the camera. Similarly, microphone-based systems involving speech recognition may not function properly in noisy environments.

As an alternative, researchers were experimenting with transforming any arbitrary surface to act as an input device. For instance, for controlling a smart home, they have experimented with the arm of a couch acting as a TV remote, or an interactive wall. They have tried many methods for building such functionality so far, with accelerometers standing out as one of the most promising solutions, as they can sense touch gestures on various surfaces without any modifications on them.

However, the sampling bandwidth of accelerometers incorporated into a surface to act as a touch-sensing device is not enough to capture more than a few relatively coarse gestures. Now, a collaboration between researchers at the Meta Reality Labs and the University of Michigan has demonstrated another method that offers the necessary bandwidth for creating user interfaces that are more advanced.

The new method relies on SAWs or surface acoustic waves rather than mechanical vibrations for sensing touch inputs. The team has also fashioned a VPU or voice pick-up unit for detecting subtle touch gestures. They have designed the VPU to conduct the surface waves into a hermetically sealed chamber that contains the actual sensor. This practically removes any interference from background noise. As the team has fabricated each VPU using the MEMS process, the sensor has the necessary high bandwidth that is typically associated with a MEMS microphone.

Although the MEMS sensor was a high-performance one, the researchers still needed a method for converting the SAWs into swipes, taps, and other gestures. A hard-coded logic would fail to convert them satisfactorily, so the team had to design a machine-learning model with an algorithm to learn from the data.

VPUs typically collect a huge amount of data, and processing this data on an edge computing device in real-time would be a challenge. The researchers dealt with this problem by calculating Mel-Frequency Cepstral Coefficients, which helped in understanding the most informative features of the data. With this analysis, the researchers could reduce the number of features they needed to consider from 24,000 to just 128. They then fed the features into a Random Forest classifier for determining the exact representation of the surface waves.

FireBeetle Drives Artificial Internet of Things

The next generation of the FireBeetle 2 development board is now available. Targeting the IoT, especially the Artificial Intelligence of Things, it has an onboard camera. According to DFRobot, the creator, the FireBeetle boasts Bluetooth and Wi-Fi connectivity, and an Espressif ESP32-S3 module.

Built around the ESP32-S3-WROOM-1-N16R8 module, the main controller of the FireBeetle provides high performance. It operates with 16MB of flash RAM, along with 8MB of pseudo-static RAM or PSRAM that allows it to store more data. The ESP32-S3 chip provides acceleration for computing neural networks and processing signals for high workloads. This makes the FireBeetle ideal for many applications like image recognition, speech recognition, and many more.

DFRobot has designed the heart of the FireBeetle, the ESP32-S3, for edge AI and low-power tinyML work. With two CPU cores, the Tensilica Xtensa LX7, both operating at 240 MHz, the ESP32-S3 also offers vector processing extensions. The design specifically targets accelerated machine learning, including workloads of artificial intelligence. In addition to the 8MB PSRAM and the 16MB Flash memory, the board also has 384kB of flash and 512kB of on-chip SRAM.

The FireBeetle development board, along with its BLE or Bluetooth 5 Low Energy and Wi-Fi connectivity, also includes an onboard camera interface driven by a dedicated power supply circuit. The camera has a 2-megapixel sensor with a 68-degree FOV or Field of Vision. There is a GDI connector, which is useful for adding a TFT display.

DFRobot offers two variants of the FireBeetle development board. One of them is the standard version, namely the FireBeetle 2 ESP32-S3, containing a PCB antenna for wireless connectivity. The second variation is the FireBeetle 2 ESP32-S3-U, and it offers a connector for rigging up an external antenna. It is possible to program both boards from Arduino IDE, ESP-IDF, and MicroPython.

It is possible to order both development boards from the DFRobot website store, The second variant is the costlier of the two, and both come with volume discounts. Although both variants come with the board and camera, the pin headers are bundled loosely but not soldered. DFRobot has published a simple project for the FireBeetle—a camera-based monitor to oversee the growth of plants.

It is possible to use the FireBeetle development board to build a DIY plant growth recorder. It allows monitoring the entire growth process of the plant, starting from seeding right up to maturity, while tracking the environmental conditions throughout. This makes it possible to identify any changes easily that could affect the health and growth of the plant, along with any fluctuations in temperature, light levels, and humidity. This information helps to organize and optimize the growing conditions of the plant, thereby ensuring that the plants get everything they need for proper growth.

The project has a screen for displaying the various parameters it is monitoring. The camera periodically captures images of the plant as it grows, storing them in the board’s memory. The board transmits real-time images and environmental data over Wi-Fi or Bluetooth for regular viewing.

Clamp-On Energy Meters

Traditional energy meters are mostly inline, meaning, the meter is in series with the circuit whose energy consumption it is monitoring. However, there is another type of energy meter that clamps onto the pipeline it must monitor, rather than break into the pipeline. This is an ultrasonic alternative and is specifically useful for energy management and billing applications that commercial, domestic, district, or shared cooling and heating systems use. Obviously, the clamp-on energy meter offers several advantages over its traditional in-line counterpart, the most significant of them being easier installation, cost savings, and maintenance benefits.

For the efficient operation of new or existing buildings, it is necessary to identify the waste, optimize the use, and accurately monitor for billing the use of energy and water. Optimizing the consumption of water and energy is essential and crucial for any type of building, whether it is a housing or accommodation, school, hospital, condo, office block, or shopping center, in the private or public sector. Mostly, energy monitoring applications associate automatic meter readings with a third-party cloud provider, thereby allowing for ease of use and seamless integration. These new type of meters are also suitable or retrofitting older and traditional energy meters in existing buildings that mostly have limited drawings and old pipework.

Managing fluid consumption requires finding out what is flowing where. The clamp-on energy meters accomplish this very easily, as there is no need to drain down the system, shut it down, or cut into the existing pipework. Therefore, using clamp-on energy meters saves a lot of time and money when retrofitting existing buildings.

There are several benefits to using clamp-on energy meters. It is possible to clamp them on a wide range of pipes, and they provide cross-correlation flow measurement systems for accuracy. These meters come with an optional RS485 serial interface and a Modbus RTU slave. They feature a 1-MHz transducer that is powerful enough to measure through poor or old thick wall pipes and larger pipes.

These new energy meters are a clamp-on and non-invasive alternative to traditional in-line meter installation. They are simple to install, simply connect power, enter the inside diameter of the pipe, adjust the sensors, and finally, clamp it on the pipe. No special tools or skills are necessary. The installation provides dry servicing, minimum downtime, and maximum availability.

The non-invasive energy meters require little or no maintenance since they have no moving parts or sensors to wear out or require calibration. Therefore, they have a longer service life with very few repair requirements. Being generally less expensive for installing and maintaining than invasive meters, the non-invasive meters do not require cutting into pipes or installing additional sensors. As it is not necessary to cut into the pipe, the risk of leaks is very much reduced. This is especially important in applications where the fluid in the pipe is expensive or hazardous. Overall, non-invasive clamp-on energy meters are a cost-effective and effective solution for measuring the energy flow in a surprisingly wide range of applications.

These meters are very useful in various types of buildings, including residential buildings, schools, hospitals, and offices.

How is a Wire Harness Made?

Many sectors, including industrial and consumer electronics, continue to use wire harnesses, and their use is increasing continuously. Therefore, there is an urgent need to understand the process of manufacturing this vital component. Wire harnesses link different electrical or electronic modules to allow the complete system to work seamlessly.

Wire harness assemblies are a bunch of wires processed with a protective sheath. They may end in different types of terminations. Harnessing is important as it organizes the wires for easy implementation. Wire harnesses must not be confused with cable assemblies, which bind multiple covered wires with a protective covering, enabling them to work in harsh environments.

The use of wire harness assemblies results in several advantages. The organized wires optimize space while helping to improve assembly times. The harness helps in customizing the appliance to its bespoke needs. The protective covering on the wires improves equipment safety while improving the life of the wires. A variety of appliances use wire harnesses extensively. These include heavy equipment, panel displays, flight simulators, control panels, vehicles, and more.

The wire harness manufacturing process begins with design. Each product requires a custom-designed harness. It is imperative to choose each component of the wire harness carefully to achieve full functionality and life.

The primary requirement is the length of each wire in the harness. As the wire may require routing through the equipment, the length of individual wires in the bunch may differ. The length of the wires may also depend on their diameter, as thicker wires require more space to bend.

Each wire must also be considered for the maximum current it will carry. In a wire harness, some wires may carry power while some may carry low-frequency signals. They will require wires of different gauges.

Once the designer determines the length and gauge of each wire, they must concentrate on the wire terminations, which are necessary to connect each wire to its starting and ending points. This depends on the end termination of the two devices the wire will be joining. The terminations may use lugs, crimps, connectors, or something similar.

Once the wires are bunched together to form the harness, it will be difficult to identify them individually. Therefore, the designer will require some means of identifying individual wires in the bunch. Ferrules are a low-cost choice for this purpose. They are available in different diameters and individually marked with numbers and alphabets. By using the same combination of ferrules with numbers and alphabets on both ends of a wire, it becomes easy to identify the wire, even after bunching it with several others. However, this identification is only necessary if the operator will connect each wire individually. They are not necessary if the termination ends in a connector.

Next, wire harnesses may require a jig to form them into the final shape necessary for implementation in the system. Once the operator arranges or dresses the wire harness in the jig, they may require a means to bunch all the wires together. This may take different forms, like a plastic wire or sheath covering the full length of the wire harness.

What is Diode Biasing?

PCB assemblies often contain numerous components. The engineer designing the board selects these components individually, based on their function in the circuit. For a successful project, it is essential to understand the basic operation of these components individually, and in relation to one another. One such component is the diode.

A diode is a semiconductor device with a PN junction. It supports current flow in only the forward direction—from the anode to the cathode—and not in the reverse. However, to allow current flow in the forward direction, a diode must be given a particular voltage to overcome the bias in its PN junction. Diode biasing is the application of a DC voltage across the diode’s terminals for overcoming the PN junction bias.

It is possible to bias a diode in two ways—forward and reverse. When forward biased, the diode allows current flow from its anode to its cathode, provided the biasing voltage is greater than the PN junction bias. However, when reverse-biased, the biasing voltage cannot overcome the PN junction bias, and the diode blocks any current flow. Reverse biasing a diode is a convenient way for using it to convert alternating current to direct current. Proper use of forward and reverse biasing also allows other functions, such as electronic signal control.

Diodes are mostly germanium or silicon-based. A diode consists of a layer of P-type semiconductor material and another layer of an N-type semiconductor material joined together. The P-type material forms the anode terminal and the N-type material forms the cathode terminal of the diode.

When fabricating a diode, the manufacturer dopes the two layers differently. They dope one of the layers with boron or aluminum to make it P-type, which gives it a slightly positive charge. The P-type semiconductor, therefore, has a deficit of electrons or an abundance of holes. They dope the other layer with phosphorus or arsenic to give it a slightly negative charge and make it N-type. Therefore, the N-type semiconductor has an abundance of electrons.

At the junction of the P-type and N-type layers, electrons and holes combine to form a sort of neutral zone. Therefore, when a current must flow, a voltage bias is necessary to push the electrons and holes through this neutral zone. The neutral zone is less than a millimeter in thickness.

A forward bias pushes holes from the P-type layer, across the neutral zone, into the N-type layer. The forward bias reduces the width of the neutral zone to allow the current to flow. The forward bias necessary depends on the material of the diode. It is 0.7 VDC for silicon diodes and about 0.3 VDC for germanium diodes.

On the other hand, a reverse bias adds more electrons to the N-type layer and holes to the P-type layer. This increases the width of the neutral zone, making it impossible for current to flow across it.

Therefore, forward biasing allows current flow through the diode from the anode to the cathode, and reverse biasing prevents current flow. Even with forward biasing, there is no current flow until the voltage is able to overcome the PN junction bias.