Tag Archives: Semiconductors

How Does Switching Affect Semiconductors?

Even though ICs rule the world of electronics, the transistor does all the work. Within each IC are millions upon millions of transistors perpetually switching on and off so that the IC can carry out its intended functions. Even if one of the multitudes of transistors were to stop switching, the IC could lose part or all of its functionality.

Circuits handling digital signals most often use transistors to switch from a high state to a low state and vice versa. It is usual to call a circuit point as being in a high state if the voltage at that point is close to the supply voltage. If the circuit point is closer to the ground or zero voltage, we generally call it as being at a low state. The time taken for the transistor to switch from a high to a low state or vice versa is its switching rate. While the transistor does not expend much energy when at either the low or the high state, the same cannot be said for the time when it is actually switching.

Under ideal conditions, a transistor should switch instantaneously. That means the transistor should take zero seconds to change its state. However, ideal conditions do not happen in reality and the transistor takes a finite time, however small, to actually switch over.

Transistors are made of semiconductor material and each junction has a finite capacitance and resistance. Junction capacitances store energy and the combination of resistance and capacitance acts to slow down switching – the capacitance must fill up or empty itself before the transistor can flip. The rate at which the capacitance fills up or empties itself depends on the junction resistance.

The situation gets worse as the switching frequency goes up. As the transistor is driven to toggle faster and faster, the junction capacitance may not get enough time to discharge or charge up fully. That defines the maximum switching rate the transistor can achieve.

Semiconductor manufacturers use various methods to reduce junction capacitances and resistances to induce these special semiconductors switch faster. Although modern semiconductors (transistors and diodes) are capable of switching at MHz or GHz scales, the cumulative effect of the tiny switching losses add up to increase the junction temperature.

Power is the product of voltage and current. When a semiconductor is in a high state, although the voltage is high, the current is negligible and consequently, the power drawn from the supply is negligible. When the semiconductor is a low state, its voltage is close to the ground level and the product of current and voltage is again negligible.

However, during switching, when the voltage is somewhere in-between the supply and ground levels, the current drawn also increases. That makes the product of voltage and current have a significant value and the semiconductor generates heat because of the power consumption. With higher frequencies, this happens more frequently and the heat accumulates to produce higher junction temperature.

If the natural process of heat dissipation can remove the accumulated heat, the semiconductor soon reaches a steady temperature. Else, heatsinks and or forced cooling methods are necessary to remove the heat accumulated.

Phosphorene Challenges Graphene as a Semiconductor

Though silicon has been the basis of semi-conductors for decades, it is facing stiff competition from other materials that promise to deliver several extras to consumers who like to enjoy more flexibility with their gadgets.

For some time, graphene, a one atom thick allotrope of carbon has been under consideration for use in electronic devices because its thin structure allows electrons to travel across it much more rapidly than they would do across silicon. However, graphene has severe limitations, as its conductivity is a little too high to be of much use in electronic devices, which need semi-conductors or materials with medium levels of conductivity. Another newly developed material dubbed phosphorene, which can form identical thin layers and is a semiconductor as well, offers a wider scope in electronics.

Phosphorene particulars

Scientists at the Technical University of Munich (TUM) have prepared a semiconducting material with black phosphorus in which a few phosphorus atoms have been swapped by arsenic atoms. Replacement of the phosphorus atoms with arsenic has caused the band gap to reduce to 0.15eV, which makes the material an effective semiconductor.

Phosphorene or black arsenic phosphorus can form very thin layers like graphene. Unlike silicon, which is hard and brittle, phosphorene is easy to manipulate into different kinds of structures and shapes. This makes possible a great range of electronic devices with considerable mechanical flexibility.

Scientists at TUM have built on technology that allows the fabrication of phosphorene with the application of high pressure. This reduces the production costs considerably. The research workers have been able to fine-tune the band gap exactly according to specific requirements by tweaking the arsenic concentration. According to Tom Nilges, who is heading the research team at TUM this has enabled them to produce a wide range of materials with diverse electronic properties that were not possible earlier.

Field Effect Transistors

American scientists from Yale University and the University of Southern California (USC) have collaborated with the researchers at TUM to build devices like field effect transistors with phosphorene. A group headed by Dr. Liu and Professor Zhou of the Electrical Engineering Department at USC has studied the transistor characteristics.

Infrared Detectors

Further exploration of the material by the scientists revealed that the material when heavily doped with arsenic could be used for infrared detection. For instance, when the arsenic concentration is as high as 83%, the band gap in phosphorene is about 0.15eV. This fact makes it an effective sensor for infrared rays of long wavelengths. Researchers expect that the new substance can be effectively used as Light Detection and Ranging (LIDAR) sensors, which find use in applications for tracing dust particles and pollutants in the atmosphere and as distance sensors in vehicles.

Anisotropic behavior

Another noteworthy feature of phosphorene is its anisotropic nature. Electronic and optical properties of the material were studied using ultra-thin films in two mutually perpendicular, x- and y-axes. It was observed that the properties were different in the two directions.

Phophorene has an edge over other newly discovered thin-layered semiconductors because it is very easy to peel off layers from a parent black phosphorus crystal.

Synthetic Diamond Manages Power

Power delivery using semiconductor devices is increasing at a rapid pace. This is evident from different forms of power delivered, whether it is controlled power through power inverters, or RF power through amplifiers. Power is necessary to operate nearly everything, such as for alternative forms of energy generation, electric vehicles, radar systems, cellular base stations and even smartphones. However, semiconductor devices need to dissipate the heat they generate, and this poses a stringent challenge for power and thermal management.

Such high-power semiconductor devices are now using a new technology in the form of GaN-on-diamond wafers and synthetic diamond heat-spreaders. The reason behind this is the excellent thermal conductivity of diamond, the highest of any material. At room temperatures, diamond conducts heat about five times better than copper does.

Any semiconductor material can use diamond heat spreaders and these lower the temperature of the semiconductor gate junction by almost 30 percent. In addition, the use of GaN-on-diamond wafers helps lower the temperatures of GaN devices further. With the gate-junction temperature going down by almost 50 percent, GaN-on-diamond devices can handle more than three times the power density than similar GaN-on-SiC can.

Manufacturers use the technique of plasma-assisted microwave CVD or Chemical Vapor Deposition for synthesizing diamond heat spreaders. With this method of growing the synthetic diamond, manufacturers make freestanding diamond wafers up to 140 mm in diameter and nearly 1 mm thick. The wafers have thermal conductivities higher than 2000W/mK, which is five times that of copper. By using microwave CVD for growing diamonds, manufacturers can engineer the properties of the diamond wafers precisely, giving them a range of thermal conductivities. This allows them to offer different cost to performance ratios for matching the specific needs of any application.

When using metalized diamond heat spreaders, manufacturers attach them to the bottom of the semiconductor die. Since they use as thin a layer of solder as is possible for attaching the heat spreader, the diamond lies within 100 to 300 microns of the gate junctions of the device. The diamond heat spreader distributes the heat equally and effectively in both lateral and vertical directions. Heat spreading in the lateral direction is particularly important for RF power amplifiers, as they typically form hot spots of up to 1 micron in diameter with intense heat density.

Manufacturers need to keep the metallization of the die and the heat spreader thin – to the extent of a few hundreds of nanometers. Metallization of the diamond has to be done carefully using a carbide-forming metal as the first layer. The solder layer used to attach the heat spreader must also be thin, preferably lower than 10 microns. With optimal integration into a package, diamond heat spreaders typically help to reduce the gate junction temperatures by nearly 30 percent, when compared to what ceramic packages do that are not using diamond heat spreaders.

The GaN-on-diamond substrates now offer new thermal management tools for GaN semiconductor devices. The reduced thermal resistance of GaN-on-diamond and diamond heat spreaders allows simpler, less expensive thermal management systems. This has a favorable impact on cooling complexity and expenses involved, also leading to better lifetimes of the entire system.

Available Methods of Marking Semiconductors

Semiconductor Markings – Available Methods

Traditionally, most components have two or three lines of identifying marks plus a company logo. Over time, the manufacturer codes have become more involved to incorporate a component’s identification plus the complete history of the process. Early on, it was the military applications that required very specific markings and identification processes. Current package markings are a by-product of those military requirements.

When a semiconductor is clearly identified, there is less room for error in the production process. Reducing errors when a component is in use for production saves time. There is also less product waste and the production process becomes more streamlined.

As the size of electronic components has decreased, the available space that manufacturers have to mark each piece has also decreased. The technology required to complete this task has become increasingly more complex.

The chief reason for the more complex codes stems from the demands of the end users. They need to have complete traceability of the product; from the history of the production cycle including the date and location of manufacture to the exact lot code. Possession of this information is critical to the end user in the event of a recall or defective components.

There are four primary methods to marking components in current use. Use of the various methods depend on the size, the type and the environment of the component production.

The methods are:
-Ink marking
-Electrolytic marking
-Pad printing
-Laser marking

In ink marking, inkjet printers are used. The technology is called ‘drop-on-demand’ which means that the flow of ink is controlled to create a pattern of ink droplets to form an image marking.

Electrolytic marking employs low voltage electric current with a stencil. The top layer of the package is etched by electricity flowing from the marking head, assisted by an electrolyte chemical. The process takes approximately 2-3 seconds to complete.

Pad printing is the most traditional of all the processes. A steel plate is etched with the image of the imprint. The ink is transferred to the plate which then is applied with pressure to the surface of the electronic component.

Laser marking is the most recent development in the marking process. It provides the greatest flexibility in the size, timing and complexity of the markings. The laser process is also the fastest method to mark electronic components; it is not uncommon for this process to print up to 300 characters per second. An additional benefit of using laser printing is the ability to produce a clean mark on many irregular surfaces.

No matter which method has been used to mark the semiconductors you use, you can be sure that much thought has been put into the decision.