Power Transistors: Overlooking the Essential

Processors, 2nm, RAM, memory, and especially VLSI are the buzzwords used in every news article, every LinkedIn post, every CV, and is currently ringing in everyone’s head. This is the new IT craze. Everyone is opting for courses which focus on designing these devices after seeing the amount of compensation industry giants like Nvidia, Qualcomm, AMD, Samsung, and others are providing. Yet have we thought what goes into powering these devices? 

Figure 1: A power MOSFET (IRFZ44N) (left) and a Central Processing Unit (right)

You must have noticed a small thing connected to every electrical appliance called a plug. It connects to a socket which is connected to the main power supply of your house. It provides 240V RMS, 50 Hz AC supply. Now what I have just written may seem like a foreign language to some of you so let us decode it.

Figure 2: MATLAB plot of AC voltage

Alternating current (AC) is a form of power transmission where electrons do not flow in one direction but oscillate to provide a sinusoidal waveform. Figure 2 shows you the idea. Sinusoidal propagation implies that the current changes direction. Now, you might think how does current flow when electrons are moving back and forth? Bear with me here, imagine that you are standing in a packed line and you get a push from the back. If you are built like a tree then maybe you won’t feel anything but if you are built normally like me, you will go forwards and hit the next poor soul in line who will hit the person in front and so on. In response you go backwards and hit the person behind you. You are oscillating in this case; your position has not changed but energy has still transferred across the line. The same principle applies here in AC. On the flip side we have direct current (DC), in this scenario, the line is continuously moving forward. RMS means the amount of AC power that produces the same heating effect as DC power, and frequency means the number of times a full wave is completed in a second. AC is far better at power distribution and generation than DC. Thermal power plants, nuclear reactors, hydroelectric sources, and wind turbines all work on the basis of mechanical input to electrical output. A generator is used for that. 

Figure 3: Difference between motor and generator is the input and output, construction is by and large the same. (Reference: Hyper Physics)

Fortunately for us AC voltage can be very easily stepped up (increased), or stepped down (decreased). Unfortunately for us it is absolutely useless in electronic appliance usage. Yes, motors can use it but even normal ceiling fans have become Brushless DC (BLDC) now. Your phone, laptop, or anything with a processor; which is basically everything, requires DC power. We have not even started on solar power, the batteries you have in your houses which you most likely mislabel as ‘inverter’, global communication systems, material characterization machines, and so much more that has shaped your world.

Switch mode power supplies (SMPS) is a category which deals with a series of circuits which fall under the electrical engineering subject of power electronics. As the name suggests, there is an element of switching involved, and no it does not mean switching power from AC to DC and vice versa but it deals with switching current paths on and off. To switch these paths on and off we cannot use mechanical switches because they will not be fast enough, we need electrical switches: hence transistors.

Figure 4: Different DC-DC simple circuit configurations (actual ones are more complicated). The item marked as ‘S’ is a transistor. (Reference: AllAboutCircuits)

Figure 5: The Vdc is your battery. The circuit is called inverter circuit as it turns DC to AC. This is a three phase DC-AC converter. (Reference: Tycorun)

For your chargers, they don’t have to handle much since a transformer steps down voltages to a safer range but then we have air conditioners, fridges, washing machines, heaters, etc. These transistors have to hold back high voltages and currents. Electric vehicles need an entire separate level of expertise in power electronics because that entire ecosystem is developing and reducing charging time, that is increasing output power to ridiculous levels. For reference, 60V has the capacity to really damage our body. At above 100V we can qualify as a conductor for a very small time period. These EV chargers work in the range of kilovolts!  An appliance of this much power being handed to a person who is uninformed or underestimates the potential (let’s see who gets this pun) of destruction that they posses is like handing a gun to a baby.

Let’s go back to the topic at hand, designing and fabricating power transistors requires a separate area of expertise and at times it requires materials far more durable than the ever-reliant silicon. Processors have the metric of stuffing as many transistors as you can on a very tiny area. The words ‘2nm’, ‘3nm’, refer to the technology node used in fabricating it. Some people may say that it is referring to the channel length (the road electrons use to complete the circuit) is wrong. A channel that small is impossible to control. These transistors have a channel length of 4-7 nm (cutting edge) to 28-30nm (ASIC, FPGAs), till 180nm (Vikram processors by SCL, Mohali).

Power transistors do not work on these standards. A channel as small as the ones used in processors means you have made a very bad conductor and have wasted quite a lot of money. Transistors of this scale have longer channel lengths and far more relaxed tolerances during fabrication. Now the challenge isn’t to control and manage data, the problem is whether it can handle power of such high magnitude without breaking down like the Berlin Wall. The channel length here is 1 micrometer for 20-100V to several micrometers for 10kV devices. For reference 1 nm is a billionth of a meter and one micrometer is a millionth of a meter. The diameter of your hair is around 100 micrometers! This may seem tiny but for the devices it is a change of orders of magnitude. Imagine a highway the size of your street road, that is what a power transistor would look like if it had the same channel length as a processor transistor. One type of traffic requires one type of road, you cannot choose a smaller one. Same goes for the other side, a larger road would mean more fuel spent, by the time you reach the destination you may even forget why you left. Signal integrity lowers with longer path so a processor requires that small road for a lower power cost.

I want to talk a bit about the state of power transistor manufacturing in India, or rather the lack of it. Recently news has come out that India has managed to make its first GaN chips. A brief introduction to GaN: Gallium Nitride is used to make High Electron Mobility Transistors (HEMT), as the name suggests it is faster and more energy efficient than silicon chips. It finds usage in high frequency devices, especially in space and defence applications. As good as the news is I want to give you all some perspective. Not taking away anything from our scientists and their accomplishments but, the first GaN HEMT device was made in 2006 by Eudyna in Japan for radio frequency applications. All work previously built towards this accomplishment. The industry works quickly to commercialize something this useful. It became available in bulk and companies like Infineon (Germany), Wolfspeed (USA), EPC (USA), and Transphorm (USA) are the major players in this device space. To manufacture this device, a process called metal organic chemical vapour deposition (MOCVD) is used. The company that manufactures these machines are Aixtron (Germany), Veeco (USA), Agnitron (USA), and others. The raw materials or the precursors for these devices to be made are supplied by: Dockweiler Chemicals (Germany), EpiValence Ltd. (UK), Shenzhen Capchem Technology Co. (China), Taiyo Nippon Sanso Corporation (Japan) and more.

My point for the huge paragraph above is that why is India so behind on technology? All the manufacturers are abroad and all the raw materials are supplied from overseas. I did not even list out wafer suppliers! How are we independent when we cannot maintain our machines without foreign help.

Figure 6: Power v/s frequency graph for choice of device material 

(Reference: Power Systems Design)

India’s first Silicon Carbide fabrication plant is openingin Bhubhaneshwar, Odisha. This device is used in low frequency (kHz) appliances like power conversion circuits. This a step in the right direction towards a more holistic approach to semiconductor manufacturing. Till now the only plants that were opening were OSATs (Outsourced Semiconductor Assembly and Testing). These work in the backend of device manufacturing, basically protecting the device with a packet and allow easy interface with the outside circuit. It is a very crucial step but we need front-end manufacturing too. Tata Electronics is opening their 28nm fabrication plant in Dholera, Gujarat. While it won’t work on power transistors, it is a very important step towards total adoption of semiconductor manufacturing and ecosystem generation in India. I won’t lie to you, the day we see India being self-reliant in semiconductor manufacturing is far off but, then again, better late than never.

Lets hope that we actually start producing the devices we use to make our 6789^th food delivery app.

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