February/March 2021

22 POWER SEMICONDUCTORS www.nexperia.com/RETs Issue 1 2021 Power Electronics Europe www.power-mag.com New Developments in RET Technology Resistor-Equipped Transistors (RETs) – also known as digital transistors or pre-bias transistors – offer many benefits, including saving space, reducing manufacturing costs and increased reliability. This article looks at their structure and design considerations, and considers the suitability of new, 80 V parts that target emerging 48 V EV systems. Burkhard Laue, Application Marketing Manager and Reza Behtash, Application Marketing Manager at Nexperia Europe Bipolar transistors are controlled using the base current. However, because the voltage drop across the base-emitter path is highly temperature-dependent, in most applications a series resistor is required to keep the base current at the desired level, thereby ensuring the stable and safe operation of the transistor. To reduce the component count and to simplify board designs, RETs combine single or dual bipolar transistors with the bias resistors which are integrated on the same die. Because these internal resistors have higher tolerances than commonly used external resistors, RETs are suitable for switching applications where the transistor operates in either on- or off-state. This is why RETs are often referred to as digital transistors. RETs are available in many voltage, current and resistance values, in NPN or PNP configuration and a variety of packages including SOT23 and SOT323 and SOT363. Bipolars for automotive applications Two common forms of RETs are shown in Figure 1. In Figure 1a, two resistors and a transistor are integrated with a single NPN transistor. The base series resistor is labeled R1 and a second resistor, R2, in parallel to the base-emitter path completes the base divider circuit. The base divider provides fine tuning and better turn-off characteristic behavior. Nexperia offers R1 values from 2.2 k Ω to 47 k Ω and R2 values from 10 k Ω to 47 k Ω . The ratio of the resistors R2/R1 can be 1, 2.13, 4.55, 10 and 21. Figure 1b shows a dual version with two transistors and four resistors. The RET structure is very similar to the well-understood, standard BJT. Figure 2 shows the resistance ‘meander’ created using a standard device processing step where a polysilicon layer is deposited on the die and then etched back, structured and patterned to form the integrated resistor layer. Figure 3 shows a cross section of a 100 mA automotive RET. Nexperia offers two types of RETs: so- called performance-based 600 mA 40 V RETs; and standard devices which are available as 50 V devices and 80 V parts which have recently been uprated, primarily to cope with new 48 V automotive systems. The 600 mA RETs are based on the company’s performance-based, low VC saturation BJTs that have a good current amplification. This gain stays high at high- collector currents, which is very important because less base energy is required, so the transistor can more easily be turned Figure 1: Two common forms of RETs, two resistors and a transistor are integrated with a single NPN transistor (a), dual version with two transistors and four resistors (b)