Whitepaper

RET devices provide robust, cost-effectivemeans of switching and driving loads directlyfrom logic devices
Availability of 80 V rated devices suits higher 48 V automotive board net voltages

By Burkhard Laue, Application Marketing Manager andReza Behtash, Application Marketing Manager at Nexperia

In response to the need for higher efficiency and higher power, many electronic systems
today in automotive, industrial and even consumer applications are shifting to fast switching
through the use of MOSFETs or wide bandgap devices, primarily silicon carbide or galliumnitride. However, this is not the whole story. Every system or sub-system, even in newautomotive 48 V designs, has a range of switching requirements. For low frequency, medium
voltage switching, Resistor Equipped Transistors (RETs) – also known as digital transistors or
pre-bias transistors - are commonly the preferred choice. Often used to provide a simple way
to switch and drive loads directly from logic devices, RETs offer benefits including reductions
in size and manufacturing costs, coupled with increased reliability and robustness. This paper
discusses the operation, structure and design considerations of Nexperia’s portfolio of RETs,and considers the suitability of new, 80 V parts that target emerging 48 V EV systems.
1) RET Operation
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 thedesired level, thereby ensuring the stable and safe operationof the transistor. To reduce the component count and tosimplify board designs, RETs combine single or dual bipolartransistors 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 areoften referred to as digital transistors. RETs are available inmany voltage, current and resistance ratings, in NPN or PNP
configuration and a variety of packages including SOT23 andSOT323 and SOT363.

Nexperia offers R1 values from 2.2 kΩ to 47 kΩ and R2 values
from 10 kΩ to 47 kΩ. The ratio of the resistors R2/R1 can be
1, 2.13, 4.55, 10 and 21. Figure 1b shows a dual version withtwo transistors and four resistors.

(a)
RI
R2(b)

6R1R2TR2TR1R212) RET Types & Structure
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 anda second resistor, R2, in parallel to the base-emitter pathcompletes the base divider circuit. The base divider provides
fine tuning and better turn-off characteristic behaviour.

45R1
23Figure 1: Two common forms of RETs, two resistors and a transistor
are integrated with a single NPN transistor (a), dual version with twotransistors and four resistors (b).

The RET structure is very similar to the well-understoodstandard BJT. Figure 2 shows the resistance ‘meander’created using a standard device processing step where apolysilicon layer is deposited on the die and then etchedback, structured and patterned to form the integrated
resistor layer. Figure 3 shows a cross section of a 100 mAautomotive RET.

3) Nexperia RET portfolio
Nexperia offers two types of RETs: so-called performancebased 600 mA 40 V RETs; and standard devices which areavailable as 50 V devices and 80 V parts which have recently
been uprated, primarily to cope with new 48 V automotivesystems.
600 mA RETs are based on the company’s performancebased, 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 on, even athigh load currents. Also, the residual voltage across thisswitch and in the on-state is lower compared to a standardtransistor, so overall they achieve a much better power
efficiency. Die size can also be reduced.
When in use, it is important that heat generation in theload path transistor is minimized, so again, it is useful if thetransistor has a good current amplification, which does
not degrade and improves with temperature over the loadcurrent. As an example, Figure 4 shows the performanceof Nexperia’s PBRN113ZT 600 mA, 40 V R1/R2 1 kΩ/10 kΩ
NPN performance-based RETs.
006aab010

104

Figure 2: The resistance ‘meander’ is created using a standard
device processing step.

hFE103
B
31E42

5nn+

102
7p+

6p

8

10
9p+
10

1. Si_Nitride passivation layer
2. Al contact metallization
3. Poly-Si layer
4. Plasma oxide
5. Thermal oxide
6. Doped emitter region
7. Doped base region
8. Epitaxial layer (collector)
9. Doped silicon substrate
10. Backmetallization

Figure 3: Cross-section of a 100 mA automotive RET.

110-1

110102103
IC (mA)

VCE = 5 V
Tamb = 100 oC
Tamb = 25 oC
Tamb = -40 oC
aaa-033227

Figure 4: Performance of PBRN113ZT 600 mA, 40 V R1/R2 1 kΩ/10 kΩ
NPN RET.

Another benefit of the Nexperia technology is the relative
tight tolerance of the R1/R2 ratio of the integrated resistors
for RETs with current values above 100 mA, which at +/-10%
are three times more accurate than other products. This
enables the input voltage safety margin to be reduced.

3RETs

4) Design considerations
While RETs offer many significant benefits, some importantdesign considerations should be noted.
VCER1/R2

VI(on)minVI(on) typVI(off)typVI(off) max

RET Type

10k/10K

2.5

1.8

1.15

0.8

NHDTC114ET

22K/22k

3

2.3

1.15

0.8

NHDTC124ETNHDTC144ET

47k/47k

5

3.3

1.15

0.8

2.2k/47k

1.2

0.81

0.595

0.5

NHDTC123JT

4.7/47k

1.4

0.95

0.625

0.5

NHDTC114YT

10k/47k

1.6

1.22

0.690

0.5

NHDTC143ZT

Table 1: Dependency of the on- and off-state input voltages on theresistor divider configuration.

The selection of a proper resistor divider option is also
important to ensure that the control voltage window of
the RET matches the driving stage.

OﬀOn
VI(oﬀ)max

VI(oﬀ)typVI(on)typInput voltage

VI(on)minaaa-033151

Figure 5: Switching characteristic of a RET.

Figure 5 shows the switching characteristic of a RET plotting
VCE versus V i. V i(off) is the input voltage the RET turns off.
The condition required for an off-state is that the collector
leakage current must be 100 µA at a collector-emitter voltageof 5 V. For safe turn-off, V(ioff)max must not be exceeded.
When determining the on-state, V i(on)min must be considered.
The circuit controlling the RET has to provide at least thisvoltage for a safe turn. On-state is defined as a collector
current of 10 mA while the collector-emitter voltage is 0.3 V.
The datasheet V i rating is valid for the test condition defined
only. If a bigger collector current shall be switched the RET
requires more base drive voltage V i(on).

5) Applications
RETs are used in many automotive and industrial applications
due to their robustness. As mentioned, they provide a simple
method that can be used to switch and drive loads directlyfrom logic devices. Some circuit examples are shown inFigure 6. In Configuration 1, the NPN RET serves as a low-side
switch, enabling the resistive load or LED, or an inductive load
which is connected directly to the battery to be switched.
The important point to note here is that the voltage swingof the logic device such as a microcontroller is enough toswitch the RET. In Configuration 2, the NPN RET is now used
as a high-side switch in the control path, and is driving a PNP
RET with the load switched directly by the logic input suchas a microcontroller. Nexperia’s standard 50 V, RETs are tobe found in very many automotive applications. One typicalapplication is converting from a low voltage, such as 3-3.3 V
for a microcontroller to the battery voltage of 12 V.

V i < VI(off)max - a RET is in off-state for every device shipped.
V i < V i(off)typ - a typical RET is in off-state
V i > V i(on)typ - a typical RET is in on-state
V i > V i(on)min - a RET is in on-state for every device shipped.
It is also important to consider the same diagram at different
temperature ranges. BJT switches become more effective
as the temperature goes up, so they will safely stay on if
temperature becomes higher. That is suitable, for example,
in an autonomous vehicle application in a hot climate;
however, if the vehicle is required to operate in a cold climate
care has to be taken to ensure that the BJT stays in on-state,because at low temperatures the BJT gain goes down andthe BJT current gain goes down and the voltage drop VBE
increases, so a higher turn-on voltage is required.
Table 1 shows the dependency of the on- and off-state input
voltages on the resistor divider configuration in RETs from
Nexperia’s NHDTC series RETs.

Vcc = 12 V / 24 V 48 V

(1)

npn RET

Logic input
e.g. uController(2)

Vcc = 12 V / 24 V 48 V

pnp RET

Logic input

npn RETe.g. uController

Figure 6: Two circuit examples using RETs:
Low-side switch (1), high-side switch (2)4RETs6) New 80 V automotive RETs
Whilst in conventional piston-driven cars, the battery voltage
can be 12 V, it can also be as high as 24 V or 25 V in trucks andup to 48 V in mild hybrid cars. Therefore, with the uptake inelectric vehicles with a 48 V board net, much more headroom
than can be provided by 40 V or 50 V RETs has become
necessary. To meet this need, Nexperia has introduced 80
V RETs to provide the safety margin required to switch theloads at these higher voltage board nets.
There is another driver. Automotive customers are now
demanding that devices meet pulse tests in accordance withISO 7637-2:2011, the standard that defines robustness against
transients along the power supply bus. At the behest of amajor Tier 1 supplier to the EV industry, Nexperia’s 80 V RETs
have been tested and withstand the required 20 V acrossbase-emitter junction. This is vital for 24 V systems in trucks
and lorries, and 48 V automotive systems.

7) Conclusion
Nexperia has many millions of RETs delivering safe, reliable
and efficient switching in automotive and many otherapplications. The company’s new range of AEC-Q101certified 80 V products has the performance, robustness
and efficiency required to provide safe operation in 48 V
board net e-mobility applications. With the move towards
autonomous vehicles, reliable switching systems are going to
be required in even more automotive applications. 80 V RETs
provide the solution.
To find out more, visit
www.nexperia.com/rest

About Nexperia
Nexperia is a leading expert in the high-volume
production of essential semiconductors that arerequired by every electronic design in the world. Thecompany’s portfolio includes diodes, bipolar transistors,
ESD protection devices, MOSFETs, GaN FETs andanalog & logic ICs. Headquartered in Nijmegen, theNetherlands, Nexperia annually ships more than
100 billion products, meeting automotive standards.
These products are recognized as benchmarks inefficiency – in process, size, power and performance —with industry-leading small packages that save valuable
energy and space. Nexperia has over 12,000 employees
across Asia, Europe and the US.
Find your regional sales office here© 2021 Nexperia B.V.
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Date of release:
August 2021