New Chip Modularizes Both Vertical Cavity Surface-Emitting Laser and MOSFET

December 05, 2020 by Jake Hertz

VCSEL usually requires an external MOSFET driver, which causes parasitic impedances. Now, ROHM has announced new R&D that puts VCSEL and MOSFET elements on the same chip.

Vertical cavity surface-emitting laser (VCSEL) technology is a form of laser emanation that has become increasingly popular in recent years. This popularity is greatly due to the many applications that benefit from a VCSEL laser source technology, such as facial recognition in smartphones (notably used in iPhones).

Further, as car manufacturers push vehicles closer to full autonomy, interest in better light and imaging technology continues to grow. 


VCSEL vs. other semiconductor light-emitting devices

VCSEL vs. other semiconductor light-emitting devices. Image used courtesy of ROHM Semiconductor


In this article, we’ll take a look at why the technology is so valued and touch on some developments occurring in the field. 


Benefits of Working With VCSEL 

VCSEL is a semiconductor-based laser diode that is unique in that it only emits light vertically from its top surface. This is important because it increases the directionality of the emitted light. In contrast, LEDs produce light from the sides and top, and edge-emitting lasers (EEL) emit light from the side.


VCSEL layout

VCSEL layout. Image used courtesy of MyVCSEL


Since the light is perpendicular to the surface of a laser, thousands of VCSELs can be processed all at one time in a wafer as opposed to edge-emitting lasers, which require dicing the wafer and building the rest of the device to test. Put more simply, VCSELs are more easily testable, resulting in a more predictable and controlled yield with low fabrication costs compared to other laser technologies. 

Finally, the unique properties of VCSEL allow them to be applied in a 2-dimensional array with hundreds of individual light sources, increasing maximum output power and longer reliability. This type of array can scale the power output to application requirements.


Issues with VCSEL in Practice

Generally, a requirement for high-accuracy laser-based sensing is to have light outputs that are high power and of short pulse width. This is one of the places where VCSELs can suffer in practice

One noted issue with VCSEL technology is that generally, the laser requires a MOSFET driver, which is not integrated onto the same piece of silicon but is instead individually mounted onto the board. The resulting parasitic impedances between the devices then have undesirable effects on power output and drive time. This makes it difficult to achieve lasers with high output power and short pulses.


ROHM’s VCSEL Improvements

To address this issue, ROHM has recently announced its new VCSEL module technology, which modularizes VCSEL and MOSFET elements into a single package.


ROHM’s new VCSEL module

ROHM’s new VCSEL module. Image (modified) and used courtesy of ROHM Semiconductor

Integrating the two devices onto the same package effectively minimizes the wiring length between elements, and hence reduces parasitic impedance. ROHM claims these new products are capable of short pulse drive (under 10ns), reducing susceptibility to external noise. At the same time, the decreased parasitics have allowed these devices to achieve up to 30% higher output over conventional solutions. 

The result of this is more accurate spatial recognition and ranging systems, where time-of-flight applications receive more reflected light and hence more accurate results. By integrating a driving MOSFET on the same package as the VCSEL module, ROHM has found a way to offer higher performance and accuracy while also saving board space. 

ROHM plans on releasing its new VCSEL module in March 2021. Meanwhile, the company plans to continue developing high-output lasers for LiDAR in vehicles and other applications.