Today’s analog foundries can integrate sensors with highly optimized analog and digital functions on a single chip. It’s possible to sense physical phenomena such as pressure, temperature, magnetic field, and light, digitize them, and apply signal processing, all in one system-on-a-chip, or SoC (Fig. 1).
This is a prime differentiator between conventional foundries and analog foundries. Conventional foundries provide state-of-the-art CMOS feature sizes, but they can only integrate and support a limited range of analog functional blocks.
For analog foundries that integrate light sensors in SoCs, the requirements for integrating optical sensors can vary for light sensitivity, supported wave length, bandwidth, and noise. Depending on the application, one or more of these factors could be important. Analog foundries can optimize their processes to match the requirements for one or more of these factors.
There are three light-sensitive devices in a CMOS process: photo diodes, photo transistors, and photo resistors. Photo diodes are commonly used as optical sensors. Silicon is light-sensitive, so every diode can be used as a photo diode to convert light into current. If light reaches the base collector junction of a bipolar transistor through a transparent case, a photo transistor can be created.
Photo transistors can be up to 100 times more sensitive than photo diodes, but they are slower in response time. For photo resistors, resistance is changed by light. Photo resistors often are used as discrete devices. Integration in a CMOS process isn’t common. Most applications use photo diodes in SoCs.
PHOTO DIODES AS OPTICAL SENSORS
Analog foundries can offer various diodes with different kinds of pn junctions (Fig. 2), doping profiles, and junction thicknesses, yielding different behaviors. Due to the modularity of foundry processes, several diffusions and well implants are available.
In X-Fab’s 0.6-µm CMOS process, the n-diffusion/p-substrate (phodn) and n-well/p-substrate (phodnw) configurations have a sensitivity peak at 700 nm. Maximum sensitivity is 0.35 mA/mW. The photo diode p-diffusion/n-well (phodp) has its highest sensitivity at 450 nm. Its overall sensitivity is lower than that of the p-substrate diodes.
PHOTO DIODE APPLICATIONS
Photo diodes are used in a wide range of applications. The availability of different doping profiles gives analog foundry users a wide selection of photo diodes for use in applications such as ambient light sensors. These sensors with standard diodes require high sensitivity for visible light. Silicon photo diodes are sensitive for visible and IR. However, the IR portion of the light should be suppressed.
An ambient light sensor (ALS) (Fig. 3) with human eye behavior is possible through a combination of two diodes. Diode dlw is sensitive for the spectrum from ultraviolet (UV) to infrared (IR) light. Diode dwplw is especially sensitive for IR light. By calculation, the IR part can be subtracted from the dlw sensitivity curve, resulting in a sensitive curve that very closely approximates the sensitivity of the human eye.
In contrast, a Blu-ray PIN diode must detect weak signals reflected from a disk surface. It requires high sensitivity and a high signal-to-noise ratio to do this because of the direct correlation between speed and bandwidth. The Blu-ray PIN diode needs high bandwidth to accommodate the current 8x speed requirement, which is expected to reach 12x in the near future.