# Review of Photodetectors for Space Lidars
## 3 Space Lidar Detectors from Short-Wave to Mid-Wave Infrared
### HgCdTe APDs for Atmospheric Lidars  
HgCdTe is a semiconductor alloy with many properties suited for infrared photodetectors. It has a >90% quantum efficiency from SWIR to long-wave infrared (LWIR). The cutoff wavelength can be tailored by the HgCdTe composition. The dark current is lower than other types of photodiodes sensitive in this wavelength range. HgCdTe photodiode arrays have been used as imaging sensors for astronomy and infrared spectrometers for decades [42]. HgCdTe APD arrays became available in recent years [43; 44]. The APD gain can be as high as several thousand, which is sufficient for the photocurrent to completely override the electronic noise of the preamplifiers. The APD gain is almost deterministic because of the ballistic nature of the electron multiplication process and the much lower efficiency for hole multiplication. They have demonstrated near-quantum limited performance in analog operation [45]. The outputs of HgCdTe APDs are like those from silicon APDs but with much lower noise. The major disadvantage of HgCdTe APDs is the low operation temperature. The APD has to be housed in a cryo-cooler. Early HgCdTe APD arrays had a relatively slow time response. The read-out integrated circuits (ROICs) are separate from the APD array. The overall electrical bandwidth is too low for high-precision ranging but adequate for atmospheric backscatter and spectroscopic surface reflectance measurements. A set of 4 \(\times\) 4 pixel HgCdTe APD arrays were designed and fabricated by Leonardo DRS for the integrated path differential absorption (IPDA) greenhouse gas lidars supported by the NASA Earth Science Technology Office (ESTO) [45]. The HgCdTe APD arrays are based on the high-density vertically-integrated photodiode (HDVIP) technology, which has a cylindrical structure for each diode, as shown in Figure 4. Each pixel consists of four APDs connected in parallel. The pixel size is 80 \(\times\) 80 \(\upmu\)m at 80 \(\upmu\)m pitch. A guard band is placed around the pixel array, which is made of identical pixels electrically connected in parallel. The outputs from all the 4 \(\times\) 4 pixels are fanned out to the side of the detector chip and wire-bonded to 16-channel TIAs in the ROIC. The ROIC chip is placed next to the APD array on the same ceramic carrier. The spectral response of these HgCdTe APD spans from visible to 4.4 \(\upmu\)m. Figure 5 shows a plot of the measured QE vs. wavelength [46]. The QE here is defined as the ratio of the number of output photoelectrons to the number of incident photons at unity APD gain. At short wavelengths, when the photon energy is many times the HgCdTe bandgap, there  
Figure 4: Illustration of HDVIP HgCdTe APD layout and a photograph of the 4 \(\times\) 4 pixel HgCdTe APD array and ROIC on the chip carrier. can be more than one pair of electron-hole generated from a single photon [47]. This is why the QE shown in Figure 5 is slightly higher than 100% near a 1 \(\upmu\)m wavelength. Although this is an artifact resulting from our QE definition, the additional photocurrent from the extra hole-electron pairs are real. This definition of QE is widely used in the lidar equation to calculate detected signals. The ROIC for these HgCdTe APD arrays is made of silicon and fabricated using a standard silicon integrated circuit foundry service. The ROICs are custom-designed such that they can be tested at room temperature but used with the APD array at a cryogenic temperature. The side-by-side arrangement allows the APD arrays and ROIC chips to be fabricated and tested separately, which reduces the risks and cost of the device production. The electrical bandwidth of the TIAs is about 8 MHz with a 20 ns pulse rise time and 50 ns fall time. The ROIC can be configured as capacitive transimpedance amplifiers (CTIAs), which integrate the photocurrent onto a small capacitor and reset periodically. The CTIA mode can be used to measure the APD output currents to below femtoamperes.