An Overview of CMOS Photodetectors Utilizing Current-Assistance for Swift and Efficient Photo-Carrier Detection

Abstract

This review paper presents an assortment of research on a family of photodetectors which use the same base mechanism, current assistance, for the operation. Current assistance is used to create a drift field in the semiconductor, more specifically silicon, in order to improve the bandwidth and the quantum efficiency. Based on the detector and application, the drift field can be static or modulated. Applications include 3D imaging (both direct and indirect time-of-flight), optical receivers and fluorescence lifetime imaging. This work discusses the current-assistance principle, the various photodetectors using this principle and a comparison is made with other state-of-the-art photodetectors used for the same application.

Keywords: CAPD; CMOS; Geiger; avalanche; current-assistance; demodulation; fast time-gated cameras.

Figure 1

Absorption percentage in Silicon as a function of wavelength for different epilayer thicknesses (a) and absorption percentage in silicon as a function of epilayer thickness for a few wavelengths (b) (Absorption coefficient data from [27]).

Figure 2

Cross-section of a simple p-i-p structure (a), the potential as a function of X (at any point in Y) (b) and the corresponding conduction and valence band energies with overlaid illustration of current-assistance operation (c).

Figure 3

Cross section of the example structure to demonstrate current-assistance principle with an overlay of simulated current flowlines.

Figure 4

Simulated timing response of the device when illuminated with a sharp light pulse (10 ps) of 850 nm for a few different ring voltages.

Figure 5

The simulated conduction band of a p-i-p photodetector shows how photogenerated minority charge carriers can be trapped and the cross-section of the p-i-p detector (inset).

Figure 6

The layout and dimensions of the fabricated current-assisted photodiode. The center cathode n⁺ region is 1.5 µm in diameter, keeping 0.75 µm space with the anode p⁺ region. The ring is biased to a negative bias voltage.

Figure 7

The simulated photo-gain and bandwidth for varying anode p⁺ region cross-section widths. The width can be used to trade bandwidth for photo-gain.

Figure 8

Measured and simulated anode and cathode current (a) and responsivity (b). V_anode = V_cathode = 0 V, V_ring = V_substrate = −5 V.

Figure 9

The normalized frequency response of the anode and cathode current at an average optical power of 360 µW, V_anode = V_cathode = 0 V, V_ring = V_substrate = −5 V.

Figure 10

3-transistor (3T) 2 tap iTOF CMOS pixel architecture.

Figure 11

iTOF sampling for a 4 tap pixel (a) and a pseudo 4 tap pixel using 2 tap pixel (b). The * sign denotes the convolution in Equation (5).

Figure 12

CAPD with a ring contact without a top polysilicon electrode (a), with a top polysilicon electrode (b), with a donut electrode (c,d) illustrates electrostatic potential under STI for device (a,e) illustrates electrostatic potential under STI for device (b,c) with a negative bias applied to the top electrode; (f–h) show measurement results of CAPD windowing function $$rec{t}_{\mathrm{CAPD}}$$ for (a–c) respectively.

Figure 13

Fluorescence lifetime (τ) definition (a) 2-window gated lifetime measurement (b) and (practical implementation of the 2-window method for a gated camera (c).

Figure 14

CAPS principle showing a top view of a CAPS sensor with detection node and four drain nodes with indication of drift field (a, left) and top view of the layout with dimensions indicated (a, right). CAPS cross section when the gate is OFF and when the gate is ON (b) and associated potential distributions at a depth of 1.3 µm (c).

Figure 15

Typical operation of a CAPS sensor showing the sensor substrate contacts and 3T readout circuit (a) and gating control signals (b). The layout of the CAPS sensor with readout circuitry is shown in (c).

Figure 16

Gating behavior of the CAPS sensor in DC (a) and instrument response function (IRF) for a 4 ns gate (b).

Figure 17

Micrograph of the 32 × 32-pixel CAPS array (a) and camera setup (b).

Figure 18

In vivo fluorescence experiment using a CAPS fluorescence-lifetime camera. A mouse bearing a xenograft tumor with EGFR expression and injected with an anti-EGFR nanobody-based NIR fluorescence contrast agent (a). Both tumor and kidneys light up under conventional fluorescence imaging (b). CAPS fluorescence-lifetime imaging reveals a fluorescence lifetime difference between the tumor and kidneys (EMIM 2020) (c).

Figure 19

Cross section of CA-SPAD-1, lines with arrows indicate the direction of photo-electrons, dotted line represents the boundary of the depletion region (a) and top-view of the layout of CA-SPAD-1 with dimensions indicated (b).

Figure 20

Light emission test to demonstrate reverse breakdown occurring at sharp corners at V_ex= 2.1 V (a) and V_ex= 2.8 V (b).

Figure 21

Cross section of CA-SPAD-2, lines with arrows indicate the direction of photo-electrons, the dotted line represents the boundary of the depletion region (a) and top-view of the layout of CA-SPAD-2 with dimensions indicated (b).

Figure 22

Light emission from “central SPAD” area at V_ex= 1.5 V (a), V_ex= 2 V (b) and V_ex= 2.5 V (c). The light emission is false-colored red.

Figure 23

Dark count rate as a function of excess bias voltage for CA-SPAD-2 (a), Timing response for different wavelengths for CA-SPAD-2 (b), Inter-avalanche histogram to characterize after-pulsing probability for CA-SPAD-2 (c), and photon detection probability (PDP) as a function of wavelength for CA-SPAD-2 (d).