Circuit design of a novel front readout circuit for SiC neutron detector with leakage current compensation

Radiation detectors have broad application prospects in environmental radiation monitoring, radiation source identification, biomedical imaging, space exploration, astrophysics, other radiation detection and imaging fields. Among the third generation semiconductors, the properties of 4H-SiC are more suitable for making high-temperature and strong radiation neutron detectors. SiC materials have become a current research hotspot. In this paper a special readout integrated circuit is designed for the extremely weak current pulse signal output by SiC trench neutron detector. A leakage current compensation circuit is designed for the influence of leakage current on charge sensitive amplifier(CSA) output baseline. The noise of CSA output is analyzed and the noise minimization is carried out. The circuit can quickly respond to the input signal and extract the energy and time information. The circuit is designed based on the DB Hitek 0.18μm CMOS process model. The layout design is achieved and the post-simulation is completed. Simulated results show that the equivalent input charge range of the system is about 2fC ∼ 20fC. The charge conversion gain is 88mV/fC. The nonlinear error is within 1.6 %. Moreover, the equivalent noise charge is 22e-. The circuit has the advantages of high gain, high linearity and low noise. Therefore, the circuit can effectively adapt to the characteristics of the charge signal output from SiC trench neutron detectors and amplify the output charge signal.

bias.As a result, the semiconductor detector is usually operated in the depletion voltage state and connected to the charge-sensitive preamplifieroutput during practical applications [4].Therefore, SiC neutron detector requires the optimized design of front-end readout ASIC in order to obtain the flux and energy spectrum of incoming neutrons.The analog front-end readout circuit determines the accuracy of the back-end data and is an indispensable key part of the whole neutron detection system.
In the last 10 years, several research institutes have developed readout ASICs for detectors.An ASIC chip for X-ray imaging was designed by S.Li et al.in Brookhaven laboratory using 0.13μm process.The single channel power consumption is only 0.6 mW, and the equivalent noise charge reaches 15e-.However, its maximum tolerable detector parasitic capacitance can only reach 100fF [5].In 2019,V.Emerson et al.developed an ASIC for the Frisch-grid detector.The linearity is better than 1%, and the energy resolution is 1.8%@238keV.However, its equivalent charge noise is 270e- [6].In 2021,Han Ning et al.from Xi'an Jiaotong University designed a readout circuit for 3D Si PIN array.The input dynamic range is 10fC ~8pC, and the integral nonlinearity is less than 1 %.The equivalent noise charge of the zero-input detection capacitor is 241.6e-[7].
At present, there are few reports on integrated circuit readout chips for SiC neutron detectors, and there is no special readout integrated circuit design for extremely narrow pulse and low charge output current pulse signals.Most of the research focuses on the preamplifier, lacking a special analysis of the amplification forming system and the monolithic integration of the readout system.
In this paper, a charge sensitive analog readout chip based on the amplitude pulse modulation of SiC neutron detector is designed according to the requirements of neutron detection system and the output signal characteristics of SiC neutron detector.The overall block diagram of the front-end readout circuit designed in this paper consists of a charge-sensitive amplifier (CSA), a leakage current compensation circuit (LCC), a pole-zero cancellation circuit (PZC), filters (CR-RC filter AND SK filter), a peak detect and hold circuit (PDH), and a time marking circuit (Discriminator).The CSA circuit is designed with low noise and leakage current compensation.The front readout circuit adopts the discriminator to detect and record the time information of incident neutrons.The discriminator generates the enable signal of PDH and the PDH exports the energy information of incident neutrons.The test results and the characteristics of the ASIC are discussed.Section2 and Section3 describes circuit techniques and prototype ASIC.Section4 gives electrical testing results.Section5 discusses concludes the paper.

2.CSA with Leakage Current Compensation
As mentioned above, the front-end readout circuitry deals with narrow pulse width and low charges signals.Since the output of SiC neutron detection is directly connected to the input of CSA, CSA is required to have sufficient excellent performance to process narrow pulse width and low charges signals.In a radiation detection system, the noise generated by the detector and CSA have the greatest impact on the overall detection system since they receive the greatest amplification.And the effect of noise is more pronounced for low charge input signals.So noise minimization for CSA is essential.At the same time leakage current can also have a serious impact on the input signal, so it is necessary to compensate leakage current.The feedback capacitance and bleeder resistor of the CSA circuit should be appropriately valued so that the CSA responds quickly enough to narrow pulse signals without attenuating the output signal.

Noise Minimization
Random electronic noise affects the accuracy of the comparison of the shaper output pulse amplitude with a set threshold, furthermore degrades the energy resolution of a detector.Noise is a very important evaluation indicator for a radiation detector front-end readout circuit.Noise is usually expressed as Equivalent Noise Charge (ENC) in radiation detection systems.The ENC is defined as the ratio of the total rms noise at the shaper output to the pulse amplitude due to one electron of the input charge.To calculate the noise in the circuit discussed, it is necessary to define the voltage (series) vn 2 and current (parallel) in 2 noise sources referred to the CSA input -see Fig. 1 [8].
When the gain of the preamplifier is sufficiently high, the noise in the detection system is predominantly influenced by the noise from the CSA input transistor.The voltage component of the noise can be represented by vn 2 .
Where k is the Boltzmann constant, T is the absolute temperature, γ is the channel inversion level coefficient ranging from 1/2 in weak to 2/3 in the strong inversion region, Kf is the flicker noise coefficient, Cox is the gate capacitance per unit area, W and L are the CSA input transistor gate width and length.
The current i2 n noise component can be expressed as follow.
Where q is the elementary charge, Id is the leakage current of the detector and CSA input tansisitor, RD is the load resistance of the detector, Rf is the feedback resistance of CSA.
The total noise at the CR-RC shaper output is calculated according to the following formulas.
Formula 5 can be obtained by calculating Formula 4.
  The ENC in the radiation detection system can be expressed as follow.
Where Vom represents the peak output amplitude when Qi=q.The noise performance of the circuit can be expressed as follow.
  Analysis using Equation 7shows that the shaping time tp of the shaper filter circuit is directly proportional to in 2 and inversely proportional to thermal noise.By performing calculations, an optimum shaping time can be determined that minimises the ENC.The dimensions (W and L) of the CSA input transistor are also related to system noise.These dimensions determine the transconductance (gm) and gate capacitance of the input transistor, so special attention must be paid to the CSA input transistor.In addition, increasing the feedback resistance (Rf) of the CSA also can reduce system noise, but at the same time reduce the overall maximum count rate of the system.

CSA Core Amplifier
The CSA Core Amplifier designed in this paper is shown in Fig. 2, and the equivalent noise at the input can be expressed as follow.
  Where vn1 to vn6 are equivalent voltage sources in series with M9, M10, M7, M8, M11 and M12 respectively.Equation 8indicates that for the folded common-source common-gate amplifier to have minimum noise, the transconductance of the input tubes M9 should be maximised.This ensures that the noise of the amplifier is determined solely by the input tubes.
CSA core amplifier of this paper is a large size PMOS transistor ( M9 ) as the input tube.It has good shielding performance, large output swing, high operating bandwidth and less introduced noise.In addition, dual power supply is used in order to avoid the influence of power supply noise.The source of the M9 tube is connected to the independent GND to reduce the noise and enhance the driving ability.Finally, the width-length ratio of M9 is 800μm/500nm.In order to obtain a higher low frequency gain, increase the current of M9 and reduce the current of M7 branch without increasing the power consumption of the system or changing the size of the device.M13, M14 and M16, M17 form a dual output source follower.The CSA circuit not only has a higher processing speed, but also achieves the purpose of cutting off the communication path with the subsequent circuits on the basis of ensuring the noise performance of the overall system [9].
Where RM8 and RM10 are the on-resistances of transistors M8 and M10 respectively, gm8 is the transconductance of M8, Rpz is the resistance in the subsequent pole-zero cancellation circuit.It can be seen that the influence of the subsequent circuitry on the CSA input is significantly reduced.The drain resistor occupies a very large area in the layout and has a low level of integration.The on-state impedance of the MOS tube is highly affected by the process and has a low gain linearity.In this paper, a high impedance circuit (HRC) is used instead of a resistor-see Fig. 4. The resistance value can be expressed by Formula (10).
HRC solves the problem of layout area caused by the circuit resistance value is too large.It will replace the resistor in parallel with the input and output terminals of CSA to realize the function of discharging charge.

Leakage Current Compensation
Leakage current entering the input end of CSA will affect the baseline position of the output end of CSA.Especially for the narrow pulse and low charge input signal.In severe cases, it will make CSA unable to work.In order to eliminate the influence of leakage current on CSA, it is necessary to add an adaptive active feedback circuit to the feedback network of CSA to compensate the leakage current.
Fig. 5 shows an adaptive active feedback circuit with CSA.The equivalent resistance of the circuit is: Where gm=gm1=gm2,gm1 and gm2 are the transconductance of transistors M1 and M2 in Figure 2-5.
The small signal analysis of active feedback circuit is carried out.
The transfer function of CSA can be obtained by formula (15).Let the right denominator of formula(16) be equal to 0.Two poles of CSA can be obtained.
In order for CAS to operate stably, the two pole frequencies of CSA should be separated as far as possible.In other words.
Discharge for C0 by Rf when RLCC>>Rf.Active feedback circuits only compensate for leakage current.The completed version of the circuit for compensation of leakage current is displayed in Fig6.The gate of M2 is linked to the CSA output, and the gate of M1 is set with the fixed voltage VREF.The CSA output voltage baseline can be maintained within a certain range of VREF value.The baseline of CSA's output voltage will rise when leakage current occurs at the input of CSA.The feedback loop of the leakage current to M2 leads to a decrease in the source-drain voltage of both M2 and M4, resulting in the charging and discharging of capacitor C.After charging, the current of the M3 tube decreases.This decrease in M3 current compensates for the increase in leakage current and minimises the impact of leakage current on M1 and M2, thus ensuring that the output voltage baseline of CSA remains relatively stable.

3.The Overall Circuit Structure Diagram
After the signal is pre-amplified through CSA, the output of CSA also necessary to extract the time and energy information of the input signal through the subsequent circuitry.For the application of a single channel radiation detection system, the function of adding a time channel is to filter out signals with lower amplitude by adjusting the comparative voltage Vref of the discriminator, so that the back-end data processing module only processes effective signals.In the radiation detection system,it is usually necessary to perform analog-to-digital conversion on the analog signal output from the front-end readout circuit in order to meet the width requirements of the ADC for the input signal, a PDH circuit must be added to the front-end readout channel to sample and maintain the amplitude information of the output signal.
The final design of the overall circuit structure is shown in Fig. 7.The output of the SiC neutron detector after coupling is connected to the input of CSA to complete the initial amplification of the signal and convert it into a voltage signal.The leakage current compensation circuit adjusts the baseline of the charge sensitive amplifier.The CR-RC rapid prototyping filter filters and amplifies the front-end output signal and further improves the signal-to-noise ratio.The time constant of CSA does not match the forming time of CR-RC shaping filter.It is necessary to adopt the pole-zero cancellation circuit to generate a new zero point.The new zero point offsets the pole of CSA to eliminate the overshoot.The first signal passes through SK-2 filter effectively reduces the width of the output waveform, so that the overall output is not easy to distort due to the accumulation of signals.It is beneficial to improve the overall counting rate of the readout system.Finally, the energy information of the output signal is output by the peak holding module.The second signal passes through the time discriminator to output time information, and the generated holding signal provides appropriate clock control for peak holding.
The layout of the circuit is shown in Fig. 8 and the area of the layout is 825 μm×445 μm.All modules in the layout design process minimize the noise as much as possible.

4.Result and Discussion
The circuit is designed based on the DB Hitek 0.18μm CMOS process model.Parasitic parameters were extracted and post-simulated using Cadence Spectre software.The output signal of the SiC trench neutron detector is replaced by a negative current pulse with an exponential decrease and the current amplitude of 0.2μA-4μA.When the leakage current flowing into the CSA input is 0 to 100nA, the change of the CSA output baseline is shown in Fig. 9.It can be seen that without an LCC circuit, the baseline variation of the CSA output voltage is very large, while with the addition of an LCC circuit, the baseline fluctuation of the output voltage is very small.Fig. 10 shows the CSA output with 2ns input current pulse width and 2μA pulse value.The circuit achieves a significantly fast time to peak of only 8ns.The output baseline of CSA is kept around 0.
Fig. 11 illustrates the output for each component of the circuit.The output waveform of the SK filter is close to the Gaussian waveform.The Gaussian waveform is beneficial to improve the readout speed [10].The output of discriminator is a square wave signal because the time discriminator is essentially a high speed and high resolution voltage comparator.The output of the discriminator marks the time information of the signal.Fig. 11 shows the width of the discriminator output waveform is approximately the same as the width of the Sk2 output waveform.The output of PDH shows that the peak value of the signal is maintained for a certain time after PDH processing.The holding signal of PDH comes from the output signal of the discriminator processed by the analog broadening circuit.The holding time should not be too long, otherwise the overall technical rate of the channel will decrease.
Fig. 12 displays the output at different pulse current values.When the input current signal amplitude is 0.2uA, the corresponding charge is about 2fC.When the input current signal amplitude is 4uA, the corresponding charge is about 20fC.It can be seen the conversion gain of the whole circuit is 88mV/fc from Fig. 12.The maximum nonlinear error is less than 1% as calculated.The root mean square of the output noise voltage amplitude is 378uV by simulation.According to formula (6), it can be calculated that ENC is 22e-.Q in Figure 12 represents input current signal amplitude.

Figure 1 .
Figure 1.Noise equivalent model The noise at the CSA output can be suppressed through subsequent filtering shaping circuits.The filtering shaping circuit in this article is a CR-RC shaper.Assuming the peak time of the shaper's output signal is tp, and its transfer function can be expressed.

Figure 2 . 3 .
Figure 2. CSA core circuit schematics Figure 3. Local small-signal model of the CSA circuit Fig.3 shows the local small-signal model of the CSA circuit.The influence of signal interference at the output end on the input end is:

Figure 5 .
Figure 5. Adaptive active feedback circuit with CSA Figure 6.LCC circuit schematics

Figure 11 .
Figure 11.Component output when Q=2uA Figure 12.Output with equal input interval 5.conclusionAn analog front-end readout circuit based on SiC neutron detector is designed in this paper.The influence of leakage current on the circuit is greatly reduced due to the addition of LCC circuit to the feedback loop of CSA.The circuit design is optimized to ensure the linearity of CSA.The post-simulation results show that the charge gain of the channel is 88mV/fC, the peak time of the signal is only 8ns, and the equivalent input noise is only 22e-.The maximum nonlinear error is less than 1%.The simulation results indicate that This design can effectively read and amplify the output current pulse signal of the trench SiC neutron detector.It can play a huge role in detecting radiation events when paired with SiC neutron detectors.