Design Rationale And Experimental Study For Highly Sensitive Tera Fet Detectors In Commercial CMOS Technology

Terahertz(THz)wave is widely used in industry,medical treatment,security check and the study of basic science on account of its unique advantages and promising proscpect.As the most crucial part of THz technology,highly sensitive and fast response detectors which can be used in room temperature catch more attention of scientists and the development of them has great significance in various fields of the successful application.Up to now,there are three kinds of detectors that are mainly used: thermal detectors,electrical detectors and optical detectors.Compared with the other two,electrical detectors are more interested by people due to their faster response,working without additional cooling system and unaffected by the low energy of THz phonon.Searching for cost-efficient devices for THz range,the charge controlled model was proposed by Dyaknov and Shur in 1996 and it proved that field-effect-transistor(FET)could be applied in frequencies far above their cutoff frequency for rectification,mixing and multiplication by the excitation of plasma waves in the transistor's channel.Thus THz FET(Tera FET)detectors have been the new exciting and promising area for THz electrical detectors.This thesis mainly focuses on the design rationale and experimental study for highly sensitive Tera FET detectors in commercial CMOS technology,including the study of theoretical model for Tera FET detectors,design methods for the antenna,development of the simulation system for Tera FET detectors model,the characterization and analysis for detectors with different design methods and the design rationale for highly sensitive Tera FET detectors.The details of the study are as folllows.1.The study of theoretical model for Tera FET detectors.In order to solve the problem of neglecting the diffusion effect in the classic kinetic theory model,this thesis developed a CMOS Tera FET Resistance-Capacitance-Inductance(RCL)model based on Boltzmann theory.And the role of diffusion motion in Tera FET model was studied,especially in conductance and responsivity.Then the differences of changing with temperature and frequency between the two models were given.Combined with 3? rules,the conditions to neglect the diffusion motion in the model were put forward.The results show that diffusion motion of the carrier density is caused by the inhomogeneity of the carrier density,which is affected by temperature and the working frequency.Here frequency plays the main role in the diffusion,while the effects caused by temperature is much smaller.For the increasing frequency,the diffusion part can be neglected when it is still below 1 THz,while the diffusion part can't be neglected when the working frequency is above 1 THz.The transistor model should contain three physical procedures of drift,scattering and diffusion part at the same time.2.Research on antenna design rationale for Tera FET.The effects of the substrate,the environment and the structure of the antenna were studied according to the measured and simulated data.The results show that the asymmetry of the environment and the structure of the antenna can both cause the tilt angle of antenna radiation pattern.In addition,the neighbor units can also affect the directivity of the antenna.Thus the symmetric structure and the wider distance between each unit should be considered in the antenna design.3.Developing a simulation system for Tera FET performance.A model for Tera FET was developed with the software of ADS based on the RCL model theory.Combined with antenna data from CST,the whole detector model was built and it could be used in detectors performance tests before manufacture.This kind of model and simulation system can solve the problems of the commercial model,which will lose accuracy in high frequency.In addition,the modular design of the model can offer an effective method for the further theoretical analysis.4.Analysises of the characterization and performance for Tera FET detectors with different design methods.The Test system for antenna radiation pattern and detector performance tests were built first,then the parameters that would affect the responsivity and the sensitivity of the detectors were measured and analyzed,including the size of the transistor,the choice of the materials and the wires on the chip.At the same time,the samples with the classic design method and the back-coupled design method were both tested.The results show that the smaller channel length is beneficial to the improvement of the sensitivity,but the choice for the channel width needs the specific analysis according to the antenna impedance.For the two design methods,the back-coupled way is more suitable for the Ga N detectors,while the classic design is still the better choice for the CMOS technology.The best sensitivity got in this thesis for CMOS Tera FET is 10 p W/ Hz.In addition,the measured data show that the wires on the chip will affect the couple ways of the detector.Thus good connection ways of the wires should also be considered besides the antenna form.5.Study on the design rationale for highly sensitive Tera FET detectors in commercial CMOS technology.Started from the parameters that affect the detectors sensitivity and based on the theoretical analysis and the measured data,the design rationale were studied in the aspects of transistor design,antenna design,the matching situation between these two parts and the environment around the antenna.The research results show that CMOS technology and the size of the transistor are the crucial points in transistor design for the transistor part.When the technology is settled down,the smaller channel length will help to get a better sensitivity,and the channel width will be fixed together with the antenna impedance.For the antenna,efficiency and the impedance are the main points.Higher antenna impedance will increase detector responsivity,but it will decrease the efficiency at the same time.Symmetric structure and proper environment will help to decrease the tilt angle of the radiation pattern and make detector have a better performance for the normal incidence THz waves.In addition,ESD protection at both the gate and drain terminals is necessary for the stability of the detector.The appearance of this kind of design rationale can offer great technical support for Tera FET detectors in commercial CMOS technology.