Research On The Characteristics Of H-terminated Diamond Field Effect Transistors

Diamond is an ultra-wide band gap semiconductor material.Due to its high breakdown field,high thermal conductivity,and high carrier mobility,it is considered as the ultimate semiconductor and can be applied to high temperature,high frequency,and high power devices.The N-type and P-type doping of diamond is difficult to be activated at room temperature,while two-dimensional hole gas can be induced in the hydrogen-terminated diamond?H-diamond?at room temperature to obtain P-type surface conductivity.Therefore,the core structure of current diamond electronic devices is H-diamond field effect transistors?FETs?.In the field of H-diamond devices,there are still many problems that need to be overcome,including how to improve the device stability,2DHG concentration,high temperature performance,and so on.Reports of the domestic research on diamond electronic devices are still few.The author carried out the preliminary research of H-diamond FETs.The main research results are as follows:1.A free-standing 2-inch,high-quality polycrystalline diamond film was grown on a molybdenum substrate by using a microwave plasma chemical vapor deposition?MPCVD?system.2.The optimal conditions for hydrogen termination process for diamond was found based on hydrogen plasma treatments on many batches of polycrystalline diamond films in MPCVD.It was found out the relationship between treating time and the diamond surface roughness,and that under the same process conditions,high-purity diamonds have a lower surface resistance,so high-quality diamond films are a necessary condition for the preparation of high-performance H-diamond devices.3.The H-diamond metal-semiconductor field effect transistors?MESFETs?were successfully prepared based on high-quality,large-area polycrystalline diamond thin films.Gold and aluminium were used to form ohmic contacts and schottky contacts on H-diamonds,respectively.The devices with 2-?m and 4-?m gate lengths demonstrated wide high-transconductance region and low on-resistance,and the characteristics meet the international level.It is found that the high channel carrier concentration of 1.56×10¹³cm^-2 and the high level mobility of 170 cm²/?Vs?over a wide range of gate voltage resulted in the wide high-transconductance region and low on-resistance.The broad transconductance peak is beneficial to achieve high frequency characteristics and highly linear microwave power amplification.The cutoff frequency?f_T?and the maximum oscillation frequency(f_max)of the MESFET with a gate length of 2?m are 1.5 GHz and 3.3GHz,respectively.4.The temperature-dependent characteristics of the H-diamond MESFET device with a gate length of 2?m prepared on domestic polycrystalline diamond were tested and analyzed.It was found that the device with no surface passivation degraded at 200°C.It may be due to that the desorption of charged ions adsorbed on the H-diamond surface at high temperature,which could lead to a decrease in the density of the 2DHG in the channel and therefore the degraded device performance.Finally,the MESFET devices was passivated by the transfer-doping dielectric MoO₃,and the output characteristics of the MESFET device were measured repeatly for three times.It was found that the device showed good stability in this iterative current-voltage sweeping after passivation.In summary,in order to obtain high-performance H-diamond devices,high-quality diamond films are required at first,and then a good hydrogen plasma treatment process is necessary to realize high-concentration and high-mobility 2DHG,and appropriate metals as the materials for ohmic contact and schottky contact.In this paper,the condition of hydrogen termination treatment was investigated,high performance long-channel H-diamond MESFETs were prepared.The high-temperature degradation of the devices was observed,and it was verified that the surface passivation of devices by MoO₃ can improve the device stability.