High temperature characteristics of bipolar mode power JFET operation

The high-temperature operating characteristics of high-voltage JFET devices operated in the bipolar mode are evaluated. Good forward blocking capability with no degradation in blocking gain is observed at up to 200°C. The on-resistance and gate turnoff time of the devices was found to double from 25°C to 200°C, and the current gain was found to decrease by 30 percent. Despite the increase in gate drive requirements with increasing temperature, these devices should still be attractive for high-speed power switching applications because their on-resistance per unit area is at least 10 time lower than that of the power MOSFET.     

The asymmetrical field-controlled thyristor

A new device structure has been developed for field-controlled thyristors. In this structure, the uniformly doped n base of the conventional device has been replaced with a very lightly doped region near the gate and a more heavily doped region at the anode. This change in the base doping profile results in a significant improvement in the tradeoff between the forward-blocking voltage capability and the on-state forward-voltage drop. In addition, the high-resistivity region around the gate area allows the device to pinch off anode current flow at zero gate bias due to the built-in potential of the gate junction. The devices can, however, be triggered to the on-state by applying a small forward gate voltage and exhibit a forward voltage drop in the on-state which is much lower than that of conventional devices. The high resistivity of the channel area between gates also results in these devices having dc blocking gains in excess of 60, which is the highest value achieved in devices of this type. Further, because these devices have been fabricated using conventional planar processing techniques, this structure is suitable for high-volume production with high processing yields.     

Electron irradiation of field-controlled thyristors

The influence of 3-MeV electron irradiation upon the characteristics of asymmetrical field-controlled thyristors has been examined for fluences of up to 16 Mrad. In addition to the lifetime reduction due to the radiation damage, carrier removal effects have also been observed in the very lightly doped n-base region of these devices. The leakage current, even after radiation at the highest fluenee, is not significantly increased and the blocking characteristies of these devices are not degraded. In fact, a small improvement in the blocking gain has been observed at low gate voltages. The electron irradiation has been found to increase the forward voltage drop during current conduction and to reduce the forced gate turn-off time. Gate turn-off times of less than 500 ns have been achieved by irradiation with a fluence of 16 Mrad. However, this is accompanied by a large increase in the forward voltage drop. Tradeoff curves between the forward voltage drop and the gate turn-off time have been obtained. From these curves, it has been determined that gate turnoff times of 1 µs can be obtained without a significant increase in the forward voltage drop for devices capable of blocking up to 600 V.     

The dV / dt capability of field-controlled thyristors

A detailed analysis of thedV/dtcapability of field-controlled thyristors is presented. It is demonstrated for the first time thatdV/dtinduced turn-on can occur in these devices due to gate debiasing as a result of capacitive gate current flow if a large series gate resistance is present in the circuit. A theoretical analysis of thedV/dtcapability is presented based upon this mechanism which predicts that thedV/dtcapability will decrease inversely with increasing gate series resistance at low values and become independent of the gate series resistance at very high values. The quantitative calculations of thedV/dtcapability that have been made by using this theory are in very good agreement with measurements taken on asymmetrical field-controlled thyristors fabricated from wafers of various thickness. The results obtained in this study allow the conclusion that thedV/dtcapability of field-controlled thyristors are superior to that of conventional thyristors.     

A high-voltage, high-temperature reverse conducting thyristor

This paper describes the structure and electrical characteristics of a silicon p-n-p-n inverter switch which has been fabricated to demonstrate the feasibility of obtaining both high-voltage and high-temperature capability in the same device while still maintaining reasonable dynamic properties. It was realized that in most inverter systems, feedback rectifiers are required, and thus the reverse blocking capability of a thyristor is not utilized. This was used to advantage in the design of a thyristor by integrating an anti-parallel rectifier into a conventional thyristor structure. Although this approach results in a thyristor having a negligible reverse blocking capability in comparison to commercially available power thyrisitors, the potential advantages of lower conduction losses, high maximum junction temperatures, and integrated antiparallel rectifier may be attractive for use in inverters. Reverse conducting thyristors designed in this fashion have been operated with reasonable turnoff times at a forward blocking voltage of 1000 volts while at case temperatures of up to 230°C.     

Breakover phenomena in field-controlled thyristors

The occurrence of breakover in the forward-blocking anode current-voltage characteristics of field-controlled thyristors is analyzed. It is demonstrated that this breakover occurs due to gate current flow which causes a debiasing of the gate potential in the presence of any series gate resistance. Based upon this mechanism, analytical expressions have been derived which describe the anode current-voltage breakover characteristics. These theoretical expressions have been found to be in very good agreement with the measured data obtained from asymmetrical field-controlled thyristors fabricated with various device thicknesses. From the analysis presented in this paper, it can be concluded that the occurrence of breakover in the forward-blocking characteristics of field-controlled thyristors can be minimized by ensuring low series gate resistances which biasing the devices and by using gate bias voltages well in excess of the minimum value required to block any desired anode voltage.     

Vertical channel field-controlled thyristors with high gain and fast switching speeds

A new, planar, surface-grid, field-controlled thyristor (FCT) structure is described. The structure is fabricated by using orientation-dependent (preferential) etching and selective vapor epitaxial growth to obtain vertical grid walls. The resulting high channel-length-to-width aspect ratio produces devices with high blocking gains and fast gate turnoff speeds. Devices have been fabricated with the capability of blocking more than 1000 V with an applied grid bias of 32 V, and simultaneously exhibiting a low forward voltage drop in the on-state. These surface-grid devices exhibit gate turnoff capability with turnoff times of less than 500 ns at a rated cathode-anode current of 1 A.     

高压高阻断增益的水平沟道场控晶闸管

水平沟道场控晶闸管(简称LFCT)是由垂直沟道场控晶闸管发展而来的,它具有开关速度快、与集成电路工艺兼容等特点。我们采用刻蚀V型槽来代替栅扩散,已制成最大正向阻断电压为200V,可关断电流为2A的LFCT,其正向电压阻断增益达40～200。     

High current density 800-V 4H-SiC gate turn-off thyristors

4H-SiC gate turn-off thyristors (GTOs) were fabricated using the recently developed inductively-coupled plasma (ICP) dry etching technique. DC and ac characterisation have been done to evaluate forward blocking voltage, leakage current, on-state voltage drop and switching performance. GTOs over 800 V dc blocking capability has been demonstrated with a blocking layer thickness of 7 μm. The dc on-state voltage drops of a typical device at 25 and 300°C were 4.5 and 3.6 V, respectively, for a current density of 1000 A/cm². The devices can be reliably turned on and turned off under an anode current density of 5000 A/cm² without observable degradation     

Temperature behavior and annealing of insulated gate transistors subjected to localized lifetime control by proton implantation

Localized lifetime control by proton implantation can result in a considerable improvement in the trade-off between device turn-off time and forward voltage when compared with the unlocalized method of electron irradiation. After a proton dose of 3 × 10¹¹cm⁻² at 3.1 MeV implanted here into insulated gate transistors, turn-off time is reduced by more than an order of magnitude compared to unimplanted devices. When the implanted devices are operated as high voltage switches at a current of 152 A cm⁻² and at a forward blocking voltage of 400 V, the following increases are observed by increasing device operating temperatures from 20 to 150°C, (a) forward voltage: 2.5 V to 2.7 V; (b) turn-off time: 0.78 μs to 1.23 μs; (c) leakage current: 20 nA to 1 mA. The physical mechanisms responsible for the qualitative temperature dependences are identified: MOS channel resistance for forward voltage, carrier capture cross-section for turn-off time, and generation and diffusion components of leakage current. Since no catastrophic or unrecoverable behavior is observed, normal device operation within the tested temperature range is possible. Isothermal annealing curves of turn-off time measured after annealing, and corresponding to a few hours annealing time, reveal that a constant turn-off time is reached after about an hour. The constant value increases with temperature, but is still below the unimplanted value after 4 h at 525°C. The turn-off time was verified to be constant even after 24 h of annealing at 200°C. Lifetime control by proton implantation seems to be more thermally stable than that caused by electon irradiation.     

High-voltage thyristors and diodes made of neutron-irradiated silicon

Neutron irradiation is a way to produce homogeneous and well-defined phosphorus doping in large diameter silicon crystals. One major application of such silicon is in the field of high-voltage power devices. The relation between resistivity and breakdown voltage determined for neutron-irradiated silicon is in good agreement with the theory of Sze and Gibbons. It is shown that by changing from conventionally doped silicon to neutron-irradiated silicon noticeable increases in the blocking capability of thyristors can be achieved without an increase of the n-base width. Production of power thyristors with blocking voltages of 3 to 5 kV has been successful, indicating that applications of neutron-irradiated silicon have already left the laboratory stage.     

Behavior of thyristors under transient conditions

The capability of gate-triggered thyristors to withstand steep wavefront, high-current pulses (i.e., di/dt capability) is a function of both junction temperature and frequency of operation. Localized internal heating occurs during turn-on and may lead to thermal runaway. The conditions required for this to occur have been determined by destructively testing many devices. The initial conducting area of a thyristor largely determines di/dt capability, which is not necessarily related to the size of the device but is a function of the design of the gate region. Gate drive is very important for determining the di/dt capability of a thyristor having a conventional gate design. Two devices which have been designed to increase the initial conducting area are discussed. One of these devices, if improperly designed, can lose its effectiveness with high gate drive. This characteristic can be studied by observing the reverse recovery current immediately following short forward current pulses.     

Power absorption capability of punch-through devices

The power absorption capability and high-current characteristics of silicon high-voltage punch-through structures were investigated. Impact ionization was observed in the devices using 100- and 75-ohmċcm base material. The transient power absorption capability of these structures was found to be less temperature-dependent than that of avalanche devices. With proper surface contouring, a power absorption capability of 48 kW/cm²at 10 µs was achieved at a junction temperature of 26°C and 38 kW at 200°C for devices made of 350-ohmċcm base material.     

Grid depth dependence of the characteristics of vertical channel field controlled thyristors

The effect of changing grid depth upon the characteristics of vertical channel field controlled thyristors has been experimentally studied. It is found that increasing the grid depth results in an exponential increase in the differential blocking gain, and a significant decrease in the turn-off time, with no detrimental effect upon the forward conduction characteristics.     

A simple method for achieving immunity to internal noise signals in thyristors

The work presents an alternative solution—with respect to the conventional cathode shunts or MOS-controlled emitter shorts—for achieving virtual immunity to the parasitic action of displacement currents in power thyristors. The simple design/technological approach developed, based upon the novel double-interdigitated or two interdigitation levels (TIL) concept with a coarse geometry, offers a fair balance between technological simplicity/cost effectiveness and overall device performance.

Based upon the presented design guidelines, two sets of gold-doped thyristors with different geometrical configurations and current/voltage-handling capabilities were produced. The measurements performed on both sets of TIL-type thyristors have shown that the devices possess an extremely high value of the maximum permissible critical rate of rise of the forward anode voltage (static dV/dt capability) even under open-gate conditions. Unlike the thyristors using conventional emitter shorts, the TIL-type devices possess a good static and dynamic turn-on/latching sensitivity and have low on-state losses at high anode current densities. The main implications of developed concepts for power thyristors design/technology are also outlined in this work.     

Annealing effects of NTD silicon for semiconductor power devices

The use of neutron transmutation doped (NTD) Si has become very important for processing high-voltage power devices. A simple annealing process is usually required to anneal the lattice defects caused by neutron irradiation. This is usually performed by the silicon supplier by annealing the silicon crystal at a low temperature (approximately 700°C). High-voltage power devices, however, require much higher processing diffusion temperature. In situ annealing of silicon, therefore, can occur during device processing. Furthermore, we demonstrate here that higher yield can be obtained using the in situ annealing. This improvement is due to the effectiveness and uniformity of annealing of thin wafers. The uniformity and blocking voltage distribution of high-voltage rectifiers and thyristors using crystal annealed and in situ wafer annealed NTD silicon are presented.     

Optimization of vertical 600 and 1500 V SOI-ESTs with low on-state voltages

For the first time, SOI-emitter switched thyristors (SOI-ESTs) with forward blocking capability of 600 and 1500 V have been fabricated. In these devices, the lateral turn-off transistor of the EST is separated dielectrically from the vertical thyristor using SIMOX Separation by Implanted Oxygen technology. By this method, the parasitic thyristor inherent to other ESTs is totally eliminated. The effects of parametrical variations on the output characteristics of the SOI-EST are discussed.     

The performance of X-band Gunn oscillators over the temperature range 30°C to 120°C

The output power of a cavity-controlled Gunn oscillator has been measured at a frequency of 10 GHz, as a function of bias voltage over the temperature range 30°C to 120°C. The measurements were made using 500-ns pulses to avoid significant changes in-device temperature during a pulse, and at a duty cycle of 20:1 to maintain a low mean input power. The device was operated in a coaxial cavity made of invar, and temperature control effected by operating the cavity in an oven. It has been found that ranges of bias voltages exist over which power output and efficiency is extremely sensitive to temperature changes, and that, depending on the particular conditions, power output can either increase or decrease. The effect of increasing temperature has also been found to reduce the bias voltage required to obtain maximum power output and efficiency.     

700-V asymmetrical 4H-SiC gate turn-off thyristors (GTO's)

Silicon Carbide (4H-SiC), asymmetrical gate turn-off thyristors (GTO's) were fabricated and tested with respect to forward voltage drop (V_F), forward blocking voltage, and turn-off characteristics. Devices were tested from room temperature to 350°C in the dc mode. Forward blocking voltages ranged from 600-800 V at room temperature for the devices tested. V_F of a typical device at 350°C was 4.8 V at a current density of 500 A/cm². Turn-off time was less than 1 μs. Although no beveling or advanced edge termination techniques were used, the blocking voltage represented approximately 50% of the theoretical value when tested in an air ambient. Also, four GTO cells were connected in parallel to demonstrate 600-V, 1.4 A (800 A/cm ²) performance     

A temperature-stable bandpass filter using dielectric resonators

A microstrip bandpass filter has been built with dielectric resonators made of a temperature-compensated material. Its unloaded Q at X band is calculated to be greater than 1200, a four-fold improvement upon conventional printed-on devices. With additional temperature compensation, a temperature coefficient less than 5 ppm/°C is shown to be feasible.