Study On Characteristics Of Terahertz Band GaN Schottky Barrier IMPATT Diode

As an emerging technology, terahertz technology has attractive applications in many areas. The first problem to solve in terahertz technology is to find terahertz radiation source. IMPATT diode is the most powerful among all the terahertz radiation sources. IMPATT diodes based on Si and Ga As can't output power high enough in terahertz band due to the limitation of material properties. GaN which has properties of wide energy gap, high electron saturation velocity, high breakdown electric field and high-temperature resistance, is suitable for terahertz wave band high power IMPATT diode. However, crystal quality of GaN is not satisfied and has many defects due to lattice and thermal mismatches in heteroepitaxy. Moreover, high concentration p type doping of GaN is almost impossible at present, so it is difficult to form fine p type ohmic contact, and the resulting contact resistance will cancel negative resistance effect. Due to the above two reasons, IMPATT diode based on GaN is not implemented, and still in the stage of simulation research. Previous researches focus on pn junction type IMPATT diode based on GaN, this paper proposes a GaN schottky barrier IMPATT diode with an avalanche termination layer. High doping concentration p type ohmic contact is avoided by substituting pn junction with schottky barrier contact. Metals have higher thermal conductivity and this is in favour of heat dissipation. Moreover, series resistance will be reduced due to the low resistivity of metals. In order to form schottky contact, doping concentration of avalanche region is relatively low. Avalanche termination layer is a thin layer with high doping concentration. Electric field decreases rapidly in avalanche termination layer and avalanche multiplication is terminated here. Compared with IMPATT diode without avalanche termination layer, avalanche breakdown occurs at a lower voltage in this IMPATT diode, thus resulting in a lower DC power and less generation of heat.Static characteristics of IMPATT diode such as avalanche breakdown voltage?profile of electric field?impact generation rate?electron drift velocity are simulated using Atlas of Silvaco TCAD. The influences of doping concentration and length of avalanche region and avalanche termination layer on static properties of IMPATT diode is researched. The research reveals that when the doping concentration of avalanche region and avalanche termination layer increase, the avalanche breakdown voltage deceases, and the doping concentration of avalanche terminator layer has much more influence than that of avalanche region does. When the lengths of avalanche region and avalanche termination layer decrease, the avalanche breakdown voltage increases, and the length of avalanche terminator layer has much more influence than that of avalanche region does. The higher the doping concentration of avalanche terminator layer, the more the decrease in electric field through avalanche terminator layer; The higher the doping concentration of avalanche region, the larger the decrease rate of electric field in drift region. An increase in the length of avalanche termination layer will cause an extra decrease in electric field through avalanche termination layer, so the electric field in drift region has a reduction. When the length of avalanche region increases, electric fields both in avalanche region and avalanche termination layer decrease simultaneously. Impact generation rate and electron drift velocity are influenced by electric field. If electric field in avalanche termination layer doesn't have an enough reduction, then electric field in drift region is so high that impact generation rate in drift region becomes too large, thus resulting in an extension of avalanche region to drift region. If electric field in drift region is not large enough, electron drift velocity doesn't saturate in drift region. An appropriate set of device parameters which lays foundation for the successive dynamic analysis is determined based on the principles that DC power should not be too large, electron drift velocity in drift region should saturate and avalanche region should not extend to drift region.Dynamic characteristics of IMPATT diode are simulated using Mixedmode of Silvaco TCAD. Bias circuit for IMPATT diode is voltage driven. Parameters of bias circuit are determined according to parameters of IMPATT diode so that the circuit matches the device. Driven by the bias circuit, the IMPATT diode generates an oscillation of 160 GHz. The generated ac current lags behind the ac voltage of about 93 °, and the ac negative resistance is about-5.7?. Variations of potential?electric field?ionization coefficient?electron concentration and hole concentration over time during one cycle are studied. In avalanche multiplication stage, electric field and ionization coefficient are relatively high, the electron concentration in avalanche region increases rapidly, the residual electrons at the end of drift region are gradually run into ohmic contact region. In drift stage, electric field and ionization coefficient are relatively low, avalanche multiplication almost stops, electron concentration in avalanche region decrease rapidly, electron pulse drift in saturation velocity towards ohmic contact region. The influences of dc bias voltage, amplitude of ac voltage, and operation frequency on RF output power and conversion efficiency are studied. When the amplitude of ac voltage increases, the performance of device has a remarkable enhancement. When the dc bias voltage decreases, the performance of device has a slow increase. The performance of the device deteriorates rapidly when the operation frequency increases.