Ge semiconductor devices for cryogenic power electronics - IV

We are taking the initial steps in developing power semiconductor devices based on the silicon-germanium (SiGe) materials system. The applications and motivation are similar to those for our development of Ge devices described elsewhere [1], namely spacecraft for cold environments as well as commercial, industrial, and defense systems that incorporate cryogenics. The SiGe materials system has proved its benefits in devices for telecommunications. It also has valuable features for power electronics and cryogenic operation. Our objective is to take advantage of the features of SiGe in combination with those of Si and Ge to develop diodes and transistors for cryogenic power operation. These features include: Si: an extensive technology base, high breakdown voltage, an excellent grown oxide. Ge: low p-n junction forward voltage, low freeze-out temperature, high mobility at low temperature. SiGe: bandgap engineering, selective placement, a developing technology base and compatibility with Si processing. The first device that we are working to develop for cryogenic power is the heterojunction bipolar transistor (HBT). These follow a standard design, using SiGe for the base region to maintain high gain over a wide temperature range from room temperature to deep cryogenic temperatures. However, we are designing the structure for high current and voltage. Initial results are encouraging, although falling short of our goal. Figure 1 is an example of the characteristics of one of our devices in liquid nitrogen. It exhibits adequate current and voltage capability for a prototype, but its current gain is only slightly larger than 1. However, the current gain increases upon cooling from room to liquid-nitrogen temperature, which is an important outcome. The charge carriers must be placed in a channel region, separated from the ionized dopants. The source of the carriers, i.e. the supply layer, must be highly doped in order to prevent carrier freeze-out at low temperatures. The first condition is necessary to minimize ionized-impurity scattering. The second is necessary because highly doped (>10 17 approximately) Si, Ge or SiGe does not freeze out. Layer design usually starts with selecting the compositions of the active layers and of the virtual substrates, which define the band offsets. Figure 1: Bipolar characteristics at liquid-nitrogen temperature, the looping and droop at high current and voltage are evidence of heating at high power (∼10 W). Vert = 20 mA/div, horiz = 5 V/div, ΔI B = 20 mA/step. Using more appropriate materials and designs we expect to improve the characteristics considerably In conjunction with the HBT work we are also developing MIS structures. Successful development of bipolar and MIS structures could then form the basis for fabrication of more complex power devices for cryogenic operation, such as the insulated-gate bipolar transistor (IGBT) and MOS-controlled thyristor (MCT).