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This thesis presents 3-D simulation of Single Event Upset (SEU) in IBM 5HP and 8HP heterojunction bipolar transistors (HBT). SiGe HBT has been attractive for space applications because of its high performance. SiGe HBTs were found robust to displacement damages and total ionizing dose radiation. But recent simulations and microbeam experiments show that SiGe HBTs are prone to single event effects. The simulated data are compared with the charge collection results from the microbeam experiments with adjustment of recombination parameters, fitting to microbeam data was, achieved on some HBTs.In general, charge collection was maximum for a strike at the emitter center and reduced for strikes outside deep trench isolation. The positional dependency of charge collection is studied, particularly regarding the drift and diffusion components. The microbeam strikes are shallow in nature due to overlayer. All charges deposited inside deep trench are col- lected, making 3-D simulation the only viable way of studying charge collection in SiGe HBTs at present. The CS junction plays a major role in charge collection. One of the obvious way of radiation hardening is removal of the CS junction. This was done by constructing the SiGe HBT on a buried oxide i.e. the intrinsic part of the device is identical to the bulk HBT and CS junction is replaced by the buried oxide. Various simulations were performed for different Si thickness above the buried oxide. It was found that the thickness variation did not have any impact on the charge collection. It was found that the charge collection is much reduced in the SOI HBT. The charge collection in SOI HBT is compared with the bulk HBT. The comparison shows that the charge collected by the collector and substrate are much smaller in SOI than bulk HBTs. But the charge collected by the emitter and base are identical in SOI and bulk HBT. This type of radiation hardening is called as Radiation Hardening By Process (RHBP), as changes in fabrication process, are made.Another type of radiation hardening is Radiation Hardening By Design (RHBD), where the changes are made in the layout only. In this work we introduce an extra shared dummy CS junction outside the deep trench. This dummy CS junction reduces the diffusive charge collected by the HBT. Design rules require a minimum distance between deep trenches of adjacent devices. The dummy collector can be filled in these space between devices. The charge collection between the bulk and dummy collector device are studied. Also the charge collection characteristic when the devices are introduced into a HBT array is studied. Charge collection in a regular 2×2 and 3×3 HBT arrays are studied. The charge collection is compared between the regular and hardened HBT arrays. It has been found that the dummy CS junction reduces the diffusive charge collection by a considerable amount thereby preventing simultaneous charge collection in multiple devices in a HBT array. The amount of charge collected by the struck device in a HBT array decreases with increasing number of adjacent devices due to the sharing of charge collection. This decrease in charge collection by the struck device becomes very gradual eventually.