Abstarct: In this research a new structure for SiGe HBT transistor has been designed and simulated. This structure has an extra pin as a Gate that changes the current gain. By applying a voltage to the Gate, the baxse width is controlled. By decreasing the baxse width, the carrier recombination rate is reduced and the emitted electrons from Emitter have higer chance to achieve to the Collector. This gives the ability to control the transistor's current gain. Adding this pin will have two approaches, one is to improve the current gain of the transistor by applying a constant voltage to this pin and the other is to modify the characteristics of the transistor such as the current gain that occurs by modulating the baxse width by applying a range of voltages from 0 to 5 volts. This 4-pin hybrid component can be used in applications such as mixers and gain controls (VGA) with a good performance. This transistor is investigated via extensive simulations. All simulations in this work have been performed using Silvaco software showing current gain variations up to 15 times. The current gain of the transistor without any Gate voltages is about 50, which increases to 750 by changing the Gate voltage from 0 to 5 volts. In the following, the effect of altring the items such as location and dimensions of the Gate plate and other parameters such as the Gate oxide permittivity and its thickness is investigated. Finally, other parameters of the transistor such as breakdown voltage and cut-off frequency had been extracted. The reason for using the HBT (SiGe) transistor is its better frequency characteristics compared to the BJT, which makes the final transistor faster and suitable for RF applications. Also, dimansions and distribution of doping had been designed to achieve the proper current gain, transision frequency and reverce breakdown voltage. In this device, the breakdown voltage is 8 volts, the cut-off frequency is 11 GHz and the Early voltage is 30 volts.