The Temperature Dependent Current Gain of Heterojunction Bipolar Transistors

A study of the thermal effects on the current gain in different heterojunction bipolar transistor structures is presented. Calculations show that the conventional heterojunction barrier in a graded base transistor is insufficient above room temperature and that a parabolically graded heterojunction is essential for the current gain at elevated temperatures. AlGaAs/GaAs HBTs have been fabricated and characterized up to an ambient temperature of 470 K. The experimental results confirm the theory. An extension of the Gummel-Poon transistor model to include the temperature dependent current gain is proposed. The improvement in correspondence with the measured DC-characteristics over the original model is shown to be substantial.