Temperature dependence of electron effective saturation velocity in GaAs is determined from do and RF characteristics of InGaP/GaAs heterojunction bipolar transistor (HBT) under different substrate temperatures (from -40 degreesC to 200 degreesC). Two approaches were utilized to extract the electron effective saturation velocity in this work. The first approach is to evaluate Kirk effect both in do current gain and cutoff frequency roll-off. The second approach is to analyze device's cutoff frequency with consideration of temperature effects in collector transit time. The deduced electron effective saturation velocity from two approaches demonstrated the similar temperature dependence. The extracted values of electron effective saturation velocity from considering collector transit time for temperatures of 200, 25, and -40 degreesC are about 6.43 x 10(6), 1.29 x 10(7), and 1.47 x 10(7)cm/sec, respectively. (C) 2004 Elsevier Ltd. All rights reserved.