Determining Transition Frequency f_T

Transition frequency f_T has become the most popular figure of merit for RF semiconductors. It is the frequency at which the short-circuited-output current gain of a common-emitter (or common-source) transistor drops to unity, i.e. there is no more current gain. The significance of f_T is that it is a measure of certain internal transistor parameters which impact high-frequency performance such as gain and noise. In particular, low-noise performance improves with higher f_T.

f_T is not measured directly but is extrapolated, and that too, using h-parameters that are derived from S-parameters. This is because short-circuits may cause transistors to self-oscillate, thus interfering with the measurement. Moreover, true short-circuits are hard to realize at high frequencies. f_T is determined from S-parameters as follows:

1. Calculate h₂₁ from S-parameters using the equation: h₂₁=-2*S₂₁/[(1-S₁₁)(1+S₁₁)+S₁₂*S₂₁].

2. Plot the log graph of h₂₁. Find the constant slope region near to unity current gain. Choose a test frequency f_test anywhere along the constant slope.

3. Calculate f_T using the formula f_T=|h₂₁|*f_test. This test frequency f_test can be used to test subsequent units, such as in a production environment.

Teoh Chin Soon

DreamCatcher Technical Training

Oct 2009