Abstract
A Heterojunction Phototransitor (HPT) is an attractive high-speed photodetector for optical communications, as it can provide high gain and high power. It can also be used to generate millimetre-wave signals using optical heterodyne for radio astronomy and wireless broad-band communications. Edge illuminated 2-terminal HPTs (2T-HPTs) integrated with an optical waveguide are designed and characterised. A small signal model is developed and it simulates the 2T-HPT frequency responses with good accuracy and supplements the measurement to determine its gain and cut-off frequency. This overcomes the requirement to measure the input photocurrent to the 2T-HPT using an exact replica of its PIN structure. The high frequency responses for the longer devices, which give higher saturation current, are limited by the loss of the transmission line. This high loss is also observed by the 2-port characterisation of the back-to-back configured 2T-HPT and gives a trade-off between the cut-off frequency and saturation current in the design. Additional analysis shows that the cut-off frequency is also limited by the large device capacitances. Using scaling of the model, the cut-off frequency is shown to increase to 59.4GHz with a gain of 39dB for a 1mum x 10mum 2T-HPT. The above result shows that the 2T-HPT could be best utilised in a periodic structure (P-TWHPT), with small device area sections connected by short transmission lines. This increases the saturation current level without reducing the cut-off frequency and gain. A P-TWHPT model is developed to simulate and analyse its frequency response. It is shown that the effects of phase matching and reverse input termination are not critical for 2T-HPT structure, as its 3dB bandwidth is limited by the base discharge time constant. A four 5mum x 10mum 2T-HPT section P-TWHPT is simulated with the phase mismatch and without reverse termination. It has a gain of 49dB and a cut-off frequency of 67.5GHz.