Sony STR-DA7100ES Technical Background page 12

Semiconductor Field Effect Transistors (MOS FETs), four per channel, for the
STR-DA7100ES. The result is another measure of signal integrity.
A detail of the power amplifier circuit board of the STR-DA7100ES. The
twelve black chips are the MOS FET output transistors for three
channels (4 MOS FETs per channel).
Sony output transistors are remarkable for their internal wiring and
bonding, their unique packaging and the way they're configured on the power
amplifier circuit board. Consider the packaging.
Conventional output transistors have two distinct sides. The bottom is
designed to radiate heat, and is usually attached to a metal heat sink. The top is
fashioned from molded plastic, from which heat cannot escape. No matter how
much cooling may be available at the top, the plastic package is too thick to
conduct heat away from the transistor.
On the left, the MOS FET output transistor used in Sony's STR-
DA9000ES was 2 to 3 mm thick. At right, the MOS FETs used in the
DA7100ES is 1 mm thick or less, for far better dissipation of heat.
ES Receivers v1.0
MOS FET Thermal Packaging
Page 12