According to the Maricopa County Office of the Medical Examiner, the former child star -- best known for his time on the sitcom "Alf" -- died from heat exposure in the setting of hepatic cirrhosis.

Basically, the hot Arizona sun led to BG's death ... but, he also had scarring of the liver caused by long-term damage to the organ. It's not clear at this time what caused the cirrhosis.

TMZ broke the story ... Benji's sister, Rebecca, told us her brother was found dead in a car in a bank parking lot in Peoria, Arizona back in June.

We're told people close to BG believe he went to the bank to deposit residual checks when he fell asleep in his car and died. Benji was with his service dog, Hans, who also died.

His sis says Benji suffered from depression, bipolar disorder and a sleep disorder that would keep him awake for days at a time.

Benji played Brian Tanner on 101 episodes of "Alf" ... later switching careers to serve in the United States Navy. He received an honorary medical discharge after 2 years in the USN.

He was 46.